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Methanol Plant Control Scheme by Taking Natural Gas as Raw Materials

Methanol production is generally to produceindustrial chemicals—methanol by making use of carbon monoxide and ammonia insynthesis ammonia production process. This type of chemical equipment is notonly to produce synthesis ammonia, but also to produce methanol. As for itsmaterial balance, feed gas shall be different from that for simply producingsynthesis ammonia; meanwhile, it is different from simply manufacturing feedgas synthesis ammonia to produce methanol in synthesis ammonia productionprocess. Therefore, ammonia-methanol coproductionrequires feed gas to not only meet synthesis ammonia production, but also havethe characteristics of methanol production, giving attention to the two thingsand not neglecting any aspect.

In ammonia-methanolcoproduction, methanol products need to consume a certain amount ofmaterials, synthesis ammonia production must require 3 moles of hydrogen and 1mole of nitrogen, and methanol production requires 2 moles of hydrogen and 1mole ofcarbon monoxide. Thecontain of carbon monoxide in feed gas must be determined according to theproportion of methanol output in the entire production, reactivity of methanolsynthesis catalyst and methanol conversion rate of carbon monoxide. Therefore,there shall be the method to adjust and change carbon monoxide in feed gas inthe manufacture of feed gas.

When producing methanol in ammonia synthesis,the proportions of hydrogen, carbon monoxide and nitrogen in feed gas becomecomplicated. After the feed gas is synthesizedinto methanol through reaction, the residual gas should just make hydrogennitrogen ratio in the relationship of 3:1 after eliminating impurities. Theamount of hydrogen consumed in the production of methanol is not onlydetermined by the amount consumed of feed gas in the reaction with carbonmonoxide, but also related to the remaining carbon monoxide in the gas afterreaction. Therefore, the manufacture of feed gas is not maintain the simple hydrogenand nitrogenratioof 3:1 inpure synthesis nitrogen production or hydrogen and nitrogen ratio of 2:1 in pure methanol production, and italso needs to adjust the proportion relationship among hydrogen,nitrogen and carbon monoxide. Of them hydrogen is the main part that needs tobe consumed by the two synthesis reactions at the same time, and it is thespecial requirements of ammonia-methanol coproduction for feed gas manufactureto maintain the final proportion balance of hydrogen and nitrogen in suchcomplex relationship.

Introduction of process flow

Introduction of Process by Taking Natural Gasas Raw Materials

Methanol production process takes natural gasas raw materials, which is mainly composed of methane and ethane, and methanolprocess is mainly divided into three sections: natural gas conversion section,crude methanol section and refine methanol section. Zinc oxide desulfurizationis carried out for raw materials—natural gas after compression, and thennatural gas and steam goes into the converter according to a certain proportionand they are converted into H2, CO and CO2 under the action of catalyst.Converted gas goes into change furnace, changed gas goes into decarburizationsystem, decarburization gas goes into alcoholization and alkylation of crudemethanol process through compressor and produce crude methanolproducts, crude methanol products goes into deecheriza, compressiondistillation and atmospheric distillation of refine methanol process to becomerefine methanol products.

Parallel alcohol process has the followingadvantages:

Synthesis ammonia and methanol systems are independent of each other,which avoids constraining and influencing each other, and raw gas production,conversion and purification can follow their own process requirements;

Thereis separate methanol production line, so you can select different synthesispressures for methanol, such as low-pressure method (5.0 ~ 8.0MPa),medium-pressure method (11.0 ~ 13.0MPa) or high-pressure method (35.0 ~30.0MPa); the synthesis purge gas of the two parts may passthrough purge gas recovery unit, such as pressure swing adsorption, so as toreturn CH4 to synthesis ammonia system or methanol system, return H2 tomethanol system for adjusting (H2-CO2) / (CO + CO2).

