I. Executive Summary
The objective is to produce 5000 MT/day of AA methanol from Montana sub-bituminous coal. The technology selected for coal gasification was the Texaco entrained-flow model. A coal-slurry enters a reactor modeled by RGIBBS in ASPEN Plus. The reactor operates at 1316 ºC and 5.17MPa using PENG-ROB thermodynamic model. 6400 MT/day of sub-bituminous coal is reacted with 4500 MT/day O2 to produce synthesis gas. ASPEN Plus was used to model the acid gas removal of syngas from a Texaco entrained-flow coal gasification process and also used to model the water-gas shift reactions. The acid gas removal process has a high selectivity of H2S to CO2, at a ratio of 17:3. The treated syngas had 3.09 MT/hr of H2S and 0.74 MT/hr of CO2 removed, respectively. This meets the requirement of a sulfur level in the syngas of 0.1 ppmv. The H2S proceeded through two Claus Reactors modeled with RGIBBS and the thermodynamic model ELECNRTL, operating at 1000 ºC and 300 ºC. In the first Claus reactor, the conversion was about 30% and the second was 97%. Overall, 2.77 MTon/hr elemental sulfur is formed through the Claus processes.
For the water-gas shift reactors, RGIBBS and PENG-ROB thermodynamic model was used to model it. The water-gas shift reactor allowed the gasifier product stream to be shifted to the desired stoichiometric ratio of 2:1 (H2: CO2) for the production of methanol. The HTS and LTS reactors operated at 370 ºC and 200 ºC and 5.17 MPa. 33.33 MT/hr and 91.67 MT/hr of steam entered the HTS and LTS reactors, respectively. The final 2:1 stoichiometric is achieved by producing 1724.4 kmol/hr of H2 and 860.4 kmol/hr of CO, necessary for the methanol synthesis step.
To model methanol synthesis, the thermodynamic model is RK-SOAVE using five RPlug adiabatic reactors operating at 10 MPa and 250 - 300 ºC. From this process, 5,316 MT/day raw methanol is produced. Ethanol, an inevitable byproduct, is also produced at 53.2 MT/day which is to be removed with water by distillation.
For the methanol refining step, a tertiary system of methanol-ethanol-water is separated using a single distillation column (RadFraq) in Aspen. The thermodynamic model selected is UNIQUAC. The distillation requires four 120 stage distillation columns and a reflux ratio of 20 in order to recover the 99.89 % mole recovery of AA methanol specification. A total condenser and a partial reboiler are used. 5,292.9 MT/day AA methanol is produced from the process.
Capcost was used to estimate the combined capital cost which was $44.49 million. The consumables costs and utility costs associated with the coal gasification process was $235 million/year. The profit from the production of 5,292.9 MT/day of methanol is annually, $592.8 million. The depreciation value with an assumed no salvage value is $2.96 million/year. The IRR was computed for 17.7 % and the payback period is about 2 years. Although the project is economically feasible, according to the results of the design, coal gasification is marginally profitable.
II. Overall Project Scope Description
As the prices for crude oil increases, methanol has been sought as an alternative source of energy. It is estimated that the United States has about 265 billion tons of coal reserves. It has been suggested that methanol may be used as a foundation for meeting future energy needs and requirements. The objective of the project is to provide a design and economic evaluation for a coal-to-methanol process and determine the economic viability of the project. The target is to produce 5000MT/day of AA methanol while meeting safety and environment regulations. The coal-to-methanol process used for this project typically incorporates the following process operations, shown in Figure 1.
Fig 1. Block diagram of coal-to-methanol process.
First, coal is pre-processed by methods such as crushing, sizing and drying to prepare it for the coal gasification process. A coal source must be selected between Martin Lake Texas Lignite, Montana Sub-Bituminous, and Illinois #6 Bituminous. During the coal conversion process, the coal is decomposed in a high pressure and temperature using steam and an oxygen supply, represented by the following equation:
where the stoichiometry of the reaction is depended on coal composition. The result is synthesis gas which then proceeds through an acid gas removal step.
