Design Of Gas Absorption Systems

General Design Procedure The design engineer usually is required to determine (1) the best solvent; (2) the best gas velocity through the absorber, or, equivalently, the vessel diameter; (3) the height of the vessel and its internal members, which is the height and type of packing or the number of contacting trays; (4) the optimum solvent circulation rate through the absorber and stripper; (5) temperatures of streams entering and leaving the absorber and stripper, and the quantity of heat to be removed to account for the heat of solution and other thermal effects; (6) pressures at which the absorber and stripper will operate; and (7) mechanical design of the absorber and stripper vessels (predominantly columns or towers), including flow distributors and packing supports. This section covers these aspects.

The problem presented to the designer of a gas absorption system usually specifies the following quantities: (1) gas flow rate; (2) gas composition of the component or components to be absorbed; (3) operating pressure and allowable pressure drop across the absorber; (4) minimum recovery of one or more of the solutes; and, possibly, (5) the solvent to be employed. Items 3, 4, and 5 may be subject to economic considerations and therefore are left to the designer. For determination of the number of variables that must be specified to fix a unique solution for the absorber design, one may use the same phase-rule approach described in Sec. 13 for distillation systems.

Recovery of the solvent, occasionally by chemical means but more often by distillation, is almost always required and is considered an integral part of the absorption system process design. A more complete solvent-stripping operation normally will result in a less costly absorber because of a lower concentration of residual solute in the regenerated (lean) solvent, but this may increase the overall cost of the entire absorption system. A more detailed discussion of these and other economical considerations is presented later in this section.

The design calculations presented in this section are relatively simple and usually can be done by using a calculator or spreadsheet. In many cases, the calculations are explained through design diagrams. It is recognized that most engineers today will perform rigorous, detailed calculations using process simulators. The design procedures presented in this section are intended to be complementary to the rigorous computerized calculations by presenting approximate estimates and insight into the essential elements of absorption and stripping operations.

Selection of Solvent and Nature of Solvents When a choice is possible, preference is given to solvents with high solubilities for the target solute and high selectivity for the target solute over the other species in the gas mixture. A high solubility reduces the amount of liquid to be circulated. The solvent should have the advantages of low volatility, low cost, low corrosive tendencies, high stability, low viscosity, low tendency to foam, and low flammability. Since the exit gas normally leaves saturated with solvent, solvent loss can be costly and can cause environmental problems. The choice of the solvent is a key part of the process economic analysis and compliance with environmental regulations.

Typically, a solvent that is chemically similar to the target solute or that reacts with it will provide high solubility. Water is often used for polar and acidic solutes (e.g., HCl), oils for light hydrocarbons, and special chemical solvents for acid gases such as CO2, SO2, and H2S. Solvents are classified as physical and chemical. A chemical solvent forms complexes or chemical compounds with the solute, while physical solvents have only weaker interactions with the solute. Physical and chemical solvents are compared and contrasted by examining the solubility of CO2 in propylene carbonate (representative physical solvent) and aqueous monoethanolamine (MEA; representative chemical solvent).

Figures 14-1 and 14-2 present data for the solubility of CO2 in the two representative solvents, each at two temperatures: 40 and 100°C.

Chemical And Physical Solvents Co2

FIG. 14-1 Solubility of CO2 in 30 wt% MEA and propylene carbonate. Linear scale.

Pco2 (kPa)

FIG. 14-1 Solubility of CO2 in 30 wt% MEA and propylene carbonate. Linear scale.

The propylene carbonate data are from Zubchenko et al. [Zhur. Prik-lad. Khim, 44, 2044-2047 (1971)], and the MEA data are from Jou, Mather, and Otto [Can. J. Chem. Eng., 73, 140-147 (1995)]. The two figures have the same content, but Fig. 14-2 focuses on the low-pressure region by converting both composition and pressure to the logarithm scale. Examination of the two sets of data reveals the following characteristics and differences of physical and chemical solvents, which are summarized in the following table:

Characteristic

Physical solvent Chemical solvent

Solubility variation with pressure Low-pressure solubility High-pressure solubility Heat of solution—related to variation of solubility with temperature at fixed pressure

Relatively linear Low

Continues to increase Relatively low and approximately constant with loading

Highly nonlinear High Levels off Relatively high and decreases somewhat with increased solute loading

Chemical solvents are usually preferred when the solute must be reduced to very low levels, when high selectivity is needed, and when the solute partial pressure is low. However, the strong absorption at low solute partial pressures and the high heat of solution are disadvantages for stripping. For chemical solvents, the strong nonlinearity of the absorption makes it necessary that accurate absorption data for the conditions of interest be available.

Selection of Solubility Data Solubility values are necessary for design because they determine the liquid rate necessary for complete or economic solute recovery. Equilibrium data generally will be found in one of three forms: (1) solubility data expressed either as weight or mole percent or as Henry's law coefficients; (2) pure-component vapor pressures; or (3) equilibrium distribution coefficients (K values).

Data for specific systems may be found in Sec. 2; additional references to sources of data are presented in this section.

To define completely the solubility of gas in a liquid, it is generally necessary to state the temperature, equilibrium partial pressure of the solute gas in the gas phase, and the concentration of the solute gas in the liquid phase. Strictly speaking, the total pressure of the system should also be identified, but for low pressures (less than about 507 kPa or 5 atm), the solubility for a particular partial pressure of the solute will be relatively independent of the total pressure.

For many physical systems, the equilibrium relationship between solute partial pressure and liquid-phase concentration is given by Henry's law:

where H is Henry's law coefficient expressed in kPa per mole fraction solute in liquid and H' is Henry's law coefficient expressed in kPam3/kmol.

Figure 14-1 indicates that Henry's law is valid to a good approximation for the solubility CO2 in propylene carbonate. In general, Henry's law is a reasonable approximation for physical solvents. If Henry's law holds, the solubility is defined by knowing (or estimating) the value of the constant H (or H').

Note that the assumption of Henry's law will lead to incorrect results for solubility of chemical systems such as CO2-MEA (Figs. 14-1 and 14-2) and HCl-H2O. Solubility modeling for chemical systems requires the use of a speciation model, as described later in this or

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  • anna
    Does loss of solvent flow in a stripper cause a high pressure event?
    7 months ago
  • Sayid
    How to determine pressure of an absorber?
    20 days ago
  • Arthur
    What are the best solvents to use in gas absorption?
    14 days ago

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