INTRODUCTION TO REFINERY PROCESS:
In refinery, for processing of Crude oil, the first step is crude distillation. Major
fractions of Crude oil are hydrocarbons, which vary in molecular weights. Their boiling
points are different. Due to the difference in boiling points Crude oil is separated in to
fractions. Vaporization of the components starts by heating in furnace pressure drop in
the lines from furnace to tower causes additional vaporization or flashing. Offer this step
flashed vapors are separated into products having specific boiling characteristics.
For narrow boiling fraction, the tower is provided with reflux and fractionating
Fractionation is more effective at low pressures than at high pressures because of the layer difference in the vapor pressure of the Components at a low pressure. This probably accounts for the relatively few plates that are used in many vacuum plants, although the wide tray spacing. This is normally used, accounts in art for the lesser number of trays.
The temperature difference between the top of the tower and the flash zone play an important role in fraction distillation processing.
The low boiling liquids from the top of the tower pass counter to vapors
containing higher boiling components from below. The Interchange, which takes place,
results in the higher components from the liquid reflux being vaporized and the heavier or
higher boiling Components from the vapors condensed. Repeated action of this nature at
each tray results in the fractions of the tower feed so that products of different boiling
characteristics can be withdrawn from successive fractionations of the tower.
Requirements for the up flow products from the fractionating tower are such that
the perfect separation between products is not necessary. The number of trays and the amount of reflux used are designed to provide the separation required for commercial utilization of products. Increasing the number of trays or increasing the amount of reflux improves the separation between the products.
The desalted crude feedstock is preheated using recovered process heat. The feedstock then flows to a direct-fired crude charge heater where it is fed into the vertical distillation column just above the bottom, at pressures slightly above atmospheric and at temperatures ranging from 650° to 700° F (above these temperatures undesirable thermal cracking may occur). All but the heaviest fractions flash into vapors. As the hot vapors rise in the tower, its temperature is reduced. Heavy fuel oil or asphalt residue is taken from the bottom. At successively higher points on the tower, the various major products including lubricating oil, heating oil, kerosene, gasoline, and uncondensed gases (which condense at lower temperatures) are drawn off.
The fractionating tower, a steel cylinder about 120 feet high, contains horizontal steel
trays for separating and collecting the liquids. At each tray, vapors from below enter
perforations under the bubble caps. The latter permit the vapors to bubble through the
liquid on the tray, causing some condensation at the temperature of that tray. An overflow
pipe drains the condensed liquids from each tray back to the tray below, where the higher
temperature causes re-evaporation. The evaporation, condensing, and scrubbing operation
are repeated many times until the desired degree of product purity is reached. Then, side
streams from certain trays are taken off to obtain the desired fractions. Products ranging
from uncondensed fixed gases at the top to heavy fuel oils at the bottom can be taken
continuously from a fractionating tower. Steam is often used in towers to lower the vapor
pressure and create a partial vacuum. The distillation process separates the major
constituents of crude oil into so-called straight-run products. Sometimes crude oil is
“topped” by distilling off only the lighter fractions, leaving a heavy residue that is often
distilled further under high vacuum.
In order to further distill the residuum or topped crude from the atmospheric
tower at higher temperatures, reduced pressure is required to prevent thermal cracking.
The process takes place in one or more vacuum distillation towers. The principles of
vacuum distillation resemble those of fractional distillation except that larger diameter
columns are used to maintain comparable vapor velocities at the reduced pressures. The
equipment is also similar. The internal designs of some vacuum towers are different from atmospheric towers in that random packing and demister pads are used instead of trays. A typical first-phase vacuum tower may produce gas oils, lubricating-oil base stocks, and heavy residual for propane de asphalting. A second-phase tower operating at lower vacuum may distill surplus residuum from the atmospheric tower, which is not used for lube-stock processing, and surplus residuum from the first vacuum tower not used for deasphalting. Vacuum towers are typically used to separate catalytic cracking feedstock from surplus residuum.
Other Distillation Towers
Within refineries there are numerous other, smaller distillation towers called
columns, designed to separate specific and unique products. Columns all work on the same principles as the towers described above. For example, a depropanizer is a small column designed to separate propane and lighter gases from butane and heavier components. Another larger column is used to separate ethyl benzene and xylene. Small “bubble” towers called strippers use steam to remove trace amounts of light products from heavier product streams.
Even though these are closed processes, heaters and exchangers in the
atmospheric and vacuum distillation units could provide a source of ignition, and the potential for a fire exists should a leak or release occur.
An excursion in pressure, temperature, or liquid levels may occur if automatic control devices fail. Control of temperature, pressure, and reflux within operating parameters is needed to prevent thermal cracking within the distillation towers. Relief systems should be provided for overpressure and operations monitored to prevent crude from entering the reformer charge.
Crude feedstock may contain appreciable amounts of water in suspension, which can
separate during startup and, along with water remaining in the tower from steam purging,
settle in the bottom of the tower. This water can be heated to the boiling point and create
an instantaneous vaporization explosion upon contact with the oil in the unit.
Atmospheric and vacuum distillations are closed processes and exposures are expected to
be minimal. When sour (high-sulfur) crudes are processed, there is potential for exposure
to hydrogen sulfide in the preheat exchanger and furnace, tower flash zone and overhead
system, vacuum furnace and tower, and bottoms exchanger. Hydrogen chloride may be
present in the preheat exchanger, tower top zones, and overheads. Wastewater may
contain water-soluble sulfides in high concentrations and other water-soluble compounds
such as ammonia, chlorides, phenol, mercaptans etc., depending upon the crude feedstock
and the treatment chemicals.
The sections of the process susceptible to corrosion include preheat exchanger (HCl and
H2S), preheat furnace and bottoms exchanger (H2S and sulfur compounds), atmospheric tower and vacuum furnace (H2S, sulfur compounds, and organic acids), vacuum tower (H2S and organic acids), and overhead (H2S, HCl, and water). Where sour crudes are processed, severe corrosion can occur in furnace tubing and in both atmospheric and vacuum towers where metal temperatures exceed 450° F. Wet H2S also will cause cracks in steel. When processing high-nitrogen crudes, nitrogen oxides can form in the flue gases of furnaces. Nitrogen oxides are corrosive to steel when cooled to low temperatures in the presence of water.
The reduced crude or the atmospheric bottoms is pumped by P-101 and is sent to
the heat exchanger E-075 to heat the reduced crude oil by cooling down the HVGO coming from the vacuum distillation tower V-109. Then it is sent to the vacuum furnace F-061 which is fired tube furnace.
In furnace feed is heated to its boiling point. The partially flashed reduced crude
is then conveyed to the flash zone of vacuum tower V-109, which operates at a very low
pressure to avoid cracking process and thus depositing coke. The vacuum unit’s low
pressure is obtained by three-stage ejector system, J-09 A, B, C. The three ejectors can be
visualized as three pumps in series with the discharge from the last pump being to the
atmosphere. In order to minimize the size of ejectors needed and condense as much
hydrocarbon vapors, three inter-condensers are like wise provided. The non-condensable
gases are discharged to the atmosphere through vent. Due to lower pressure in vacuum
tower the residence time of reduced crude in vacuum furnace is reduced.
The first side stream product, Gas oil is pumped by centrifugal pumps, P-8 to three separate places
1- A part of this stream is returned directly to the tower just below the total draw-off
tray as top pump-back reflux.
2- The rest of this stream is cooled by a water cooler, E-13, from where the flow is
again split and
(a) Part is sent to the top of the tower at the plate, having the same
temperature as this stream
(b) The rest is sent to the storage tank.
The second side stream product LVGO is pumped by centrifugal pump P-9, through a water cooler, E-14 to storage tank.
The third stream product, HVGO, is pumped by P-11. Part of this stream product through a shell and tube heat exchanger, E-5, and a water cooler E-16 to storage tank, and the rest from P-11 is returned as a flash zone reflux to vacuum tower, V-
109 to wash the oil of asphaltic type products. This stream including the washings is pumped by centrifugal pump P-14 and is mixed with the vacuum furnace feed, Reduced crude from atmospheric tower and is thereby recycled to the flash zone of the vacuum tower, V-109. Vacuum bottoms, from vacuum tower are pumped by centrifugal pump, P-15 through a shell and tube heat exchanger, E-6 and a water cooler, E-17 to butane De asphalting plant.
Butane De-asphalting unit also known as ROSE process, which is commonly used in refineries for the separation of DAO from asphalts.
Vacuum residue is cooled down in cooler C-112 and pumped to the asphaltene separator A-079. The feed pump P-101 boosts the vacuum residue to a sufficiently high pressure to feed the asphaltene separator. The feed feeds the top distributor of the asphaltene separator. Solvent required for the extraction enters the bottom distributor of the asphaltene separator, providing countercurrent flow. Asphaltenes are insoluble in the extraction solvent at the extraction conditions and therefore drop out of solution and exit through the bottom of the asphaltene separator. Slightly less than one volume of dissolved solvent per volume of asphaltenes exits as an asphaltene/solvent solution. This asphaltene/solvent solution flows to the asphaltene stripping section where the dissolved solvent is stripped from the asphaltene product.
