Cryogenic Demethanizer — Engineering Fundamentals
Turboexpander cycle, GSP / RSV / ISS variants, Fenske–Underwood–Gilliland sizing, side reboilers.
1. Why turboexpander cryogenics
Lean-oil and Joule-Thomson plants top out at ~65 % ethane recovery; modern cryogenic gas plants using a turboexpander reach 90 %+. The principle: nearly isentropic expansion of the dehydrated feed gas across an expander cools it to −100 to −150 °F while extracting shaft work to recompress the residue gas, then a single distillation column separates the chilled feed into methane-rich vapor (overhead) and ethane-plus liquid (bottoms).
Compared to the rich-oil demethanizer (which uses a circulating lean absorber oil to scrub heavies into liquid), the cryogenic column is a true distillation — methane and ethane are separated by relative volatility on actual fractionation trays, not by partition into a third solvent.
2. ISS, GSP, RSV — which cycle?
| Cycle | Typical C2 recovery | Notable feature |
|---|---|---|
| ISS (Industry Standard / single side reflux) | 80 % | Pre-cooled feed splits; turboexpander to column top. |
| GSP (Gas Subcooled Process) | 85–92 % | Split flow: part to top condenser, part to expander. Most common today. |
| RSV (Recycle Split Vapor) | 92–98 % | Adds a residue-gas recycle compressor for extra reflux. |
| IPSI / OHR / SCORE | 95–99 % | Licensed variants of RSV with additional integration. |
This calc treats the column as a single-feed, single-side-condenser stage count — adequate for screening between the cycles before committing to detailed Aspen / HYSYS modelling. The R/R_min handle absorbs the effect of choosing GSP (R/R_min ≈ 1.2) vs RSV (R/R_min ≈ 1.05) without changing the underlying equations.
3. Fenske–Underwood–Gilliland
For a binary key system (C1 light key, C2 heavy key) with all C3+ rolling to bottoms, the classical short-cut sequence applies cleanly:
- Fenske gives minimum stages at total reflux from the product compositions.
- Underwood gives minimum reflux at infinite stages from feed-and-distillate compositions; the binary form drops the multi-component θ-solve into a closed equation.
- Gilliland (Eduljee 1975 fit) interpolates between the two limits at the operating R/R_min.
Multiply N by 1/η (η = 0.65 typical for cryogenic sieve / valve trays) to get actual trays. A 10-15 m column with 20-30 actual trays is a normal result.
4. Choosing α at column conditions
The single most important number in the short-cut is α (C1/C2). It is strongly P- and T-dependent because methane is supercritical at all demethanizer conditions (T_c = −116 °F < column T everywhere) while ethane is well below its critical (T_c = 90 °F).
| P (psia) | T (°F) | α (C1/C2) |
|---|---|---|
| 200 | −140 (top) | ~5.5 |
| 250 | −40 (mid) | ~4.0 |
| 250 | 60 (bottom) | ~3.0 |
| 400 | −140 (top) | ~3.5 |
Standard practice is to use the geometric average α over the column. The default 4.5 in this calc corresponds to GSP at 250 psia. For higher-pressure RSV columns (350–400 psia) drop α toward 3.5–4.0 — N grows accordingly.
5. Side reboilers & energy integration
A real cryogenic demethanizer rarely has a single bottom reboiler. To minimize the cold-end exergy penalty (heating up a −60 °F liquid only to send it to a +60 °F reboiler) the column uses two or three side reboilers at progressively higher T, each integrated with the warm-end feed-gas chiller. The total reboiler duty reported here lumps all heat input into one number — it's accurate for screening but not for heat-exchanger placement.
6. References
- GPSA Engineering Data Book, §16 — Hydrocarbon Recovery.
- Lindsay, R.J.; Brown, G.E. (1975). "Demethanizer overhead design." Hyd. Proc. 54(7).
- Fenske, M.R. (1932). "Fractionation of straight-run Pennsylvania gasoline." Ind. Eng. Chem. 24, 482.
- Underwood, A.J.V. (1948). "Fractional distillation of multicomponent mixtures." Chem. Eng. Prog. 44, 603.
- Gilliland, E.R. (1940). Ind. Eng. Chem. 32, 1101. Eduljee, H.E. (1975) fit, Hyd. Proc. 54, 120.
- Pitman, R.N.; Hudson, H.M.; Wilkinson, J.D. (1998). "Next-generation cryogenic NGL recovery — GSP, RSV, RSVE." Proc. 77th GPA Convention.