The pharmaceutical industry has already been mentioned as a user of refrigeration in freeze-drying processes. An approach that seems popular in that industry is also to chill a fluid and distribute this cold fluid (glycol antifreeze or alcohol, for example) to the location where refrigeration is needed. Low-temperature alcohol may chill the jacket of a vessel, or glycol may be sent to air-cooling coils located in a room where filling of powder is taking place that must be maintained at a low humidity.
The chemical, petrochemical, and oil-refining industries often need largesized refrigeration plants. Some important operations often requiring refrigeration are:
– separation of one gas from another by liquefying more of one gas
– condensation of gases, such as capturing gas venting from a liquid storage tank
– solidification of one substance in a mixture to separate it from others
– maintenance of a stored liquid at low temperature to control pressure in the containing vessel
– removal of reaction heat
– humidity control for hygroscopic chemicals
Two major concepts in providing the refrigeration are direct and indirect. In direct refrigeration the product itself is the refrigerant and is compressed, condensed, and throttled to form liquid at a low temperature. The petrochemical industry frequently uses propane, ethane, and ethylene, for example, as refrigerants. Often these chemical plants are large and traditionally use highcapacity centrifugal compressors as shown in Figure 1.6. The facility shown is in a liquefied petroleum gas fractionation plant where 8 two-stage centrifugals compress propane in the refrigeration cycle. An alternate approach is to provide indirect chilling through the use of a skid-mounted package that employs ammonia or a suitable halocarbon refrigerant. The ammonia or halocarbon refrigerant from the package serves the heat exchanger performing the cooling, or in some cases a pump delivers a low-temperature secondary refrigerant to the heat exchanger. The skid-mounted package in Figure 1.7 provides lowtemperature refrigeration through a combination of two circuits, one using R-22 and the other carbon dioxide.
A list of just a few of the industries and the temperature range of their refrigeration needs is: precooling air prior to separation10, -100°C (-148°F); condensation of hydrocarbon vapors11 taking place at -55°C (-67°F); and in the manufacture of such chemicals as ammonia/urea, aniline dyes, butadiene, butyl rubber, chlorine, pesticides/herbicides, synthetic rubber, vinyl chloride, and xylenes, as well as in the recovery of solvents.