Page 171 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

5.2 Absorption 145

A common and relatively simple procedure for estimating flooding velocity and
minimum column diameter is to use a generalized flooding and pressure drop
correlation. One version of the flooding and pressure drop relationship for a packed
tower is the Sherwood Chart [12]. Readers are referred to literature for in-depth
understanding on this topic in engineering design of a packed bed wet scrubber.

5.2.4 Chemical Absorption

In many engineering applications, the resistance in the liquid phase mass transfer is
reduced by converting the dissolved gas into other materials. This reduction in the
solute concentration in the liquid allows more gases to be absorbed at a much lower
consumption of the liquid absorbent. For example, base solvents are used for the
capture of acidic gases. The most common acid gases include sulfur dioxide (SO 2 ),
hydrogen chloride (HCl), and hydrogen fluoride (HF). Nitric oxides and carbon
dioxide formed in most combustion processes are also mildly acidic. Common
alkalis include lime, soda ash, and sodium hydroxide. Sodium hydroxide is usually
fed in solution. One classic example is de-SO 2 by spray of limestone or sodium
hydroxide. The alkali requirements are usually calculated based on the quantities of
acidic gases captured and the molar ratios necessary for the corresponding chemical
reactions.
Consider the packed-bed wet scrubber again and assume that the fresh liquid
solvent contains little dissolved gas of concern. At steady state, the mass transfer
rate (in mole/s) within the liquid phase is

N ¼ k x aV x i x ss Þ ð5:52Þ
ð
3
where V the total reactor volume (m ) and aV together is the interfacial contact area
2
for mass transfer (m ); x and x ss are the mole fraction of the target gas at gas–liquid
interface and that in the bulk liquid at steady state, respectively.
The target gas transferred into the liquid is either physically stored in the liquid
or consumed by chemical reactions:

N ¼ L x ss þ kx ss ð5:53Þ
1 x ss

Again L is the mole flow rate (in mole/s) of the solute free liquid entering the
tower. k (mole/s) is the chemical reaction coefficient corresponding to x ss . When k is
available based on the mass concentration c ss rather than x ss , a conversion between
units is needed. The first term on the right-hand side of Eq. (5.53) stands for amount
of physically dissolved gas; the last term for chemically absorbed gas.