MANUFACTURING PROCESS OF DIMETHYL ETHER

1.1  INTRODUCTION

Future energy demand especially in the Pacific & Asian regions is to be huge. Therefore limited energy supply as well as environment issue caused by consumption of fuel would be substantial obstacles to realize constant economic growth in these regions. Dimethyl ether will be a solution of secure energy supply and environmental conservation.

Di methyl ether is a colourless gas at an ambient condition and easily liquefied under high pressure. Since its physical and chemical characteristics are very similar to those of LPgas, it is an easy substitute for LP gas It is also a clean substitute for motor fuel because a DME fueled vehicle emits neither soot nor toxic gases.

 

1.2  HISTORY

During the OPEC (organization of petroleum exporting countries) oil embargos of the mid 1970’s to early 1980’s, the high prices of petroleum prompted development of alternatives to fuels derived from crude oil. Up to that time, only two processes of fuel synthesis had any commercial significance. The first was the Bergius process that used oil/coal slurry and an iron catalyst to produce synthetic crude oil. The second was the Fisher-Tropsch process, which produced hydrocarbons from coal. Both of these processes produced hydrocarbons with poor selectivity and quality. This problem was overcome by Mobils methanol-to-gasoline (MTG) process of methanol conversion over a highly selective zeolite catalyst. The MTG process makes possible the synthesis of a high quality, high octane gasoline without the need for expensive post-production processing. If oil prices stay high, MTG may be a competitive method of producing gasoline.

The MTG process was discovered by accident by researchers at Mobil Corporation. They had been trying to use zeolite ZSM-5 to convert methanol into a fuel additive. The process instead produced dimethyl ether, which with increasing space time next produced olefins (alkenes), and finally paraffin’s (alkanes) and aromatics. This mixture of paraffin’s and aromatics is commonly known as gasoline. Chang and other Researchers at Mobil Corporation beginning in the mid 1970s extensively studied MTG. It reached its commercial zenith in the early 1980s when a 14,000 barrel/day (gasoline product) facility was scheduled to be built in New Zealand (Meyers, 1984). However, when oil prices dropped again in the mid 1980s, MTG was no longer as economic and has not been used on a significant scale since The scientists from the Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, won Gold Medal and a bonus grant at the 49th World Exhibition of Invention, Research and Industrial Innovation (“Eureka”) in Brussels for their work “Synthesis of dimethyl ether”. The dateless dream of several generations of chemists has finally become true. The Moscow scientists have thought up how to produce motor fuel not from oil but from oil gas.

1.3   PROPER TIES OF DIMETH YL ETHER

 

1.3.1  PHYSICAL PROPERTIES OF DME:

It is a simplest aliphatic ether, (C2H6O), structure CH3-O-CH3 and molecular weight 46.07. It is a colorless, flammable gas with a slight ethereal odour. It is Non-toxic, non-carcinogenic and less corrosive gas. The physical properties of DME are.

 

Molecular weight 46.069 g
Normal boiling point -24.9 oC
Normal freezing point -141.5 oC
Critical temperature 400 K
Critical pressure 53.7 Kpa
Critical volume 0.178 m3
Liquid density at 667 [kg/m3]
Temperature 25°c heat of vaporization at normal boiling point 21,520
Standard enthalpy of formation of vapour at 25°c [kj.kmol-1] -184.18

 

1.3.2  CHEMICAL PROPERTIES OF DME:

Dimethyl ether may decompose to methane, ethane, formaldehyde, carbon dioxide or carbon monoxide based on reaction conditions and catalysts used. It may also act as methylating agent as it reacts with benzene to form toluene, xylene and multi-methylated benzene under catalyzation of aluminium silicate. Dimethyl ether reacts with carbon monoxide and carbon dioxide to form acetic acid/methyl acetate and methoxy-acetic acid respectively. It reacts with HCN to form acetonitrile. It forms complex with boron trifluoride. (CH3)2OBF3.

1.4  SAFETY & STORAGE

 

1.4.1 PRECAUTIONS TO BE TAKEN IN HANDLING:

Secure cylinder when using it as storage tank inorder to protect from falling. Use suitable hand truck to move cylinders. Minimize breathing of vapors and avoid prolonged or repeated contact with skin. Wear proper protective equipment. If ventilation is not sufficient, wear proper respiratory equipment. Do not use near ignition sources. Do not puncture or incinerate.

 

1.4.2 STORAGE REQUIREMENT:

Store in cool dry, well-ventilated area away from all sources of ignition. Empty container may contain residues, which are hazardous. Do not store at temperature above 120°F. Store in well ventilated areas. Keep valve protection cap on cylinders when not in use.

1.5  USES

  • Ethers are most common of all anesthetics used. Two particular disadvantages of ethers are its low boiling point -24.9°C, less than that of the body temperature, which makes its use difficult to use in hot climates and the risk of explosion in operating theaters resulting from its great inflammability as well as its low boiling point.
  • Dimethyl Ether has found commercial use as a refrigerant. It is easy to liquefy and possesses good refrigerant effect.
  • Dimethyl ether is absolutely harmless to aerosphere as. it may decompose within 24 hours in the troposphere.
  • Dimethyl ether =99.5% is used as aerosol propellant, foaming agent, solvent, as an extraction agent etc.
  • Readily form complexes with inorganic compounds, e.g., boron trifluoride. It is an excellent methylating agent in the dye industry
  • Recently, the use of DME as a fuel additive for diesel engines has been investigated due to its high volatility (desirable for cold starting) and high cetane number. Dimethyl ether has much lower saturated vapor pressure than LPG under identical temperatures. It has good combustion property, featuring high thermal efficiency, low COx/NOx emission and free of residue and black smoke. It can be mixed with LPG, LNG and water gas to facilitate combustion. Dimethyl ether =95% can be used as fuel directly, and the CO, NO and hydrocarbon exhausted are 55%, 83%, 4% lower than that of gasoline respectively. On the other hand its cetane value is 27% high than that of diesel fuel, indicating higher burst power over diesel fuel.

1.6  STATUS IN PAKISTAN

Dimethyl ether is used as aerosol propellant, foaming agent, solvent, as an extraction agent etc. Dimethyl Ether has found commercial use as a refrigerant. It is easy to liquefy and possesses good refrigerant effect.

In Pakistan, unfortunately it is used only as anesthetic agent on small scale. It is totally imported from foreign countries which is expensive, because we have not any plant producing DME.

MANUFACTURING PROCESS OF DIMETHYL ETHER:

2.1  GENERAL METHODS OF PREPARATION

 

  1. A. FROM ALCOHOL:

Di methyl ether may be prepared from methyl alcohols in acidic media, i.e. by heating excess of alcohol with conc. sulphuric acid or glacial phosphoric acid at low temp. One molecule of water is removed from two molecule of alcohol-hence the reason regarding ethers as the anhydrides of alcohols:

 

H2SO4, -H2O

2CH3OH                        (CH3)2O

 

Dehydration of alcohols to ether may also be carried out by passing the alcohol vapor over a heated catalyst such as alumina, aluminum phosphate, zeolite etc.e.g.,

 

2CH3OH               CH3OCH3 + H2O

 

  1. BY WILLIAMSON’S SYNTHESIS:

In this method sodium or potassium alkoxide is heated with an alkyl halide, e.g., Sodium methoxide reacts with methyl halide to give Methyl ether

 

CH3ONa + CH3Cl                             CH3OCH3 + NaCl

 

The proposed mechanism is given by as follows

 

CH3O + CH3Cl       CH3 -O-CH3Cl

CH3 -O- CH3Cl       CH3 – O – CH3 + Cl

 

Also methyl or ethyl group, methyl ethyl sulphate, respectively can be used instead of the corresponding alkyl halide, e.g.,

Sodium methoxide reacts with di-methyl sulphate to give Methyl ether and sodium methyl sulphate.

 

CH3ONa + (CH3)2SO4             CH3 -O-CH3 + CH3NaSO4

 

 

 

2.2 INDUSTRIAL METHOD OF PREPARATION

DME is now manufactured with two processes, which are methanol synthesis and methanol dehydration process. In order to use DME as fuel, it must be produce in large cost in large quantities. Almost 10,000 ton/year are manufactured in JAPAN and 150,000 ton/year worldwide.

 

2.2.1 DIRECT DME SYNTHESIS FROM HYDROGEN & CARBONMONOXIDE:

Direct synthesis of DME is done by Hydrogen & carbonmonoxide using coal as raw material.

There are mainly two.overall reaction routes that synthesize DME from synthesis gas:(synthesis gas: H2+CO gas), reaction (a) and (b). The reaction (a) synthesizes DME in three steps, which are methanol synthesis reaction (c)

Dehydration reaction (d) and

Water-gas shift reaction (e).

When the shift reaction does not take place, reaction (c) and (d) are combined to the reaction (b), which is the other DME synthesis route. Haldor Top-soe A/S and some other direct DME synthesis follow reaction (b) [3].

 

Reaction formulas concerning

DME synthesis Reaction                            Reaction (heat kj/mol)

 

  • 3CO + 3H2 CH3 – O – CH3 + CO2                         -246
  • 2CO+4H2 CH3-O-CH3 + H20                             -205
  • ICO + 4H2 2CH3OH                                  -182
  • 2CH3OH CH3OCH3 + H2O                     -23
  • CO + H2O CO2+H2                               -41

 

Since both reaction (a) and (b) generate two molecules of products from six molecules of synthesis gas, the higher reaction pressure gives higher synthesis gas con-version.

2.2.2 JFE DIRECT DME SYNTHESIS PROCESS:

 

JFE deals with direct DME synthesis from natural gas.The process consists of mainly three sections, synthesis gas preparation (auto-thermal reformer), DME synthesis (slurry reactor), and separation/purification (CO2, DME, methanol distillation columns).

Natural gas is converted to synthesis gas with O2(steam) and by-product CO2 in an auto-thermal reformer (ATR). The synthesis gas is compressed and fed to the DME slurry reactor. The effluent from the reactor is DME, by-product CO2, small amount of methanol and un reacted synthesis gas. DME and other by­products are chilled and separated as a liquid from un reacted gas. In this separation DME works as a solvent to re-move CO2 from unreacted synthesis gas, which is recycled to the reactor. DME and other by-products are fed to the distillation columns. DME and methanol are purified. CO2 is recycled to the ATR and converted to synthesis gas. Methanol is also recycled to the DME reactor and finally converted to DME.

 

2.2.3 MANUFACTURE OF DME FORM METHANOL:

DME is produced by the catalytic dehydration of methanol over zeolite catalyst (ZMS-5). The reaction is as follows:

 

2CH3OH                     (CH3)2O + H20

 

In the temperature & pressure range of normal operations, 250–364oC,

14.7 bar, there are no side reactions. This is slightly exothermic reaction with heat release 11,770KJ/Kmol.

