As a result of the lower reflux, LPG product escapes with the residue gas. With the rise in LPG content in the feed gas, the deethanizer reboiler duty also increases, which boosts overhead temperature, resulting in lower reflux to the absorber and to the deethanizer column. In different cases, as LPG content in the feed gas increases from 10 wt% to 17 wt%, C 3 recovery decreases from 99 wt% to 66.8 wt%, as shown in Fig. For measuring LPG recovery, C 3 recovery is considered because C 3 recovery falls more significantly than does C 4 recovery. To measure increasing C 3 + content in feed gas, higher LPG content (C 3 + C 4) is used. These parameters review, in detail, the effect of LPG richness-i.e., increased volumes of C 3 + and heavy hydrocarbons in the feed gas.įor the sake of comparison, the cases are simulated while considering the same key process parameters, as shown in Table 2. Different cases that consider different gas compositions are shown in Table 1. This situation causes a significant fall in LPG recovery and reduces refinery revenues.Ĭase study. The deethanizer overhead temperature also rises due to the increasing reboiler duty. Due to higher temperatures in the low-temperature separator, heavier hydrocarbons do not condense, which decreases the turboexpander performance.
As C 3 + content in feed gas increases, the overall refrigeration in the unit decreases, which reduces LPG recovery. LPG is recovered from the debutanizer column (9) as top product, whereas NGL is recovered from the bottom.Įffect of LPG richness in feed. Liquid from the deethanizer bottom reboiler (8) containing C 3 + content then routes to the debutanizer column (9) for LPG recovery. Lean gas (C 2 –) from the top of the absorber column commingles with gas from the deethanizer reflux drum (7) after heat recovery and serves as residue gas. The deethanizer overhead partially condenses using the deethanizer multipass plate and frame condenser (6), and it flashes in the deethanizer reflux drum (7). It is then routed to the deethanizer column (3) after heat recovery. This reflux liquid acts as lean oil and absorbs most of the C 3 + content from the gas. The turboexpander outlet then routes to the bottom of the absorber column (5), where gas moves upward and is washed with reflux from the deethanizer, which contains heavier hydrocarbons. This step adds refrigeration to the system, which boosts LPG recovery. Light hydrocarbons are expanded via the turboexpander (4), which produces cryogenic temperatures of less than –100☏.
Heavier hydrocarbons condense as liquid in the low-temperature separator (2) and are routed to the deethanizer column (3). In the LPG recovery unit, feed gas enters the inlet multipass plate and frame exchanger (1) and is cooled through heat exchange with the unit’s chilled streams. 1 shows a schematic of the LPG recovery unit.įig. Turboexpander technology with an absorber column is conventionally used for >99% propane recovery in many licensed processes, with several variations.Ī turboexpander reduces the pressure of the inlet gas to the outlet pressure, and it produces a cryogenic temperature, which aids in the recovery of NGL and LPG from the feed gas stream to meet hydrocarbon dewpoint specifications. The LPG recovery unit plays a vital role in boosting revenue by increasing liquids recovery from the feed gas. Technology is improving day by day to increase the profitability of gas processing plants.
Hani, Energy Enterprise Associates, Karachi, Pakistan Use refrigeration to boost liquids recovery from LPG-rich gas