During acid gas removal, the hydrogen sulfide and carbon dioxide are recovered in a solvent absorption process. Then, the H2S enters a Claus unit where is it converted into elemental sulfur according to the following equation (eq. 2):
The elemental sulfur may be further used in other chemical applications. Some of the CO2 is removed in this process and it helps prepare the syngas for the water-gas shift reaction. Next, the synthesis gas must be prepared by going through a water-gas shift reactor to achieve the desired syngas ratio (H2:CO) of 2:1. The water-gas shift reaction is a commonly used method for further enhancing the yield of hydrogen. The governing equation for water-gas shift reactor is as follows:
A methanol reactor is used to convert the synthesis gas coming out of the water-gas shift reactors into the desired methanol product. The amount of water formed in methanol synthesis shows that this amount is more than what it has been calculated thermodynamically bases on water gas shift reactor. As a result the surplus of water must result from the direct synthesis of methanol from CO2 which produced water (3,4). In addition, water gas shift reaction is a slow reaction compared to methanol formation reaction. As a result the contribution of water gas shift reactor to the amount of water formed is negligible. Since hydrogenation of CO only results in methanol formation, it can be conclude that the water formed will be the result of the methanol formation from CO2. The reaction for methanol synthesis is given as (eq. 3,4):
The equilibrium constant for the methanol synthesis reaction is shown in the following expression (eq.6):
III. Design Basis, Principles and Limitations
A. Coal Selection
Due to the objective of creating methanol as a reasonable alternative to crude oils, minimizing the cost was crucial in the selection of the coal. These include choosing the feedstock which is the cheapest based on the amount of carbon present. Due to environmental regulation, the gas, liquid, and solid effluents must be treated. Hydrogen Sulfide (H2S), carbon dioxide (CO2), and ash have high costs associated with their removal. The figures comparing the different types of coal are featured in Appendix A.
The main concept considered in choosing the type of coal is the carbon percentage. Based on the dry basis alone, the Martin Lake Texas Lignite is the best choice. However the coal received with high percentage moisture increases the amount of coal required. Martin Lake coal has the highest percentage of moisture, which will require a higher amount of coal to make the required methanol, which results in higher production costs. The amount of sulfur present must also be considered. With the data given, the Montana Sub-Bituminous has the lowest percentage of moisture as well as sulfur. This will lead to less sulfur which needs to be treated in the acid gas removal step. The total cost for each source has been calculated in Appendix A which it can be concluded that Montana coal will be the best choice.
B. Gasifier Selection
Coal gasification is a process by which coal is converted into a gaseous form composed of its component parts and oxygen, primarily forming CO and H2. Coal gasification processes are generally classified as one of three categories: moving bed, fluid bed and entrained flow gasifiers. Each type of gasification process can be characterized by several properties: oxidant demand, steam demand, process temperature and the actual physical method of processing which may or may not lead to complications such as clogging and various safety concerns.
A moving-bed gasifier uses a bed of coal moved downward through a reactor by gravity. A counter-current flow of oxygen is pumped in through the bottom of the reactor. Combustion occurs in the bottom of the reactor moving heat to the central region of the reactor where gasification occurs. This type of reactor uses the smallest amount of oxidant of the three, and because the gases leaving the gasifier make contact with the cooler fuel, the syngas leave the reactor at a lower temperature than the other two processes. While this process is the cheapest in terms of oxidant and safest due to the low temperature range, the syngas is heavily contaminated with hydrocarbons which must subsequently be removed.
Fluid bed gasifiers are characterized by the quick mixing of fuel particles with oxidant and steam in a fluid bed. All three components are pumped in from the bottom of the reactor and are mixed with materials already undergoing gasification. This mixing leads to a uniform distribution of solids and gases within the reactor. Only non-caking coals can be used in these reactors, making this group’s choice of Montana sub-bituminous coal incompatible with this reactor type.