 

2.3  PROCESS DESCRIPTION

Fresh methanol is combined with recycled reactant form the top of distillation column T-202. Then this stream is send to vaporizer E-201 to vaporize which is then send in reactor cooler E-202, where it is   superheated by the reactor effluents, prior to being sent to a fixed bed reactor R-201, operating between 250°C and 400°C. The single pass conversion in the reactor is limited to 80% due to equipment constraints. The reactor effluent  which contains DME, methanol & water are cooled in reactor cooler E-202 prior to being sent to the first of two distillation columns T-201. DME (99.5%) product is taken overhead from the first column T-201. The bottom product which mainly contains water & methanol & some fraction of DME is send to 2nd column T-202 which separates water from the DME & unreacted methanol. The methanol & DME is recycled back to the front end of the process, while the water is sent to waste treatment to remove trace amounts of organic compounds.

PFD for Production of Dimethyl Ether by Methanol:

MATERIAL AND ENERGY BALANCE:

3.1MATERIAL BALANCE CALCULATION

INFORMATION AVAILABLE

 

Methanol fresh feed is available (98.6% wt purity) is available which is to be converted into DI METHYL ETHER (containing methanol= 3%Kmol)

 

Conversion per pass=80%

 

Water is only by-product.

 

OVERHEAD PRODUCT AND BOTTOM PRODUCT OF DME TOWER

 

Composition of DME in Top product = 97%

 

Composition of Methanol in Top product = 3%

 

All the water goes into bottom product.

 

OVERHEAD PRODUCT AND BOTTOM PRODUCT OF METHANOL TOWER

 

Composition of DME in Top product = 2.1%

 

Composition of Methanol in Top product = 97%

 

Composition of water in Top product = 0.9%

 

All the DME goes into top product.

 

FEED COMPOSITION GIVEN TO REACTOR

 

Composition of DME = 0.456%

 

Composition of methanol = 98.38%

 

Composition of water = 1.164%

 

 

 

3.1.1 MATERIAL BALANCE CALCULATIONS FOR REACTOR

BASIS

100 kmol/hr feed to the reactor.

 

COMPOSITIONS IN F

 

XM,F=0.9838

 

XDME, F =0.00456

 

XW, F =0.01164

 

MOLES IN F STREAM

 

NDME, F =0.456

 

NM, F =98.38

 

NW, F =1.164

 

REACTION INVOLVED

 

 

2CH3OH(g )                                               (CH3)2O(g ) +H2O(g )

 

This applies that for 100% conversion

 

1Kmol of methanol=1/2 Kmol of DME

98.38Kmoles of Methanol=49.91Kmol

 

But for 80% conversion

 

Kmol of DME formed=49.19×0.8 = 39.35Kmol

Kmol of DME in G-stream = DME formed + Kmol of DME in F-stream

=39.35+0.456=39.808 Kmol

Kmol of Methanol consumed during reaction= 39.35×2

=78.704 Kmol

 

Kmol of Methanol remaining = 98.38-78.704

=19.676Kmoles

 

Kmol of water produced=39.35kmol

 

Kmol of Methanol in G-stream = 19.676Kmoles

 

Kmol of water in G-Stream = moles of water in F-stream+ moles of water produced   =39.35+1.162

=40.512Kmol

 

Kmol of G-Stream = kmol of methanol+ kmol of DME+ kmol of water

=100kmol

 

COMPOSITION IN G-STREAM

 

XDME,F = 0.3935

 

XM,F= 0.19676

 

XW,F=0.40512

 

FOR STREAMS H, X,Y ,Z AND I

 

As

Stream-G=Stream-H=Stream-X= Stream-I

 

So Molar flow rates and compositions remains constant for all these streams.

Now

 

Stream-X= Y+Z

Where

Y=Z

 

So

Y=50kmol

 

Z=50 kmol

 

Compositions of Y=Z=X

 

3.1.2 MATERIAL BALANCE CALCULATIONS FOR DIMETHYL ETHER TOWER

 

COMPOSITION IN I-STREAM

 

XDME,I = 0.3935

 

XM,I= 0.19676

 

XW,I=0.40512

 

COMPOSITION IN J-STREAM

 

XDME,J =0.97

 

XM,J= 0.03

 

All the water goes into bottom product i.e. K-Stream

 

OVERALL MATERIAL BALANCE AROUND DME TOWER

 

100= J+K

 

DME losses are 1%.

 

Moles of DME in I-stream=39.35 Kmol

Moles of DME in J-stream=39.35×0.99 = 38.96 Kmol

 

Moles of DME in K-stream=39.35×0.01 = 0.3935 Kmol

 

Moles of J-stream= moles of DME in J-stream/mole fraction of DME in J-stream

 

Moles of J-stream= 38.96/0.97 = 40.16 Kmol

Now from overall balance around DME tower we have;

 

K=100-J

K=100-40.16  =59.84 Kmol

 

Moles of methanol in K-stream = K – (moles of water in K + moles of DME in K)

 

Moles of methanol in k-stream = 59.84-(40.512+0.3935)

= 18.94kmol

 

COMPOSITION OF K-STREAMDME

 

Mole fraction=0.3935/59.84=0.0065

Mole percent=0.65%

 

METHANOL

 

Mole fraction=18.94/59.84=0.3219

Mole percent=31.65%

 

WATER

 

Mole fraction=40.512/59.84=0.6715          Mole percent=67.7%

 

3.1.3 MATERIAL BALANCE CALCULATIONS FOR METHANOL TOWER

 

K-Stream

 

DME =0.65%

 

METHANOL=31.65%

 

WATER=67.7%

 

L-Stream

 

DME=2.1%

 

METHANOL=97%

 

WATER=0.9%

 

OVERALL MATERIAL BALANCE AROUND METHANOL TOWER

 

K=L+M

 

59.84=L+M

 

All the remaining DME and methanol goes to L stream an all the water goes to M-Stream

So Kmol of DME in K= Kmol of DME in L=0.3935 kmol

i.e.

NDME,L=0.3935 Kmol

Now

L x XDME,L=NDME,L

 

L=0.3935/0.021 =18.7 Kmol

 

So from overall balance around METHANOL tower

 

59.84=18.7 + M

 

M=41.1 Kmol

 

Now

 

Methanol in M-Stream= Methanol in K-Stream-Methanol in L-Stream

 

Methanol in L-Stream=0.97×18.7 =18.139 Kmol

 

Methanol in M-Stream=18.94-18.139 =0.801 Kmol

 

COMPOSITION OF M-STREAM

 

METHANOL=0.801/41.1 = 0.019

 

WATER=0.981

 

N-Stream =M-stream=41.1 kmol

 

3.1.4MATERIAL BALANCE ACROSS FEED TANK

 

Stream B= Stream C=stream D=stream E= Stream F=100 kmol

 

MOLES OF STREAM  -B

 

NM,F=NM,B=0.456 kmol

 

NDME,F=NDME,B=98.38 kmol

 

COMPOSITION OF STREAM –B

 

XM,F=XM,B=0.9838

 

XDME,F=X­DME,B=0.00456

 

XW,F=XW,B=0.01164

 

 

 

OVERALL MATERIAL BALANCE AROUND FEED TANK

 

A + L = B

 

A = 81.3 k moles

 

 

COMPONENT BALANCE

 

Methanol Balance;

 

XM,AA + XM,LL = X M,BB

 

X M,A*(81.3) + 18.139 = 98.38=0.986

 

Water Balance;

 

(XW,A81.3) + (0.009×18.7) = 1.164 = 0.016

 

 

 

Moles of A

 

NM,A = 80.16 kmol

 

NW,A = 1.138 kmol

 

Composition of A

 

XM,A = 0.986

 

XW,A = 0.014

MATERIAL BALANCE DATA SHEET

STREAM PHASE Mole%

DME                    M                W

Moles (Kmol/hr)

DME                   M                   W

FLOW RATE  (Kmol/hr)
A L —– 98.6 1.4 ———– 249.6 3.54 253.1
B L 0.456 98.38 1.164 1.42 306.3 3.62 311.3
C L 0.456 98.38 1.164 1.42 306.3 3.62 311.3
D L 0.456 98.38 1.164 1.42 306.3 3.62 311.3
E V 0.456 98.38 1.164 1.42 306.3 3.62 311.3
F V 0.456 98.38 1.164 1.42 306.3 3.62 311.3
G V 39.35 19.675 40.512 122.5 61.25 126.11 311.4
H V 39.35 19.675 40.512 122.5 61.25 126.11 311.4
X V 39.35 19.675 40.512 122.5 61.25 126.11 311.4
Y V 39.35 19.675 40.512 61.27 30.63 63.1 155.7
Y-1 L 39.35 19.675 40.512 61.27 30.63 63.1 155.7
Z V 39.35 19.675 40.512 61.27 30.63 63.1 155.7
Z-1 L 39.35 19.675 40.512 61.27 30.63 63.1 155.7
I L 39.35 19.675 40.512 122.5 61.25 126.11 311.4
J L 97 3 ——- 120.3 3.75 ———- 125
K L 0.65 31.67 67.7 1.21 59.1 126.2 186.4
L L 2.1 97 0.9 1.22 56.5 0.52 58.22
M L ——- 1.9 98.1 ——— 2.45 126.45 128.9
N L ——- 1.9 98.1 ——— 2.45 126.45 128.9

 

3.2  ENERGY BALANCE

 

BASIS:

 

Reference Temperature = 250C

 

Reference Pressure         =101.325KPa

 

The equations used:

 

ΔH  = ∑HoR – ∑HR + ∑HP

 

HoR = Heat of reaction at the standard conditions.

 

ΔH  = CpΔT + VΔP

 

Where Cp is specific heat capacity at given temperature and pressure.

 

AVAILABLE INFORMATION:

 

Methanol and water (0-15 wt%) at approximately 100°F and 20 Psia is drawn from a surge tank (Tl) and pressurized to approximately 225 Pisa. After being pressurized the stream passes through a pre heat cross exchanger (El) where it is heated to just below it’s vaporization temperature. Then it enters a kettle type vaporizer (VI), is completely vaporized and leaves at a temperature of approximately 310-325 “F depending on the composition of the stream. This stream then proceeds to a second cross exchanger (E2) and is heated to approximately 450°F, which is the required temperature for the reaction to take place in the reactor (RI). Approximately 80% of the methanol is converted to DME resulting in a temperature rise of approximately 200°F. After leaving the reactor and passing through the two cross exchangers (reactor feed E2 then vaporizer pre-heat El) the reactor product stream is further cooled (E3) to a temperature of -190 F. This stream is then fed at about 150 psia to DME distillation column (Cl).

