A common feature of a cryogenic plant is the separation and/or Liquefaction of Gases at process conditions which may be at elevated pressures, but always involving very low temperatures. There are many industrial cryogenic processes which operate at temperatures in the region of –165°C to –195°C at their coldest point, with some operating as low as –269°C. Consequently, the conservation of cold becomes a dominant feature in the design of such processes, which focuses on highly efficient heat exchange. However, a typical cryogenic process has many elements to it, the cryogenic section being only a part of the whole flowscheme. Figure 1 illustrates a typical cryogenic plant which consists of a pretreatment section, a cryogenic section and a compressor/ expander section which provides refrigeration for the process. In many instances, feed compression may be required as in the case of air separation. Another aspect of cryogenic plant is the use of aluminum and stainless steel for the cold sections of the plant to avoid embrittlement problems encountered with carbon steel.
The prepurification section is usually needed upstream of a cryogenic plant because most feed gases will contain constituents that may freeze inside or even corrode the cryogenic equipment and will therefore require removal. Wet hydrocarbons can form hydrates at temperatures above 0°C (typically 5-15°C) and in such cases, water removal may be necessary. Freezable components include H2O and CO2 and NH3. These are usually removed from the feed gas by absorption, chilling, permeation, reversing heat exchangers, or combinations of these in a prepurification section.
Table 1 gives a list of typical gases treated by cryogenic units. Typical impurities are listed for each of these gases. In general, these impurities are removed upstream to less than 1 ppm. However, in some cases the liquids formed in the process dissolve the freezable components to a significant extent, making it possible to allow several hundred ppm to enter the cryogenic unit. Each case has to be considered carefully by the cryogenic unit designer, who uses reliable solubility and equilibrium data to determine the levels acceptable in the cryogenic unit. The impurity concentration in the feed gas determines the nature of the pretreatment process used. Table 2 shows a very general guide to pretreatment processes used for various impurity levels; each case must be carefully analyzed to select the proper scheme. Impurities such as HCl, NH4Cl, Hg and arsenic compounds are generally found in very low concentrations and catalytic methods are often used to remove them.
The clean, dry gas is admitted to the cryogenic part of the plant, which cools, condenses and separates the desired components in the gas at low temperatures. In order to produce low temperatures, it is imperative that all parts of the cryogenic unit are designed as efficiently as possible (see Liquefaction of Gases). Exergy analysis [Tomlinson et al. (1990)] shows how important process efficiency is in reducing power consumption and improving product yields.
Both reciprocating and rotating compression equipment is used in feed compression and refrigeration cycles in a cryogenic plant. The major difference between compressor equipment for cryogenic plant compared with noncryogenic plant is in the lubrication system. Many gas compressor preceding cryogenic processes are oil-free on the process side. If oil-lubricated compressors are used, then oil-removal systems are included after the machines. For these types of reciprocating compressors, piston rings are made of composite materials, which include graphite, thereby promoting lubrication. Packing glands are purged with dry gases to form a seal between the oil-lubricated crankcase side and the process end. In centrifugal machinery, a dry-seal system is provided to prevent process gas being contaminated with oil so that the metal bearings are allowed thorough lubrication while the process gas is kept dry.
Expansion turbines are used extensively to provide efficient refrigeration. These require dry operation for the process gas in a similar manner to 'dry' centrifugal compressors, and therefore employ similar lubrication and seal systems.
Reciprocating expanders are also used today in helium liquefiers, where flows are relatively small. Also, design of rotary expanders for low molecular weight gases to achieve a high efficiency is difficult without using very high speeds.
One of the most important aspects of a good cryogenic plant design is the effective use of heat exchangers. This is clearly brought out by exergy analysis during the conceptual design stage. Cryogenic plants generally utilize three types of heat exchangers: shell-and-tube, wound-coil, and plate-fin. The most widely used is the plate-fin. To appreciate why this is so, the three types of heat exchanger are compared in Table 3 in terms of area per unit volume, maximum design pressure, practical approach temperatures, number of streams handled in one unit, and materials of construction. The materials of construction have cryogenic service in mind. Another effective heat exchanger that can be applied in cryogenic service is the etched plate compact heat exchanger. This is particularly suitable where pressures exceed 10 MPa.
A cutaway drawing of a plate-fin heat exchanger is shown in Figure 2.
Distillation is extensively used in cryogenics to produce pure components from mixtures of gases. There are some significant differences in cryogenic plant mass transfer equipment which aim to make the units as compact as possible. Consequently, sieve trays with very low tray spacing (80 to 100 mm) are extensively used in air separation and other areas. Structured packing is also seeing a significant increase in applications in cryogenic service. Table 4 gives a summary of mass transfer equipment used in cryogenic plant.
To improve efficiency of cryogenic processes, process intensification is always seriously examined during process selection. An excellent example where this has succeeded is in the use of plate-fin heat exchangers to carry out simultaneous heat and mass transfer. Figure 3 shows a section of plate-fin heat exchanger used for distillation purposes. This is particularly suitable for a situation where a lower molecular weight gas contains a few percent of heavier components. The partly-cooled vapor may enter at point A, any condensate is knocked out in V1, the vapor then enters the reflux exchanger, E1, at B, and is cooled by refrigerant DE and product XY. The exchanger is always mounted vertically so that the feed stream cools in an upward direction to C. As it cools, condensate forms. Instead of the condensate passing up with the main gas flow, it runs back down the heat exchanger to the bottom and B. As it runs back, it is warmed by the upcoming vapor stream which tends to boil-off "light ends". Therefore, the liquid that emanates from the base B contains only small amounts of "light ends" and most of "the heavy ends". This liquid is approaching the equilibrium condition of the vapor entering at B and not the condition at the cold end of the exchanger C. This is the important factor of the reflux exchanger, which makes it more efficient than straight cooling and partial condensation. Several equilibrium stages can be catered for in a single long core, at the same time providing refrigeration at each theoretical stage.
The designer has to cater for heat transfer and distribution problems for plate-fins exchangers, as well as mass transfer. In counter-flow of vapor and liquid an appropriate flooding correlation is required.
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