The lighter DAO is soluble in the solvent at the extraction condition. This DAO/solvent
solution, containing the majority of the solvent, exits the top of the asphaltene separator
as rich solvent. Operating temperature, solvent composition, solvent/oil ratio, and, to a
lesser extent, pressure in the asphaltene separator affect product yield and quality. Since
certain primary process parameters (i.e., solvent/oil ratio, solvent composition, and
operating pressure) are fixed or set at relatively constant values, the asphaltene separator
operating temperature is used as the primary performance control variable. The DAO
yield is effectively controlled by the asphaltene separator operating temperature. Higher
operating temperatures result in less DAO product extracted overhead. Lower operating
temperatures produce more DAO, but of a poorer quality. The asphaltene separator
overhead DAO/solvent solution (i.e., rich solvent) is heated above the critical temperature
of the pure solvent by exchanging heat with steam in the exchanger E-103. Increasing the
temperature of the solvent above its critical temperature takes advantage of the solvent’s
low-density properties in this region. As the temperature increases above the critical
point, the density of the solvent significantly decreases to values approaching those of
dense gases. At this increased temperature, the DAO is virtually insoluble in the solvent,
and a phase separation occurs. Approximately 90 percent of the solvent from the rich
solvent stream is recovered by this supercritical phase separation. The DAO separator D-
120 operating conditions are set to achieve the density difference needed for good
separation. Pressure is controlled by adjusting recycle solvent flow to the high pressure
system from the recycle solvent pump. Temperature is controlled by adjusting the steam
flow to the DAO separator preheater. The recovered solvent leaves the DAO separator as
lean solvent, also known as circulating solvent. Heat is recovered from the lean solvent in
the cooler C-113. The solvent is then circulated back through the solvent cooler for
temperature control of the asphaltene separator overhead. Sufficient excess duty is
available to provide cooling for swings in feed temperature. The recycle solvent from the
recycle solvent pump combines with the large volume of circulating solvent from the
solvent cooler. The combined flow enters the solvent circulation pump CU-110, which
boosts the pressure back to the asphaltene separator operating pressure, thus making up
for the pressure drop in the circulating solvent loop. Flow valves downstream of the
pump provide adequate control for splitting solvent between the top and bottom
distributors of the asphaltene separator. Before entering the DAO stripper St-057, the
DAO solution is heated in the DAO stripper heater. The heater provides sufficient heat to
the system to maintain the recommended operating temperature in the DAO stripper.
Heat is provided by steam. The DAO is contacted with superheated steam in the stripper
to strip any remaining solvent to low levels in the product stream. Steam reduces the
partial pressure of the solvent in the stripper, thus allowing more solvent to vaporize from
the DAO liquid. The steam temperature should be at or above the recommended
operating temperature of the stripper. Colder steam can cool the DAO and impair
stripping performance. Wet steam can cause foaming and operational problems. The
asphaltene is then fed to the asphaltene stripper St-111 on liquid-level control from the
asphaltene extractor A-079. Before entering the asphaltene stripper, the asphaltenes flow
through the asphaltene stripper heater E-102. The heater provides sufficient heat to the
system to maintain the recommended operating temperature in the asphaltene stripper.
Heat is provided by steam. The asphaltene is contacted with superheated steam in the
stripper to strip the remaining solvent to low levels in the product stream. Steam reduces
the partial pressure of the solvent in the stripper, thus allowing more solvent to vaporize
from the asphaltene liquid. The steam temperature should be at or above the
recommended operating temperature of the stripper. Colder steam can cool the asphaltene
product and impair stripping performance. Wet steam can cause foaming and operability
problems. The asphaltene stripper overhead solvent vapor and steam flow through the
stripper condenser C-115, where the solvent and steam are condensed. The condensed
solvent and water are separated in the LP solvent drum. The water is removed on level
control and sent to the sour water system. The condensed solvent is pumped by the LP
solvent pump to the solvent surge drum before being recycled to the process. The
asphaltene product exits the stripper bottom and is pumped on level control by the
asphaltene pump. Positive displacement P-104 pumps are usually required to handle the
highly viscous material. The operating temperature maintains the asphaltenes at a
viscosity suitable for pumping. Colder temperatures may cause pumping and handling
problems. This asphalt is sent to mixer M-031 where it is mixed with water, kerosene to
produce paving grade 60/70 asphalt. Then this hot asphalt is sent to the pelletizing
section. Hot liquid asphalt is pumped to a pelletizer vessel Pt-50 at the optimal
temperature required for successful pelletization. Liquid asphaltenes are converted to
droplets in the vapor space of the pelletizer vessel using a proprietary high capacity feed
distributor. The surface hardened pellets fall in to a water bath and on to a vibrating
screen where the pellets are dewatered. The pellets may be transported to a silo or pit or
other loading facilities by a conveyor. The pellets are near spherical with an expected size distribution between 1 and 3 mm and have good grindability, storage and transportation characteristics as indicated by the high Hargrove Grindability Index (HGI), storage test temperature and low friability. The high angle of repose provides high capacity on conveyors. The small amount of residual moisture on the pellets helps to minimize dust formation during transport.
Design of the HEAT EXCHANGER E-075
A heat exchanger is a heat transfer device that is used for transfer of internal thermal energy
between two or more fluids available at different temperatures. In most of the exchangers the fluids are separated by a heat transfer surface and ideally they don’t mix.
Heat exchangers are used in process, food, power, petroleum, transportation, air
conditioning, refrigeration, cryogenics, heat recovery, alternate fuels, and other industries.
Common example of heat exchangers familiar to us in day to day use are automobile
radiators, condensers, evaporators, air pre heaters, cooling towers, oil coolers, furnaces gas ovens etc.
Classification Heat Exchanger:
In general industrial heat exchangers are classified according to there:
2) Transfer processes
3) Degrees of surface compactness
4) Flow arrangements
5) Pass arrangements
6) Phase of the process fluid
7) Heat transfer mechanism
Short description of classification of heat exchanger is
v Classification according to Construction
According to construction heat exchangers are:
1) Tubular heat exchanger (double pipe, shall and tube, coiled tube)
2) Plate heat exchanger (gas kited, spiral, plate coil, lamella)
3) Extended surface exchangers (tube fin, plate fin)
4) Regenerators (fixed matrix, rotary)
v Classification according to Transfer Process
These classifications are:
1) Indirect Contact (double pipe, shall and tube, coiled tube)
2) Direct contact (cooling towers)
v Classification according to Surface Compactness
A compact heat exchanger incorporates a heat transfer surface having a high area density,
which is the ratio of heat transfer area (A) to its volume (V) it is somewhat 700 m2/m3.
They can often achieve higher thermal effectiveness than shall and tube exchangers.(95%
vs. 60-80% for STHE) which makes them particularly useful in energy intensive industries.
v Classification according to Flow Arrangement
The basic flow arrangements in a heat exchanger are
1) Parallel flow
2) Counter flow
3) Cross flow
The choice of a particular flow arrangement is dependent upon the required exchanger effectiveness, fluid flow paths, packaging envelope, allow able thermal stress, temperature levels etc.
v Classification according to Pass Arrangement
A fluid is considered to have made one pass if it flows through a section of heat exchanger
through its full length once. There are either single pass or multi pass , in a multi pass
arrangement the fluid is reversed and flows through the flow length two or more times. The
multi pass arrangements are possible with compact shall and tube and plate exchangers.
v Classification according to Phase of Fluid
These classifications is made according to the phase of the fluid I-e gas-gas, liquid-liquid e.g. Gas-liquid etc.
v Classification according to Heat transfer Mechanisms
The basic heat transfer mechanism employed for heat transfer from one fluid to another are
1) Single phase convection, forced or free
2) Two phase convection (condensation or evaporation) by force or free convection
3) Combined convection or radiation.
Heat Exchanger Selection Criteria:
When selecting a heat exchanger for a given duty the following points must be considered
1) Material of construction
2) Operating pressure and temperature
3) Flow rates
4) Flow arrangements
5) Performance parameters–thermal effectiveness and pressure drops
6) Fouling tendencies
7) Types and phases of fluids
8) Maintenance, inspection, cleaning, extension, and repair possibilities
9) Overall economy
10) Fabrication technique
11) Intended applications
Shell and Tube Heat Exchanger:
The most commonly used heat exchanger is shell and tube type. It is the “work horse” of
industrial process heat transfer. These exchangers are being used in 90% of the process
industries now days.
Advantages of Shell and Tube Exchanger than other Exchangers
It is used for high heat transfer duties.
It occupies less space.
Its compactness is more.
Its maintenance is easy.
It can be fabricated with any type of material depend up fluid properties.
Furnace is a equipment in which chemical energy of fuel or electrical energy is converted into
heat which is then used to raise the temperature of material, called the burden or stock, placed with in the furnace.
Furnace is a device for generating the control heat with the objective of performing work.
