Cryogenic separation processes

Depending on the composition of the synthesis gas, two different kinds of processes are technically applied for the separation of raw synthesis gas into the components H2 and CO  

With both processes almost aU desired CO-purities are achievable according to the requirements, the purity of the raw hydrogen can be improved with a downstream pressure swing adsorption. 

For raw synthesis gas from steam reformers with a higher H2/CO-ratio and a higher residual methane content, the methane scrubbing for the extraction of pure CO and crude hydrogen is more suitable than the condensation process. The process flow diagram is shown in Fig. 5.11. In the first, so-called scrubbing 

The nitrogen serves as solvent for the removal of CO, simultaneously the H2/N2 ratio of almost 3, required for the ammonia synthesis, is adjusted (see Fig. 5.12). 

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The dephlegmator process recovers a substantially higher purity C2+ hydrocarbon product with 50—75% lower methane content than the conventional partial condensation process. The C2+ product from the cryogenic separation process can be compressed and further separated in a de-ethanizer column to provide a high purity C3+ (LPG) product and a mixed ethylene—ethane product with 10—15% methane. Additional refrigeration for the deethanization process can be provided by a package Freon, propane or propylene refrigeration system. 

Most commercial oxygen at present is obtained from air by cryogenic separation processes. Although design of oxygen manufacturing plants and process conditions may vary depending on production capacity, purity desired, and cost, basic steps are similar. 

Alternatives to the cryogenic separation process include the membrane separation process and adsorption processes. The adsorption processes include pressure swing adsorption (PSA)10 and vacuum swing adsorption (VSA). These methods are noncryogenic and produce a vapor product only. This reduces the cost of production considerably when the local use of gas-phase product is the primary objective. 

The hydrogen upgrading in refineries is traditionally carried out by means of PSA and cryogenic separation processes. However, the application of membrane systems for this type of separation is rapidly growing toward the commercial level, owing to the advantages related to the low capital costs, low energy requirements, and modularity. 

The cmde product from the gasifier contains CO2 and H2S, which must be removed before the gas can be used to produce chemicals. The Rectisol process is used to remove these contaminants from the gas. This is accompHshed by scmbbing the product with cold methanol which dissolves the CO2 and H2S and lets the H2 and CO pass through the scmbber. The H2S is sent to a Claus sulfur plant where over 99.7% of the sulfur in the coal feed is recovered in the form of elemental sulfur. A portion of the clean H2 and CO are separated in a cryogenic distillation process. The main product from the cryogenic distillation is a purified CO stream for use in the acetic anhydride process. The remaining CO and hydrogen are used in the methanol plant. 

An enrichment is defined as a separation process that results in the increase in concentration of one or mote species in one product stream and the depletion of the same species in the other product stream. Neither high purity not high recovery of any components is achieved. Gas enrichment can be accompHshed with a wide variety of separation methods including, for example, physical absorption, molecular sieve adsorption, equiHbrium adsorption, cryogenic distillation, condensation, and membrane permeation. 

The process begins with a gasification process that converts coal into carbon monoxide and hydrogen. Part of this gas is sent to a water-gas shift reactor to increase its hydrogen content. The purified syngas is then cryogenically separated into a carbon monoxide feed for the acetic anhydride plant and a hydrogen-rich stream for the synthesis of methanol. 

Cryogen-free electromagnets, 23 856 Cryogenic adsorption processes, 73 461 Cryogenic air separation, 77 275-278, 752-753... 

Besides absorption processes, adsorption processes, cryogenic separation or membrane separation can be applied. Adsorption processes are based on the physical attachment and bonding of components from the gas mixture on the surface of solid sorbents. As with absorption a distinction can be made between physical and chemical adsorption the first one is referred to as pressure-swing adsorption (PSA), where... 

The redox process (RP) development was eventnally abandoned as other technologies such as pressure swing absorption (PSA) and cryogenic separation began to dominate. In recent years there has been a renewed interest in developing the RP. 

Finally, it is possible to obtain a pure hydrogen stream through several techniques, such as PSA, cryogenic distillation or membrane separation. PSA and cryogenic distillation processes are commercially available separation techniques [14]. 

The process to separate the four Cg aromatics by distillation is very difficult because of their closeness in boiling points (table 11.3), so that only o-xylene can be separated by distillation. / -Xylene has a unique high melting point, and can be separated by cryogenic crystallization, but this is an expensive process that requires refrigeration. What is desired is an economic separation process that singles out jo-xylene among these four compounds. 

As can be seen in Table III the Synthol process produces a large amount of C to C- hydrocarbons which are predominantly olefins. At Sasol tne C stream from the cryogenic separation unit is fed to a standard ethylene plant. The feed is first dried and then fractionated to remove the small amounts of C-. and C products. The C stream is then selectively hydrogenated to remove acetylene... 

Purification of Air Prior to Liquefaction. Separation of air by cryogenic fractionation processes requires removal of water vapor and carbon dioxide to avoid heat exchanger freeze-up. Many plants today are using a 13X (Na-X) molecular sieve adsorbent to remove both water vapor and carbon dioxide from air in one adsorption step. Since there is no necessity for size selective adsorption, 13X molecular sieves are generally preferred over type A molecular sieves. The 13X molecular sieves have not only higher adsorptive capacities but also faster rates of C02 adsorption than type A molecular sieves. The rate of C02 adsorption in a commercial 13X molecular sieve seems to be controlled by macropore diffusion 37). The optimum operating temperature for C02 removal by 13X molecular sieve is reported as 160-190°K 38). 

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