Chapter 6, Problem 6.78P

Dehydration of natural gas is necessary to prevent the formation of gas hydrates, which can plug valves and other components of a gas pipeline, and also to reduce potential corrosion problems. Water removal can be accomplished as shown in the following schematic diagram:

Natural gas containing $80{\text{lb}}_{\text{m}}{\text{H}}_{\text{2}}\text{O}/{10}^6\text{SCF}$gas $\left[{\text{SCF=ft}}^3\left(\text{STP}\right)\right]$enters the bottom of an absorber at a rate of $4.0\times {10}^6$SCF/day. A liquid stream containing triethylene glycol (TEG, molecular weight = 150.2) and a small amount of water is fed to the top of the absorber. The absorber operates at 500 psia and 90°F. The dried gas leaving the absorber contains $10{\text{lb}}_{\text{m}}{\text{H}}_{\text{2}}\text{O}/{10}^6\text{SCF}$gas. The solvent leaving the absorber, which contains all the TEG—water mixture fed to the column plus all the water absorbed from the natural gas, goes to a distillation column. The overhead product stream from the distillation column contains only liquid water. The bottoms product stream, which contains TEG and water, is the stream recycled to the absorber.

(a) Draw and completely label a ?owchart of this process. Calculate the mass ?ow rate (lb_m/day) and volumetric ?ow rate (ft³/day) of the overhead product from the distillation column.

(b) The greatest possible amount of dehydration is achieved if the gas leaving the absorption column is in equilibrium with the solvent entering the column. If the Henry's law constant for water in TEG at 90°F is 0.398 psia/mol fraction, what is the maximum allowable mole fraction of water in the solvent fed to the absorber‘?

(c) A column of in?nite height would be required to achieve equilibrium between the gas and liquid at the top of the absorber. For the desired separation to be achieved in practice, the mole fraction of water in the entering solvent must be less than the value calculated in Part (b). Suppose it is 80% of that value and the ?ow rate of TEG in the recirculating solvent is 37 lb_mTEG/lb_mwater absorbed in the column. Calculate the ?ow rate (lbm/day) of the solvent stream entering the absorber and the mole fraction of water in the solvent stream leaving the absorber.

(d) What is the purpose of the distillation column in the process? (Hint: Think about how the process would operate without it.)