A furnace is ‘an enclosed place in which heat is produced by the combustion of fuel, as for reducing ores or melting metals, for warming a house, for baking pottery, etc’. The range of operation and the condition under which these processes must be carried out cover a very wide field, and the types of furnaces are equally diverse. Furnaces may operate over a range of temperature from 300 ºF or there about, to upwards of 3000 ºF and they may be intermittent or continuous in operation.
- Furnace that is operating at temperature below 1200 ºF(650 ºC) are commonly called
- In ceramic industry furnaces are called ‘kilns’.
- In the petrochemical and CPI (Chemical Process Industries) furnaces may be termed as
‘heaters’, ‘kilns’,’ after burners’, ‘incinerators ‘or ‘destructors’.
Types of Furnaces:
The classification of the furnace can be done as
1) Based on process:
2) Based on method of heating:
Direct heating type
Indirect heating type
3) Based on type of fuel used:
Solid fuel fired furnace
Liquid fuel fired furnace
Gaseous fuel fired furnace
Multi fuel fired furnace
4) Based on draft control:
Selection Criteria Of Furnace
The selection of a furnace is based upon the following points.
- Kind of product to be fired.
- Quantity to be produced.
- Firing temperature.
- Kind of fuel
- Condition of load.
- Economics is the final factor.
Design of Furnace
When we talk about furnace design it means we want to find
- Size require for the given heat duty
- Number of tubes require
- Arrangement of tubes
- Flue gas temperature
- Amounts of fuel air steam
Methods for Designing:
There is no universally applicable method for the furnace design for all types of the furnaces
specially fuel used determent the design method applicable, there are four known design
Method of Lobo and Even
Method of Wilson Lobo and Hattel
The Orrak- Hudson equation
Wallenberg Simplified Method
Here we shall consider only method of lobo and Evans. This is a trial and error method which make use of the overall exchange factor () and a Stefan-Boltzmann type equation. It has a good theoretical basis and is used extensively in refinery furnace design work. It is also recommended for oil or gas fired boilers.
As in all trial and error solutions, a starting point must be assumed and checked. For orientation purposes, we shall estimate the number of tubes required in the radiant section by assuming an average flux (permissible average radiant rate) Btu/ (hr) (ft2 of circumferential tube area).
The principle of fuel economy is the same in all furnaces, and involve
- a) The complete combustion of fuel
- b) The rejection of the products of combustion at the lowest practicable temperature
- c) The reduction of external losses by means of suitable insulation
Calculating Exchange Factor:
Exchange factor depends upon the emissivity of the source. Source is combustion gases, normally CO2, H2O, CO, N2, H2, SO2 .There is great difference of emissivity of these gases. Diatomic gases such as N2, H2 has very less Emissivity. So they are neglected in calculation. Furnaces are operated with sufficient excess air to eliminate CO. small amount of Sulphur in the fuel is neglected. Therefore we consider only Emissivity of CO2 and H2O.
Instead of composition of CO2 and H2O we get partial pressure of CO2 and H2O from graph, where it is plotted against %age excess air.
So to find exchange factor we need concentration of CO2 and H2O which is obtained by partial pressure of both gases and flame length.
Stefan Boltzman Law:
Stephan Boltzman stated that total radiation from a perfect black body is proportional to the 4th power of the absolute temperature of the body.
E = T4
Where = 0.173*108 Btu/ hr ft2 0F.
So equation becomes Q/A = 0.173*108 T4
Same amount of heat is absorbed by another black body.
Denominator of equation is not simply area but it is effective heat transfer surface. For industrial furnaces normally heat receiver is composed of multiple tubes, which are disposed on walls, roof, or floor of the furnace or some time in the center of the furnace.
Common case is one in which tubes are located in single row in front of refractory wall. In order to find effective heat transfer surface of the tube we use method of Hottel. In this method it is assumed that heat source is parallel to the tube rows. It is also assumed that both planes are infinite so there end effects are eliminated. In this method tube rows are replaced by a plane and its area is called cold plane area, ACP.
ACP = No. of tubes* length of tubes*center to center distance between tubes
At factor is multiplied with cold plane area in order to get effective heat transfer surface.
At = α ACP
α is that factor it is called effectiveness factor obtained by the figure. Abscissa of the graph is ratio of center to center distance to outside diameter of tubes. Different curves showing different arrangements of tubes i.e. single and double arrangement.
Finally STEFAN BOLTZMAN law equation becomes
Q/A = 0.173 F [(T1 / 400) – (T2 / 400)] a ACP
Mean Beam Length:
In order to calculate exchange factor we need geometry of radiating body. Here our radiating body is combustion product of fuel. So the geometric shape of the gas mass must also be considered.
Mean beam length is defined as the average depth of the blanket of flue gas in all directions for each of the point on the bounding surface of the furnace and is use instead of cubical measure of volume.
Beam length for various geometric shapes has been determined by Hottel as shown in table.
Dimension Ratio of Rectangular Furnaces
1-1-1 to 1-1-3
1-2-1 to 1-2-4
1-1-4 to 1-1-α
1-2-5 to 1-2-8
Mean Beam Length
2/3 (Furnace volume in ft3)1/3
1.0* smallest dimension
1.3 * smallest dimension
Many factors have to be considered when selecting engineering materials, but for
chemical process plant the overriding consideration is usually the ability to resist
corrosion. The process designer will be responsible for recommending materials that will
be suitable for the process condition. He must also consider the requirements of the
mechanical design engineer; the materials selected must have sufficient strength and be
easily worked. The most economical material that satisfies both process and mechanical
requirement should be selected; this will be the material that gives the lowest cost over
the working life of the plant, allowing for maintenance and process safety. Other factors,
such as product contamination and process safety, must also be considered. The
mechanical properties, those are important in the selection of materials.
D-Difficult, Special Technique needed, S-Satisfactory
The tensile strength (tensile street) is a measure of the basic strength of material. It is the
maximum stress that the material will withstand, measured by a standard tensile test the older
name for this propeliy, which is more descriptive of the property, was Ultimate Tensile
The design stress for a material, the value used in any design calculation, is based on the
tensile strength, or on the yield or proof stress. The proof stress is the stress to cause a
specified permanent extension, usually 0.1 percent the tensile testing of materials is covered by BS 18:
Stiffness is the ability to resist bending and buckling. It is a function of the elastic modulus of the materials and the shape of the cross section of the member.
Toughness is associated with tensile strength, and is a measure of the material’s
resistance to crack propagation. The crystal structure of ductile materials such as steel, aluminum and copper, is such that they stop the propagation of a crack by local yielding at the crack tip. In other materials, such as the cast irons and glass, the structure is such that local yielding does not occur and the materials arc brittle. Brittle materials are weak in tension but strong in compression under compression any incipient cracks present are closed up. Various techniques have been developed to allow the use of brittle materials in situations where tensile stress would normally occur. For example, the use of priestesses’ concrete, and glass-fiber reinforced plastic in pressure vessels construction.
The surface hardness, as measured in standard test, is an indication of materials
ability to resist wear hardness testing is covered by British Standards: BS 240, 4175
427. This will be an important property if the equipment is being designed to handle
abrasive, lids, or liquids containing suspended solids which are likely to cause
Fatigue failure is likely to occur in equipment subject to cyclic loading; for example, rotating equipment, such as pumps and compression, and equipment subjected to pressure cyc1ing.
Creep is the gradual extension of a material under a steady tensile stress, over a prolonged Period of time. It is usually only important at high temperatures; for instance, with steam and gas turbine blades. For a few materials, notably lead, the rate of creep is significant at moderate temperatures. Lead will creep under its own weight at room temperature and lead linings must be supported of frequent intervals. The creep strength a materials is usually as the stress to cause rupture in 10000 hours, at the test temperature.
Effect of Temperature On The Mechanical Properties:
The tensile strength and elastic modulus of metals decrease with increasing temperature. For
example, the tensile strength of mild steel (low carbon, C< 0.25 percent) is 450 N/mm2 at 2SDC
falling to 210 at SOODC and the value of Yong’s modulus 200,000 n/mm2C falling to 150,000
n/mm2C if equipment is being designed to operate at high temperatures, materials that retain
their strength must be selected. The stainless steels are superior in this repeat to plain carbon
steels. Creep resistance will be important if the material is subjected to high stress at elevated
temperature, special alloys, such as inconel (international Nickel Co.,) are used for high
temperature equipment such as furnace tubes.
The conditions that cause corrosion can arise in a variety of ways for this brief discussion on
the selection of materials it is convenient to classify corrosion into the following categories:
1) General wastage of materials uniform corrosion.
2) Galvanic corrosion dissimilar metals in contact.
3) Pitting localized attack
4) Intergranular corrosion.
5) Stress corrosion
6) Erosion – corrosion
7) High temperature oxidation
8) Hydrogen embitterment
Metallic corrosion is essentially an electrochemical process. Four components are necessary to
set up an electrochemical cell:
1) Anode the corroding electrode.
2) Cathode the passive, non – corroding electrode.
3) The conducting medium the electrolyte the corroding fluid.
4) Completion of the electrical circuit through the material.
Cathode area can arise in many ways:
- ii) Corrosion products
iii) Inclusions in the metals, such as slag.
- iv) Less well aerated areas.
