Exiting liquid

FIG. 13-42 General adiabatic countercurrent cascade for simple absorption or stripping.

is the effective or average stripping factor for component i. When the entering streams are at the same temperature and pressure and negligible absorption and stripping occur, effective component absorption and stripping factors are determined simply by entering-stream conditions. Thus, if K values are composition-independent, then

(Ai)e = 1/(Si)e = (Ln+1/KilTN+!, Pn + x}V0) (13-44)

When entering-stream temperatures differ and/or moderate to appreciable absorption and/or stripping occurs, values of Ai and Si should be based on effective average values of L, V, and Ki in the cascade. However, even then Eq. (13-44) with TN+1 replaced by (TN+1 + T0)/2 may be able to give a first-order approximation of (Ai)e. In the case of an absorber, LN+1 < Le and V0 > Ve will be compensated to some extent by Kj|(TN+1 + T0)/2, P)) < Kl[Te, P). A similar compensation, but in opposite directions, will occur in the case of a stripper.

Equations (13-40) and (13-41) are plotted in Fig. 13-43. Components having large values of Ae or Se absorb or strip respectively to a large extent. Cooresponding values of OA and OS approach a value of 1 and are almost independent of the number of equilibrium stages.

An estimate of the minimum absorbent flow rate for a specified amount of absorption from the entering gas of some key component K for a cascade with an infinite number of equilibrium stages is obtained from Eq. (13-40) as

The corresponding estimate of minimum stripping-agent flow rate for a stripper is obtained as

Example 2: Calculation of Kremser Method For the simple absorber specified in Fig. 13-44, a rigorous calculation procedure as described below gives results in Table 13-9. Values of O were computed from component-product flow rates, and corresponding effective absorption and stripping factors were obtained by iterative calculations in using Eqs. (13-40) and (13-41) with N = 6. Use the Kremser method to estimate component-product rates if N is doubled to a value of 12.

Assume that values of Ae and Se will not change with a change in N. Application of Eqs. (13-40), (13-41), and (13-39) gives the results in the last four columns of Table 13-10. Because of its small value of Ae, the extent of absorption of C1 is unchanged. For the other components, somewhat increased amounts of absorption occur. The degree of stripping of the absorber oil is essentially unchanged. Overall, only an additional 0.5 percent of absorption occurs. The greatest increase in absorption occurs for n-C4, to the extent of about 4 percent.

Component |
N = 6 (rigorous method) |
N = 12 (Kremser method) | ||||||||

(lb • mol)/h |
(Oi)A |
(©i)S |
(Ai)e |
(Si)e |
(lb • mol)/h |
(O)a |
(O)s | |||

(vi)6 |
(•i)1 |
(vi)12 |
(•i)1 | |||||||

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