1500 TPD, methanol plant. Unit has Selas reformer, that feeds a common compression and methanol converter reaction system. Main components include (2) Lurgi reactors, a Clark compressors, GE turbines, SS and Adm Brass condensors and exchangers.
GENERALThe plant has been designed for the Low Carbon Concept process route producing 1360 STD of refined methanol. It is based upon production of methanol using only natural gas feedstock. Synthesis gas obtained from natural gas by the Low Carbon Concept process route contains a large excess of hydrogen over carbon compared to the relative amounts required for stoichiometric methanol synthesis. SELAS PROCESS FEED SYSTEM
Methanol cannot be synthesized (or converted) from natural gas directly in the Bishop process. The components of natural gas (methane, ethane, propane, etc.) must first be converted into intermediate compounds before they can then be synthesized into methanol in the Lurgi converters. The process used to form these intermediate compounds (carbon monoxide, carbon dioxide, and hydrogen) is steam reforming. Since the reforming reaction is endothermic (requires heat to sustain it), the reaction is carried out in what is termed a primary reformer which is actually a fired furnace. Natural gas-fired burners provide heat to the Selas Reformer. Since sulfur is a reformer catalyst poison, a desulfurizing system is in place to "scrub" the incoming natural gas for process feed. Two vessels filled with activated carbon are operated in series, and a third vessel contains zinc oxide. This scrubbing system removes primarily hydrogen sulfide (H2S). The process feed to the Selas Reformer is a mixture of 240 psig steam, "scrubbed" natural gas, and (at times) gaseous carbon dioxide. The carbon dioxide (CO2) is vaporized from liquid CO2 held in refrigerated storage tanks. The ratio of process steam to the carbon components of the process feed is called the steam to carbon ratio. This is controlled to prevent the formation of elemental carbon in the reformer tubes. This can result when insufficient oxygen (available in the steam, water) is present in the reforming reaction.
The process gas is passed through reformer tubes filled with a nickel in an aluminum-oxide base catalyst. The resulting product is a mixture (primarily) of hydrogen, carbon monoxide, carbon dioxide, and water. This is called "reformed gas." The reformed gas from the Selas Reformer passes through two waste heat boilers, a boiler feed water preheater, a set of fin fans, a knock out pot, trim cooler, knock out pot, and then to the suction of the Booster compressor.
The steam and natural gas are on flow ratio control (steam to carbon ratio). Both the steam and natural gas flows are temperature and pressure compensated. The ratio controller is tied into the reformer shutdown system.
The reforming reaction is endothermic or requires heat to sustain it. The Selas Reformer is a side-fired furnace with downflow in the process tubes. Natural gas-fired burners provide heat to the Selas Reformer, and an induced draft (ID) fan pulls combustion air into the firebox through openings in the burner assembly. The furnace is divided into two sections, east and west. Each section contains ninety catalyst filled tubes (4 1/2 in. X 40 ft.) in two staggered rows. There are eight rows of burners on the east and west side of each section, with each row containing twenty-four duradiant burners (768 total burners).
The combustion gas from each section goes into a common flue gas duct at the top of the furnace. The flue gas goes through a preheater section where the inlet process gas is preheated to reduce the heat load needed on the reformer tubes. After the preheater, is the steam superheater (HE-1984) where steam generated in the process waste heat recovery boilers downstream is superheated. The flue gas then goes through the induced draft fan (C-176) and then joins with the exhaust gas from the G.E. gas turbine. The combined flue gases then go to the economizer (HE-2607) where boiler feed water to all the waste heat boilers is preheated. After the economizer (HE-2607), the flue gases are ducted to the 240# steam boiler (HE-2728) and the BFW economizer (HE-2729).
SELAS WASTE HEAT BOILERS
The Selas waste heat recovery system serves two purposes. The process gas out of the Selas is approximately 1,575 F. Therefore, the primary purpose is to reduce this temperature to cause the water to separate from the reformed gas. This water is recovered in knock out pots and put into the process condensate system where it will be treated and put into the MS II Unit boiler feed water system. The second purpose of the heat recovery system is to heat up the boiler feed water needed to generate steam to operate equipment throughout the unit as well as to supply steam export to the Boilerhouse. This preheated boiler feed water is used in the MS I Area and the rest is returned to the Boilerhouse. Preheated boiler feed water is used in order to reduce the heat load required to produce steam.
The Selas waste heat recovery system utilizes heat from the reformed gas to preheat the Boiler Feed Water and produce steam. As the reformed gas is cooled from approximately 1,575 0F to 1000 F, water is removed via knock out pots. The condensate is collected in the unit process condensate system where it is treated and used in the MS II Area for boiler feed water. Preheated boiler feed water from the Selas Area is used in the MS I Area and the rest is returned for use at the Boilerhouse. Preheated boiler feed water reduces the heat required to produce steam. The steam generated is used in the unit and the excess is exported to the plant.