Entrained flow reactors are plug-flow type reactors where a mixture of finely ground coal and steam flow co-currently through a high temperature process to obtain good conversion. The gasifiers accept ash conditions which are in the slagging range. An entrained-flow gasifier may handle any type of coal as feedstock, and produce clean, tar-free gas. However, coals with a high moisture and ash content will increase the demand for added oxidant, which increases the cost. The carbon conversion is almost 100% because the ash leaves the reactor as slag and is very low in carbon. Since this process is compatible with sub-bituminous coal, and produces high carbon conversion, it was determined to be the best fit for the process.
C. Acid Gas Removal
In this design the Selexol process is used to remove hydrogen sulfide from the syngas following the initial coal gasification process. The Texaco process was chosen and since it is a high pressure and high temperature system, the following processes were dealt with accordingly. Further discussion about the selection is detailed in Progress Report 2. According to literature, usually 95% of hydrogen sulfide gets removed while syngas flow is in contact with counter current of Selexol solvent. The Selexol solvent also remove 15% of the carbon dioxide in the syngas. The selectivity of H2S to CO2 is 17:3. H2S and COS stripped from Selexol solvent along with sour gas produced in water treatment unit to the Claus sulfur plant for recovery of sulfur element. The inlet syngas is contacted counter current in packed bed with Selexol solvent (5). The temperature of syngas gets reduced in absorber. The treated syngas flows through knock out drum to remove solvent mist, and then it gets heated in the heat exchanger by the entering flow of syngas. The final separator also serves to separate the contaminated water for further treatment and the elemental sulfur which is valuable for further processing in other chemical applications.
D. Water-Gas Shift Reactors
The water-gas shift reaction is commonly used method for further enhancing the yield of hydrogen from industrial process such as the gasification of heavy carbonaceous materials or steam reforming of natural gas. Synthesis gas mixtures containing mostly hydrogen and carbon monoxide are generated at elevated temperatures through the combustion of natural gas and coal (2). Steam is added to CO-H2 feed mixture before entering the water-gas shift reactor to convert CO to CO2 and additional H2.The water-gas shift reaction is commonly run at low temperature since thermodynamic equilibrium favors high conversion of CO and steam to CO2 and H2 in the presence of catalysts that enhance the reaction rate (3).
Although the equilibrium favors formation of product at lower temperatures, the reaction kinetics is faster at elevated temperatures. For this reason, the catalytic water-gas shift reaction is initially carried out in a high-temperature shift (HTS) reactor at 350-370°C. Conversion in the HTS reactor is limited by the equilibrium composition at the high temperature. To achieve higher conversions of CO to H2, the gas leaving the HTS reactor is cooled to 200-220°C and passed through a low-temperature shift (LTS) reactor (4). Most of the CO shift reaction out in industry, are converting 90% CO to H2 in the first HTS reactor, and the 90% of the remaining CO is converted in LTS reactor. In the coal gasification process, the water-gas shift is used to achieve the stoichiometric 2:1 ratio of syngas (H2:CO), necessary for methanol synthesis.
E. Methanol Synthesis
Rising pressure favors CO and CO2 conversion. However, at about 60 bars, 99% of carbon monoxide in synthesis gas is converted on good catalysts and only the conversion of carbon dioxide still increases notably at higher pressure (5). Low temperatures favor the equilibrium of methanol formation and higher temperatures cause an improvement in the reaction kinetics. As a result both factors are utilized in that methanol catalyst is operated at the lowest possible temperature as long as it still exhibits its full activity. The deterioration in the methanol equilibrium at the end of the reaction caused by the temperature increase can be counteracted by increasing the recycle ration. In industries, for uniting high reaction speed and favorable methanol equilibrium, isothermal reactor are mostly uses. Another thing that affects the CO and CO2 conversion is dependent upon the space velocity at which the catalyst is operated. The reactor feed gas under standard conditions related to the catalyst volume present in the reaction chamber which is known as space velocity, i.e. m3 gas/ m3 catalyst. The recycle ration also influences conversion. Higher recycles gas ration causes greater dilution of the reacted gas and higher conversion of the carbon oxide due to low percentage of methanol in the reactor(2).
