Absorption/Lean Oil

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The oldest kind of gas plants are absorption/lean oil plants, where a kerosene type oil is circulated through the plant as shown in Figure 9-1. The “lean oil” is used to absorb light hydrocarbon components from the gas. The light components are separated from the rich oil and the lean oil is recycled.

Simplified flow diagram of an absorption plant.

Typically the inlet gas is cooled by a heat exchanger with the outlet gas and a cooler before entering the absorber. The absorber is a contact tower, similar in design to the glycol contact tower. The lean absorber oil trickles down over trays or packing while the gas flows upward countercurrent to the absorber oil. The gas leaves the top of the absorber while the absorber oil, now rich in light hydrocarbons from the gas, leaves the bottom of the absorber. The cooler the inlet gas stream the higher the percentage of hydrocarbons which will be removed by the oil.

Rich oil flows to the rich oil de-ethanizer (or de-methanizer) to reject the methane and ethane (or the methane alone) as flash gas. In most lean oil plants the ROD unit rejects both methane and ethane since very little ethane is recovered by the lean oil. If only methane were rejected in the ROD unit, then it may be necessary to install a de-ethanizer column downstream of the still to make a separate ethane product and keep ethane from contaminating (i.e., increasing the vapor pressure of) the other liquid products made by the plant.

The ROD is similar to a cold feed stabilizing tower for the rich oil. Heat is added at the bottom to drive off almost all the methane (and most likely ethane) from the bottoms product by exchanging heat with the hot lean oil coming from the still. A reflux is provided by a small stream of cold lean oil injected at the top of the ROD. Gas off the tower overhead is used as plant fuel and/or is compressed. The amount of intermediate components flashed with this gas can be controlled by adjusting the cold lean oil reflux rate.

Absorber oil then flows to a still where it is heated to a high enough temperature to drive the propanes, butanes, pentanes and other natural gas liquid components to the overhead. The still is similar to a crude oil stabilizer with reflux. The closer the bottom temperature approaches the boiling temperature of the lean oil the purer the lean oil which will be recirculated to the absorber. Temperature control on the condenser keeps lean oil from being lost with the overhead.

Thus the lean oil, in completing a cycle, goes through a recovery stage where it recovers light and intermediate components from the gas, a rejection stage where the light ends are eliminated from the rich oil and a separation stage where the natural gas liquids are separated from the rich oil.

These plants are not as popular as they once were and are rarely, if ever, constructed anymore. They are very difficult to operate, and it Is difficult to predict their efficiency at removing liquids from the gas as the lean oil deteriorates with time. Typical liquid recovery levels are:

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