Mem Guard Unit
Mem Guard unit consist of 5 vertical vessels. Mem Guard is basically a temperature swing absorber. It has packing of adsorbent on which water and heavier hydrocarbons are adsorbed. MG3 MG5 and COSMOX are the materials of packing. The two main steps of Mem Guard are adsorption and regeneration.
Adsorption: In this step gas from the main feed line enters from the top of the vessel. Moisture and heavier hydrocarbons are adsorbed on the surface of the adsorbent. Bone dry gas leaves from the bottom of the vessel. Gas leaving has moisture content less than 1 ppm. Among the 5 vessels 3 are in adsorption step while the rest are in regeneration step.
Regeneration: In this step the moisture from the packing material is removed in order to get it ready again for the adsorption of moisture. Regeneration step has two phases. The first is heating and then cooling. Two vessels are in regeneration step. One is in heating phase and one is in cooling phase. A slip line taken from the bone dry gas is taken. It is about 15-18 MMSCFD. This gas is at a temp of 75 F. It is passed from the top of the vessel which has completed its heating phase and is now in cooling phase. This will cool the vessel from 550 F to 75 F and moves to get heated in a heater at a high temperature of 550 F and then moves back to the Mem Guard unit. Now this gas will enter the last one vessel which is in heating phase.
In the regeneration step, after adsorption, vessel is first regenerated by the heated gas which enters from the bottom and leave from the top evaporating and taking all the moisture from the surface of the packing.
If we observe a vessel, every vessel is first in adsorption step then in heating phase and lastly in cooling phase. And if we observe the slip line he gas first enters the vessel which is in cooling phase and after being heated at high temp it moves to the vessel which is in heating phase.
Regeneration Gas Cycle: Hot gas after passing through the vessel for removing the moisture content moves to the cooler. Cooler is a fin tube Air cooler. This cools the air to a lower temperature. 3 fans are installed and the system is over designed so that if one fan is not working the cooler can still cool the gas. The gas is then passed through a scrubber to remove and condensed liquid. A boost up 50 psi is required to enter the gas in the main feed line again as after passing through number of vessel, pressure has dropped. A blower is installed after the scrubber to boost up the pressure by 50 psi. After achieving a pressure of 720 psi this gas in injected in the discharge line of inlet separator as it has the same composition as that of the raw feed gas. In case of failure of blower, the line has a lower pressure so it can’t be injected in the raw gas feed line. So, it is sent to the inlet of slug catcher as there the pressure is much less than this line.
Mem Guard is an automated system. All the valves open and close automatically. A control loop is installed in the system which closes and opens the valves after proper time. A total of 113 minutes are required for adsorption heating and cooling the vessel in which vessel is in adsorption state for 51 minutes. After 51 minutes the valve will close automatically and then valve of hot gas will open automatically. This technology increases the efficiency and life of the membrane unit a lot.
Bone dry gas from the Mem Guard unit moves to the primary membrane for the removal of carbon dioxide and hydrogen sulfide up to the desired limits.
Internals of Mem Guard Vessels: They are filled with the packing of the adsorbent. The material of adsorbent is MG3 MG5 and COSMOX. They adsorb the moisture on their surface. Below and above the packing are the layers of ceramic ball to avoid any loss of material moving away with the gas. Above the upper layer and below the lower layer are the supporting plates to support the packing material and ceramic balls.
Any material if passed through the ceramic ball or any dust particle if present in the gas is removed by passing the bone dry gas through two Particle Filters installed in parallel fashion, to remove it and protect our membrane unit. It is a vertical vessel and a filter is installed in it. A total of 45 filter cartridges are installed in each vessel. Gas from the bottom of the particle filter vessel enters and moves up passing through the filters and clean gas moves from the top to the membrane unit. When the pressure drop across the filters increases from a limit then filters are removed.
Membranes are thin semi permeable barriers that selectively separate some compounds from others. CO2, which falls into the category of acid gases, is found naturally in the Qadirpur gas stream up to the level of 6.5 %.The purpose of the Qadirpur gas processing to reduce the CO2 contents up to less than 2 %. In addition the removal of this acid gas is necessary because
- Acid gases are extremely poisonous and very harmful to humans.
- In combination with water, it is highly corrosive and rapidly destroys pipelines and equipment’s.
- CO2 also reduces the heating value of a natural gas stream and wastes pipeline capacity.
- In LNG plants, CO2 can causes freezing in the low-temperature chillers.
A wide variety of acid gas removal technologies are available. In Qadirpur gas plant membrane process is using for CO2 removal and gas sweetening. The Separex™ membrane system is designed to provide 500 MMSCFD of gas with < 2 % CO2. The membrane section consists of the existing membrane tubes, which contain the membrane elements configured for peak performance Membrane Material In Qadirpur Gas Plant Currently, the only commercially viable membranes used for CO2 removal are polymer based, e.g. Cellulose acetate, Polyamides, Polysulfide, Polycarbonates, and Polyetherimide. The most widely used and tested material is Cellulose acetate as used in UOP’s membrane system in Qadirpur. Cellulose acetate membranes were initially developed for reverse osmosis but are now the most rugged CO2 removal membrane available.
Membrane Elements: One of the forms of the membrane element is in the form of flat sheet. The flat sheets are combined into a spiral-wound element. In the spiral-wound arrangement, two flat sheets of membrane with a permeate spacer in between are glued along three of their sides to form an envelopes (leaf) that is open at one end. Many of these envelopes are separated by feed spacers and wrapped around a permeate tube with their open ends facing the permeate tube. More than one element are joined together and inserted into a tube. Multiple tubes are then mounted on skids in either a horizontal or vertical orientation. Skids with horizontal tubes are installed in Qadirpur plant.
Principle of CO2 Removal: The membranes used for CO2 removal do not operate as filters, where small molecules are separated from larger ones through a medium with pores. Instead, they operate on the principle of solution-diffusion through a non-porous membrane. Feed gas enters along the side of the membrane and passes through the feed spacers separating the envelopes. The CO2 1st dissolves into the membrane and then diffuses through the non-porous membrane. Because the membrane does not have pores, it does not separate on the basis of molecular size. Rather, it separates based on how well different compounds dissolve into the membrane and then diffuse through it. As the gas travels between the envelopes, CO2, and other highly permeable compounds permeate into the envelopes. These permeated compounds have only one outlet: they must travel within the envelope to the permeate tube. The permeate gas enters the permeate tube through holes drilled in the tubes. From there, it travels down the tube to join permeate from other tubes .Any gas on the feed side that does not get a chance to permeate leaves through the side of the element opposite the feed position. Because CO2, H2, He, H2S and water vapors, for e.g. permeate quickly, they are called “fast gases”. CO, N2, CH4, C2H6 and other hydrocarbons permeate less quickly and so are called “slow gases”. The membranes allow selectively removal of fast gases from slow gases.
The pre-treated feed gas enters the membrane skid, where the CO2 content is reduced to the sale gas specification. As the feed gas passes through the membrane tubes, the CO2 is selectively removed and collected in the low pressure permeate pipe. The residual gas is sent to the sale gas header. The residual header system combines the feed from the individual tubes, banks, and skids. The sales gas from the 1st membrane stage is cross-exchanged with the feed in the pre-cooler. As the sales gas is cooler than the feed gas, this provides heat recovery, which increases the overall efficiency of the system and lowers the chilling duty required. The sales gas is then sent to the gas metering system.