F. Methanol Refining
After methanol formation, water and the formation of dimethyl ether must be separated. The boiling points of water and higher alcohols are greater than that of methanol, so distillation is required. In industry, to produce pure methanol of grade AA two column distillation is required, which always consists of an extraction (topping) column, and a refining column. This is done to reduce the amount of methanol wasted and to get waste water which does not require treatment before discharging it. The column is heated by a re-boilers installed at the bottom which in most cases feeds on low pressure steam. Water, methanol, and low boilers are condensed at the bottom of column and refluxed while the dissolved gasses and the uncondensed low boilers are withdrawn from the system.
A major limitation in achieving grade AA methanol is that the ethanol affects the water/methanol mixture. At higher water concentrations, ethanol becomes more volatile that methanol which occurs in the stripping section of the distillation column. There will be some methanol loss, which is associated with the reflux ratio. Methanol loss decreases with an increasing reflux ratio. In turn, a high heat requirement, using LP steam is needed.
IV. Technology Selection Criteria and Conclusions
A. Texaco Entrained-Flow Gasifier
The entrained-flow gasifier which uses the top-fired coal-water-slurry-feed gasifier is used in the Texaco process. In addition to being the most inexpensive design available, this process is ideal for producing syngas for further processing. There is no requirement of additional added steam in this process, since the water present in the slurry will vaporize in the high-temperature reactor.
In the Texaco process, slurry is prepared by combining crushed coal feedstock and water in a slurry tank. The slurry is introduced with oxygen through the feed-injector (burner) into the refractory-lined gasifier. The gasification process occurs generally around 1500 ºC, which is in the slagging temperature range. The pressure requirement is between 30-70 bar.
The syngas is cooled in a water quench because it is suitable for providing conditions for CO shift reaction, required in the coal-to-methanol process. In using the quench process for syngas cooling, the gaseous effluent leaves through the bottom of the reactor with the liquid ash and enters the cooling vessel. In the Texaco quench setup, a total quench occurs where the leaving cooled gas is entirely saturated with water and at a temperature between 200 to 300 ºC. Once cooled, the liquid ash is solidified into slag and is separated by a lock hopper. The water that leaves the water is recycled and may be used again in feed preparation.
B. Coal-Water Slurry Preparation
To prepare the feed, crushed coal at concentrations of 60-70 wt% is combined with water. It is not necessary to dry the coal using the Texaco entrained-flow process, which is an energy requirement. Drying also introduces the risk of tar formation, which is one advantage of entrained-flow gasifiers which produce tar-free gas. Since only a small amount of water is used in the gasification process, the rest of the water is vaporized and heated to about 1500 ºC. To reduce the oxygen requirement, the slurry is preheated to around 150 ºC. The benefits include increasing the carbon conversion, and the efficiency is comparable to dry-coal feed gasifier.
C. Total Water Quench for Syngas Cooling
After the gasification process, the synthesis gas must be scrubbed to remove contaminants before further processing. Some of these contaminants include sulfur, chlorine and slag. To clean up the gas, the gas effluent must be cooled before it may be processed. The gas must be cooled quickly so the ash particles may become dry and solidify into slag. It is inevitable the entrained-flow ash will go through the critical temperature range where is becomes sticky, leading to unwanted side-effects, such as leaving contaminant residue on the sides of the reactor wall. The main issue is transition temperature range between slagging and non-slagging conditions.
A total quench is used because it is an effective and inexpensive way to cool the synthesis gas. Once the gaseous effluent is condensed, the water leaves as a contaminated stream for further cleaning and recycled in coal-water slurry feed preparation. Particles and water soluble gases such as NH3 depart with the water. Since one of the desired products from gasification is hydrogen, the total quench process is also suitable because there is no additional steam requirement. The gaseous feed, once the solids are removed in the dry solids removal step, is prepared for the subsequent CO shift reaction. The water introduced from the condensation process shifts the CO shift reaction to the right (eq.3), thus increasing the CO2 and the H2/CO ratio of the synthesis gas.