- v) Areas of differential concentration
- vi) Deferentially strained areas.
Selection for Corrosion Resistance:
In order to select the correct material of construction, the process environment to which the materials will be exposed must be clearly defined. Additional to the main corrosive chemicals, the following factors must be considered:
1) Temperature affects corrosion rate and mechanical properties.
4) Presence of trace impurities stress corrosion.
5) The amount of aeration differential oxidation cells.
6) Stream velocity and agitation erosion corrosion
7) Heat-transfer rates differential temperatures
The conditions that may arise during abnormal operation such as at start up and shout down, must be considered, in addition to normal, steady state operation
Commonly Used Materials of Construction:
The general mechanical properties, corrosion resistance, and typical areas of use of some of the materials commonly used in the construction of chemical plant are given are for a typical, representative, grade of the materials or alloy. The multitude of alloys used in chemical plant construction is knows by a variety of trade names, and code numbers designated in the various national standards. For the full detail of the properties and compositions of the grade available in a particular class of alloy, and the designated code numbers, reference should be made to the appropriate national code, to the various handbooks, or manufacturer’s literature.
Iron and Steel:
Low carbon steel (mild Steel) is the most commonly used engineering materials. It is cheap; is available in wide range of standard forms and sizes, and can be easily worked and welded. It has good tensile strength and ductility.
The carbon steels and iron are not resistance to corrosion, except in certain specific environment, such as concentrated sulfuric acid and the caustic alkalis. They are suitable for use with most organic solvents, except chlorinated solvents; but traces of corrosion products may cause discoloration.
Mild steel is susceptible to stress Corrosion cracking in certain environments. The corrosion
resistance of the low alloy steels (less than 5 percent of alloying elements), where the alloying
elements are added to improve the mechanical strength, and not for corrosion resistance, is not
significantly different from that of the plain carbon steels.
The stainless steels are the most frequently used corrosion resistance materials in the chemical
industry. To impart corrosion resistance the chromium content must be above 12 percent and
the higher the chromium content. The more resistance allows the corrosion in oxidizing
conditions. Nickel is added to improve the corrosion resistance in non – oxidizing
A wide range of stainless steel is available, with composition tailored to give the properties required for specific application. They can be divided into three broad classes according to their microstructure.
1) Ferritic: 1320 percent Cr, <0.1 percent C, with no nickel
2) Austenitic: 18-20 percent Cr, 7 percent Ni
3) Matensitic: 12-10 percent Cr, 0.2 to 0.4 percent C, upto 2 percent Ni
Structure desired for corrosion resistances, and it is these grades that are widely used in the chemical industry, Their Properties are:
Type 304 (the so-called 18/8 stainless steels): the most generally used stainless Steel it contains
the minimum Cr and Ni that give a stable austenitic structure. The carbon content is low
enough for heat treatment not to be normally needed with thin sections to prevent weld decay.
Typed 302L: low carbon version of type 304 (< 0.03 percent C) used for thicker welding section, where carbide precipitation would occur with type 304.
Type 321: a stabilized version of 304, established with titanium to prevent carbide precipitation
during welding. It has slightly higher strength than 304L, and is more suitable for high
Type 347: Stabilized with niobium.
Type 316: in this alloy, molybdenum is added to improve type the corrosion resistance In reducing condition, such as in dilute sulfuric acid, and, in particular, to solutions continuing chlorides.
Type 316L: a low carbon version of type 316, which should be specified if Welding or heat treatment is liable to cause carbide precipitation in type 316.
Type 309/310: alloy with a high chromium content, to give greater resistance to oxidation at
high temperature. Alloys with greater than 25 percent Cr and susceptible to embattlement due to sigma phase formation at temperatures above 5000C. Sigma phase in an intermetallic
compound, Fe Cr.
The austenitic stainless steels have greater than the plain carbon steels, particularly at elevated temperatures.
As was mentioned above, the austenitic stainless steel, unlike the plain carbon steels, do not become brittle at low temperatures. It should be noted that the thermal conductivity of stainless steel in significantly lower than that of mild steel.
Typical at 1000 C values are, type 304 (18/8) 16 W/mo C mild steel 60
W/moC Auslentic stainless steel arc non magnetic in the annealed condition.
Aluminum and Its AI Allows:
Pure aluminum lacks mechanical strength but has higher resistance to corrosion than its alloys.
The main structural alloys used are the Duralumin (Dural) range of aluminum copper alloys
(typical composition 4 percent Cu, with 0.5 percent Mg) which have a tensile strength
equivalent to that of mild steel. The pure metal can be used as a cladding on Dural plates, to
combine the corrosion resistance of the pure metal with the strength of the alloys. The
corrosion resistance of aluminum is due to the formation of thin oxide film (as with the
stainless steels). It is therefore, most suitable for use in strong oxidizing conditions. It is
attached by mineral acids, and by alkalis; but is suitable for concentrated nitric acid, greater
than 80 percent. It is widely used in the textile and food industries, where the mils steel would
cause contamination. It is also used for the storage and distribution of dematerialized water.
A wide range of paints and other organic coatings is used for the protection of mild steel
structures; paints are used mainly for protection from atmospheric corrosion. Special
chemically resistance paints have been developed for chemical process equipment. Chlorinated
rubber paints and epoxy based paints are used. In the application of paints and other coatings, good surface preparation is essential to ensure good adhesion of the joint film or coating.
Design for Corrosion Resistance:
The Life equipment subjected to corrosive environments can be increased by proper attention to design details. Equipment should be designed to drain freely and completely the internal surfaces should be smooth and free from crevasses corrosion products and other solids can accumulate. But joints should be use in preference-to lap joint. The use of dissimilar metals in contact should be avoided, or care taken to ensure that they are effectively insulated to avoid galvanic corrosion.
Fluid velocities and, turbulence should be high enough to avoid the deposition of solids, but not so high as to cause erosion corrosion.
Instrumentation and Control:
The full usefulness of refinery equipment can be attained only by the use of automatic control instruments, and the use of instruments has contributed much to the productivity of refinery work-men, Instrumentation, in the broad sense, serves two functions, Measurement and control. Instrumentation is the fundamental requisite to process control, whether that control is affected automatically, Semi automatically or manually.
Instrumentation generally and process control specifically are directly responsible for processes which are complex, integrated, and large, both physically and in terms of through put. Faster, more critical and more hazardous operations can be carried out because of suitable control equipment.
Various control instruments are: –
1- Top temperature controller
2- Recording Pyrometer at vaporizer and at inlet and outlet of pipe still.
3- Pressure at inlet and out let of pipe still, at charge pump, and in tower.
4- Rate of flow of crude oil
5- Level controls at bottom of tower and the side-draw plates.
In the control of a continuously operating system, certain variables must be fixed or the control
of others is almost impossible. Of prime importance are: –
1- The rate of feed
2- The Temperature feed
3- The top temperature of the tower.
With these variables fixed, the control of the others is well with in the capability of a competent operator. Even an expert operator is almost helpless, if these three variables are not fixed. A plant can be operated with these controls alone, but the operation will be much more efficient if other controls are utilized.
A flow sheet showing the suggested instrumentation and control of various units has been drawn and included in this chapter. The following indicators and recording instruments are selected for the measurement of temperature. Pressure and level and are placed at the appropriate process lines.
1- Temperature indicator
2- Pressure indicator
3- Level indicator
Basic Types of Process Measurement
Measurement of temperature plays an important role in petroleum processes, since, in addition to the obvious purpose of determining sensible heat, it offers an excellent indication of petroleum processes, since, in addition to the obvious purpose of determining sensible heat, it offers and excellent indication of material state and composition.
In fractionating columns, temperature measurement is found at feed points and overhead, bottom and intermediate draw-off points. Where intermediate refluxes exist temperature is measured at the reflux entry point.
Furnace is furnished with temperature measurement throughout. On the process side, such measurements are made at the inlet and outlet of convection tubes, the outlet of radiant tubes, and (where a tube bank is extremely long) at intermediate points. Each pass naturally requires its own set of measurement, except that inlet temperature is common to all passes. On the hot side measurements are made in radiant section, in the gases leaving the radiant section and entering the convection section and is the outlet flue gases.
Heat exchangers may have temperature measurement on the inlet and outlet of both the streams.
Most temperature measurements in petroleum processes are made by means of
thermocouples to facilitate bringing the measurements to a centralized location. Locally at the
equipment, bimetallic or filled system thermometers are used. For some transmitting applications filled system types are used.
Like a temperature, pressure is a valuable indication of material state and composition. Infect, these two measurements considered together are the primary evaluating devices of petroleum materials.
In fractionating columns, pressure measurement is of primary importance. Occasionally, the pressure difference between top and bottom may be made to indicate tower loading.
Furnace requires pressure measurement for process consideration and to serve as a
guide to the condition of tubes. This is particularly important if the heat addition represents severe cracking of the charge. Pressure measurements also serve cracking of the charge. Pressure measurements also serve to maintain a rough split through multi pass furnaces without individual flow control.