In ammonia-methanol coproduction,under the circumstance that the production capacity of “total ammonia” isunchanged, methanol production capacity is shown by alcohol ammonia ratio(methanol production / total ammonia production), which can be adjusted withina certain range, and the adjustment way is generally to change the proportion ofH2/CO in feed gas, accurately, it is to adjust (H2-CO2): (CO + CO2) = F.Therefore, in ammonia-methanol coproduction, in addition to means to adjusthydrogen nitrogen ratio in synthesisnitrogen, there must be control means to adjust F value. In general, it usuallyadjusts the CO content in feed gas through changing CO conversion rate intransformation reaction or setting up a shortcut at inlet or outlet of changefurnace during ammonia-methanol coproduction, so as to adjust alcohol ammoniaratio within a certain range.

Ammonia-methanol coproduction is a link ofammonia synthesis process, so methanol production may affect the ammoniasynthesis and the production of the entire system, for example, activity ofcatalysts, starting, stopping and operating changes of methanol synthesis toweras well as other reasons may cause changes to CO + CO2 content in copper washgas, lead to fluctuations of copper wash load and may affect the normalproduction of ammonia synthesis tower; the gas-liquid separation of methanolsynthesis tower may affect the composition of copper solution; under abnormalmethanol production or accident conditions, the production of synthesis ammoniashould be maintained.

Main control scheme

Methanolprocess is mainly divided into three sections: feed gas conversionsection, crude methanol section and refine methanol section.

Feed GasConversion Section

Typical control is water / carbon ratioadjustment at the inlet of conversion furnace, and the general ratio isdesigned to be controlled within 2.5 to 3.5. If water / carbon ratiois too high, it will increase energy consumption and reduceproduction; if water / carbon ratio is too low, the catalysts in converter willbe charred and damaged, and therefore accidents will be caused. General water /carbon ratio control loop is designed to be single-loop ratio control, ratioalarm and interlock protection system are designed. Anti-surge control systemand compressor interlock protection system is set up for natural gas compressorsystem. Other flow, pressure, temperature and level are mostly controlled bysingle loop, which is more practical and reliable.

Crude MethanolSection and Refined Methanol Section

It mainly uses temperature, level and flow andpressure control, the control scheme is relatively simple, mostly controlled bysingle loop, which is practical and reliable.

In the methanol production process, in order toensure the quality of intermediate product (crude methanol) and the finalproduct (refined methanol) as well as the continuous operation of technologicalprocess, automatic measurement must be carried out for all process parameters.As for main raw materials crude methanol and auxiliary materials includingcaustic soda, soft water, and heating steam, electric power and cooling wateras well as finished product of methanol, reliable data is provided throughright measurement as accounting and comparison basis for various technical andeconomic indicators. Distillation process is the process that rising steam andfalling liquid carry out heat exchange and mass transfer at slab, which ismainly composed of three balances:

①Material balance—balance of feed quantity and produced quantity.

② Heatbalance—balance of heat input and heat output.

③Vapor-liquid balance—balance of rising steam and falling liquid.

Components in liquid mixture are separatedaccording to their different boiling temperatures, in order to maintain theboiling temperatures of these components at different locations ofrectification tower constant, the temperature of heat source forheating—steam shall be unchanged, therefore, the heating steam pressure isadjusted at 0.35Mpa + 0.01 inoperation; secondly, the flows should be controlled, such as feed quantity,water filling quantity, return flow and so on, so as to ensure the normaloperation of tower operation and obtain qualified products.

In the design of distillation tower unit,automatic control is carried out for important operation indicators, such asautomatic adjustment of steam pressure, automatic adjustment of bottom level,automatic display of bottom level, the finished mining temperature is reachedthrough 26th board of distillation tower with remote control manual station;the main flows are controlled, such as crude alcohol into rectificationtower after deecheriza, crude alcohol into pre-tower, pre-tower returnflow, main tower return flow, produced quantity, pre-tower water fillingquantity and so on, electric remote metal rotameter isused; the extraction of head product and high boiling point materials as wellas lye flow added into pre-tower should be controlled. There are more than 50control points including temperature, pressure, flow, level and analysis. Thecorrect temperature indication is an important means of operation, sotemperature control points account for more than 50%. In the design, moreadvanced explosion-proof f platinum RTD is generally selected as meter element.