D. Reactor Design and Heat Containment
A refractory-lined reactor is suitable for a single-stage Texaco process entrained-flow gasifer. This is a simple and inexpensive design. There is a burner at the top of a simple cylindrical reacting vessel where the feedstock is introduced. There is also an outlet at the bottom of the chamber where the syngas and slag leaves. The advantage of having such a simple method for control is that there is no chance of slag buildup. The total quench process occurs in the water quench chamber, within the gasifier.
Additionally, by using a brick-wall as the insulating material, it has a high heat capacity. In contrast to a membrane wall, refractory lining is cheaper because of less construction and maintenance cost. Also, there is not as much heat loss in a refractory brick-lined vessel. Refractory lining is suitable to meet the demands of the gasification process, which requires temperatures of around 1500 ºC. The design usually consists of an inner layer of brick (Al2O3), middle layer of alumina, and outer well-insulating layer of silica fire brick. This design is suitable to withstand the high temperature as well as provide insulation. The estimated lifetime of a refractory-lined vessel is between 25,000 and 40,000 hours due to the corrosion and eventual cracking effects, which is important to consider in the final project economics.
E. Selexol Solvent for Acid Gas Removal
To treat the effluent syngas from the gasifier after the coal gasification process, the Selexol process has been selected to remove acid gas (H2S, CO2). The Selexol process uses a physical solvent to remove the acid gas by absorption, and is ideal for removing sulfur compounds and removal of CO2. The solvent used is a mixture of dimethyl ethers polyethylene glycol (CH3(CH2CH2O)nCH3).
There are many advantages associated to using the Selexol solvent process. The solvent has a low vapor pressure which limits its losses to the treated gas, and a low viscosity to limit pressure drop. The solvent does not react with the gases, and is non-corrosive. Also, this process is acceptable to use because it is non-toxic, which is a safety and environmental concern.
F. Methanol Synthesis
In this design an adiabatic is selected for the methanol synthesis in contrast to an isothermal design, due to its higher capacity and lower cost. To design the methanol synthesis process for this project, the LURGI low pressure process has been used. Some of the reactor feed gas is preheated to almost reaction temperature in a heat exchanger and enters the adiabatic Plug Flow Reactor from the top, and it flows through its tubes filled with copper/ zinc-oxide/ alumina (Cu-Zn-Al) catalyst from top to bottom. The reaction of the carbon oxides with hydrogen occurs during this. The reactor gas leaves the reactor at the bottom, and it flows through the heat exchanger, where it preheats the reactor feed gas. Then it passes into the condensation and cooling section, which is usually consist of air cooler and a cooling water cooler. At this point methanol, water and other condensable gas components condensed at the ambient temperature. The crude methanol is separated from the downstream separator and passes through a flash tank to distillation unit.
The recycled gas is routed to the recycle compressor after the purge gas is withdrawn through a pressure controller to bring up to makeup gas pressure and to ensure it is mixed. Plants built to the LURGI process operate in the pressure range of between 5 and 10 MPa and with recycle gas ratio of 3.0 to 4.0 (4). The heat produced from the reaction in the reactor is removed by a circulating water system. Water at about boiling, flows the bottom section of the reactor. The heat is transferred from the tubes where the reaction takes place. The steam mixture leaves the water section of the reactor again at the top and rises back to the steam drum. This method will provide a constant control of the temperature for the catalyst as the temperature rise due to the reaction. Generally the pressure drop across the overall synthesis loop is only 0.35 to 0.4 MPa.