Pumps, compressors, and other process equipment associated with pressure changes in the process material are furnished with pressure-measuring devices. Thus, pressure measurement becomes and indication of energy increase or decrease, degree of agitation across mixing valves, operation of the equipment, etc.
Most pressure measurements in petroleum processes are by elastic elements devices, either direct connected for local use or transmission type to a centralized location.
The measurement most representative of continuous process operation and economics is flow. In petroleum processing, flow control predominates over all other types, hence the importance of proper and sufficient flow measurement.
At fractionating column, flow is of primary importance on feed and reflux streams. Furnace charge flows, either over all or through individual passes, are almost always controlled. All manually set streams require some flow indication or some easy means for occasional sample measurement.
For accounting purposes, feed and product streams are metered. In addition, utilities to
individual and group equipment are measured to determine operating costs and equipment
Most flow measurements in petroleum processes are by variable head devices. To a lesser extent, variable area and positive displacement types are used, as are the many other available types as special metering situations arise.
While most petroleum processes are classified as continuous, this is not literally so with regard to liquid streams. In fact, many process vessels, notably column and drums, actually serve to interrupt the passage of liquid in some manner, depending upon the nature of the manipulation made upon the floor stream to the following item of process equipment. Except for a few special fractionating operations, the top pump around is always flow controlled. The question of level recording vs. level controlling depends largely upon the manner in which liquid product is withdrawn.
Fractionating column bottoms serve as temporary inventory for liquid bottoms product and require the same considerations as drums. Level measurement is of extreme importance on equipment whose specific function is to separate liquid and vapor. In addition, level measurement and control assure the proper liquid flow into and out of the equipment in which material is completely evaporated or condensed.
Most level measurements in petroleum processes are by external devices such as gage glasses, level displacers, and ball floats. Less frequently and depending upon special circumstances, internal displacers and ball floats are used.
A central room has become an integral part of all modern petroleum units. The grouping of recording and controlling instruments facilitates the work of the operator and concentrates responsibility for the operation of the plant. Although the first cost is high, it reduces the number of operators required to man the plant and provides an appreciable saving in operating cost. In dealing with emergency shutdowns, such as those arising from explosions and fires, the facility afforded by centralized control is vital.
On the other hand control instruments and the lines between the control room and the equipments are costly. For this reason the less important measurements should be given preference over recording instruments except where a record of the operating conditions is of definite service. A large number of instruments divide the attention of the operator, and hence careful thought should be given to the selection of truly useful operating data.
Air, water and soil are vital to life on this planet. We must protect these
resources and use them wisely – our survival as a species depends on them. Man
appeared on this earth relatively recently as compared to other forms of life. In the
beginning his effect on the bio-sphere remained relatively small. With the
invention of agriculture, he has made his first move to manipulate’ nature.
Exploitation of nature and its resources continued to grow with the increasing lust
of man to dominate the planet and mould the eco-system to Suit his whims and
fancies. The impact on this indiscriminate exploration, without regard to the basic
principle that sustained the eco-system, has resulted in the present degradation of
the environment to such an extent that even human survival has been jeopardised.
This has resulted in great interest in the environment and many new words, such as
ecology, environment, photochemical smog; greenhouse effect, etc. have become
part of vocabulary.
The plants, animals and micro-organisms that live in a defined zone (it can
range from dessert to an ocean) and the physical environment in which they live
comprise together in eco-system. The initial source of all the energy used by the
eco-system is the sun. Green plants capture solar energy during photosynthesis and
store it in chemical form for subsequent use by the plant itself or by any other
organisms that consume the plants. Some animals live on these organisms and for
plants they themselves become prey to some other higher animals and men. This
chain is called a food chain. There also exists a microorganism, which derives their
energy from waste products and other dead organism in the system and converts complex organic molecules into simpler inorganic forms, which in turn are taken up by the other organism as nutrients. Eco-system has the means of producing both energy and materials for life going on continuously. If this balance in the ecosystem is disturbed, biological communities can die out or essential services will not be performed, such as self-purification of river.
Man’s activities can upset the balanced operation of the eco-system in many ways. The discharge of pollutants to water streams will reduce their capacity for self-purification resulting in elimination of many species of organisms and plants. The emissions of toxic compounds in the atmosphere will affect not only vegetation, birds and animal life but also physical health of man himself. For creating more spaces for living in hilly areas, unwanted deforestation is going on. It is, therefore, essential that activities of man should be so controlled that unnecessary stress and strains are not put on the environment.
A variety of definitions of air pollution have been devised based on
philosophical, theoretical, practical interpretations of their authors. Any process,
which adds to or subtracts from the usual constituents of air may alter its physical
or chemical properties sufficiently to be detected by occupants of the medium. It
is usual to consider as pollutants only those substances added in sufficient
concentration to produce a measurable effect on man, animal, vegetation and
material. Pollutants may, therefore, include almost any natural or artificial
composition of matter capable of being airborne – solid particulates, liquid
droplets, gases or various admixtures of these forms.
The pollutants can be classified in two general groups:
(a) Those emitted directly from identifiable sources; and
(b) Those produced in the air by interaction among to or from primary
pollutants or by reaction with normal atmosphere constituents.
Primary emissions include particulate matter, sulfur compounds, organic
compounds, nitrogen compounds, carbon compounds, halogen compounds,
radioactive compounds, etc. Secondary pollutants are formed by inter-action
between various primary pollutants and/or normal constituents of air. Few examples of these are formation of sulfuric acid mist, smoke etc. The pollutants in the atmosphere cause various effects on the environment, which are:
- Reduction in visibility
- Damage to the materials
- Damage to vegetation
- Physiological effects on men and animals
- Psychological effects.
Air Pollutants from Refining Operations
Major air pollutants that may be emitted from refining operations are sulfur
compounds, hydrocarbons, nitrogen oxides, particulates including smoke and
carbon monoxide. Other emissions are aldehydes, ammonia and odors.
Sulfur compounds. Sulfur dioxide constitutes the maximum proportion of sulfur
compounds emitted from refineries: The main sources of sulfur dioxide emissions
are from combustion operations such as fired heaters, boilers and catalytic
cracking regenerators, an from sulfur dioxide extraction plants. Refinery flares,
incinerators and decoking operations are other minor sources. The amount of
sulfur dioxide emissions depends upon the type an amount of fuel burnt and its
Other sulfur compounds emitted from refineries include hydrogen sulfide, sulfur trioxide and mercaptans from treatment processes. However, these emissions are in very small amounts. Quantities since they result mainly from evaporation of leaks and vent losses from storage.
Hydrocarbons: The emissions of hydrocarbons result mainly from evaporation of light-oils during storage and handling of crude and petroleum products and from leaks. Source hydrocarbon emissions include loading -facilities, sampling, storage tanks, waste water separators, and blow down systems, air-blowing operations, and compressor engines. The extent of these emissions depends upon design, maintenance and operating practices.
Oxides of nitrogen: Combustion of fuel in fired heaters and boilers and in
internal combustion engines used to drive compressors and electric generators are
the main sources of nitrogen oxides emissions in the petroleum refineries. Nitrogen
oxides are also released from catalytic cracking regenerators and from CO boilers.
Combustion of fossil fuels produces nitrogen oxides partly from the
combination of atmospheric nitrogen with excess oxygen in the furnace
atmosphere, and partly from combination of nitrogen present in the fuel. The
formation of nitrogen oxides is mainly dependent on the flame temperature, the
residence time at this temperature, and the excess air present in the flame.
Particulates: The major sources of emissions of particulates in petroleum
refineries are catalytic cracking regenerators. The minor sources include asphalt
oxidizers, sludge burners, emergency flares, and boilers and furnaces during soot
blowing and emergency operations, and incomplete combustion.
Carbon monoxide: The only significant source of carbon monoxide in refineries
is the catalytic cracking regenerator. This, however, is eliminated in units having a
CO boiler. The minor sources include internal combustion engines used to drive
compressors and electrical generators and incinerators. Incomplete combustion in
furnaces and boilers also generates carbon monoxide emissions.
air, ultimately forming sulfate particulates.
Emissions of oxides of sulfur are expressed in terms of sulfur dioxide. Knowing the quantity of fuel used, based either on flow meter readings or actual tank gauges, and the sulfur content of the fuel from actual laboratory analyses, the emissions of sulfur dioxide may be easily calculated. Actual monitoring of stack gases in such cases may not be necessary.
Petroleum refineries differ in the type of processing schemes employed, type
of units used in a given processing scheme, the type of crude or crudes processed,
varieties of end products, location, source of power, utilities requirements, and
operating and housekeeping practices. All these factors have a bearing in varying
degrees on the quantities of emissions from each refinery. Consequently, it is
desirable to undertake individual refinery measurements or calculations to estimate
emissions, rather than employing generalized emission factors for this purpose.
