Hot-gas recycle process

Developments in the Hot-Gas-Recycle Process, Bureau of Mines Rep. Invest. (1965) 6609. 

L. Seglin Why has Lurgi selected the hot gas recycle process for methanation rather than the isothermal reactor (ARGE) design which they used for the Fischer-Tropsch plant in SASOL s plant in South Africa ... 

Foreseeable improvements that will increase operability and decrease operating costs of Fischer-Tropsch processes are the development for the fluidized-iron process of a catalyst that will not accelerate the reaction 2CO = C02 + C and will not be appreciably oxidized during the steady-state life of the catalyst and the development of a more active and mechanically stable catalyst for the oil-circulation process so as further to reduce Ci + C2 production. The hot-gas recycle process could be made operable by use of a catalyst that will be less active but more resistant to thermal shock which occurs during regeneration to remove carbon deposits, and during operation at lower end-gas recycle rates. The powdered catalyst-oil slurry process recently has been satisfactorily operated in a pilot plant by K6lbel and Ackerman (21). Although the space-time yield in this operation was low (10 to 20 kg. of C3+ per cubic meter of slurry per hour), the Ci + C2 production was less than one third of that... 

The purpose of the so-called hot gas recycle process (without intermediate removal of products) with a ratio of recycle gas to fresh feed of about 100 1 was to remove the exothermic reaction heat (700 kcal. per cubic meter of synthesis gas) from the catalyst bed to heat exchangers outside of the reactor. The temperature increase within the catalyst was limited to 10°C. Michael (I.G. Farbenindustrie) carried out these experiments. 

The hot gas recycle process made it necessary to use iron catalysts of adequate mechanical strength. Sintered catalysts showed better resistance against the erosive influence of fast moving gases than highly active precipitation catalysts. 

The hot-ga -recycle process was tested in a large pilot plant with a reactor having a catalyst volume of 140 cu ft and a bed depth of less than 3 ft to keep the pressure drop reasonable. 

The same catalyst, when used at a higher temperature as in the hot-gas recycle process, yields chiefly hydrocarbons, with only 5-10% alcohols, whereas in the synol process a very high recycle ratio of about 100 w ith drying of the gas on each cycle yields as much as 70% conversion to alcohols. The total conversion is over 90% to alcohols plus hydrocarbons, with only a very small amount of carbon dioxide produced. It is likely that the formation of the normal straight-chain alcohols, which constitute the bulk of the synol alcohol product, precedes the formation of olefins on this iron catalyst at 20 atmospheres in the temperature range 190-325 C. 

The separate-gas recycle method was developed to overcome the difficulties of the mixed hot gas recycle process. The CO2 and NHs can be compressed separately without difficulties caused by carbamate formation. Processes of this type were developed by Inventa (Switzerland) and CPI-AlHed (United States). The principle of the processes is that CO2 in the gas mixture from the decomposers is absorbed selectively in a solvent sudi as monoethanolamine (ME. The NH3 remaining after CO2 removal is compressed and recycled to the synthesis reactor. The CO2 is desorbed from the MEA solvent by heating, and it is recycled separately... 

The chapter by Eisenlohr et al. deals with the results of large scale pilot operations using a newly developed high-nickel catalyst with hot-gas recycle for temperature control. This and other work, conducted by Lurgi Mineraloeltechnik GmbH, with South African Coal and Oil Limited (SASOL), are the bases of the methanation process which Lurgi is proposing for commercial plants. 

L. Winsor What are the estimated potential advantages of steammoderated methanation over hot or cold gas recycle processes ... 

Synthane A coal gasification process using steam and oxygen in a fluidized bed. An unusual feature is the large volume of hot gas recycled. Developed by the U.S. Bureau of Mines from 1961. A pilot plant, designed by the C. E. Lummus Company, was built at Bruceton, PA, in 1976. 

Table V shows the salient features of several Fischer-Tropsch processes. Two of these—the powdered catalyst-oil slurry and the granular catalyst-hot gas recycle—have not been developed to a satisfactory level of operability. They are included to indicate the progress that has been made in process development. Such progress has been quite marked in increase of space-time yield (kilograms of C3+ per cubic meter of reaction space per hour) and concomitant simplification of reactor design. The increase in specific yield (grams of C3+ per cubic meter of inert-free synthesis gas) has been less striking, as only one operable process—the granular catalyst-internally cooled (by oil circulation) process—has exceeded the best specific yield of the Ruhrchemie cobalt catalyst, end-gas recycle process. The importance of a high specific yield when coal is used as raw material for synthesis-gas production is shown by the estimate that 60 to 70% of the total cost of the product is the cost of purified synthesis gas.

Carbon deposition apparently is proceeding at an appreciable velocity even at 190-225° C. under the conditions of the synol process. The formation of carbon at 300-325 C. in the hot-gas recycle or the fluidized iron catalyst processes, therefore, is not peculiar to the elevated temperatures used in those processes, although its rate is higher at the higher temperatures. 

Investigations of the FTS in small pilot plants at Bruceton was initially under the direction of Dr. J.H. Crowell, followed by Homer E. Benson in 1947 and Joseph H. Field in 1958. Benson and Field were major contributors to the development of the oil circulation and the hot-gas-recycle FTS processes, and the hot carbonate COp-scrubbing process. Martin D. Schlesinger studied the FTS in sflirry and fluidized-bed reactors. 

Still another process, called BI-GAS, was developed by Bituminous Coal Research in a 73 t/d pilot plant in Homer City, Peimsylvania. In this entrained-bed process, pulverized coal slurry was dried and blown into the second stage of the gasifier to contact 1205°C gases at ca 6.9 MPa (1000 psi) for a few seconds residence time. Unreacted char is separated and recycled to the first stage to react with oxygen and steam at ca 1650°C to produce hot gas and molten slag that is tapped. 

The BP Chemicals polymer cracking process is based at Grangemouth in Scotland and uses mixed plastics as the raw material. The reactor uses a fluidised bed which operates at 500 °C in the absence of air, and under these conditions the plastics crack thermally to yield hydrocarbons. These vaporize and are carried away from the bed with the fluidising gas. Solid impurities such as metals from PVC stabilisers accumulate in the bed or are carried away in the hot gas to be captured by a cyclone further along in the plant. PVC decomposes to HCl and this is neutralized on a solid lime absorbent to yield CaCl2 which is disposed of in landfill. The purified gas is cooled to condense most of the hydrocarbon which can be employed as commercially useful distillate feedstock. The light hydrocarbons which are less easy to condense are compressed, reheated and recycled as fluidising gas. 

Figure 4.2 presents a simplified flow diagram of the ENCOAL Liquid from Coal (LFC) process. The process upgrades low-rank coals to two fuels, Process-Derived Coal (PDF ) and Coal-Derived Liquid (CDL ). Coal is first crushed and screened to about 50 mm by 3 mm and conveyed to a rotary grate dryer, where it is heated and dried by a hot gas stream under controlled conditions. The gas temperature and solids residence time are controlled so that the moisture content of the coal is reduced but pyrolysis reactions are not initiated. Under the drier operating conditions most of the coal moisture content is released however, releases of methane, carbon dioxide, and monoxide are minimal. The dried coal is then transferred to a pyrolysis reactor, where hot recycled gas heats the coal to about 540°C. The solids residence time... 

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