G. Methanol Refining
A single column is sufficient for refining methanol in contrast to the two-columns in industry. For the purposes of the project, the methanol specifications is simplified to deal with removing water and ethanol which is heavier than the product, methanol. Although using a 1 column design requires more LP steam due to the high reflux ratio, it is adequate for the production of 5000 MT methanol/day. The distillate and bottom composition are calculated in Progress Report 4 Appendix. In order to develop a distillation sequence, it is necessary to make at least one preliminary estimates of column operation pressure and condenser type (total or partial). These estimates are facilitated by the use of algorithm given in figure 7-16 in Separation book by Seader (5). The bubble point pressure (PD) had been calculated by a single flash run in Aspen for the distillate at 490 C. This pressure is calculated to be 120 psia. As a result total condenser had been used. For the selected condenser pressure, 10 psia added to estimate the bottoms pressure, and the bubble point temperature had been computed with Aspen to be 2000 F. Fenske equation had been used to calculate the minimum number of equilibrium stage Nmin to be 49 to separate the two key components. However for the ease of calculation, it is assumed that relative volatility between two key components are constant throughout the column. Underwood equation has been used to calculate the minimum reflux ration, Rmin. Since more than one component appears in one product, this case will be Class 2 separation. Two roots and had been calculated to satisfy the condition of. These roots had been used to calculate Rmin to be 3.12. The Gilliland Correlation had been used to calculate the actual number of equilibrium stage, N for a specified ratio of actual reflux ration to minimum reflux ratio.
V. Process Performance Summary
For a coal gasification plant, 6400 MT/day of Montana sub-bituminous coal reacts with 4500 MT/day of O2 in a Texaco-entrained flow gasifier. Synthesis gas is created in this process and there is an overall conversion of 99% in each gasifier. About 3 gasifiers are required in order to accommodate for the amount of coal required. Next, the sulfur components are removed from the syngas. One of the byproducts from the Claus Reactors is liquid sulfur, which is a very valuable commodity which may be sold as a foundation for other chemical processes.
The synthesis gas then proceeds through a high temperature water gas shift reactor as well as a low temperature one. The overall conversion which shifts the syngas ratio H2:CO (2:1) is around 54.10%. Next, methanol is synthesized through a series of Plug Flow Reactors. Overall, and confirmed by literature, the conversion is around 90%. Finally, the raw methanol goes through a distillation process and achieves an isolation of 99% AA grade methanol, which results in the production of 5,292.9 MT/day methanol. From the project economics analysis, the process was determined to be economically feasible.
VI. Project Economics Summary
Capcost was used to estimate the capital costs of the equipment using a 2006 CEPCI index value of 478.7. Since it is assumed that the plant has not been built yet, the values f or capital costs includes the base as well as the building costs, taken from the grassroots cost. Next, the consumable costs such as process steam, coal and oxygen are considered. A large amount of costs is also associated with heating between each of the stages.
Coal Gasification
The most significant costs associated with the coal gasification process are the consumable costs for coal and oxygen. Since 6400 MT/day of coal is required, there needs to be 3 gasifiers to handle the capacity.
Acid Gas Removal
Since the Selexol process is a fraction of the cost compared to the overall process, the costing for it was taken as a percentage of the overall capital costs. The Claus-reactors were sized and cost estimated, and again, the O2 required for the process turned out to be a high consumable cost requirement. Also, the heat requirements were high due to the high pressure systems.
Water Gas Shift Process
Two water gas shift reactors at high and low temperature were required for the high pressure system, a factor in using the Texaco-entrained flow process.
Methanol Synthesis
Five Plug Flow Reactors were required to attain the required production of methanol, which resulted in a higher capital cost. Also not included, but should be taken into consideration is the amount of catalyst which is required to ensure conversion. There was a great heat requirement in between each PFR, which is shown in Table 8.
Methanol Distillation
According to Capcost, the estimated capital cost for four a 120-stage distillation columns is $16.32 million. The operating cost for this step was high due to the high reflux ratio for a tertiary system where the volatilities were low. The operating cost from the reboiler and condenser of the 4 distillation columns were $92.1 million/year.
Overall
For the coal gasification process, it was estimated that the consumables annual cost was $235 million/year, to account for the needed 6400 tons of Montana Sub-Bituminous and 4500 tons of O2 required by the Texaco entrained-flow gasification model and other oxygen and steam requirements. The combined capital cost was $44.49 million. The projected profit from the 5292.9 MT/day of methanol is about $593 million each year. Most of the costs associated with the coal gasification plant are due to the purchasing of coal and oxygen.
