Separating one or a limited number of components from a gas mixture is usually done by adsorption or
absorption columns. In the simulations for this question, you will put together a simulation with a gas/liquid
absorption column. You need to complete part a) before starting b). Note that two simulation files are needed.
Each question uses imperial units, and you should set up your HYSYS output to display the imperial unit system.
a) A wet natural gas feed having a flow rate of 100 lb mol/h at 90°F and 60 psia was fed to an gas-liquid
absorber which has a total of four stages where 110.4 lb mol/h of lean oil at 90°F and 60 psia is used to
absorb the hydrocarbon components. Table 1 and 2 shows the composition of wet natural gas feed and
lean oil respectively. After the absorption process, the lean oil has become the rich oil, enriched with
hydrocarbons. The residue gas then leaves the top of the absorber at 60 psia. Note that you are required
to use bottom-up approach for stage numbering and select Peng Robinson as the fluid package.
Table 1: Composition of wet natural gas feed
Component Methane Ethane Propane n-Butane n-Pentane
Mole fraction 0.285 0.158 0.240 0.169 0.148
Table 2: Composition of lean oil feed
Component n-Octane n-Butane n-Pentane
Mole fraction 0.93 0.02 0.05
Develop a HYSYS simulation and determine:
i) The total molar flow rate of product gas with its molar composition
ii) The total molar flow rate of rich oil with its molar composition
iii) The recovery of propane (in mole percentage) in rich oil
b) In a second scenario, the lean oil which is at 90°F and 60 psia is first passed through a cooler to cool down
to 70°F prior entering the absorber. Note that the remaining properties remain unchanged.
i) What can you comment on the percentage of propane recovery in rich oil (in mole percentage)
when your temperature of lean oil is lower?
ii) If 90 mole percent of propane from wet gas is to be recovered in rich oil, what temperature of lean
oil would be needed to achieve this specification? Run a series of simulations using the case study
tool to show the recovery as a function of lean oil temperature.
iii) Develop a HYSYS spreadsheet that has as its objective function, the recovery of 90 mole percent.
Show via the case study that HYSYS identifies the optimum temperature to achieve this recovery.
Note that you are not to aim for anything but the 90 mole %.