 

SPECIFIC HEAT-CAPACITIES OF  COMPOUNDS USED:

 

Dimethyl Ether = 83.3KJ/kgmol-K

Methanol          = 144.4 KJ/kgmol-K

Water                = 77.75 KJ/kgmol-K

ENERGY BALANCE AROUND FEED TANK:

Where ΔHA = 0 KJ/kgmol-K (as feed is at reference conditions)

ΔHL   = mCpΔT + VΔP

= 2.34×105

V       =   5.99kmol/hr

P       =   275kpa

So,

ΔHB   = 2.38×105

 

ENERGY BALANCE ACROSS HEAT EXCHANGER E-1

Temperature of C          = 37oC

Pressure of C                 = 1551kpa

Temperature of D          = 153 oC

Temperature of H          = 287 oC

Pressur of X         = 1047kpa

 

So, By applying equation:

ΔH    = CpΔT + VΔP

ΔHc   = 2.385×105

ΔHd   = 2.239×105

ΔHh   = 1.955×105

ΔHx   = 2.1×105

Tx    = 145.4oC

Tc      = 37 oC

Heat duity = ΔHd – ΔHc = 4.5×106

ENERGY BALANCE ACROSS HEAT EXCHANGER E-2

In this case input stream is reactant in saturated vapor state i-e

Te          = 154.9oC

Pe      = 1540kpa

Also;

Pf       = 1535kpa

And it is given that this vapor stream is to be heated upto 250oC

So; by applying equation;

ΔH = CpΔT + VΔP = 1.9×105kj/kgmol

Now by applying ;

Heat releases by hot fluid = heat absorbed by cold fluid

(Tg – Th)=mCp(Tf – Te)

 

So;

Th      = 287.7oC

And

Ph      = 1050kpa

So; by applying equation;

ΔH = CpΔT + VΔP

ΔHh = 2.00×105kj/kgmol

ENERGY BALANCE AROUND VAPORIZER E-4

In the case of a vaporizer we know that D liquid feed is coming at saturated condition and

The purpose of this equipment is to add latent heat only.

The feed is entering at the conditions;

Te      = 153oC

Pe      = 1544kpa

So; enthalpy at these conditions is;

ΔHd   = 2.24×105

The purpose of vaporizer is to add latent heat only;

Where;

λ        = 2.8×104 kj/kgmol

so;

Q       = mλ

Q       = 6.9×107 kj/hr

And the enthalpy of E;            ΔHe = CpΔT + VΔP + mλ        ΔHe = 2.40×105

ENERGY BALANCE ACROSS E3:

Temperature of X          = 145.4oC

Pressure of        X          = 1047kpa

The cooling water is entering at STP conditions.

Temperature of Y          = 88oC

Pressure of  Y                = 1034kpa

So, By using ideal situation that there are negligible energy losses;

Heat evolved by hot fluid = Heat absorbed by cold fluid

8.3×106       =       Qc

From this equation we can find out;

Flow rate of cold fluid = 1999kgmol/hr

Out-let temperature of  Cold fluid = 80oC

ENERGY BALANCE ACROSS REACTOR:

Energy Balance across the reactor is given by the formula:

ΔH  = ∑HoR – ∑HR + ∑HP

HoR = Heat of reaction at the standard conditions.

∑HoR                                  2.72×103    MJ/hr

∑HR                                    5.91×104     MJ/hr

∑HP                                    6.00×104      MJ/hr

Difference              3620            MJ/hr

Reference Temp.      = 25oC          Reference pressure = 101.2kpa

ENERGY BALANCE ACROSS DME COLUMN

In this case I is coming from unit E-3 and it value has been already calculated.

P   = 1034kpa

T   = 88oC

ΔHi = 2.37×105

Now the composition of J is 98.7% and we know that it is top product and leaving at saturated Temperature so from Due point calculation it’s temperature is

Tj = 45.5oC

Pj = 999kpa

So by assuming reflux ratio of 3

Its volumetric flow rate = 125 kgmol/hr

Thus by applying equation:

ΔH = CpΔT + VΔP

ΔHj = 2.01e5 kj/kgmol

 

Now:

Tk = 155.8oC

And by taking Bubble-Point calculations

Pk = 1069kpa

Thus by applying the equation:

ΔH = CpΔT + VΔP

ΔHk = 2.6×105 kj/kgmol

ENERGY BALANCE ACROSS METHANOL COLUMN:

In this case K is coming from the DME tower and its value of enthalpy is already known.

 

ΔHk = 2.58×105

Also other known things are as follows;

Tl   = 92oC

Tm = 141oC

Pk = 372kpa

Where;

Mass flow rate of k                  = 186.3kmol/hr

Mass flow rare of L                 = 59.99kmol/hr

Mass flow rate of M                = 126.3kmol/hr

Hence by assuming reflux ratio = 3

Cold water flow rate at 25oC and 1atm= 1610 Kmol/hr

Saturated steam at 10atm for Reboiler = 186.4Kmol/hr

Condenser Heat duty = 2.9×106  KJ/hr

Reboiler Heat duty = 6.7×106 KJ/hr

Using:

ΔH = CpΔT + VΔP

ΔHL = 2.35×105kgmol/hr

ΔHM = 2.74×105kgmol/hr

ENERGY BALANCE DATA SHEET:

Stream Phase Temp.

(oC)

Pressure

(Kpa)

Enthalpy

(KJ/Kgmol)

A L 25 101.3 0
B L 37 138 4.3×105
C L 37 1551 4.3×105
D L 150 1544 2.1×106
E V 154 1540 2.2×106
F V 250 1539 4.2×106
G V 373 1055 7.8×106
H V 311 1048 6.1×106
X V 257 1041 2.3×106
Y V 257 1041 1.2×106
Z V 257 1041 1.2×106
Y-1 L 88 1041 1.1×106
Z-1 L 88 1041 1.1×106
I L 88 1027 2.2×106
J L 56.43 1024 4.7×106
K L 105 1036 1.3×106
L L 86 276 4.3×105
M L 129 290 1.0×106
N L 49 283 2.3×105

 

REACTOR DESIGN:

FIXED BED CATALYTIC REACTORS

4.1  INTRODUCTION

Fixed-bed catalytic reactors have been aptly characterized as the  workhorses of me process industries. For economical production of large amounts of product, they are usually the first choice, particularly for gas-phase reactions. Many catalyzed gaseous reactions are amenable to long catalyst life (1-10 years); and as the time between catalyst change outs increases, annualized replacement costs decline dramatically, largely due to savings in shutdown costs. It is not surprising, therefore, that fixed-bed reactors now dominate the scene in large-scale chemical-product manufacture.

4.2   TYPES OF FIXED BED REACTOR

Fixed-bed reactors fall into one of two major categories:

  • Adiabatic
  • Non-adiabatic.

A number of reactor configurations have evolved to fit the unique requirements of specific types of reactions and conditions. Some of the more common ones used for gas-phase reactions are summarized in Table(4.1) and the accompanying illustrations. The table can be used for initial selection of a given reaction system, particularly by comparing it with the known systems indicated.

Table 4.1:   Fixed-Bed Reactor Configurations for  Gas-Phase Reactions

 

Classification Use Typical Applications

Single adiabatic bed

Moderately exothermic or

endothermic non-equilibrium

limited

Mild hydrogenation

Or dehydration

 

Radial flow Where low AP is essential

and useful where change

in moles is large

Styrene from ethylbenzene
Adiabatic beds in series with intermediate cooling or heating High conversion, equilibrium

limited reactions

 

SO2 oxidation

Catalytic reforming

Ammonia synthesis

Hydrocracking Styrene from ethylbenzene

Multi-tabular

non-adiabatic

Highly endothermic or

exothermic reactions requiring

close temperature control to

ensure high selectivity

Many hydrogenations

Ethylene oxidation to

ethylene oxide, formaldehyde

by methanol oxidation, phthalic anhydride production

 

4.3   SELECTION OF REACTOR TYPE

After analyzing different configuration of fixed bed reactors we have concluded that for our system the most suitable reactors is single adiabatic bed / massive adiabatic bed reactor. Because dehydration of methanol is mildly exothermic reaction so no cooling or heating is required.

 

Factors to be account in the selection of reactor are as follows:

  1. Pressure drop.
  2. Heat-Transfer control.
  3. Catalyst separation.
  1. Cost.

 

Reactor Selection

Characteristics Fixed bed reactor Bubble column slurry reactor
Conversion High Low
Heat control Relatively difficult Relatively easy
Catalyst Separation No need Need
Heat Control Difficult Easy
Catalyst attrition No Major Problem
Maximum volume 400m3 50m3
Working pressure High Low
Investment cost Low High
Working cost High High

 

Infect every reactor has its own significance. Some of which are given as below.

Feed Stream

4.4  CONSTRUCTION & OPERATION OF REACTOR

 

A single catalyst bed is supported by inert ceramic balls. Slotted reactor outlet prevents flow from bypassing lower portion of catalyst bed. Inert packing at the inlet of the bed prevents the movement of catalyst by high velocity gas, thermal shocks & also prevents scale from collecting in the bed. For proper flow distribution only a single inlet baffle is adequate because ceramics balls also helps for flow distribution.

IN INDUSTRIES

 

Basically there are two Industries producing DME in the world one of them is in Koria and other one in china and both of them are using catalytic fixed bed reactor working at adiabatic conditions. Mostly it is used in industries like Petroleum oil Refineries, in water treatment plants, in Catalytic cracking and catalytic dehydration.etc.

 

So The reactor selected is Adiabatic Catalytic Fixed-Bed Reactor.

 

Reaction

 

The only reaction taking place in our reactor is as follows:

2CH3-OH→CH3-OCH3 + 2H2O                   ∆H= -1.1×104kj/kgmol

Conversion

 

The conversion of the reactor depends upon the following factor.

  1. GHSV( gas hourly space velocity)
  2. Partial pressure of methanol.

C/(1 – C) = k x 1/(GHSV)

k = (1/ GHSV) x pH

Where C is %age conversion of Methanol.

 

Catalyst

 

Basically there are two types of the catalyst that can be used in our system.

  1. ðAl2O3.
  2. H-ZSM5.

 

The catalyst to be selected is H_ZSM5.

WHY?

As it is hydro-phobic one and in our feed water is the major portion of feed so we prefer it on the other one.

 

Properties of Catalyst

 

The properties of our catalyst are as follows:

 

Characteristics Values
Porosity 0.45
Catalyst size 5µm
Pore size 100–300 µm
Maximum temperature of retention 500oC

 

Reactor Internals:

The reactor Internals are as follows:

  1. Inlet distributors
  2. Two fixed-Beds
  3. out let
  4. Spacing Between two beds.

 

 

4.5.1  DESIGN PROCEDURE

       Dimetrhyl ether = 0.65

Methanol           = 98.69%

Water                 = 0.66%

T                       = 250 oC ,

P                       = 14.7 bar

 Dimethyl ether = 57.32%

Methanol         = 19.83%

Water               = 22.85%

T                     = 364 oC ,

P                      = 14.2 bar

Material Balance of Reactor

 

 

 

Feed Feed composition Product Product Composition
Methanol 98% Methanol 19.6%
DME 1.6% DME 39.6%
H2O 0.4% H2O 40.8%

 

 

Mass in   =   Mass out

 

9926kg/hr  =   9926kg/hr

 

ENERGY BALANCE ACROSS THE REACTOR

 

Energy Balance across the reactor is given by the formula:

 

ΔH  = ∑HoR – ∑HR + ∑HP

HoR = Heat of reaction at the standard conditions

∑HoR                  2.72exp3    MJ/hr                                         

∑HR                    5.91exp4 MJ/hr                                     

∑HP                          6.00exp4    MJ/hr

Difference                3620 MJ/hr

 

Reference Temp.      = 25oC

Reference pressure = 101.2kpa

 

OPERATING AND DESIGN CONDITIONS

 

As we know that the design of the equipments is usually done at the conditions 10% more than the operating conditions so the operating and design conditions are as follows.

