Liquefaction refrigerants

Deep refrigeration Ice making Thermal expansion Compatibility of materials Gas liquefaction Refrigerants and gases System control and monitoring Vapor-compression circuits Absorption systems... 

A. Latham Jr. and H. O. McMahon, "Low temperature research is aided by simplified helium liquefaction," Refrigerating Engineering (June, 1949). 

Where the distance to the customer is very large, or where a gas pipeline would have to cross too many countries, gas may be shipped as a liquid. Gas has to be chilled to -160°C in a LNG plant to keep it in liquid form, and is shipped in refrigerated tankers. To condition the gas for liquefaction any COj, HjS, water and heavier hydrocarbons must be removed, by the methods already discussed. The choice of how much propane and butane to leave in the LNG depends upon the heating requirements negotiated with the customer. 

Gas can be condensed by (a) mechanically refrigerating it, (b) compressing and expanding it, using turboexpanders, or, (c) pressure effects such as by Joule-Thomson cooling and overcoming the vapor pressure. The liquefaction of methane can involve all three of these effects. These effects can be separately evaluated to show the effectiveness of each in producing liquid. 

Figure 3-1. This example flow scheme for methane liquefaction uses compression, turboexpansion, and mechanical refrigeration.

In summary, starting with 105°F gas at atmospheric pressure, the theoretical work necessary to liquify one pound of methane is 510.8 Btu or 352 hp/MMcfd. The simplified liquefaction process, as illustrated, uses a turboexpander/compressor and a small propane refrigeration unit. The 41.25% efficiency breaks down as follows one-fourth contributed by the turboexpander/compressor at 35.8% efficiency one-sixteenth contributed by the mechanical propane refrigeration unit at 43% efficiency, at a moderate temperature where its efficiency is high and a large fraction—eleven-sixteenths—contributed at 58.2% efficiency by compression and Joule-Thomson condensation energy. 

An expansion turbine (also called turboexpander) converts gas or vapor energy into mechanical work as the gas or vapor expands through the turbine. The internal energy of the gas decreases as work is done. The exit temperature of the gas may be very low. Therefore, the expander has the ability to act as a refrigerator in the separation and liquefaction of gases. 

Because much of the world lacks the natural gas resources and transportation pipelines of the United States, remote natural gas must be liquefied and transported by ship. Gas-rich countries want to capture stranded gas by liquefying and shipping it to gas-poor regions as LNG. The gas-poor countries enter into contracts so that a long-term supply is available to warrant the investment in the electricity-generating infrastructure. The overall investment is enormous, not only in the liquefaction plant, but in the refrigerated tankers and the regasification plant at the deliveiy site. 

Rather than use the simple cycle shown in Figure 24.44 for the liquefaction of natural gas, much more complex arrangements using multiple cycles (with both pure and mixed refrigerants) and cascade systems can be used. 

As noted already, the electrical energy needed to run the refrigeration unit for liquefaction of chlorine is about 335 kj kg-1, equivalent to 93 kWh per tonne of chlorine. The annual power consumption will be 19.5 million kWh. At a power cost of 5c per kWh, the annual cost is close to US 1.0 million. There will also be an operating cost associated with the incremental cooling water usage that has not been included. 

The separation section of a gas oil cracker looks like a small refinery, as you can see in Figure 5 or in Figure 5—5. In addition to the fractionators and treaters used in the purification section of the simpler ethane cracker, there are facilities to separate the heavier coproducts. In the front end of the separator facilities in Figure 5-4, the cold box option for handling the liquefaction of the gases is shown. Temperatures as low as -220°F are achieved in this super-refrigerator. At those low temperatures, Freon wont do the job. Liquid air, methane, ethylene, or ammonia are often used as the refrigerant in much the same way Freon has been used in an air conditioner. 

Absorption Air-Conditioning Brayton Gas Refrigeration Cycle Stirling Refrigeration Cycle Ericsson Cycle Liquefaction of Gases Nonazeotropic Mixture Refrigeration Cycle Design Examples Summary... 

---