Reference no: EM13876999
Nitrogen gas can be injected into oil wells to increase the recovery of crude oil (enhanced oil recovery). Usually, natural gas is produced with the oil and it is desirable to recover the nitrogen from the gas for reinjection into the well. Furthermore, the natural gas must not contain more than 3 mol% nitrogen if the natural gas is to be put into a pipeline. A total of 170.000 SCFH (based on 60SF and 14.7 psia) of natural gas containing 18% N2, 75% CH4, and 7% C2H6 at 100°F and 800 psia is to be processed for Nr removal. A two-step separation process has been proposed consisting of (1) membrane separation with a nonporous glassy polyimide membrane, followed by (2) pressure-swing adsorption using molecular sieves to which the permeate gas is fed. The membrane separator is highly selective for N2 (SPN2,CH4 = 16) and completely impermeable to ethane. The pressure-swing adsorption step selectively adsorbs methane, giving 97% pure methane product in the adsorbate, with an 85% recovery of CH4 fed to the adsorber. The non-permeate (retentate) gas from the membrane step and adsorbate from the pressure-swing adsorption step are combined to give a methane stream that contains 3.0% N2. The pressure drop across the membrane is 760 psia. The permeate at 20°F is compressed in two stages to 275 psia and cooled to 100°F before entering the adsorption step. The adsorbate gas, which exits the adsorber during regeneration at 100°F and 15 psia, is compressed in three stages to 800 psia and cooled to 100°F before being combined with nonpermeate gas to give the final pipeline natural gas.
(a) Draw a process-flow diagram of the separation process using appropriate symbols from Tables. Include the gas compressors and heat exchangers. Label the diagram with all of the data given above, and number all process streams.
(b) Compute by material balances, using the data above, the component flow rates of N2, CH4, and C2H6 in lbmol/h for all process streams entering and exiting the two separation operations. Place the results in a material-balance table similar to Table.