Sulfur oxides: The main source of sulfur oxides is fuel combustion. Any organic
sulfur contained in a fuel is oxidized to sulfur dioxide or sulfur trioxide during
combustion. Normally about 97 percent of the fuel sulfur is converted to sulfur
dioxide and the balance to sulfur trioxide during combustion. Eventually, however,
considerably more of the sulfur dioxide is oxidized to sulfur trioxide in the ambient
In refineries employing sulfur dioxide extraction units, sulfur dioxide may be released directly into the atmosphere by way of leaks, vents, drains and in drying tower operations. An overall material balance on the unit including sulfur dioxide storage, plus sulfur content in the feed, extract and raffinate streams will indicate the net escape of sulfur dioxide to the atmosphere.
Hydrocarbons: These emissions cannot be directly measured except in the case of
un – burnt hydrocarbons released during combustion operations. These may be
measured by direct stack monitoring. The major sources of hydrocarbon emissions,
however, are evaporation losses from storage tanks and from oil separators. These
losses can only be estimated by empirical formulae based on extensive testing and
Oxides of nitrogen: Emissions of oxides of nitrogen from refinery furnaces and
boilers depend upon the fuel nitrogen content, and firing conditions, which depend
upon the design of the furnace, type of firing and excess air. Consequently,
emissions will differ from furnace to furnace, and hence these emissions must be
determined by individual stack sampling for any degree of accuracy. However,
since refinery furnaces in general are relatively small, and have low flame
temperatures and residence time, nitrogen oxides emissions from refineries are
generally low. Consequently individual monitoring will not appreciably enhance the quality of data obtained, as compared to emissions estimated by using generalized emission
Particulates: The major sources of particulates in refineries are catalytic cracking regenerators. Emissions from these regenerators may be estimated from the daily catalyst loss data. The losses from regenerators depend upon operating conditions and type of catalyst recovery equipment used. Consequently use of emission factors will not give a true picture of these losses.
The minor sources include asphalt air blowing operations, fired heaters and
boilers, sludge burners and emergency flares. Asphalt air blowing operations give
rise to blowing losses, which are dependent upon blowing temperatures. However,
the off-gas is generally water scrubbed to recover the entrained oil droplets. The
net loss therefore is of the order of 0.1 to 0.2 percent of the blowing charge.
Carbon monoxide: The only significant source is the catalytic cracking
regenerator when CO boiler is not employed. No emissions of carbon monoxide
are released when CO boilers are used. Where CO boilers do not exist the
emissions can be accurately calculated based on stack gas analysis for carbon
monoxide content and the quantity of air used for regeneration, which is measured
Air Pollution Control Techniques and Options
The control of air pollution is an expensive and complex problem because
the character and quantity of refinery atmospheric emissions vary greatly from
refinery to refinery. Controlling factors include crude oil capacity, type of crude
oil, complexity of processing scheme employed, operating practices followed, and
the degree of maintenance and good housekeeping procedures in force. However,
refinery air pollution control techniques exist which should permit refineries to
operate in any community without constituting a serious air pollution problem. A
realistic control strategy requires the evaluation of the various factors involved.
These factors include the type and number of pollutants, the technical feasibility,
size of the installation, cost benefits, commercial and possibility of creating other
The majority of refinery emissions occur during combustion in providing power and heat for processing operations. These include combustion of fuel in boilers for steam generation, combustion of fuel in fired heaters, and combustion of carbon during regeneration of cracking Catalysts. The combustion processes pose general problems such as the presence in the combustion gases of sulfur oxides and particulates which relate to the quantity of fuel burnt, and the production of nitrogen oxides and other minor pollutants. The control techniques and options available are discussed below from a general standpoint.
Control of Emissions From Refinery Process Gases
Nearly all refinery processes generate gases, which generally contain hydrogen
sulfide or other low molecular weight sulfur compounds. These gases are normally used as fuel in fired heaters and boilers. Sulfur dioxide emissions therefore result if the sulfur compounds are not removed. The most common procedure for removal of hydrogen sulfide and light mercaptans involves scrubbing the gases with an absorption solvent such as aqueous amine solution. The amine solution extracts the hydrogen sulfide, and is then regenerated by stripping hydrogen sulfide with heat and/or steam. The regenerated solution is reused for further absorption. The hydrogen sulfide recovered from this process can be converted to either sulfuric acid or elemental sulfur.
The use of this process however depends upon the quantity of gas and the sulfur content of the gas. The conversion of hydrogen sulfide to elemental sulfur also requires a certain minimum availability for the process to be viable.
Control of Emissions From Fuel Combustion
Combustion of fuel and residual fuel oils in particular can be a significant source
of sulfur dioxide emissions from refineries. The major options available in this
case are changing of fuel type or improving fuel oil quality. Changing fuel type
usually means switching to a cleaner type of fuel such as from residual to
distillate, distillate to naphtha, naphtha to gas. Cleaner fuel can also be obtained
by switching from high sulfur to a natural low sulfur fuel oil or crude. While the
investment required for fuel switching may be comparatively lower relative to other options, this often means competing for scarce supplies of the cleaner fuel with substantial cost debits. However, on a short-term basis this may be the only viable alternative. Availability, applicability and cost are critical factors in employing this method.
The quality of fuel oil can be improved by treatment of the fuel to remove potentially polluting substances, prior to combustion. Hydrogen processing is a prime example of such an option. Technology for hydrodesulphurization of both distillate and heavy residual fractions crude is available. Hydro treating reduces both sulfur and nitrogen but ratio of sulfur removal is higher. These processes, however, are extremely expensive, and are normally justifiable from product quality considerations rather than emission reduction.
Other major emissions from fuel combustion are oxides of nitrogen and
particulates or smoke. Improved combustion techniques can substantially reduce
both the formation of nitrogen oxides as well as emissions of smoke, carbon
monoxide and hydrocarbons. Proper burner maintenance, good atomization of
liquid fuels, optimum excess air levels, and correct stack temperatures are all-
important factors in reducing emissions. Various techniques include staged
combustion, flue gas recirculation, water and steam injection, and fluidized-bed –
Vent gas flares, on the other hand, may be a major source of smoke. Flares are
necessary as a safety measure to handle losses of hydro car ban vapors. These are
released from process units in emergencies caused by compressor failures,
excessive pressure in the units, line breaks, leaks, power failures and fires. Since
large surges of gas cannot be vented to furnaces or her enclosed burning
equipment, flares are usually designed to safely dispose of these gases.
Smoke from these flares results from an inadequate supply of air in the
combustion zone. Several techniques have been developed that virtually eliminate
smoke.The most widely used; injection of large quantities of steam into the
combustion zone with uniquely designed flare tips. However, occasions when
large quantities of gases are released to the flare are very rare, since they result
only from emergencies. Under normal operating conditions, most refinery flares
will be practically non-existent except for the burning of the pilot, and these do
not produce any smoke.Control of emissions from catalyst regeneration. Catalytic
cracking regenerators are a major potential source of particulate matter emissions
and also small amounts of sulfur oxides, and oxides of nitrogen, as well as some
aldehyde and ammonia. Normally two-stage conventional cyclones are located
within the regenerator vessel for catalyst recovery. Additionally, a third external
cyclone stage or an electrostatic precipitator may be used before discharge of the
flue gas to the atmosphere. As regard hydrocarbon emissions from catalytic
cracking regenerators, the amount is generally small. Moreover, these
hydrocarbons are completely consumed where cracking units are equipped with
Control of carbon monoxide emissions. The only significant source of carbon
monoxide emissions in petroleum refineries is the catalytic cracking regenerator.
Concentrations of carbon monoxide in the regenerator flue gases may be in the
range of 6 to 10 percent. These flue gases are generally released high-in the air
and at high temperatures, and result in very low ground level concentrations.
Carbon monoxide emissions from catalytic cracking units can be eliminated
by incinerating the flue gases in waste heat CO boilers at temperatures of 1100 to
1400 K. The heat of combustion of the carbon monoxide and other combustibles,
and the sensible heat of the regenerator gases, are recovered by generating steam
or heating the oil charged to the cracking unit. Carbon monoxide is completely
oxidized to carbon dioxide. In addition, traces of aldehydes, hydrocarbons and
cyanides are also destroyed. While these can be easily installed on new plants,
retrofitting in existing refineries could present problems of location and
availability of space. The high temperature regenerator using molecular sieve
zeolite catalysts eliminates carbon monoxide emission and may be adapted to
Control of emissions in storage. Hydrocarbons are the products of a refinery and
hence there is an obvious economic incentive to prevent their loss to the
atmosphere. Many air pollution control measures are therefore necessarily
employed as accepted good practice. Hydrocarbon emissions in quantities normally released in refinery operations are invisible and non-toxic. Hydrocarbon vapor losses are the principal contaminants. In addition, objectionable odors may be caused by emissions of sulfur or nitrogen compounds. All these are associated with the movement and storage of oil and gas. However, the control methods developed, and in general use, are capable of sufficiently reducing or eliminating their emissions into the atmosphere.
Emissions from storage vessels are generally caused by evaporation of liquids or liquefied gases. The control of emissions from oil and gas storage facilities results in reduction or elimination of fire hazards, and the recovery of valuable products. This reduction is normally achieved by the use of floating roof or pressure storage for light hydrocarbons.
