High-pressure process streams

Where high-pressure gas or liquid process streams are throttled to lower pressures, energy can be recovered by carrying out the expansion in a suitable turbine. 

The economic operation of processes which involve the compression and expansion of large quantities of gases, such as ammonia synthesis, nitric acid production and air 

The energy recoverable from the expansion of a gas can be estimated by assuming polytropic expansion see Section 3.13.2 and Example 3.17. 

The design of turboexpanders for the process industries is discussed by Bloch et al. (1982). 

Consider the extraction of energy from the tail gases from a nitric acid adsorption tower, such as that described in Chapter 4, Example 4.4. 

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High-pressure process streams can be throttled by the positioned disc-type slide gate valve. 

Physica.1 Absorption. Whereas chemical absorption rehes on solvent reactions to hold acid gas components in solution, physical absorption exploits gas—hquid solubiUties. The amount of absorption for these solvents is direcdy proportional to the partial pressure of the acid gas components. Thus these processes are most appHcable in situations involving high pressure feed streams containing significant concentrations of acid gas components. To favor absorption, lower temperatures are often employed. Some processes require refrigeration. 

When a large flow of gas is reduced from a high pressure to some lower pressure, or a high-temperature process stream (waste heat) is available at moderate pressures, turboexpanders should be considered to recover the energy (see Figure 7-22). The turbine can drive a compressor... 

If a gas or vapor process stream is available at a high pressure and downstream conditions do not require this high pressure, the stream can be expanded to provide useful cooling. The cooling might allow partial condensation for recovery of the less volatile components in a mixture, or to... 

Gas expanders are used to recover energy from high pressure process gas streams in a plant when the lower pressure is adequate for further processing. Power calculations are made in the same way as those for compressors. Usually several hundred horsepower must be involved for economic justification of an expander. In smaller plants, pressures are simply let down with throttling valves (Joule-Thomson) without attempt at recovery of energy. 

The CNG process removes sulfurous compounds, trace contaminants, and carbon dioxide from medium to high pressure gas streams containing substantial amounts of carbon dioxide. Process features include 1) absorption of sulfurous compounds and trace contaminants with pure liquid carbon dioxide, 2) regeneration of pure carbon dioxide with simultaneous concentration of hydrogen sulfide and trace contaminants by triple-point crystallization, and 3) absorption of carbon dioxide with a slurry of organic liquid containing solid carbon dioxide. These process features utilize unique properties of carbon dioxide, and enable small driving forces for heat and mass transfer, small absorbent flows, and relatively small process equipment. 

Hydraulic turbines are used for recovering energy from high-pressure liquid streams. A common process application is an absorber-stripper combination. In this apphcation, a gas is absorbed in a solvent at a high pressure, where absorption is favored. Then, the solvent is stripped of the absorbed components at a low pressure, where stripping is favored, to recover the solvent. Thus, the energy of the high-pressure solvent stream from an absorber can be partially recovered by a hydraulic turbine. There are three types of hydraulic turbines, the Pelton-wheel turbine, the Francis turbine, and the propeller reaction turbine, an axial type turbine. The propeller reaction turbine is used in hydroelectric applications and will not be considered further. The Pelton-wheel and Francis tur-... 

Tims tlie Linde process is a limiting case of tlie Claude process, obtained when none of tlie high-pressure gas stream is sent to an expander. 

Figure 10.17 shows an example of the separation efficiency of the SSF membrane for SMROG-PS A waste gas.64 It plots the rejection ( ) of the more selectively adsorbed components of the gas mixture (i = C02, CH4/CO) as a function of H2 recovery (aH2). The rejection of component i is defined by the ratio of the molar flow rate of that component in the low-pressure permeate stream to that in the feed stream. The recovery of H2 is defined by the ratio of the molar flow rate of H2 in the high-pressure effluent stream to that in the feed stream. The plot also shows the ratio of the membrane area (A) needed to process a given flow rate (F) of the feed gas. These data are sufficient to design the membrane for a given feed gas composition and flow rate.69... 

The microbial contamination of raw materials can also occur in the form of viruses [78, 79] or mycoplasma. In these instances, conventional 0.2 pm-rated filters are ineffective. Newly developed 100-nm filters have been claimed by the vendors to remove substantial amounts of mycoplasma, but breakthrough may occur at high operating pressures or in high-osmolahty process streams. 

The optical cells described so far have mainly been used as batch-type reactors to study discontinuous fluid-phase reactions. Continuously operated high-pressure processes in tubular reactors are easily monitored by transmission-type cells, like that shown in Fig. 4.3, which are introduced into the product stream and perhaps also into the feed flux. The spectroscopic analysis may be performed on the main stream or by choosing some suitable by-pass arrangement. 

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