1. Control of material quantity into tower anddischarge quantity at the bottom of pre-tower

Control Principles:

① When selecting main tower feed quantity tocontrol system flow (discharge liquid quantity at the bottom of pre-tower),pre-tower feed quantity uses bottom level control.

② When selecting pre-power feed quantity tocontrol system output, the discharge liquid quantity at the bottom of pre-toweruses bottom level control.

2. Control of return flow

Return flow is bottom evaporation capacity,which is not only the key parameter for distillation tower but also the majoradjusting object in production. It is directly related to feed quantity, andthe return flow ratio is often shown by the ratio of return flow and feedquantity. Usually, it is controlled through adjusting heating quantity ofreboiler at the bottom of tower, but the heating quantity ofreboiler is affected by condensation water level, so it should be controlled bythe discharge quantity of condensation water. Now the feed quantity increases,the ratio of return flow and it decreases, the level of reboiler will beautomatically lowered, that is, condensation water quantity increases, theheating quantity of reboiler increases, so the return flow increasesaccordingly to create a new balance.

3. Relationship between condensation water addingquantity and bottom temperature

The temperature at pre-tower is controlled bycondensation water adding quantity. If the bottom temperature is too low, thecondensation water entering quantity can be increased, and if the temperatureis too high, the entering quantity can be reduced.

4. Adding quantity of lye

The adding quantity of lye is mainly controlledby the PH value of crude methanol, and the lye adding valve is controlled byreproduction monitoring instrument. When PH value is too high, the quantity ofadded lye should be reduced; when PH value is too low, the quantity of addedlye should be increased.

5. Control of feed quantity

Main tower feeding and pre-tower bottomdischarging is the major parameter of rectification capacity of main tower, andother parameters change with it.

6. Control of return flow

Return flow is one of the key parameters ofmain tower operation, and it is also the main adjusting object. Generally,return flow ratio should be controlled at about 2, which is controlled throughadjusting the heating quantity of reboiler at the bottom of tower.

7. Produced quantity of refined methanol

The purchased quantity of refined methanol isdirectly related to the temperature distribution of the tower, and theimbalance of produced quantity and feed quantity directly affect the quantityof methanol. When the produced quantity is balanced with feed quantity, the materialcomposition and temperature of plates at each layer are basically constant, sothe quality of methanol produced complies with the standard. When producedquantity is less than feed quantity, there is methanol beyond the provision inthe discharge liquid, the produced valve must be turned up; when producedquantity is more than feed quantity, the water content in methanol produced ishigher than product standard, the produced valve should be tuned down.

8. Discharge capacity of bottom residual liquid

In order to maintain the normal bottom levelwhen discharging bottom residual liquid, it is controlled by drain valve at thebottom of tower.

Rectification system is mainly composed ofpre-tower and main tower, so an automatic control system can be establishedthrough connecting the automatic system of pre-tower and main tower. System-wideself-control main tower feed adjustment is taken as the control adjustment ofoutput. If output adjustment selects pre-tower feed control, main tower feed iscontrolled by pre-tower bottom level. When methanol output increases, the maintower can receive the signal to increase feed only through turning up the maintower feed valve, pre-tower bottom will receive the signal to reduce level andautomatically adjust crude methanol feed, so as to ensure the balance of bottomlevel. The increase in feed quantity of crude methanol makes the originalreturn flow ratio out of balance. The condensation waterlevel adjustor is automatically adjusted through ratio controller,and the condensation water level is adjusted through drain valve of condensationvalve, so as to achieve a new balance. The increase in feed quantity may causeschanges to the temperature at the bottom of pre-tower, the condensation feedingquantity controlled by bottom temperature will automatically track theadjustment, so as to make the bottom temperature maintain within the ratedindicators. PH value detector will automatically adjust the quantity of addedlye.