Coal to Derivative Chemicals

Primary chemicals produced directly from synthesis gas (syngas), i.e., carbon monoxide (CO), hydrogen (H₂) and methanol (MeOH) can be used as intermediates to manufacture a variety of derivative chemicals such as olefins, acetic acid, formaldehyde, ammonia, urea and others. Examples include carbonylation reaction of CO with MeOH to produce acetic acid, which in turn, can further react with additional MeOH to produce methyl acetate.  Methyl acetate can then undergo further carbonylation reaction with CO to form acetic anhydride, a raw material to convert cellulose into cellulose acetate, which is a component for photographic film and other coating materials. This process, in essence, is what is being practiced at Eastman Chemicals’ Kingsport plant.  Most of the coal-to-derivative chemicals productions are based on proprietary catalytic systems, with specific process know-how. Selected coal-to-derivative chemical processes are briefly presented here. Additional information can be found in the cited references.

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Figure 1: Coal to Chemical Routes

Coal-to-Acetic Acid and Derivatives
Acetic acid (CH₃COOH), an important industrial chemical, can be produced from coal indirectly through carbonylation of MeOH over rhodium or iridium catalysts (with various iodide or other additives) according to the following reaction:

The rhodium catalyzed process is highly selective (>98% acetic acid) and operates under mild reaction pressure (~ 500 psia) in a liquid phase reactor. Technology licensors include Monsanto/BP, Celanese, BP, and Chiyoda, the latter three vendors represent an improved version of the original Monsanto/BP technology.

Figure 2 shows a simplified block flow diagram (BFD) of the Eastman Chemicals’ coal-to-chemicals facility producing MeOH from coal derived syngas, followed by converting MeOH into acetic acid and its derivatives of methyl acetate and acetic anhydride.

Figure 2: Eastman Coal to Acetic Acid & Derivative Chemical BFD^[3]

With the Eastman facility, acetic acid is reacted with MeOH to form methyl acetate (CH₃COOCH₃), which is further reacted with CO to produce acetic anhydride ([CH₃CO]₂O).  The catalytic reactions for these additional derivatives are shown in Figure 3.

Figure 3: Eastman Coal to Acetic Acid & Derivative Chemistry

Coal-to-Formaldehyde
Formaldehyde can be produced from coal indirectly through dehydrogenation and partial oxidation of MeOH using a silver catalyst, based on the following reactions:

Equilibrium conversion and potential side reactions are highly temperature dependent. The overall reaction temperature is controlled by the quantity of air (oxygen) used, and the addition of inerts, such as water and/or nitrogen.

Figure 4 shows a typical flow scheme of a MeOH oxidative dehydrogenation process producing commercial grade formaldehyde. A mixture of methanol and water is mixed with air and recycled gas, and the total feed mixture is vaporized by heat exchange against hot reactor effluent. The vaporized feed mixture is fed into the catalytic reactor to form formaldehyde.  Excess reaction heat is removed by generating steam.  The reactor effluent, after cooled by heat exchanging with incoming feed, is scrubbed with water in the absorber to remove the formaldehyde product as a 55% solution.  Water can be added to produce commercial grade formaldehyde at 37% concentration. A portion of the product gas leaving the top of the absorber is recycled, and the remainder is incinerated. Typical overall formaldehyde yield is in the range of 92 to 95%.

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Figure 4: A Typical Methanol Oxidative Dehydrogenation Process of Producing Commercial Grade Formaldehyde

Coal-to-Ammonia and Urea
Figure 5 shows a simplified BFD of the Farmland Coke-to-Ammonia plant, to illustrate the typical flow scheme of a gasification-based ammonia and urea production process. All component technologies are commercially proven. Per Figure 5, high purity H₂ is produced from coke gasification. It is combined with a high purity nitrogen (N₂) stream extracted from the air separation unit (ASU) to produce ammonia according to the following reaction:

The reaction is carried out typically between  370 and 540 °C, and at high pressure (> 2,000 psi) over an iron-based catalyst to achieve reasonable conversion per pass.  Ammonia synthesis is highly exothermic, and the reaction is equilibrium limited.  Per pass conversion is increased by allowing the reaction to take place in successive catalyst beds, arranged as two to four adiabatic conversion stages.  Some form of intercooling and/or dilution is applied between stages to allow the reaction to continue.

Ammonia production technology is commercially proven.  Companies that supply the proprietary commercial technology and/or process include Haldor Topsoe, KBR/Haliburton, Linde/Lurgi and Uhde.  

With the Farmland ammonia plant, part of the CO₂ captured from the Selexol acid gas removal process is purified and reacted with a stream of the ammonia product to produce urea, according to the following reaction:

Technology licensors offering ammonia-to-urea technology include: Snamprogetti, Stamicarbon, and Toyo Engineering Corp.

Figure 5: Farmland Coke-to-Ammonia Block Flow Diagram^[3]

Coal-to-Olefins
Olefins, such as ethylene and propylene, can be produced from coal gasification indirectly by catalytic cracking of MeOH, commonly called the methanol-to-olefins (MTO) process.  

Figure 6 shows a simplified flow diagram of UOP/HYDRO’s MTO process. Other technology licensers include ExxonMobil and Lurgi, using different types of catalyst systems and process know how. Per Figure 6, fresh MeOH feed is combined with recycled water and fed to a fluidized-bed catalytic reactor, equipped with a catalyst regeneration and recycle reactor as shown. The reactor operates at typically 350 °C and at 30 psig. With UOP/HYDRO’s proprietary catalyst system, claimed methanol conversion is quite high, and the process is 80% selective for ethylene and propylene. The produced ethylene/propylene ratio can be altered from 1.5 to 0.6, depending on operation conditions.

Reactor effluent leaving the reactor is cooled to condense most of the water and unreacted methanol for recycling. Spent catalysts from the reactor are routed to the regenerator where the coke deposits are burned off with air. Regenerated catalysts are recycled. The cooled reactor effluent is compressed to remove CO₂ and water, followed by further compression to high pressure to liquefy the hydrocarbon mixture for separation by distillation. The final product from distillation separation typically consists of polymer grade ethylene and propylene, a methane-rich fuel gas, plus small amount of ethane, propane, butane, pentane and higher molecular weight liquids.  

Other MTO technology licensors include Exxon/Mobil and Lurgi, of which the ExxonMobil process is very similar to that of UOP/HYDRO except perhaps with the use of a different catalyst formulation. Lurgi’s process is optimized for propylene production, and is being marketed as a methanol to propylene (MTP®) process.

Figure 6: Simplified Process Flow Diagram for UOP/Hydro MTO Process^[1]

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