Cryogenic processing

Final Purification. Oxygen containing compounds (CO, CO2, H2O) poison the ammonia synthesis catalyst and must be effectively removed or converted to inert species before entering the synthesis loop. Additionally, the presence of carbon dioxide in the synthesis gas can lead to the formation of ammonium carbamate, which can cause fouHng and stress-corrosion cracking in the compressor. Most plants use methanation to convert carbon oxides to methane. Cryogenic processes that are suitable for purification of synthesis gas have also been developed. 

K. D. Timmeibaus and T. M. Flynn, Cryogenic Process Engineerings Plenum Piess, New Yoik, 1989. 

As in the case of the salt complexation processes, the cryogenic systems require prepuriftcation of the feed gas. Bulk water, hydrogen sulfide, and carbon dioxide are removed by standard techniques. Final removal of these materials is accompHshed by adsorption. After prepuriftcation, the gases are ready for cryogenic processing. 

Augmenting refrigeration in various cryogenic processes such as the recovery of ethylene... 

For many years, turboexpanders have been used in cryogenic processing plants to provide low-temperature refrigeration. Power recovery has been of secondary importance. Expander efficiency determines the amount of refrigeration produced and, in gas process plants, the amount of product usually depends on the available refrigeration. Accordingly, there is a large premium on efficiency and, of course, on reliability. 

Cryogenic processes using turboexpanders facilitate high levels of ethylene recovery from refinery gas while producing byproducts of hydrogen- and methane-rich gas. In a cryogenic process, most of the ethylene and almost all of the heavier components are liquified and ethylene is separated from this liquid. 

As stated earlier, turboexpanders are normally used in cryogenic processes to produce isentropic expansion to cool down the process gas. Two common applications are natural gas processing plants and chemical plants. In natural gas processing plants, turboexpanders are installed to liquify heavier hydrocarbon components and produce lean natural gas with specified dew point limits to meet required standards. 

A methane gas stream taken off the demethanizer process, and still at 350 psig, is compressed via byproduct energy from the turboexpanders and raised to 410 psig. The gas is then introduced into a 15,000 hp compressor and raised to 850 psig for delivery back to the El Paso Natural Gas Company. The 60 psig boost by each turboexpander represents a 15% reduction in required horsepower. This amounts to considerable energy saved and is yet another reason why the turboexpander is useful in a cryogenic process of this type. 

Chapter 9 discusses the refrigeration and cryogenic processes used u-remove specific components from a gas stream, thereby reducing its Btu content. 

Improved control over heat and mass transfer as well as residence time by micro-channel processing often allows one to increase the reaction temperature of cryogenic processes without losing selectivity. It often leads to improved selectivity. 

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