Some of the important assumptions for the project economics included a fixed tax rate of 40%, an inflation rate of 3%, project life of 20 years and a straight line depreciation of 15 years. The depreciation value with an assumed no salvage value is $2.96 million/year. Graph 1 depicts a simplified cumulative cash flow which reveals that the project has a payback after 2 years.
In conclusion, the coal gasification is economically feasible, as revealed from the IRR value of 17.7%. Although the project economics is simplified and doesn’t include other factors such as the fluctuating price of methanol and other unforeseen costs, it is marginally profitable.
In conclusion, the coal gasification is economically feasible, as revealed from the IRR value of 17.7%. Although the project economics is simplified and doesn’t include other factors such as the fluctuating price of methanol and other unforeseen costs, it is marginally profitable.
VII. Process Description and Process Flow Diagram
A. Coal Gasification
An entrained-flow Texaco process was selected to model the coal gasification. A coal-slurry is required for this process, so the wet coal is mixed in a separator with water to achieve 70 weight % coal and 30 weight % water. A pump is used to increase the pressure to 5.17 MPa, required by the gasifying reactor. Since coal is a solid, a DECOMP RYIELD is used to break the coal into its individual components. Oxygen at 4500 MT/hr is reacted with the coal-slurry at 6400 MT/day in a reactor operating at 5.17MPa and 1316 ºC to produce synthesis gas and slag. An RGIBBS reactor with the PENG-ROB thermodynamic model was used. A heat exchanger cools the gas mixture to 710 ºC and separators are used to removed slag and trace amounts of water. The slag proceeds off the plant for further use, such as being used for concrete. The diagram of the process is given in Figure 1 and the mass balance is given in Table 10.
Table 10. Mass Balance for Coal Gasification Process
B. Acid Gas Removal
First, ASPEN plus was used to achieve the desired composition of syngas by first using a separator to model the Selexol process. ELECNRTL was the thermodynamic model used to model the sour water treatment. The Selexol solvent has a selectivity ratio of H2S to CO2 of 17:3. In this step, 3.09 MT/hr H2S and 0.74 MT/hr CO2 was removed. The H2S is combusted in the first Claus reactor, which operates at 1000 ºC and 350 Pa, with a conversion of 30%. Oxygen was added at 1.38 MT/hr. The acid gas stream goes through a separator which condenses the elemental sulfur. Finally, the gas continues through the second Claus reactor which operates at 300 ºC and 350 Pa. Both reactors are modeled with RGIBBS. Almost all (97%) of the elemental sulfur is formed, and goes through another separator at 2.77 MT/hr. The water is separated from the sulfur for further waste processing, and the sulfur is available for other chemical applications.
Figure 2. Acid Gas Removal Flow Diagram
Figure 2 shows the process flow diagram for the acid gas removal. The Selexol process is modeled with a separator and the Claus reactions are modeled with two RGIBBS reactors. Table 11 shows the mass balances for the acid gas removal process. In the appendix, the ASPEN diagram for this portion may be viewed.
Table 11. Mass Balance for Acid Gas Removal
C. Water Gas Shift Process
After the initial Selexol separation process, the treated syngas continues through the water-gas shift reactors. PENG-ROB is used as the thermodynamic model because it is suitable for gases as well as high temperature and high pressure systems. Before it may be processed, a valve is used to reduce the pressure from 6.27 MPa to 5.17 MPa. A heater then prepares the gas by heating it from 46 ºC to 371 ºC. First, 33.33 MT/hr of HHP steam at 120 ºC and is added and the syngas undergoes the water-gas shift reaction to achieve the desired 2:1 (H2:CO) ratio. The HTS reactor operates at 370 ºC and 5.17 MPa. Before entering the LTS reactor, a heat exchanger cools the gas to the operational conditions at 200 ºC. HHP Steam enters the LTS reactor 91.67 MT/hr. The syngas is then cooled with water to 150 ºC, necessary to meet the temperature range requirements for methanol synthesis reaction (373-673K). Finally, for methanol synthesis preparation, the feed goes through a separator which removes all the liquid water.