 

 

Operating and design conditions Values
Operating Temperature 250oC
Design Temperature 275oC
Operating Pressure 1535kpa
Design Pressure 1788kpa

DESIGN OF THE REACTOR

 

GHSV                               =             vol. flow rate of  feed

                                                                     vol. of catalyst

So,

Volume of Catalyst          =    Volume flow-rate of feed /GHSV

Where;

GHSV                               =    20hr-1

And

Flow rate of feed              =     9926kg/hr

So,

Volume of Bed 1              =    50m3

Volume of Bed 2              =    48m3

 

In this case we are dividing the total volume into two beds as for the case of single bed the height of bed becomes greater than 10m that can cause the problem of channeling.

 

THE TOTAL VOLUME OCCUPIED BY THE REACTOR IS;

 

Volume occupied by:

Distributor

Upper and lower spacing.

Head collector

Taking 15% for each bed:

Total volume of reactor=113m3

no of bed in reactor       =2

 

NOW L/D RATIO FOR OUR REACTOR THAT IS GIVEN IN THE PATENT IS = 8

 

So; By using The formula for the cylindrical volume;

 

V   = л r2L

L   = 21m        D  = 2.6m

 

 

PRESSURE DROP CALCULATIONS

 

Using Ergun equation:

 

δP/L = (1- Ø)/ Ø3 x (G/Dpδ+gc){(150(1-Ø)µ/Dp)+1}

 

Where;

δP = pressure drop

L = Length of reactor

Ø = porosity of the catalyst

G = Mass velocity of the feed

δ  = density of the fluid

µ = Viscosity of the feed

Now by putting the relevant values in the equation;

δP in bed 1 = 124kPa

δP in bed 2 = 113kPa

 

MECHANICAL DESIGN

 

MATERIAL OF CONSTRUCTION

 

Selection criteria:

Tensile strength and stiffness.

Effect of temperature rise.

Resistance to corrosion.

Cost.

 

The material of construction is Nickel-Base alloy.

 

REASONS FOR SELECTION ARE:

 

Better tensile strength and stiffness

Not much affected by temperature rise.

Resistant to corrosion.

Relatively cheaper.

Can withstand at extremely high pressure.

 

SHELL WALL THICKNESS

 

E= Pi – Di/(2jf – Pi )

Design temperature = 275oC

Design Pressure        = 1688kpa

Design stress             = f = 86.5*10-6/m2

Full radio graphic       = j = 1

Internal Diameter     = Di =2.6m

Taking 2mm as corrosion allowances:

Total thickness          = 163mm (about)

 

TYPES OF HEAD

 

  1. Torispherical

 

 

The head selected for our reactor is Hemispherical

.

TYPE OF SUPPORT

 

  1. Supports available for vertical columns:
  2. Conical skirt.
  3. Straight skirt.

 

The support selected is Conical Skirt support.

SPECIFICATION SHEET:

Equipment Fixed-Bed reactor
Equipment Code CRV-100
Function Catalytic dehydration of Methanol
Height (m)  21.1
Inside Diameter (m)  2.6
Wall Thickness (mm) 3
No. of Catalyst Beds  2
Design Temperature (°C)  275
Operating Temperature (°C)  250
Design Pressure (KPa)  1688
Operating Pressure (KPa)  1535
Catalyst  HZSM-5
Catalyst pore Size  30oA
Material of Construction  Nickel base Alloy
Head  Hemispherical
Support  Conical skirt

DISTILLATION COLUMN:

Distillation is defined as:

A process in which a liquid or vapour mixture of two or more substances is separated into its component fractions of desired purity, by the application and removal of heat.

TYPES OF DISTILLATION

EXTRACTIVE DISTILLATION:-

Extractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-vlatile component, the solvent, that forms no azeotrope with the other components in the mixture. Used for mixtures having a low value of relative volatility, nearing unity. Mixtures having a low relative volatility can not be separated by simple distillation because the volatility of both the components in the mixture is nearly the same causing them to evaporate at nearly the same temperature to a similar extent, whereby reducing the chances of separating either by condensation.

FLASH DISTILLATION

In the flash evaporation process is when the liquid is preheated and is then subjected to a pressure below its vapor pressure causing boiling or flashing to occur. Sea water is first heated in tubes and is then put in a chamber with a vapor pressure lower than in the heating tubes and the liquid evaporates. The vapors flash off the warm liquid and the salts exit with the remaining water. The process seems inefficient because the evaporation of a small amount causes the temperature to drop dramatically.

For example, when only 7.1% of the liquid is evaporated off, the temperature drops 40 degrees, from 100 to 60 degrees Celcius. However, since the design is so simple, it has become a close competitor to multiple-effect evaporation economically. This is true especially in larger plants.

STEAM DISTILLATION:-

Steam distillation is a special type of distillation (a separation process) for temperature sensitive materials like natural aromatic compounds.

Steam distillation is employed in the manufacture of etherial oils for, for instance, perfumes. In this method steam is guided over the plant material containing the desired oils. It is also employed in the synthetic procedures of complex organic compounds.

VACUUM DISTILLATION:-

Vacuum distillation is a method of distillation whereby the pressure above the solution to be distilled is reduced to less than one Atmosphere (unit) causing evaporation of the most volatile liquid(s) (those with the lowest boiling points.) Vacuum distillation is used with or without heating the solution; some distillation processes use both vacuum and thermal action.

Vacuum distillation works on the principle that boiling occurs when the vapor pressure of a liquid exceeds the ambient pressure (atmospheric pressure above it or pressure in the distillation apparatus.) In standard thermal distillation, the vapor pressure is increased. In vacuum distillation, the ambient pressure is decreased.

AZEOTROPIC DISTILLATION:-

Azeotropic distillation is any of a range of techniques used to break an azeotrope in distillation. A common distillation with an azeotrope is the distillation of ethanol and water. Using normal distillation techniques, ethanol can only be purified to approximately 95% (hence the 95% (190 proof) strength of some commercially available grain alcohols).

PLATE/TRAY TYPE COLUMNS :-

To optimize the mass transfer between vapor and liquid in thermal separations, various types of plates are used. Usually, trays are horizontal, flat, perforated to offer vapor passages, specially prefabricated metal circular sheets, which are placed at a regular distance in a vertical cylindrical column. On these elements, the reflux liquid circulates and encounters the vapor which must pass through it.

 

TRAYS HAVE TWO MAIN PARTS:

1) The part where vapor (gas) and liquid are being contacted;  the contacting area. And

 

2) The part where vapor and liquid are separated, after  having been contacted; the down comer area.

TRAY DEVICES:-

In the design of the distillation column, the most important step is estimation of number of stages (plates) or trays.

CLASSIFICATION OF TRAYS:-

Classification of trays is based on:

 

  • Type of plate used in the contacting area
  • Type and number of down comers making up the down comer area
  • Direction and path of the liquid flowing across the contacting area of the tray
  • Vapor (gas) flow direction through the (orifices in) the plate
  • Presence of baffles, packing or other additions to the contacting area to improve the separation performance of the tray.

SELECTION CRITERIA OF  ANY TRAY:-

The principal factors to consider when comparing performance of three types of plate i.e., sieve plate, valve tray, bubble-cap tray are as follows:-

COST:-

Cost of plate depends upon material of construction used. For mild steel, the ratio of cost between plates is

 

Sieve Plate     :    Valve Plate    :    Bubble-Cap Plate 

3         :          1.5            :             1.0  .

CAPACITY:-

There is little difference in the capacity rating of the three types (the diameter of the column required for a given flow rate).The ranking is

Sieve tray     >      valve tray    >      bubble-cap tray

OPERATING RANGE:-

Operating range means the range of vapor and liquid rates over which the plate will operate satisfactorily. Bubble-cap plates have a positive seal for liquid and so operate at very low vapor flow rates. Sieve trays rely on the vapor flow rate through holes to hold the liquid on the plate. And cannot operate at very low vapor rates. But with good design, it will give satisfactorily results.

PRESSURE DROP:-

 

It is an important parameter for plate design, particularly for vacuum process. It depends upon the plate design.On the basis of pressure drop the ranking is

 

Bubble-cap tray   >    valve tray    >    sieve tray

 TYPE OF  PLATES USED IN THE CONTACTING AREA:-

 

There are mainly three types of trays.

  1. Sieve Trays.
  2. Valve Trays.
  3. Bubble Cap Trays.

SIEVE TRAYS ( PERFORATED TRAYS):-

ADVANTAGES:-

  • They are simple to design.
  • These trays are very cheap.
  • Sieve trays give lower pressure drop.
  • It gives high efficiency.
  • They give us efficient mass transfer.

 DISADVANTAGES:-

  • When misoperation occurs, mass transfer ceases.
  • Due to misoperation, efficiency drops because of weeping.
  • Misoperations are foaming and forth formation depending upon the nature of liquid.
  • It is also sometimes unacceptable for low liquid loads when weeping has to be minimized.

 VALVE TRAYS:-

 ADVANTAGES:-

  • Although slightly more expensive than Sieve Trays.
  • They can operate over a wide range of pressure.
  • It gives low pressure drop.
  • Valve trays may be uses for a wide range of flow rates.
  • Because of their flexibility and price, they are replacing bubble cap trays.
  • It can operate at large capacities.
  • Their cost is less than bubble cap trays.
  • In this, entrainment and weeping are controllable.

 

DISADVANTAGES:-

  • It pressure drop is greater than sieve tray.
  • Their cost is 20% higher than sieve plates.

 ADVANTAGES:-

  • It is widely used due to its performance.
  • Entrainment is controlled here.
  • They are capable of dealing with very low liquid rates.
  • Weeping is also controlled in bubble cap trays.
  • They are useful for operation at low reflux.
  • They are required to provide maximum tray flexibility.

DISADVANTAGES:-

  • Bubble Caps are a relatively high cost than both sieve and valve trays.
  • Capacity of bubble cap trays is also lower than both sieve and valve trays.
  • It gives high pressure drop.
  • They cause problems in large columns.
  • Their construction is very complicated.