Control of Emissions By Dispersion
The dispersion of excess pollutants by use of taller stacks is an another means
for abatement of pollution levels. This technique has been successfully used in
controlling ground level concentrations. It is the simplest and often the most
economic. Tall stacks, however, may be a hazard to aircraft or entirely inadequate
for certain local topographical and meteorological conditions. Further, since tall
stacks do not eliminate but only disperse the pollutants, if the natural scavenging
processes are not completely effective, they may contribute to regional pollution
problems. They may also transfer the problem from one locality to another.
Standard dispersion models may be used for calculating stack heights required.
In many refineries sulfur removal from some refinery streams is a part of
refining, from consideration of product quality rather than pollution abatement. These process units generate a potentially rich source for sulfur recovery units. Two important areas in this respect are hydro fining and amine extraction. Both these yield sour gases rich in hydrogen sulfide. Based on the availability of hydrogen sulfide, a sulfur plant may be an attractive option to reduce emissions as well as recover a useful product.
The classical method of sulfur recovery is the Claus process. The process is
based on producing elemental sulfur, by first converting one-third of the hydrogen
sulfide to sulfur dioxide and then using this to combine with the remaining
hydrogen sulfide in the presence of a catalyst to form-sulfur. Plant size is an
important consideration in determining the viability of a sulfur recovery unit. An
incinerator is normally provided to burn the residual gases prior to discharge to the
Increased Energy Efficiency
Increased efficiency and effective utilization of energy results in the pollution
reduction. Increased heat recovery and heat integration help to reduce fuel
consumption, thereby not only reducing pollution, but also manufacturing cost.
Various measures for enhancing energy efficiency of fired heaters include addition
of convection sections, air preheating by flue gas and improved burner designs.
Condensate recovery and low pressure flash steam saves both fuel and water in
boilers. Energy conservation, therefore, helps to reduce pollution at source and
avoids costly and cleanup processes. This should normally be the first area of
attack in any pollution control program, since it also helps conserve national
A regular and efficient preventive maintenance program comes high on the list of good operating practices. On-stream inspection prevents emergencies and leaks. Similarly, proper sampling and loading procedures will also avoid loss. Use of proper pump seals and glands will avoid leakage from these sources.
A routine periodic check up of safety valves will prevent gas leakage to flare, as well as liquid blow down, which ultimately increase flare losses. Similarly, floating roof tank seals must be regularly inspected for damage and leaks. Regular loss surveys can pinpoint sources of loss and leaks.
Proper furnace operation, burner cleaning and adjustment of atomizing steam
and excess air are important in ensuring high efficiency of combustion, which will
result in less fuel usage with consequent reduction in emissions. Excess air levels
can be easily controlled by regular check of flue gas for oxygen content. Simple
portable analyzers are easily available for this purpose. Complete combustion and
minimization of particulate matter can be effectively controlled by proper atomization of oil fired. Correct usage of atomizing steam and proper fuel temperature are essential for good atomization.
Control of Emissions Through Reduction Of
Reduction of hydrocarbon losses is achieved by exercising utmost vigilance at
each stage of the refinery operation. The factors which are important in loss reduction efforts; are:
(a) Reporting of losses as accurately as possible so that the problem is
(b) Use of modern technology to detect and quantify the losses.
(c) Maintenance of facilities in the best mechanical conditions as this is
essential to a low level of hydrocarbon losses.
Various measures for reduction of hydrocarbon losses are listed below:
(a) Installation of LPG evaporator system for fuel gas pressure control
and reducing flare loss.
(b) Maximization of gas consumption in furnace/boilers to reduce flare
loss by close coordination between gas producing units and consuming
(c) Provision of alarm/visual signal system on refinery fuel gas pressure
system for quick action, control and adjusting gas consumption in units
to reduce flare loss.
(d) Action at the change over time of coke chamber operation/CRU
Mogas/aromatic operation to adjust gas consumption as per gas
(e) Increased LPG recovery and steady LPG production.
(f) Installation of vapor recovery system for loading of LPG in bulk
(g) Provision of block valves on safety relief valves to facilitate their
inspection/repair of passing valves/maintenance during operation.
(h) Provision of sufficient flow meters/integrators for carrying out daily
fuel gas balance in units, and monitor flare loss.
(i) Use of instruments like anemotherm to detect leakages and
measure flare gas flow.
Standards for Gaseous Emissions:
The objective of the standards for gaseous emissions from refineries is to fix
tolerance limits for emissions of S02, NOx, hydrocarbon vapors etc. keeping in view the ambient air quality requirement. One commonly used criterion for emission standards is to require the use of best practicable means to control emissions. The word ‘practicable’ implies not only technological practicability but also economic, sociological and political practicability.
Standards for gaseous emission from refineries taking into account the factors like the type of crude processed, complexity of the refinery etc. have been formulated. These include:
- Methods of sampling and analysis of atmosphere and emission
- Quality standards for source emissions
- Code of practice for control of air pollutant at sources.
Any Organization has a legal and moral obligation to safeguard the health and welfare
of its employees and the general public. Safety is also good business; the good
management practices needed to ensure safe operation will also ensure efficient
All manufacturing processes are to some extent hazardous, but in chemical processes there are additional, special, hazards associated with the chemicals used and the process conditions. The designer must be aware of these hazards, and ensure, through the application of sound engineering practice, that the risks are reduced to acceptable levels. Safety and loss prevention in process design can be considered under the
Following broad headings:
Identification and assessment of to hazards
Control of the hazards for example, by containment of flammable and toxic
Control of the process, Prevention of hazardous deviations in process variables
(Pressure, temperature, flow), by provision of automatic control systems,
interlocks, alarms, trips, together with good operating practices and management.
Limitation of the loss. The damage and injury caused if an incident occurs:
pressure relief, plant layout, provision of fire-fighting equipment
Intrinsic and Extrinsic Safety
Processes can be divided into those that are intrinsically safe, and those for which
the safety has to be engineered in an intrinsically safe process is one in which safe operation is inherent in the nature of the process, a process which causes no danger, or negligible danger under all foreseeable circumstances (all possible deviations from the design operating conditions). The term inherently safe is often preferred to intrinsically safe, to avoid confusion with the narrower use of the term intrinsically safe as applied to electrical equipment.
Clearly, the designer should always select a process that is inherently safe whenever it is recital, and economic, to do so. However, most chemical manufacturing processes are, to a greater or lesser extent, inherently unsafe, and dangerous situations can developed is the process deviate from the design values.
The safe operation of such processes depends on the design and provision of engineered safety devices, and on good operating practice, to prevent a dangerous situation developing, and to minimize consequences of any incident that arises from the failure of these safeguards.
The term “engineered safety” covers the provision in the design of control systems, alarms, trips, pressure-relief devices, automatic shut -down systems, duplication of key equipment services; and fire-fighting equipment, sprinkler systems and blast walls, to contain any fire or explosion.
Kletz discusses the design of inherently safe process plant in a booklet published by the Institution of chemical Engineers, Kletz (1984). He makes the telling point that what you do not have cam10t leak out: so cam10t catch fire, explode or poison anyone.Which is a plea to keep the inventory of dangerous material to the absolute minimum required for the operation of the processes.
Most of the materials used in the manufacture of chemicals are poisonous, to
some extent. The potential hazard will depend on the, inherent toxicity of the material
and the frequency and duration of any exposure. It is usual to distinguish between the
short-term effects (acute) and the long-term effects (chronic). A highly toxic material that
causes immediate injury, such as phosgene or chlorine, would be classified as a safety
hazard. Whereas a material whose effect was only apparent after long exposure at low concentrations, for instance, carcinogenic material, such as vinyl chloride, would be classified as industrial health and hygiene hazards. The permissible limits and the precautions to be taken to ensure the limits are met will be very different for these two classes of toxic materials. Industrial hygiene is as much a matter of good operating practice and controls as of good design.
The inherent toxicity of a material is measured by tests on animals. It is usually expressed as the lethal dose at which 50 percent of the test animals are killed, the LD50 (lethal dose fifty) value.
In fixing permissible limits on concentration for the long term exposure of
workers to toxic materials, the exposure time must be considered together with the
inherent toxicity of the material. The Threshold limit Value” (TL V) is the most
commonly used guide for controlling the long-term exposure of workers to contaminated air. The TL V is defined as the concretion to which it is believed the average worker could be exposed to, day by day, for 8 hours a day, 5 days a week without suffering harm. It is expressed in ppm for vapors and gases, and in mg/m3 (or grains/fe) for dusts and liquid mists
Control Substances Hazardous To Health
The designer will be concerned more with the preventive aspects of the use of
hazardous substances. Points to consider are:
- Substitution: Of the processing route with one using less hazardous material. Or,
substitution of toxic process materials with non-toxic, or less toxic materials
- Containment: Sound design of equipment and piping, to avoid leaks. For
Specifying welded points in preference to gas kited flanged joints (liable to leak).
- Ventilation: Use open structures, or provide adequate ventilation systems.
- Disposal: Provision of effective vent stacks to disperse materials vented from
pressure relief devices, or use vent scrubbers.
- Emergency equipment: escape routes, rescue equipment respirators, safety
showers and eye baths.
