Shaftwork targeting gas phase cryogenic systems with power recovery from surplus cold streams

Many chemical processes require refrigeration over a broad range of low-temperatures. Gas phase refrigeration systems can provide cooling down to cryogenic temperatures, using autorefrigeration to cool the refrigerant before it is expanded. The economics of these systems are dominated by the shaftwork supplied to the compressors and the capital cost of the rotating equipment, both the compressors and turbo-expanders. A shaftwork targeting method has already been developed for gas phase refrigeration systems, which require a continuous cooling load from ambient to some minimum temperature. The method involves the use of a number of gas phase refrigeration loops which are matched against the Grand Composite Curve. In the original work, only cooling is required in each stage, such as for a liquefaction process. However, there are many processes often involving a cold separation unit, where one or more cold streams may require reheating. With condensation refrigeration systems, shaftwork can be reduced by using these cold sinks to condense refrigerant, reducing the amount which needs to be condensed at ambient temperature and higher pressures. With gas phase systems, it is also possible to recover power from these cold sinks. A targeting method has therefore been developed to assess this potential. The simulation results show that the power recovery is favourable only when the mechanical equipment runs at high efficiencies. Hence, process-to-process heat transfer seems to be a better option.