BASIC DISTILLATION EQUIPMENT AND OPERATION

MAIN COMPONENTS OF DISTILLATION COLUMNS

Distillation columns are made up of several components, each of which is used either to transfer heat energy or enhance material transfer. A typical distillation contains  Column internals such as trays/plates and/or packings which are used to enhance component separations everal major components

 

  • A vertical shell where the separation of liquid components is carried out
  • A reboiler to provide the necessary vaporisation for the distillation process
  • A condenser to cool and condense the vapour leaving the top of the column
  • A reflux drum to hold the condensed vapour from the top of the column so that liquid (reflux) can be recycled back to the column

The vertical shell houses the column internals and together with the condenser and reboiler, constitute a distillation column. A schematic of a typical distillation unit with a single feed and two product streams is shown below:

BASIC OPERATION AND TERMINOLOGY

The liquid mixture that is to be processed is known as the feed and this is introduced usually somewhere near the middle of the column to a tray known as the feed tray. The feed tray divides the column into a top (enriching or rectification) section and a bottom (stripping) section. The feed flows down the column where it is collected at the bottom in the reboiler.  Thus, there are internal flows of vapour and liquid within the column as well as external flows of feeds and product streams, into and out of the column. The vapour moves up the column, and as it exits the top of the unit, it is cooled by a condenser. The condensed liquid is stored in a holding vessel known as the reflux drum. Some of this liquid is recycled back to the top of the column and this is called the reflux. The condensed liquid that is removed from the system is known as the distillate or top product.

Heat is supplied to the reboiler to generate vapour. The source of heat input can be any suitable fluid, although in most chemical plants this is normally steam. In refineries, the heating source may be the output streams of other columns.  The vapour raised in the reboiler is re-introduced into the unit at the bottom of the column. The liquid removed from the reboiler is known as the bottoms product or simply, bottoms..

DESIGN OF DISTILLATION COLUMN

In industry it is common practice to separate a liquid mixture by distillating the components, which have lower boiling points when they are in pure condition from those having higher boiling points. This process is accomplished by partial vaporization and subsequent condensation

.5.1 CHOICE BETWEEN PLATE AND PACKED

      COLUMN

Vapour liquid mass transfer operation may be carried either in plate column or packed column. These two types of operations are quite different. A selection scheme considering the factors under four headings.

  1. Factors that depend on the system i.e. scale, foaming, fouling factors, corrosive systems, heat evolution, pressure drop, liquid holdup.
  2. Factors that depend on the fluid flow moment.
  • Factors that depends upon the physical characteristics of the column and its internals i.e. maintenance, weight, side stream, size and cost.
  1. Factors that depend upon mode of operation i.e. batch distillation, continuous distillation, turndown, and intermittent distillation.

The relative merits of plate over packed column are as follows:

  1. Plate column are designed to handle wide range of liquid flow rates without flooding.
  2. If a system contains solid contents, it will be handled in plate column, because solid will accumulate in the voids, coating the packing materials and making it ineffective.
  • Dispersion difficulties are handled in plate column when flow rate of liquid are low as compared to gases.
  1. For large column heights, weight of the packed column is more than plate column.
  2. If periodic cleaning is required, man holes will be provided for cleaning. In packed columns packing must be removed before cleaning.
  3. For non-foaming systems the plate column is preferred.
  • Design information for plate column are more readily available and more reliable than that for packed column.
  • Inter stage cooling can be provide to remove heat of reaction or solution in plate column.
  1. When temperature change is involved, packing may be damaged.
  2. Plates are mostly used for large diameter more than 0.6m

For this particular process, “DME, Methanol, Water ”, I have selected plate column because:

  1. System is non-foaming.
  2. Temperature is high.
  • Diameter is greater than 0.6 meter.

5.2   CHOICE OF PLATE TYPE

There are four main tray types, the bubble cap, sieve tray, ballast or valve trays and the counter flow trays. I have selected sieve tray because:

  1. They are lighter in weight and less expensive. It is easier and cheaper to install.
  2. Pressure drop is low as compared to bubble cap trays.
  • Peak efficiency is generally high.
  1. Maintenance cost is reduced due to the ease of cleaning.

5.3   DESIGNING STEPS OF DISTILLATION

         COLUMN

  • Calculation of Minimum Reflux Ratio Rm.
  • Calculation of Optimum reflux ratio
  • Calculation of Theoretical number of stages.
  • Calculation of Actual number of stages.
  • Calculation of Diameter of the column.
  • Calculation of Weeping point
  • Calculation of Pressure drop.
  • Calculation of Thickness of the shell.
  • Determination of Flow Parameters
  • Calculation of the Height of the column.

SPECIFICATION SHEET

Identification:

Item                      Distillation column

Item No.

No. required         2

Tray type             Sieve tray

Function:    Separation of DME from methanol and Water

Operation: Continuous

Material handled

Feed Top Bottom
Mass Flow Rate 9926.2 kg/hr 5752.5  kg/hr 4150.5kg/hr
Di-methyl Ether 57.28% 99.14% 0.053%
Methanol 19.69% 0.79% 45.5%
Water 23.02% 0.063% 54.4%
Pressure 1034 kPa 1028kPa 1043kPa
Temperature 88oC 45.48oC 155.8oC

DESIGN DATA:

No. Of Trays = 18                                                Active Holes = 5600

Pressure = 1035 kPa                                   Weir Height = 50 mm

Height of column =9.44 m                          Weir Length = 1.10 m

Diameter of column = 1.43 m                       Reflux Ratio = 1.067

Hole size = 5 mm                                         Tray Spacing = 0.46 m                                                   Pressure Drop Per Tray = 0.65 kPa               Active Area = 1.224 m2

Tray Thickness = 5 mm                                Flooding = 81 %

5.4  DESIGN CALCULATIONS OF DISTILLATION COLUMN :

FEED STREAM-I

 

 

 

   
       

OVERHEAD PRODUCT J

 

Component Moles (kgmol/hr) Mole Fraction
Di-methyl Ether 123.375 0.987
Methanol 1.4125 0.0113
Water 0.2 0.0016
  125  

 

 

BOTTOM PRODUCT K

 

Component Moles (kgmol/hr) Mole Fraction
Di-methyl Ether 0.048 0.00026
Methanol 59.578 0.3198
Water 126.65 0.6798
  186.3  

 

RELATIVE VOLATILITY(µ)

Relative volatilities of all the components are found by using partial pressure data, obtained from temperature by using Antoine equation.

 

Relative volatility of Di-methyl Ether = 2.6

Relative volatility of Methanol = 1.6

Relative volatility of Water = 1

Heavy Key   =   Water

 Light Key    =  Di-methyl Ether

STREAMS SPECIFICATIONS

Streams I J K
Pressure 1034Kpa 1028Kpa 1043Kpa
Temperature 88oC 45.48oC 155.8oC

 

As the Feed (stream-I) is entering the column at saturated conditions so;

q =  1

 

Now to find out reflux ratio R we use Underwood method:

 

µ1 xF1       +

µ1 – θ

µ2 xF2           +

µ2 – θ

µ3 xF3        

µ3 – θ

 

=    1 – q

       

 

Θ2 – 0.882 θ + 1.213 = 0

 

 

1 < θ <2.6

 

By hit and trial method

 

θ = 1.13

 

 

REFLUX RATIO

 

 

µ1 xD1       +

µ1 – θ

µ2 xD2         +

µ2 – θ

µ3 xD3        

µ3 – θ

 

= 1+ RMIN

 

RMIN = 0.821

 

 

R=1.3 * RMIN

 

 

R = 1.067

Calculation to Find No. of Stages

 

Ideal number of Stages can be found by Lewis Matheson Method, Using R = 1.067

 

Ln = R x D = 133.375 Kg mol/ hr
Vn = Ln + D = 258.375Kg mol/ hr
Lm = Ln + F = 444.675Kg mol/ hr
Vm = Lm – W = 258.375Kg mol/ hr

 

 

Top Operating Line

 

 

Yn =   xn+1 + xD

 

 

Di-methyl Ether

 

Yn =  0.516 xn+1  +  0.478

 

Methanol

 

Yn =  0.516 xn+1  +  0.00546

 

Water

 

Yn =  0.516 xn+1  +  0.00077

 

Bottom Operating Line

 

Ym =   xm+1 + xw

 

 

Di-methyl Ether

 

Ym = 1.72 xm+1   –   0.00019

 

 

Methanol

 

Ym = 1.72 xm+1   –   0.231

 

Water

 

Ym = 1.72 xm+1   –   0.49

 

 

After doing Plate to Plate calculations, the Ideal No. of plates were found to be

 

    Nideal = 12 

 

 

And 5th plate from the top is the feed plate.

OVER ALL COLUMN EFFICIENCY:

 

 

Where

 

μL = viscosity of liquid at mean tower temperature

μ w = viscosity of water at 293 k

Xf = Mol fraction of components in feed

Thus

Eo  =  66.67%

 

5. ACTUAL NUMBER OF STAGES:

 

 

Actual number of stages =     Ideal number of stages

Eo

 

Actual number of stages          = 18

 

Location of the feed plate =   5.00   =  7

                                              0.66

 

Thus 7th plate from top is the Feed plate.

 

MECHANICAL DESIGN OF SIEVE TRAYS

 

NET AREA REQUIRED

 

Maximum volumetric flow rate of vapors =  q=   Vm / dv

 

=    1.176 m3 / s (dv=1.34kg/m3)

Net area required, An    =    qv / Uf

 

An   =   1.412  m2

 

COLUMN CROSS SECTIONAL AREA

 

Column area  =   A=  An / 0.88

 

Ac = 1.61 m2

 

DIAMETER

 

Diameter = Dc = (4*Ac / 3.14) 0.5

 

= 1.43 m

 

DOWNCOMER AREA

 

Ad = 0.12 * Ac

=  0.1932 m2

NET AREA AVAILABLE

 

A= Ac – Ad

 

= 1.4168 m2

ACTIVE AREA

 

Aa = Ac – 2Ad

 

= 1.224 m2

 

HOLE AREA

 

A=  0.11 *  Aa

 

=  0.135 m2

 

 

 

 

WEIR HEIGHT AND WEIR LENGTH

 

Weir height  = hw   = 50 mm

 

Plate thickness = 5 mm

 

Hole diameter = dh = 5 mm

 

(Ad/Ac) * 100 = 12

 

From Figure 11.31, page 573, Coulson & Richidson, vol; 6, at (Ad/Ac)*100= 12

 

 

Weir length = lw / D=  0.77

 

lw = 1.10 m

 

WEIR LIQUID CREST

 

how(max) = 750 (Lm / dl*lw)2/3

 

= 750 (2.718/874.46 * 1.115)2/3

 

how(max) = 11.04  mm

 

hw + how  =  50 + 11.04

 

=   61.04  mm

 

From Figure 11.30, page 571, Coulson & Richidson, vol; 6, at hw + how = 61.7 mm K2 = 30.4

 

WEEP POINT

 

Uh(min) = [K2-0.9(25.4-dh)]/dv0.5

     Uh(min)  = 3.96 m/s

 

Actual Uh (min) based on active hole area is given as:

 

Actual Uh (min)   =  0.7*Vw/dv*Ah

 

Actual Uh(min)    =5.89  m/s

 

As, actual minimum velocity >       Uh(min)

 

So  No  Weeping.