Figure 3. Water-Gas Shift Diagram
Figure 2 depicts the flow starting after the Selexol solvent separator. Table 12 features the mass balances for each of the streams in mass flows. From the molar flows, the final H2 and CO ratio is 2:1 is achieved by producing 1,724.4 kmol/hr of H2 and 860.4 kmol/hr of CO.
Table 12. Mass Balance for Water-Gas Shift Reactors
D. Methanol Synthesis
To model the methanol synthesis, five adiabatic PFR reactors was used and RK-SOAV was selected as the thermodynamic model. This property is appropriate for gas-processing operations. The temperature approach method was used and set to equal less than 50 ºC of the equilibrium temperature. It operates at 10 MPa and 250 - 300 ºC. Although the reaction is favorable at lower temperatures, it is limited to the Cu-Zn-Al catalyst requirements for methanol production. The coolers and recycle streams are implemented to improve the final yield of methanol while reducing the amount of unwanted byproducts such as ethanol from being produced. The produce was calculated according to the assumption that for every 100 parts of methanol being produced, 1 part of ethanol was produced. In turn, for the production of 5,316 MT/day raw methanol, 53.2 MT/day ethanol was produced as a side product. The process flow diagram is revealed in Figure 1 and the mass balance for the methanol synthesis system is shown in Table 13. The appendix gives data and mass balances from the individual PFRs.
Figure 4. Methanol Synthesis Flow Diagram
Table 13. Mass Balance for Methanol Synthesis
VIII. Major Equipment List and Preliminary Sizing
To size the coal gasifiers, it was done by volume. There were 3 coal gasifiers required to accommodate the required amount of coal to meet the 5,000 MT/day of AA methanol. For the water gas shift reactors, the volume for each of the HTS and LTS reactors was calculated to be 1,060 m3. The five PFRs were sized to have a diameter about 2m and width of 10m. For the distillation column, it was sized with 1 feet spacing in between the trays. The height was calculated to be 124 feet and a diameter with 16.6 feet. For more information about the calculations, the appendix provides details.
IX. Environmental and Process Safety Considerations and Analysis
A. Equipment Safety
The gasification reactor requires a thick (50-70cm) insulating refractory wall, which exists as protection between the outer shell and the inside high-temperature reacting vessel. There are many toxic gases present in the gasification process, such as CO, H2S, COS, ammonia and HCN. Although the plant is designed to protect the personnel as much as possible, employees are still required to be updated on the safety information and be properly trained in safety handling.
B. Oxygen Precautions
In a gasification plant, there is gas which becomes flammable or even explosive when it is within the upper and lower flammability limits. Thus, the start-up and shutdown processes must be handled carefully. During start-up, the coal is ignited and at all times, a mixture of combustible gas and oxygen in the reactor must be prevented.
When oxygen exists in concentrations greater than 21 %, extreme oxygen fires have the possibility of occurring. In order to minimize the sustainability of a fire in an oxygen environment, the ignition sources and materials are selected for their capability to minimize the effect. The materials selected are chosen to be copper or stainless steels. The oxygen is kept in a fireproof wall, and has an elaborate monitoring system which causes the oxygen compressor to stop working, depressurize and be flooded with nitrogen to render it inert.
When oxygen exists in concentrations greater than 21 %, extreme oxygen fires have the possibility of occurring. In order to minimize the sustainability of a fire in an oxygen environment, the ignition sources and materials are selected for their capability to minimize the effect. The materials selected are chosen to be copper or stainless steels. The oxygen is kept in a fireproof wall, and has an elaborate monitoring system which causes the oxygen compressor to stop working, depressurize and be flooded with nitrogen to render it inert.
C. Nitrogen Precautions
During shutdown, the gasifier is blasted with nitrogen to avoid the effects of corrosion. It is also used in the blanketing and transport of coal, and used to dilute fuel gas. One of the key issues with nitrogen is the lack of smell, and inhaling it quickly leads to unconsciousness. In addition to adequate ventilation, air masks are always suggested when maintaining the gasifier.
Source: Sheida Saeidi, Josh McElfresh and Joyce Stillman - University of California, La Jolla, California
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