Chapter 4, Problem 4.47P

In an absorption tower (or absorber), a gas is contacted with a liquid under conditions such that one or more species in the gas dissolve in the liquid. A stripping tower (or stripper) also involves a gas contacting a liquid, but under conditions such that one or more components of the feed liquid come out of solution and exit in the gas leaving the tower.

A process consisting of an absorption tower and a stripping tower is used to separate the components of a gas containing 30.0 mole% carbon dioxide and the balance methane. A stream of this gas is fed to the bottom of the absorber. A liquid containing 0.500 mole% dissolved CO₂ and the balance methanol is recycled from the bottom of the stripper and fed to the top of the absorber. The product gas leaving the top of the absorber contains 1.00 mole% CO₂ and essentially all of the methane fed to the unit. The CO₂-rich liquid solvent leaving the bottom of the absorber is fed to the top of the stripper and a stream of nitrogen gas is fed to the bottom. Ninety percent of the CO₂ in the liquid feed to the stripper comes out of solution in the column, and the nitrogen/CO₂ stream leaving the column passes out to the atmosphere through a stack. The liquid stream leaving the stripping tower is the 0.5(K)% CO₂ solution recycled to the absorber.

The absorber operates at temperature T_a and pressure P, and the stripper operates at T_sand P_s. Methanol may be assumed to be nonvolatile—that is, none enters the vapor phase in either column— and N₂ may be assumed insoluble in methanol.

1. In your own words, explain the overall objective of this two-unit process and the functions of the absorber and stripper in the process.

The streams fed to the tops of each tower have something in common, as do the streams fed to the bottoms of each tower. What are these commonalities and w hat is the probable reason for them?

Taking a basis of 100 mol/h of gas fed to the absorber, draw and label a flowchart of the process.

For the stripper outlet gas, label the component molar flow rates rather than the total flow rate and mole fractions. Do the degree-of-freedom analysis and write in order the equations you would solve to determine all unknown stream variables except the nitrogen flow rate entering and leaving the stripper. Circle the variable(s) for w hich you would solve each equation (or set of simultaneous equations), but don't do any of the calculations yet.

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Calculate the fractional CO₂ removal in the absorber (moles absorbed/mole in gas feed) and the molar flow' rate and composition of the liquid feed to the stripping tower.

Calculate the molar feed rate of gas to the absorber required to produce an absorber product gas flow rate of lOOOkg/h.

Would you guess that T_s would be higher or lower than T_a? Explain. (Hint: Think about what happens when you heat a carbonated soft drink and w hat you want to happen in the stripper.) What about the relationship of P_s to P_a?

What properties of methanol would you guess make it the solvent of choice for this process? (In more general terms, what would you look for when choosing a solvent for an absorption-stripping process to separate one gas from another?)