 

 

DRY PLATE DROP

 

Maximum vapor velocity through holes = Uh(max) = qv / Ah

 

= 14.11 m / s

 

(Ah/Aa)*100 = (0.11)*100 = 11

 

From Figure 11.34, page 576, Coulson & Richidson, vol; 6, at (Ah/Aa)*100 = 11

Co = 0.85

h=  51(Uh / Co)2 (dv / dl)

 

= 22.37 mm

REISDUAL DROP

 

                   hr = 12.5 * 1000 / dl

 

= 14.29 mm

 

ht      =  hd + hr + (hw +how)

 

= 75.33 mm

 

TOTAL PLATE PRESSURE DROP

 

∆Pt = 9.81 * 10-3 * (ht) * dl

 

=  646.2Pa = 0.6462 kPa

 

DOWNCOMER LIQUID BACKUP/ LIQUID HEIGHT IN DOWNCOMER

 

Take hap =  hw-10  =  40 mm  = 0.04 m

 

Area under Apron = Aap =  hap*lw

 

=  0.0446 m2

 

As Aap is less than Ad = 0.174 m2 so use this value of Aap in the following equation

 

hdc = 166 * (Lw / dl * Aap)2

 

= 11.57mm

 

HD = ht + hdc + (hw + how)

 

=147.94 mm = 0.147m

 

 

RESIDENCE TIME

 

tr = Ad * hbc * dl /  Lm

 

= 9.14 sec

 

As residence time is greater than 3 sec, which is satisfactory.

 

ENTRAINMENT

 

Percentage Flooding = ( un / uf ) * 100

 

un = Vm/dv*An  = 0.999 m/s

 

Liquid Vapor Flow Factor = FLV =     V / δL]0.5

 

FLV = 0.062

 

Flooding velocity = uf = K1     

From Figure 11.27, page 570, Coulson & Richidson, vol; 6, at FLV­ = 0.062 K1 = 0.09

 

Flooding velocity = uf =1. 2 m/s

 

Percentage Flooding = (0.999 / 1. 2) * 100

 

= 81%

 

From Figure 11.29, page 570, Coulson & Richidson, vol; 6, at FLV = 0.062

 

Y = 0.047

 

Which is well below 0.1 (Satisfactory Result)

 

NUMBER OF HOLES

 

Number Of Holes = Ah / ah

 

= 0.135 / 1.964*10-5

 

Number Of Holes  =  6873

HEIGHT OF COLUMN:

 

No. of plates = 18

 

Tray spacing = 1.5 ft = 0.457 m

 

Distance between 18 plates = 7.769 m

 

Top clearance = 0.7769 m

 

Bottom clearance = 0.7769 m

 

Tray thickness = 5mm/plate

 

Total thickness of the plates = 0.005 * 18

 

= 0.009m = 0.0295

 

Total height of the column = Hc =  7.769 + 0.7769 + 0.7769 + 0.009

 

=  9.33 m

NOMENCLATURE OF DISTILLATION COLUMN

 

Aa = Active Area

Ad = Downcomer Area

Ah = Hole Area

An = Net Area available for vapor liquid disengagement

Ac = Column Area

Aap =Area of Apron

Ah = Hole Area

Ap = Area of Perforations

Co = Orifice coefficient

Dc = Column diameter

dl = Density of liquid

dv = Density of Vapors

dh = Hole diameter

DPt =Total Pressure Drop,Pa(N/m2)

Eo =Overall column Efficiency

F = Flow rate of feed

Flv = Liquid Vapor flow rate factor

DH = Clearances for top and bottom

hap = Height of Apron

Hc =Height of Column

HD=Height of liquid back up in Downcomer

hd = Pressure drop through dry plate

hdc = Head Loss

HK = Heavy Key

hr = Residual head

Hs = Plate spacing

ht = Total plate pressure drop,mm liquid

hw = Weir height

how = weir crest

K1 =Constant obtained from figure

K2 = Constant obtained from figure

LK = Light Key

Lm = Mean molal liquid flow rate of top and bottom

Ln = Flow Rate of liquid at top

Lw = Flow Rate of liquid at bottom

lw = Weir Length

Rmin =Minimum Reflux

tr = Residence time

Uc = Vapor velocity

Uf = Flooding vapor velocity

Uhmin = Minimum vapor velocity through holes

Vm = Mean Molal vapor flow rate of top and bottom

Vn = Vapor Flow Rate at top

Vw = Vapor Flow Rate at bottom

xD = Mass fraction of distillate

xF = Mass Fraction of Feed

xW = Mass Fraction of bottom

mf,avg = Viscosity at the average temperature of top and bottom

q = Root of equation

aavg =Ratio of relative volatility of light to heavy key at average column temperature

am=Relative volatility of methanol

aw= Relative volatility of water

aDME= Relative volatility of DME

heat exchanger design steps:

6.1   INTRODUCTION:

 

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 heat exchangers, the fluids are separated by a heat-transfer surface, and ideally they do not mix. Heat exchangers are used in the process, power, petroleum, transportation, air conditioning, refrigeration, cryogenic, heat recovery, alternate fuels, and other industries. Common examples of heat exchangers familiar to us in day-to-day use are automobile radiators, condensers, evaporators, air pre-heaters, and oil coolers.

 

In our project a number of heat exchangers are used . Here we will discuss heat exchanger used as

 

  • Condenser

 

  • Vaporizer

 

  • Cooler

 

All of these are shell and tube heat exchangers.

6.1.1  Selection Guide To Heat Exchanger Types

Type Significant feature Applications best suited Limitations Approximate relative cost in carbon steel construction
Fixed tube sheet Both tube sheets fixed to shell. Condensers; liquid-liquid; gas-gas; gas-liquid; cooling and heating, horizontal or vertical, reboiling. Temperature difference at extremes of about 200 oF Due to differential expansion. 1.0
Floating head or tubesheet (removable and nonremovable bundles) One tubesheet “floats” in shell or with shell, tube bundle may or may not be removable from shell, but back cover can be removed to expose tube ends. High temperature differentials, above about 200 oF extremes; dirty fluids requiring cleaning of inside as well as outside of shell, horizontal or vertical. Internal gaskets offer danger of leaking. Corrosiveness of fluids on shell-side floating parts. Usually confined to horizontal units. 1.28
U-tube;

U-Bundle

Only one tube sheet required. Tubes bent in U-shape. Bundle is removable. High temperature differentials, which might require provision for expansion in fixed tube units. Easily cleaned conditions on both tube and shell side. Bends must be carefully made, or mechanical damage and danger of rupture can result. Tube side velocities can cause erosion of inside of bends. Fluid should be free of suspended particles. 0.9-1.1
Double pipe Each tube has own shell forming annular space for shell side fluid. Usually use externally finned tube. Relatively small transfer area service, or in banks for larger applications. Especially suited for high pressures in tube (greater than 400 psig). Services suitable for finned tube. Piping-up a large number often requires cost and space. 0.8-1.4
Pipe coil Pipe coil for submersion in coil-box of water or sprayed with water is simplest type of exchanger. Condensing, or relatively low heat loads on sensible transfer. Transfer coefficient is low, requires relatively large space if heat load is high. 0.5-0.7
Plate and frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids, slurries, high heat transfer. Not well suited for boiling or condensing; limit 350-500 oF by gaskets. Used for liquid-liquid only; not gas-gas. 0.8-1.5
Spiral Compact, concentric plates; no bypassing, high turbulence.

 

Cross-flow, condensing, heating. Process corrosion, suspended materials. 0.8-1.5

 

 

 

 

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. They are the first choice because of well-established procedures for design and manufacture from a wide variety of materials, many years of satisfactory service, and availability of codes and standards for design and fabrication. They are produced in the widest variety of sizes and styles. There is virtually no limit on the operating temperature and pressure.

Applications

Shell and tube heat exchangers are frequently selected for such applications as:

  1. Process liquid or gas cooling
  2. Process or refrigerant vapor or steam condensing
  3. Process liquid, steam or refrigerant evaporation
  4. Process heat removal and preheating of feed water
  5. Thermal energy conservation efforts, heat recovery
  6. Compressor, turbine and engine cooling, oil and jacket water
  7. Hydraulic and lube oil cooling
  8. 6.1.1  Selection Guide To Heat Exchanger Types
    Type Significant feature Applications best suited Limitations Approximate relative cost in carbon steel construction
    Fixed tube sheet Both tube sheets fixed to shell. Condensers; liquid-liquid; gas-gas; gas-liquid; cooling and heating, horizontal or vertical, reboiling. Temperature difference at extremes of about 200 oF Due to differential expansion. 1.0
    Floating head or tubesheet (removable and nonremovable bundles) One tubesheet “floats” in shell or with shell, tube bundle may or may not be removable from shell, but back cover can be removed to expose tube ends. High temperature differentials, above about 200 oF extremes; dirty fluids requiring cleaning of inside as well as outside of shell, horizontal or vertical. Internal gaskets offer danger of leaking. Corrosiveness of fluids on shell-side floating parts. Usually confined to horizontal units. 1.28
    U-tube;

    U-Bundle

    Only one tube sheet required. Tubes bent in U-shape. Bundle is removable. High temperature differentials, which might require provision for expansion in fixed tube units. Easily cleaned conditions on both tube and shell side. Bends must be carefully made, or mechanical damage and danger of rupture can result. Tube side velocities can cause erosion of inside of bends. Fluid should be free of suspended particles. 0.9-1.1
    Double pipe Each tube has own shell forming annular space for shell side fluid. Usually use externally finned tube. Relatively small transfer area service, or in banks for larger applications. Especially suited for high pressures in tube (greater than 400 psig). Services suitable for finned tube. Piping-up a large number often requires cost and space. 0.8-1.4
    Pipe coil Pipe coil for submersion in coil-box of water or sprayed with water is simplest type of exchanger. Condensing, or relatively low heat loads on sensible transfer. Transfer coefficient is low, requires relatively large space if heat load is high. 0.5-0.7
    Plate and frame Composed of metal-formed thin plates separated by gaskets. Compact, easy to clean. Viscous fluids, corrosive fluids, slurries, high heat transfer. Not well suited for boiling or condensing; limit 350-500 oF by gaskets. Used for liquid-liquid only; not gas-gas. 0.8-1.5
    Spiral Compact, concentric plates; no bypassing, high turbulence.

     

    Cross-flow, condensing, heating. Process corrosion, suspended materials. 0.8-1.5

     

    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. They are the first choice because of well-established procedures for design and manufacture from a wide variety of materials, many years of satisfactory service, and availability of codes and standards for design and fabrication. They are produced in the widest variety of sizes and styles. There is virtually no limit on the operating temperature and pressure.

