Gas recycle process

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

Nickel catalysts were used in most of the methanation catalytic studies they have a rather wide range of operating temperatures, approximately 260°-538°C. Operation of the catalytic reactors at 482°-538°C will ultimately result in carbon deposition and rapid deactivation of the catalysts (10). Reactions below 260°C will usually result in formation of nickel carbonyl and also in rapid deactivation of the catalysts. The best operating range for most fixed-bed nickel catalysts is 288°-482 °C. Several schemes have been proposed to limit the maximum temperature in adiabatic catalytic reactors to 482°C, and IGT has developed a cold-gas recycle process that utilizes a series of fixed-bed adiabatic catalytic reactors to maintain this temperature control. 

Description of the Cold-Gas Recycle Process and Pilot-Plant Equipment... 

The diagram of the IGT cold-gas recycle process includes four reactor stages in series (Figure 3). Note that the concept can be applied to any number of stages. 

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

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 ... 

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.

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... 

In none of the technically applied processes complete conversion of the reactants is achieved in a single pass. The conversion of propene, for example, can be as low as 25-30% per pass (LPO gas recycle process) or amount to more than 95 % (cobalt high-pressure process) as a consequence of several process variables (temperature, pressure, catalyst, reactant concentration, and residence time of reactants). Consequently, in every case an unconverted portion remains which, after separation from the product, has to be recycled (or partially vented to avoid accumulation of inerts) or used in a second stage. For LPO processes with limited olefin conversion, especially, several solutions have been proposed for arranging single reactors in series as a cascade [147]. 

Later, the efficiency of the gas recycle process was improved by a switchover to the liquid recycle process, to which most plants have been converted in the meantime. Catalyst solution and aldehyde products leave the reactor as liquid and are... 

In commercial applications of propene hydroformylation the process underwent several modifications predominantly aimed at improvements in product/catalyst separation. The very first version of the process, which was later named the gas recycle process , effected the removal of the product aldehydes from the catalyst solution by applying a large gas recycle in order to evaporate the aldehydes [146, 196, 197]. The catalyst solution consisted of high-boiling aldehyde condensation products (dimers, trimers, and various other aldehyde consecutive products), in which an excess of TPP and the rhodium complex itself was dissolved [198, 199]. In order to keep the volume of this reaction mixture constant, the reaction conditions had to be maintained in a manner which allowed continuous evaporation of the aldehyde products generated by the hydroformylation reaction... 

Table 10. UCC gas recycle process composition of product streams [201].

The process scheme corresponds to the gas recycle process already described. Propene and synthesis gas (H2/CO = 55 45) are fed to a stainless steel tank reactor with thorough mixing. Aldehydes are withdrawn by a recycle gas stream, condensed by partial cooling, and freed from dissolved gases in a stabilizer column [220]. The combined gaseous streams from these operations are re-compressed and sent to the reactor. A vent stream is used to control the level of propane in the gas loop. Due to the application of the recycle, the syngas actually entering the reactor is extremely rich in H2. Typical data are shown in Table 13 [221]. 

SGR. Abbreviation for steam gas recycle process for shale oil recovery. 

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... 

Figure 3. Process type I stripping reactor (gas recycle process Union Carbide)...

One important advantage of the gas recycle process is that all the catalyst always remains in the reactor and, thus, all the catalyst works under identical process... 

An important advantage of the liquid recycle process over the gas recycle process is that the product can be removed from the catalyst solution using vaporization parameters independent of the reaction parameters. This additional degree of... 

Figure 27.1 describes the gas recycle process. In this case ... 

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