Process Principles, Before it can be fed to the gasification reactor, coal must be ground, Chemical Engineering

Process Principles, Chemical Engineering

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Before it can be fed to the gasification reactor, coal must be ground to meet specifications on particle size. The grinding is accompanied by the generation of coal dust that can represent a loss of raw material and a hazard to both personnel and equipment. This problem is minimized by recognizing that one of the primary reactions in coal gasification is between coal and water, and that grinding and separating the resulting coal particles by size can be performed on a coal-water mixture as well as on dry coal. The ratio of dry coal to water in the feed to the gasifier is to be 2 kg coal/kg water, and this ratio is set with the water added to the grinding and classification operations. The reactor in which gasification is to be carried out is an entrained flow gasifier. Coal-water slurry is fed to the gasifier along with high-pressure oxygen. Both feed streams may be assumed to be at 25°C. The reactor is operated adiabatically at 6300 kPa absolute. At these conditions, only trace quantities of hydrocarbon byproducts other than methane are present in the gas leaving the gasifier, and essentially all of the reactive carbon, hydrogen, and sulfur in the coal are reacted. The gas and molten ash (slag) leaving the gasifier is fed to a wash tank where the gas is bubbled through water and the slag is solidified. The gas leaving the wash tank is fed to a scrubber where it contacts a water spray. Small amounts of residual particulate matter are removed from the gas, which is also cooled from 220°C to 170°C. The water with which the gas is contacted has been pumped from the wash tank, through a heat exchanger, and into the scrubber. The gas-liquid mixture formed in the scrubber is separated into saturated gas and liquid streams at 170°C and 5960 kPa absolute, and the water is returned to the wash tank. Solids collected in the wash tank are allowed to settle, and are then removed along with the water condensed from the gas leaving the gasifier. The composition of the gas leaving the gasifier is given in Table 3. The ratio of hydrogen to carbon monoxide in the gas leaving the scrubber must be adjusted to satisfy stoichiometric requirements associated with downstream reactions. The adjustment is made by contacting a portion of the gas leaving the scrubber with a catalyst that increases the rate at which the water-gas shift reaction occurs. The shift reactor consists of a fixed bed of cobalt-molybdenum catalyst pellets and operates at 300°C and 5615 kPa absolute.

The product from the water-gas shift reactor and the unreacted portion of the synthesis gas stream obtained from the gasifier are blended and cooled by generating steam in waste-heat boilers. Water is removed completely from the cooled gas in a molecular sieve drier. The dry gas is then fed to an acid gas removal system where all of the containing compounds are separated and sent to a sulfur recovery reactor, and 99.9% of the CO2 is separated and either vented or recovered for use elsewhere in the plant. The acid gas removal system consists of two absorption columns, both of which operate at 5000 kPa absolute, and a solvent regeneration section. The gas enters the first absorption column and contacts refrigerated methyl alcohol flowing countercurrently to the gas. In the column, some CO2 and essentially all of the H2S are absorbed in the methyl alcohol. The gas leaving this column is sent to a second absorption column where most of the remaining CO2 is absorbed in additional methyl alcohol. The gas exiting the second column is at -30°C and contains 0.1% of the CO2 that entered the first absorber with the crude coal gas. There are two separate operations in the regeneration section of the acid gas removal system. In the first, the methyl alcohol from the first absorber is flashed (abruptly exposed to a lower pressure, causing a significant fraction of the absorbed gases to come out of solution). Essentially all of the absorbed CO2 and H2S are recovered in the off-gas, and the regenerated methyl alcohol is recirculated to the first absorber. The off-gas from the flash tank is a mixture consisting of 15 mole% H2S and 85 mole% CO2. In the second operation, methyl alcohol from the second absorber is fed to a stripping column where it flows downward to a reboiler. Indirect steam heating (heat transferred through a heat transfer surface) is used to vaporize a portion of the methyl alcohol, which is returned to the column and strips the CO2 from the liquid methyl alcohol. The regenerated liquid methyl alcohol is cooled and recirculated to the second absorber. Sulfur emissions are potential health hazards, and safe disposal of the H2S separated from the product gas is an important provision of the proposed plant. To accomplish this, the hydrogen sulfide is converted into elemental sulfur in a Claus process. In this process, the H2S rich gas discharged from the flash tank is split, with one-third going to a furnace where the hydrogen sulfide is burned at 1 atm with a stoichiometric amount of air to form SO2.

H2S+ 1.5O2?SO2+H2O

The hot gases leave the furnace and are cooled prior to being mixed with the remainder of gas from the flash tank. The mixed gas is then fed to a catalytic reactor where hydrogen sulfide and SO2 react to form elemental sulfur.

2H2S+SO2?2H2O + 3S

The gases leave the reactor at 380°C and are cooled to 130°C by generating steam at 205kPa from saturated liquid at 205kPa. Liquid sulfur is condensed and recovered as the hot gases are cooled at atmospheric pressure.

As an intermediate step in the production of acetic anhydride, carbon monoxide and carbon dioxide are reacted with hydrogen to produce methanol.

CO + 2H2 ? CH3OH

CO2 + 3H2 ? CH3OH + H2O

These reactions are carried out at 200°C and 4925 kPa absolute. The feed to the methanol reactor is formed by combining recycle from the reactor with a fresh feed stream containing 5% excess hydrogen, based on complete conversion of CO and CO2 to methanol. The composition of the gas leaving the acid gas removal system is adjusted to meet the fresh feed requirement on excess hydrogen. This is accomplished by splitting the gas from the second absorption column into two streams, one of which is sent to a low-temperature separation unit while the second is blended with the hydrogen-rich stream exiting this unit. The separation is designed to condense 95% of the inlet carbon monoxide and to operate at 5000 kPa

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Process Principles, Chemical Engineering

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