    Applications

    Shell and tube heat exchangers are frequently selected for such applications as:

    1. Process liquid or gas cooling
    2. Process or refrigerant vapor or steam condensing
    3. Process liquid, steam or refrigerant evaporation
    4. Process heat removal and preheating of feed water
    5. Thermal energy conservation efforts, heat recovery
    6. Compressor, turbine and engine cooling, oil and jacket water
    7. Hydraulic and lube oil cooling6.3  VAPORIZERS

      Vaporizers are heat exchangers, which are specially designed to supply latent heat of vaporization to the fluid. In some cases it can also preheat the fluid then this section of vaporizers will be called upon preheating zone and the other section in which latent heat is supplied; is known as vaporization zone but he whole assembly will be called a vaporizer.

      Vaporizers are called upon to fulfill the multitude of latent-heat services which are not a part of evaporative or distillation process.

      There are two principal types of tubular vaporizing equipment used in industry: Boilers and Vaporizing Exchangers. Boilers are directly fired tubular apparatus, which primarily convert fuel energy into latent heat of vaporization. Vaporizing Exchangers are unfired and convert latent or sensible heat of one fluid into the latent heat of vaporization of another. If a vaporizing exchanger is used for the evaporation of water or an aqueous solution, it is now fairly conventional to call it an Evaporator, if used to supply the heat requirements at the bottom of a distilling column, whether the vapor formed be steam or not, it is a Re-boiler; when not used for the formation of steam and not a part of a distillation process, a vaporizing exchanger is simply called a vaporizer. So any unfired exchanger in which one fluid undergoes vaporization and which is not a part of an evaporation or distillation process is a vaporizer.

       

      6.4   TYPES OF VAPORIZERS

      Some common types of vaporizers are

      • Vertical vaporizer
      • Indirect fluid heater
      • Tubular low temperature vaporizer
      • Electrical resistance vaporizer
      • Cryogenic vaporizer

       

      The commonest type of vaporizer is the ordinary horizontal 1-2 exchanger or one of its modifications, and vaporization may occur in the shell or in the tubes. If steam is the heating medium, the corrosive action of air in the hot condensate usually makes it advantageous to carry out the vaporization in the shell.

       

      In the case of vaporizer, however, operation is often at high pressure, and it is usually too expensive to provide disengagement space in the shell, since the inclusion of disengagement space at high pressures correspondingly increases the shell thickness. For this reason vaporizers are not usually designed for internal disengagement. Instead some external means. Such as an inexpensive welded drum, is connected to the vaporizer where in the entrained liquid is separated from the vapor.

       

      Forced and Natural – circulation Vaporizer. When liquid is fed to is fed by forced circulation. The circuit consists of a 1-2 exchanger serving as the vaporizer and a disengaging drum from which the un-vaporized liquid is withdrawn and recombined with fresh feed. The generated vapor is removed form the top of the drum.

      The vaporized may also be connected with a disengaging drum without the use of a recirculating pump. This scheme is natural circulation. It requires that the disengaging drum be elevated above the vaporizer. The advantages of forced circulation or natural circulation are in part economics and a part dictated by space. The forced-circulation arrangement requires the use of a pump with its continuous operating cost and fixed charges. As with forced-circulation evaporators, the rate of feed recirculation can be controlled very closely. If the installation is small, then use of a pump preferable. If a natural-circulation arrangement is used pump and stuffing box problems are eliminated but considerably more headroom must be provided and recirculation rates cannot be controlled so readily.

       

      The vaporization of a cold liquid coming from storage, the liquid may not be at its boiling point and may require preheating to the boiling point. Since the shell of a forced-circulation vaporizer is essentially the same as any other 1-2 exchangers, the preheating can be done in the same shell as the vaporization. If the period of performance of a vaporizer is to be measured by a single overall dirt factor, it is necessary to divide the shell surface into two successive zones, one for preheating and one for vaporization.

      The true temperature difference is the weighted temperature difference for the two zones, and the clean coefficient is the weighted clean coefficient.

       

      Vaporizers tend to accumulate dirt, and for his reason higher circulation rates and large dirt factors will often be desirable. Preference should be given to the use of square pitch and a removable tube bundle. Although it may reduce the possibility of using a 1-2 vaporizing exchanger for other services, the baffle spacing can be increased or staggered form inlet to outlet to reduce the pressure drop of the fluid vaporizing in the shell.

       

      Vaporizing Processes

       

      Vaporizers are called upon to fulfill the multitudes of latent-heat services which are not a part of evaporative or distillation processes.

      • Vaporizers are heat exchangers which are specially designed to supply latent heat of vaporization to the fluid.
      • So, any unfired exchanger in which one fluid undergoes vaporization and which is not a part of an evaporation or distillation process is a vaporizer.
      • Unlike Boilers (which use Fuel energy) they convert the Latent or sensible heat of one fluid into the latent heat of Vaporisation of another fluid.

      In the power-plant evaporator, for example, the upper 50 to 60 percent of the shell is used for the purpose of the disengaging the liquid entrained by the bursting bubbles at the surfaces of the pool. The disengagement is further implemented by the use of a steam separator in the shell. The mechanical design and the thickness of the evaporator shell, flanges, and the tube sheets are based upon the product of the shell-side pressure and the diameter of the shell. In majority of instances the pressure or vacuum is not great and the shell, flanges, and the tube thickness are nor unreasonable.

      In case of vaporizer, however, operation is often at high pressure, and it is usually too expansive to provide disengagement space at high pressure correspondingly increases the shell thickness. For this purpose vaporizers are not usually designed for internal disengagement. Instead some external means, such as an inexpensive welded drum, is connected to the vaporizer wherein the entrained liquid is separated from the vapor.

       

      Classification of Vaporizing Exchanges

       

      There is a greater hazard in the design of vaporizing exchangers than any other type of heat exchanger. For this reason it is convenient to set up a classification based on the method of calculation as employed for each distinct type of service. Each of the common classes below is distinguishable by some difference in calculation.

       

       

      A: Forced circulation vaporizing exchangers

       

      1: Vaporization in the shell

      • Vaporizer or pump-through reboiler with isothermal boiling
      • Vaporizer or pump-through reboiler with boiling range
      • Forced circulation evaporator or aqueous-solution reboiler

       

      2: Vaporization in the tubes

      • Vaporizer or pump-through reboiler with or without boiling range
      • Forced circulation evaporator or aqueous-solution reboiler

      B: Natural circulation vaporizing exchangers

       

      1: Vaporization in tubes

      • Kettle reboiler
      • Chiller
      • Bundle-in-column reboiler
      • Horizontal thermosyphon reboiler

       

      2: vaporization in the tubes

      • Vertical thermosyphon reboiler
      • Long-tube vertical evaporator

       

      SELECTION

       

      • The major distinguishing feature of a Kettle type vaporiser is its over-sized shell which allows adequate space required for the disengagement of vapours.
      • It has a negligible pressure drop in the shell side and it is horizontal so there is no need to give hydrostatic head.

      These factors make it the optimum selection for our process

      7.1   FACTORS AFFECTING CHOICE OF A

               PUMP:

      Many different factors can influence the final choice of a pump for a particular operation. The following list indicates the major factors that govern pump selection.

      • The amount of fluid that must be pumped. This factor determines the size of pump (or pumps) necessary.
      • The properties of the fluid. The density and the viscosity; of the fluid influence the power requirement for a given set of operating conditions, corrosive properties of the fluid determine the acceptable materials of construction. If solid particles are suspended in the fluid, this factor dictates the amount of clearance necessary and may eliminate the possibility of using certain types of pumps.
      • The increase in pressure of the fluid due to the work input of the pumps. The head change across the pump is influenced by the inlet and downstream reservoir pressures, the change in vertical height of the delivery line, and frictional effects. This factor is a major item in determining the power requirements.
      • Type of flow distribution. If nonpulsating flow is required, certain types of pumps, such as simple reciprocating pumps, may be unsatisfactory. Similarly, if operation is intermittent, a self-priming pump may be desirable, and corrosion difficulties may be increased.
      • Type of power supply. Rotary positive-displacement pumps and centrifugal pumps are readily adaptable for use with electric-motor or internal-combustion-engine drives; reciprocating pumps can be used with steam or gas drives.
      • Cost and mechanical efficiency of the pump.

      PUMP P-201

      The duty of P-201 is to pump methyl alcohol (99.5%) from 1 bar to 15.3 bar with a flow rare of 5838.1 Kg/hr. for this purpose the best choice is Positive displacement pump because the required pressure is to much high.

      PUMP P-202

      The duty of pump-202 is to pump a mixture of Dimethyl ether& methyl alcohol  with slight pressure development and the flow rate required is 4166.67 Kg/hr. Centrifugal pump is most suitable pump for such a service i.e. high flow rate and low pressure development.

      PUMP P-203

      The duty of pump-203 is to pump a mixture of Dimethyl ether, methyl alcohol & water with slight pressure development and the flow rate required is 1486 Kg/hr. Centrifugal pump is most suitable pump for such a service i.e. high flow rate and low pressure developmentINSTRUMENTATION & CONTROL

      Measurement is a fundamental requisite to process control. Either the control can be affected automatically, semi-automatically or manually. The quality of control obtainable also bears a relationship to the accuracy, re-product ability and reliability of the measurement methods, which are employed. Therefore, selection of the most effective means of measurements is an important first step in the design and formulation of any process control system.

       

      8.1   TEMPERATURE MEASUREMENT AND

               CONTROL

      Temperature measurement is used to control the temperature of outlet and inlet streams  in  heat exchangers, reactors, etc.

      Most temperature measurements in the industry are made by means of thermo-couples to facilitate bringing the measurements to centralized location. For local measurements at the equipment bi-metallic or filled system thermometers are used to a lesser extent. Usually, for high measurement  accuracy,  resistance  thermometers  are used.

      All these meters are installed with thermo-wells when used locally. This provides protection against atmosphere and other physical elements.

       

      8.2 PRESSURE MEASUREMENT & CONTROL

      Like temperature pressure is a valuable indication of material  state  and  composition.  In  fact,  these  two measurement considered together are the primary evaluating devices of industrial materials.

      Pumps, compressor and other process equipment associated with pressure changes in the process material are furnished with pressure measuring devices.  Thus pressure measurement becomes an indication of energy increase or decrease.

      Most pressure measurement in industry are elastic element devices, either directly connected for local use or transmission type to centralized location.  Most extensively used industrial pressure element is the Bourderi Tube or a Diaphragm or Bellows gauges.

       

      8.3   FLOW MEASUREMENT AND CONTROL

      Flow-indicator-controllers are used to control the amount of liquid. Also all manually set streams require some flow indication or some easy means for occasional sample measurement. For accounting purposes, feed and product stream are metered. In addition utilities to individual and grouped equipment are also metered.