The waste heat recovery system consists of the following: HE-1542, HE-2955, HE-2607, HE-2551 (boiler feed water heat exchangers), HE-2728 (240 psig boiler), HE-2954, HE-1470, HE-1790, and HE-1501 (600 psig boiler), and HE-1984 (600 psig steam superheater). SELAS PRECOOLING BOOSTER COMPRESSOR
Reformed gas is delivered to the Booster compressor, C-327, at 104 F and 105 psig after being cooled in the waste heat recovery section of the Selas Reformer.
The reformed gas from the Selas Reformer goes through two waste heat boilers, a boiler feed water preheater, a set of fin fans, knock out pot, trim cooler, knock out pot, and then to the suction of the Booster compressor (C-327). After the Booster compressor, the heat generated in compression must be removed by means of a boiler feed water heat exchanger, fin fans, trim cooler, a knock out pot, and then to the Make-Up Gas (MUG) compressor suction.
The speed of C-327 is controlled by a pressure controller (PIC-204) which adjusts the governor on the compressor driver, PT-1220, to maintain a constant suction pressure to the compressor. This allows the compressor to compensate for small to moderate changes in the Selas reformed gas flow and still maintain a constant backpressure on the Selas Reformer. In order to prevent surging from occurring, a certain minimum flow must be supplied to the compressor suction. This is accomplished by a flow-controlled bypass (antisurge valve PDIC-203) from the discharge of C-327 back to its suction upstream of the cooling equipment. The compressor, in this manner, pumps enough gas to avoid the combination of conditions that could allow it to surge.
MAKE-UP GAS (MUG) COMPRESSION PROCESS DESCRIPTION
PROCESS GAS TO MUG
The combining of process gas and process steam and passing it through catalyst-filled tubes while heating them externally at high temperatures will result in reformed gas. Reformed gas is composed primarily of hydrogen, carbon dioxide, and carbon monoxide. Once the reformed gas exits the Selas Reformer, the objective is to lower the temperature of the reformed gas and remove water saturating the reformed gas. This objective has to be completed before the reformed gas from the Selas Reformer flows into the suction first stage of the MUG (C-328). In order to cool the reformed gas, it flows through a series of heat exchangers, fin fans, waste heat boilers, and water knock out vessels.
The reformed gas flows through the shell while methanol flows through the tubes which is acting as heat for the finishing column (T-288). This process helps reduce the cost of using 40# steam to heat up T-288. The reformed gas then flows into the waste heat reboiler knock out vessel (V-2353) where some hot condensate is separated from the reformed gas. The condensate now becomes process condensate that flows to the decarbonator (T-334) in preparation for the deaerator (V-2364) to be used as BFW. The overhead of V-2353 is reformed gas that flows to two heat exchangers fixed side by side, HE-2541 and HE-2780. In the purge gas preheater (HE-2541) reformed gas flows through the tubes and purge gas from the Lurgi loop to the expanders flows on the shell side. The purge gas firing preheater (HE-2780) has reformed gas on the tube side and purge gas on the shell side. The purge gas from the expanders is being preheated for the Davy Reformer to be used for firing fuel in the furnace. The reformed gas flow then goes to the reformed gas air cooler (HE-2545) which is made of ten fin fans. MS UNIT LURGI AREA PROCESS DESCRIPTIONLURGI CONVERTERS
The purpose of the Lurgi converter loop is to produce methanol using synthesis gas as the raw material. Synthesis gas from the discharge of the MUG compressor (C-328) is combined with synthesis gas from the high pressure separator (V-2359) at the suction of the recycle compressor (C-329). The discharge pressure of C-329 is controlled at approximately 846 psig.
The discharge flow from C-329 is preheated in interchanger HE-2553 (shell side) and fed to the two converters (V-2357 and V-2358) which are operated in parallel. The synthesis gas flows through the tubes of the converters, which are filled with a pellet-shaped catalyst. This catalyst is zinc/copper in an aluminum oxide base. The product is a gas stream containing methanol and synthesis gas, which was not converted due to catalyst efficiency.
Heat is evolved during this reaction is (exothermic) and is transferred to boiler feed water on the shell side of the converters. The boiler feed water comes from the Lurgi steam drum (V-2361) located above the converters. The water flows through downcomers, picks up heat in the converters, and the steam flows through risers to V-2361. The temperature of the water in the reactor shell is controlled by varying the steam pressure in steam drum V-2361 with the pressure controller (PRC-630). The product from the converters is passed through the interchanger (HE-2553) on the tube side where it is cooled by synthesis gas on the shell side. The converter product is further cooled by a final crude methanol condenser (HE-2827). Following the condenser, the flow goes to the high pressure separator (V-2359) where the liquid methanol is dropped out and the resulting synthesis gas stream is taken overhead. To provide pressure control of the converter loop, some synthesis gas from the V-2359 overhead stream is removed or "purged" from the loop. The volume of purge gas removed is controlled using PIC-231 valve based on the discharge pressure of C-329. The remaining synthesis gas from V-2359 overhead is sent to the suction of C-329. The liquid from V-2359 goes to the low pressure separator (V-2360). The overhead of V-2360 goes to the Boilerhouse as low pressure gas for firing their boilers. The base of V-2360 crude methanol goes to T-280 and T-288 via HE-2557 and HE-2569 for sales grade methanol, and to T-279 for MO grade methanol. MS UNIT PURIFICATION AREA PROCESS DESCRIPTION"A" TRAIN PURIFICATION (T-280 & T-92)
The purpose of T-280 is to purify crude methanol to sales grade specification. This is done by reducing the concentration of water, ethanol and light ends.