In addition, good plant operating practice would include:
- Written instruction in the use of the hazardous substances and the risks involved
- Adequate training of personnel
- Provision of protective clothing:
- Good housekeeping and personal hygiene.
- Monitoring of the environment to check exposure levels. Consider the installation
of permanent instruments fitted with alarms.
- Regular medical check-ups on employees, to check for the chronic effects of toxic
The term “flammable” is now more commonly used in the technical literature than
“inflammable” to describe materials that will bum. The hazard caused by a flammable material, depends on a number of factors:
- The flash – point of the material.
- The auto ignition temperature of the material.
- The energy released in combustion
The flash – point is a measure of the ease of ignition of the liquid. It is the lowest
temperature at which the material will ignite from an open flame. The flash – point is a function of the vapor pressure and the flammability limits of the material. It is measured in standard apparatus, following standard procedures Both open and closed cup apparatus is used- Closed cup flash-points are lower, than open cup” and the type of apparatus used should be started clearly when reporting measurements.
Auto Ignition Temperature
The auto ignition temperature of a substance is the temperature at which it will ignite
spontaneously in air, without any external source of ignition. It is indication of the maximum temperature to which a material can be heated in air, for example, in drying operations.
The flammability limits of a material are the lowest and highs concentration in
air, at normal pressure and temperature, at which a flame will propagate through the mixture. They show the range of concentration over which the material will bum in air, if ignited. Flammability limits are characteristic of the particular material,-and differ widely for different material.
A flammable mixture may exist in the space above the liquid surface in a storage tank. The vapor space above highly flammable liquids is usually purged with inert gas (nitrogen) or floating head tanks are used- in a floating head tank a “piston” floats on top of the liquid, eliminating the vapor space.
Flame arresters are fitted in the vent of equipment that contains flammable – materials to
prevent the Propagation of flame through the vents. Various types of proprietary flame arresters are used in general, they work on the principle of providing a heat sink.Usually expanded metal grids or plates, to dissipate the heat of the flame. Traps should also be installed in plant ditches to prevent the spread of flame. These are normally liquid V-legs, which block the spread of flammable liquid along ditches.
An explosion is the sudden, catastrophic, release of energy, causing a pressure
wave (blast wave). An explosion can occur without fire, such as the failure though overpressure of a stream boiler or an air receiver.
When discussing the explosion of a flammable mixture it is necessary to
distinguish between detonation and deflagration. If a mixture detonates the reaction zone
propagates at supersonic velocity (approximately 300 mls) and the principle heating
mechanism in the mixture is shock compression. In a deflagration the combustion
process is the same as in the normal burning of a gas mixture, the combustion zone
propagates at subsonic velocity, and the pressure build-up is slow. Whether detonation or
deflagration occurs in a gas-air mixture depends on a number of factors: including the
concentration of the mixture and the source of ignition. Unless confined or ignited by a
high-intensity source (a detonator) most materials, will not detonate. However, the
pressure wave (blast wave) caused by a deflagration can still cause considerable damage.
Certain materials, for example, acetylene, can decompose explosively in the absence of oxygen; such materials are muscularly hazardous.
Sources of Ignition
Though precautions are normally taken to eliminate sources of ignition on
chemical plants, it is best to work on the principle that a leak of flammable material will ultimately find an ignition source.
The sparking of electrical equipment, such as motors, is a major potential source
of ignition, and flameproof equipment is normally specified. Electrically operated instruments, controllers and computer systems are also potential sources of ignition of flammable mixtures.
Hazardous areas are those, where explosive gas-air mixtures are present, or may be expected to be present, in quantities such as to require special precautions for the construction and use of electrical apparatus. Non-hazardous areas are those where explosive gas-air mixtures are not expected to be present. Three classification are defined for hazardous areas:
Zone 0: Explosive gas-air mixtures are present continuously or present for long periods
Specify: Intrinsically safe equipment
Zone I: explosive gas-air mixtures likely to occur in normal operation
Specify: Intrinsically safe equipment, or flame – proof enclosures: enclosures
eighth pressurizing and purging.
Zone 3: Explosive gas-air mixtures not likely to occur during normal operation,
but could occur for short periods
Specify: Intrinsically safe equipment, or total enclosure, or non-sparking apparatus.
The movement of any non-conducting material, powder, can generate static electricity,
producing sparks precautions must be taken to ensure that all piping is properly earthed (grounded) and that electrical continuity is maintained around flanges. Escaping steam, or other vapors and gases, can generate a static charge. Gases escaping from a ruptured vessel can self-ignite from a static spark.
Open flames from process furnaces and incinerators are obvious sources of ignition and
must be sited well away from plant containing flammable materials
It is the usual practice on plants handling flammable materials to control the entry on to
the site of obvious sources of ignition, such as matches, cigarette lighters and battery operated equipment. The use of portable electrical equipment, welding, spark-producing tools and the movement of petrol-driven vehicles would also be subject to strict control. Exhaust gases from diesel engines are also a potential source of ignition
The radiation emitted by radioactive materials is harmful to living matter. Small
quantities of radioactive isotopes are used in the process industry of various purposes, for example, in level and density, measuring instruments, and for the non-destructive testing of equipment.
Over-pressure, a pressure exceeding the system design pressure, is one of the
most serious hazards in chemical plant operation. Failures of vessels, or the associated piping, can precipitate a sequence of events that culminate in a disaster.
Pressure vessels are invariably fitted with some form of pressure-relief device, set
at the design pressure, so that (in theory) potential over-pressure is relieved in a
Three basically different types of relief device arc commonly used:
Directly Actuated Valves: Weight or spring-loaded valves that open at a predetermined pressure, and which normally close after the pressure has been relieved. The system pressure provides the motive power to pirate the valve.
Indirectly Actuated Valves: Pneumatically or electrically operated valves, which are activated by pressure-sensing instruments.
Thin doses of material that are designed and manufactured to fall at a predetermined
pressure, giving a fail bore opening for flow.
Relief valves are normally used to regulate minor excursions of pressure; and bursting doses as safety devices to relieve major over-pressure. Bursting doses are often used in conjunction with relief valves to protect the valve from corrosive process fluids during normal operation.
When designing relief venting systems it is important to ensure that flammable or
toxic gases are vented to a safe location. This will normally mean venting at a sufficient height to ensure that the gases are dispersed without creating a hazard. For highly toxic materials it may be necessary to provide a scrubber to absorb and “kill” the material, for instance, the provision of caustic scrubbers for chlorine and hydrochloric acid gases. If flammable materials have to be vented at frequent intervals; as, for example, in some refinery operations, flare stacks are used.
The rate at which material can be vented will be determined by the design of the complete venting system, the relief device and the associated piping. The maximum venting rate will be limited by the critical (sonic) velocity, whatever be the pressure drop. The design of ventilation systems to give adequate protection against over-pressure is a complex and difficult subject particularly if two-phase flow is likely to occur. For complete protection the venting system must be capable of venting at the same rate as the vapor is being generated. For reactors, the maximum rate of vapor generation resulting from a loss of control can usually be estimated. Vessels must also be protected against over-pressure caused by external fires. In these circumstances the maximum rate of vapor generation will depend on the rate of heating.
Unless designed to withstand external pressure a vessel must be protected against
the hard fonder-pressure, as well as over-pressure. Under-pressure will normally mean
vacuum on the inside with atmospheric pressure on the outside. It requires only a slight
drop in pressure below atmospheric pressure to collapse a storage tank. Though the
pressure differential may be small, the force on the tank roof will be considerable. For
example, if the pressure in a lO-m diameter lank falls to 10 milli bars below the external
pressure, the total load on the tank roof will be around 80,000 N (8 tone). It is not an uncommon occurrence for a storage tank to be sucked in (collapsed) by the suction pulled by the discharge pump, due to the tank vents having become blocked. Where practical, vacuum breakers (valves that open to atmosphere when the internal pressure drops below atmospheric) should he fitted.
Excessively high temperature, above that for which the equipment was designed, can
cause structural failure and initiate a disaster. High temperatures can arise I from loss of control of reactors and heaters, and, externally, from Open fires. In the design of processes where high temperatures are a hazard, protection against high temperatures is provided by:
- Provision of high-Temperature alarms an interlocks to shut down reactor feeds, or healing systems, if the temperature exceeds critical limits.
- Provision of emergency cooling systems for reactors, where heat continues to be generated after shut down; for instance, in some polymerization system.
- Structural design of equipment to withstand the worst possible temperature excursion.
The selection of intrinsically safe heating systems for hazardous materials.
Steam and other vapor heating system, are intrinsically safe as the temperature cannot exceed the saturation temperature at the supply pressure. Other heating systems rely on control of the healing rate to limit the maximum process temperature. Electrical healing systems can be particularly hazardous.
To protect against structural failure water-Sludge systems are usually installed to
keep vessels and structural steel work cool in a fire.
The lower sections of structural steel columns are also often lagged with concrete or other suitable materials.
Excessive noise is a hazard to health and safety, long exposure to high noise levels can
cause permanent damage to hear gin.
- Emergency equipment: escape routes, rescue equipment respirators, safety