      Most flow measures in the industry are/ by Variable Head devices. To a lesser extent Variable Area is used, as are the many available types as special metering situations arise.                            .

       

       

      8.4   CONTROL SCHEMES OF DISTILLATION

              COLUMN

       

      GENERAL CONSIDERATION

       

      8.4.1 OBJECTIVES

      In distillation column control any of following may be the goals to achieve

      1. Over head composition.
      2. Bottom composition
      3. Constant over head product rate. .
      4. Constant bottom product rate.

       

      8.4.2 MANIPULATED VARIABLES

      Any one or any combination of following may be the manipulated variables

      1. Steam flow rate to reboiler.
      2. Reflux rate.
      3. Overhead product withdrawn rate.
      4. Bottom product withdrawn rate
      5. Water flow rate to condenser.

       

       

      8.5   LOADS OR DISTURBANCES

      Following are typical disturbances

      1. Flow rate of feed
      2. Composition of feed.
      3. Temperature of feed.
      4. Pressure drop of steam across reboiler
      5. Inlet temperature of water for condenser.

       

      8.6   CONTROL SCHEME

      Overhead product rate is fixed and any change in feed rate must be absorbed by changing bottom product rate. The change in product rate is accomplished by direct level control of the reboiler if the stream rate is fixed feed rate increases then vapor rate is approximately constant & the internal reflux flows must increase.

       

      ADVANTAGE

      Since an increase in feed rate increase reflux rate with vapor rate being approximately constant, then purity of top product increases.

       

      DISADVANTAGE

      The overhead reflux change depends on the dynamics of level control system that adjusts it.

      9.1  INTRODUCTION

      A HAZOP survey is one of the most common and widely accepted methods of systematic qualitative hazard analysis. It is used for both new or existing facilities and can be applied to a whole plant, a production unit, or a piece of equipment It uses as its database the usual sort of plant and process information and relies on the judgment of engineering and safety experts in the areas with which they are most familiar. The end result is, therefore reliable in terms of engineering and operational expectations, but it is not quantitative and may not consider the consequences of complex sequences of human errors. The objectives of a HAZOP study can be summarized as follows:

      • To identify (areas of the design that may possess a significant hazard potential.
      • To identify and study features of the design that influence the probability of a hazardous incident occurring.
      • To familiarize the study team with the design information available.
      • To ensure that a systematic study is made of the areas of significant hazard potential.
      • To identify pertinent design information not currently available to the team.
      • To provide a mechanism for feedback to the client of the study team’s detailed comments.

       

      9.2  STEPS CONDUCTED IN HAZOP STUDY

       

      • Specify the purpose, objective, and scope of the study. The purpose may be the analysis of a yet to be built plant or a review of the risk of unexisting unit. Given the purpose and the circumstances of the study, the objectives listed above can he made more specific. The scope of the study is the boundaries of the physical unit, and also the range of events and variables considered. For example, at one time HAZOP’s were mainly focused on fire and explosion endpoints, while now the scope usually includes toxic release, offensive odor, and environmental end-points. The initial establishment of purpose, objectives, and scope is very important and should be precisely set down so that it will be clear, now and in the future, what was and was not included in the study. These decisions need to be made by an appropriate level of responsible management.
      • Select the HAZOP study team. The team leader should be skilled in HAZOP and in interpersonal techniques to facilitate successful group interaction. As many other experts should be included in the team to cover all aspects of design, operation, process chemistry, and safety. The team leader should instruct the team in the HAZOP procedure and should emphasize that the end objective of a HAZOP survey is hazard identification; solutions to problems are a separate effort.
      • Collect data. Theodore16 has listed the following materials that are usually needed.

       

      • Process description.
      • Process flow sheets.
      • Data on the chemical, physical and toxicological properties of all raw materials,, intermediates, and products.
      • Piping and instrument diagrams (P&IDs).
      • Equipment, piping, and instrument specifications.
      • Process control logic diagrams.
      • Layout drawings.
      • Operating procedures.
      • Maintenance procedures .
      • Emergency response procedures.
      • Safety and training manuals.
        • Conduct the study. Using the information collected, the unit is divided into study “nodes” and the sequence diagrammed in Figure , is followed for each node. Nodes are points in the process where process parameters (pressure, temperaturechange between nodes as a result of the operation of various pieces of equipment’ such as distillation columns, heat exchanges, or pumps. Various forms and work sheets have been developed to help organize the node process parameters and control logic information. When the nodes are identified and the parameters are identified, each node is studied by applying the specialized guide words to each parameter. These guide words and their meanings are key elements of the HAZOP procedure. They are listed in Table(9.1). Repeated cycling through this process, which considers how and why each parameter might vary from the intended and the consequence, is the substance of the HAZOP study.

         

        • Write the report. As much detail about events and their consequence as is uncovered by the study should be recorded. Obviously, if the HAZOP identifies a not improbable sequence of events that would result in a disaster, appropriate follow-up action is needed. Thus, although risk reduction action is not a part of the HAZOP, the HAZOP may trigger the need for such action. The HAZOP studies are time consuming and expensive. Just getting the P & ID’s up to date on an older plant may be a major engineering effort. Still, for processes with significant risk, they are cost effective when balanced against the potential loss of life, property, business, and even the future of the enterprise that may result from a major release.

         

        HAZOP Study of Storage Tank for Methyl Alcohol:

        A HAZOP study is to be conducted on methyl alcohol storage tank, as presented by the piping and instrumentation diagram show in fig(9.2).

        In this scheme, methyl alcohol is unloaded from tank trucks into a storage tank maintained under a slight positive pressure until it is transferred to the process. Application of the guide words to the storage tank is shown in Table(9.2) along with a listing of consequences that results from process deviation. Some of the consequences identified with these process deviations have raised additional questions that need resolution to determine whether or not a hazard exist.

        Deviations from operating conditions What event could cause this deviation Consequences of this deviation on item of equipment under consideration Process indications
        Level:

        Less

         

         

         

         

         

         

         

        More

         

         

         

        Temperature:

        Less

         

        More

         

        Tank runs dry

         

        Rupture of discharge line

        V-3 open or broken

        V-1 open or broken

        Tank rupture (busting of vessel)

         

        Unload too much from column

        Reverse flow from process

         

        Temperature of inlet is colder than normal

        Temperature of inlet is hotter than normal 

        External fire

         

        Pump cavitates

         

        Reagent released

         

        Reagent released

        Reagent released

        Reagent released

         

        Tank overfills

         

        Tank overfills

         

         

        Possible vacuum

         

        Region released

         

        Tank fails

         

        LIA-1

        FICA-1

        LIA-1, FICA-1

        LIA-1

        LIA-1

        LIA-1

         

        LIA-1

         

        LIA-1

      •  Environmental Impacts

        Dimethyl ether is heavier than air and may travel along the ground; distant ignition possible, and may accumulate in lowered spaces causing a deficiency of oxygen. It can form explosive peroxides under the influence of light and air. On combustion, forms irritating fumes. Reacts with oxidants.

        10.1 HEALTH HAZARD INFORMATION

        10.1.1 ACUTE HEALTH EFECTS:

        The following acute (short term) health effects may occure immediately or shortly after exposure to dimethyl ether:

        • Vapour can cause eye, nose & throat irritation.
        • High exposure can cause headache, dizziness, lightheadness, & even loss of consciousness.
        • Skin contact with liquid dimethyl ether can cause severe frostbite.

         

        10.1.2 CHRONIC HEALTH EFFECT:

        The following chronic (long term) health effects can occure at some time after exposure to dimethyl ether & lodt for months & years:

        • Cancer Hazard:

        According to the information presently available Dimethyl ether has not been tested for its ability to cause cancer in animals.

         

         

        • Reproductive Hazard:

        According to the information presently available Dimethyl ether has not been tested for its ability to effect reproduction.

        • Other long term effects:

        Dimethyl ether has not been tested for other chronic (long term) health effects.

        10.1.3 MEDICAL:

        Medical Testing :

        There is no special test for this chemical . However, if illness occure or overexposure is suspected, medical attention is recommended.

        Any evaluation should include a careful history of past & present symptoms with an exam. Medical tests that look for damage already done are not a substitute for controlling exposure.

        11.1  COST ESTIMATION

         

        An acceptable plant design must present a process that is capable of operating under conditions which will yield a profit.0^ Since, Net profit total income-all expenses

         

        It is essential that chemical engineer be aware of the many different types of cost involved in manufacturing processes. Capital must be allocated for direct plant expenses; such as those for raw materials, labor, and equipment. Besides direct expenses, many other indirect expenses are incurred, and these must be included if a complete analysis of the total cost is to be obtained. Some examples of these indirect expenses are administrative salaries, product distribution costs and cost for interplant communication.

         

        11.2 ESTIMATION OF EQUIPMENT COST

        Equipment Cost (Rs.)
        Vaporizer                             E-201 975000
        Reactor Cooler                    E-202 68640
        Condenser                           E-203 422500
        Condenser                           E-204 166400
        Rebioler                              E-205 1487200
        Condenser                          E-206 585000
        Reboiler                             E-207 3650400
        Exchanger                          E-208 624000
        Distillation Column           T-201 895000
        Distillation Column          T-202 1097980
        Reactor                              R-201 8359000

        11.3   ESTIMATIONOF TOTAL CAPITAL

                  INVESTMENT    Direct Cost (Rs)

         

        Purchased equipment cost       =       Rs. 18331120

        Purchased equipment installation = 0.47 ´ 18331120 = Rs. 8615626.4

        Instrumentation & Process Control = 0.12 ´18331120 = Rs. 2199734.4

        Piping (installed) = 0.66 ´18331120 = Rs. 12098539.2

        Building (Including Services) = 0.18 ´ 18331120 = Rs. 3299601.6

        Yard improvements = 0.1 ´ 18331120 = Rs. 1833112

        Service facilities (installed) = 0.7 ´ 18331120 = Rs. 12831784

        Land = 0.06 ´ 18331120 = Rs. 1099867.2

        Total direct plant cost = Rs. 60309384.8

         

        Indirect Cost 

        Engg & Supervision = 0.33 ´ 18331120 = Rs. 6049269.6

        Construction expenses = 0.41 ´ 18331120 = Rs. 7515759.2

        Total Indirect Cost = Rs. 13565028.8

        Total Direct & Indirect Cost    = Rs. 73874413.6

        Contractor’s fee = 0.05 ´  73874413.6 = Rs. 3693720.7

        Contingency = 0.1 ´  73874413.6 = Rs. 7387441.36

         

        Fixed Capital Investment = Total direct + indirect cost + contigency +

        Contractor’s fee

        = Rs. 84955576

        Total Capital Investment = F.C.I + W.C.

        Now

        W.C        = 0.15 (T.C.I)

        = 0.15 (84955576+ W.C)

        W.C  = Rs. 14992160

        T.C.I = 84955576 + 14992160

                           = Rs. 99947737

About Admin

Leave a Reply

Your email address will not be published. Required fields are marked *