Crude methanol feed to T-280 is preheated in HE-2557 and HE-2569. Preheating the feed reduces the amount of steam required to maintain the columns temperature profile. To control internal corrision of the column, caustic is added to the feed to maintain a pH of 10. Samples of the residue flow are taken and analyzed for pH concentrations.
The sales grade methanol is removed from the upper portion of the tower at tray 56 as a sidestream flow via HE-2558 and HE-1927 and is sent to the rundown storage vessels. The overhead vapors taken off from the top of T-280 are condensed as they pass through a series of fin fan air coolers and heat exchangers. The condensed liquid is then fed back to T-280 at tray 60 as reflux. T-280 O.H. receiver (V-1387) non-condensable vapors are taken off the top of V-1387 via the "A" train DME compressor and sent via the burn line to the Boilerhouse as firing fuel. The ethanol and other impurities are removed from T-280 by means of an upper draw off locaated at trays 26 & 30 and from a lower draw off located at trays 5 & 7, this drawn off material is sent to the MO grade storage vessel (V-323). The water removed from T-280 base as a residue flow, is pumped to T-92. At T-92 methanol is stripped from the residue streams from T-280 and T-288. The overhead vapor either directed into T-280 when in operation or to T-288 which is in continious opertation as T-92. The hot residue flow from T-92 is pumped through HE-2557 and HE-2569 to preheat the methanol feed for T-280 and T-288. The T-92 residue flow then goes to the Waste Water Treatment Plant (WWTP). T-279 DEGASIFIER
The purpose of T-279 is to remove dissolved gases in the crude methanol liquid from the low pressure separator (V-2360). The methanol liquid exits the low pressure separator (V-2360) at approximately 90 psi. The methanol from V-2360 flows to T-279 as feed via LCV-608. T-279 pressure is controlled by "A" Train DME Compressor (C-256) suction at approximately 5.0 psi. As the crude methanol liquid enters T-279, saturated gases flash overhead into V-1386 where the liquid is batched back into T-279 base. The gases in V-1386 go overhead through cooling water exchanger HE-1490 to further condense any remaining liquid in the gas before it enters the "A" train DME compressor (C-256) suction liquid knock out vessel. The overhead gas from V-2469 enters the C-256 suction at approximately 5.0 psi and is compressed to approximately 25 psi before entering the low pressure purge gas line that serves as boiler fuel to the main Boilerhouse. The liquid from V-2469 is pumped to MO grade methanol storage vessel V-323 with the base of T-279, or the flow can be diverted to T-279 along with T-280 and T-288 side draws. T-288 PURIFICATION COLUMN
The purpose of T-288 is to purify crude methanol to sales grade specification. This is done by reducing the concentration of water, ethanol and light ends. Crude methanol is fed to T-288 after it has been preheated in HE-2557 and HE-2569. This helps to reduce the required steam for the tower. T-288 has two waste heat reboilers, HE-2539 and HE-2540, that receive their heat from hot DPG reformed gas, and are able to supply approximately 64% of the normal reboiler requirements. Steam reboiler HE-2578, which uses 40# steam, is used as a trim reboiler to supply the remaining 32% of the heat. HE-2578 is sized to handle the full distillation required if reformed gas is not available.
Caustic is added to the feed (preheated crude methanol) to reduce corrosion in the tower. This caustic flow is added to maintain a 10 pH in the tower residue stream. The sales grade methanol is removed from the upper portion of the tower (at trays 70, 72, 74 & 76) as a sidestream flow and sent to the rundown storage vessels (V-1390, V-1391 & V-1392). The overhead vapor is taken off the top of T-288 and condensed in the overhead condensers (HE-1575 & HE-1576) and sent to the overhead receiver V-104. The liquid in V-104 is fed back to T-288 as reflux (at tray 79). The noncondensables in V-104 are taken off the top of the vessel via the "B" train DME compressor (C-332) and sent to the Boilerhouse to be used as fuel.
The ethanol and other impurities are removed from the side of T-288 by means of an upper draw-off (at trays 22, 26, 30 & 36) and a lower draw-off (at trays 6 ,8 & 10) and are sent to the MO grade storage vessel, V-323. The water is removed from the base of T-288 as a residue flow is sent to T-92. At T-92 methanol is stripped from the residue streams from T-280 and T-288. The overhead vapor either directed into T-280 when in operation or to T-288 which is in continious opertation as T-92. The hot residue flow from T-92 is pumped through HE-2557 and HE-2569 to preheat the methanol feed for T-280 and T-288. The T-92 residue flow then goes to the Waste Water Treatment Plant (WWTP).
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