Cryogenic distillation, product separation

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. 

A sharp separation results in two high purity, high recovery product streams. No restrictions ate placed on the mole fractions of the components to be separated. A separation is considered to be sharp if the ratio of flow rates of a key component in the two products is >10. The separation methods that can potentially obtain a sharp separation in a single step ate physical absorption, molecular sieve adsorption, equiHbrium adsorption, and cryogenic distillation. Chemical absorption is often used to achieve sharp separations, but is generally limited to situations in which the components to be removed ate present in low concentrations. 

Reactive absorption is probably the most widely applied type of a reactive separation process. It is used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. It is also often employed in gas purification processes, e.g., to remove carbon dioxide or hydrogen sulfide. Other interesting areas of application include olefin/paraffin separations, where reactive absorption with reversible chemical complexation appears to be a promising alternative to the cryogenic distillation (62). 

Sharp separation consists of obtaining splitting of the mixture into products with a high recovery of target components. The sharpness is defined as the ratio of key component concentrations in products. This should be better than 10. Potential techniques are physical absorption, cryogenic distillation, molecular sieving, as well as equilibrium adsorption when the molar fraction of the adsorbate is less than 0.1. Chemical absorption may also be applicable when the component concentration is low. 

Krypton and Xenon are valuable gases present in very low concentrations in air. U.S. 6,662,593 assigned to Air Products describes a cryogenic distillation process for air separation with recovery of a stream concentrated in Kypton and Xenon. What would be the cost of producing purified Krypton and Xenon by this method Consider reactive methods for separating Krypton and Xenon from the concentrated stream as well as the methods suggested in the patent. 

With the proper choice of membrane material, a membrane separator can achieve significant separations in a single stage. Indeed, as we shall see, it usually turns out that multistage operations with membranes are not economical (compared to cryogenic distillation). Thus most industrial applications of membranes do not require very high purity products. 

Split generation and their sequencing can be managed by means of heuristics. General heuristics are presented in Table 7.12, which are inspired by the use of distillation. Firstly must be removed corrosive, hazardous, and any troublesome materials. This is particularly true for gas phase separations. For example, components that freeze, as water and CO2, may foul the equipment in cryogenic distillation. Then, the next should be splits that separate directly a product. The same priority is valid for the removal of the most plentiful component, which reduces the cost of downstream operations. Finally, if possible, a 50/50 split is the best solution for further sequencing. 

A more radical approach is to develop membrane processes that combine hydrogen generation and separation in a single reactor. Further research on membranes is also needed to yield a new and cheaper process for the separation of oxygen from air. The present process of cryogenic distillation is too costly for the bulk production of hydrogen as a fuel. 

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