Heat-absorption effects

The simplest possible case occurs when (1) both the operating and the equilibrium hues are straight (i.e., there are dilute solutions), (2) Henry s law is valid y /x = yifXj = m), and (3) absorption heat effects are negligible. Under these conditions, the integral term in Eq. (14-20) may be computed by Colburn s equation [Trans. Am. Jn.st. Chem. Eng., 35,211 (1939)] ... 

In many practical situations involving nearly complete cleanup of the gas, an approximate result can be obtained from the equations just presented even when solutions are concentrated or when absorption heat effects are present. In such cases the driving forces in the upper part of the tower are very much smaller than those at the bottom, and the value of mGM/LM used in the eqiiations should be the ratio of the slopes of the equilibrium line m and the operating line Lm/Gm iu the low-concentration range near the top of the tower. 

Overview One of the most important considerations involved in designing gas-absorption towers is to determine whether or not temperatures will vaiy along the length of the tower because of heat effects, since the sohibility of the solute gas normally depends strongly upon the temperature. When heat effects can be neglected, computation of the tower dimensions and required flows is relatively straight-... 

There are, however, a number of well-known systems in which heat effects definitely cannot be ignored. Examples include absorption of ammonia in water, dehumidification of air with concentrated H9SO4, absorption of HCl in water, and absorption of SO3 in H9SO4. Another interesting example is the absorption of acetone in water, in which the heat effec ts are mild but not neghgible. 

Temperature of Fresh Solvent The temperature of the entering solvent has surprisingly httle influence upon the degree of absorption or upon the internal-temperature profiles in an absorber when the heat effects are due primarily to heat of sohition or to solvent vaporization. In these cases the temperature profile in the hquid phase apparently is dictated solely by the internal-heat effects. 

Temperature and Humidity of Rich Gas Cooling and consequent dehumidification of the feed gas to an absorption tower can be very beneficial. A high humidity (or relative saturation with solvent) limits the capacity of the gas phase to take up latent heat and therefore is unfavorable to absorption. Thus, dehumidification of the inlet gas prior to introducing it into the tower is worth considering in the design of gas absorbers with large heat effects. 

In concentrated wstems the change in gas aud liquid flow rates within the tower and the heat effects accompanying the absorption of all the components must be considered. A trial-aud-error calculation from one theoretical stage to the next usually is required if accurate results are to be obtained, aud in such cases calculation procedures similar to those described in Sec. 13 normally are employed. A computer procedure for multicomponent adiabatic absorber design has been described by Feiutnch aud Treybal [Jnd. Eng. Chem. Process Des. Dev., 17, 505 (1978)]. Also see Holland, Fundamentals and Modeling of Separation Processes, Prentice Hall, Englewood Cliffs, N.J., 1975. 

Absorption recovers valuable light components such as propane/propylene and butane/ butylene as vapors from fractionating columns. These vapors are bubbled through an absorption fluid, such as kerosene or heavy naphtha, in a fractionating-like column to dissolve in the oil while gases, such as hydrogen, methane, ethane, and ethylene, pass through. Absorption is effectively performed at 100 to 150 psi with absorber heated and distilled. The gas fraction is condensed as liquefied petroleum gas (LPG). The liquid fraction is reused in the absorption tower. 

Steady-State Gas Absorption with Heat Effects... 

Figure 4.18. Steady-state gas absorption with heat effects. Mole ratio and mole fraction contents are related by...

Chapter 3 concerns the dynamic characteristics of stagewise types of equipment, based on the concept of the well-stirred tank. In this, the various types of stirred-tank chemical reactor operation are considered, together with allowance for heat effects, non-ideal flow, control and safety. Also included is the modelling of stagewise mass transfer applications, based on liquid-liquid extraction, gas absorption and distillation. 

Fig. 4.18 Steady-state gas absorption with heat effects.

AMMONAB - Steady-State Design of a Gas Absorption Column with Heat Effects System... 

---