Lean oil plants

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 ab.sorb light hydrocarbon components from the gas. The light components are separated from the rich oil and the lean oil is recycled. 

These are higher than for a lean oil plant. It is possible to recover a small percentage of ethane in a refrigeration plant. This is limited by the ability to cool the inlet stream to no lower than -40°F with normal refrigerants. 

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 loan oil retlux rate. 

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

Many operations in petrochemical plants require the absorption of components from gas streams into lean oils or solvents. The resultant rich oil is then stripped or denuded of the absorbed materials. The greatest use of this operation utilizes hydrocarbon materials, but the basic principles are applicable to other systems provided adequate equilibrium data are available. 

The selection of lean oil for an absorber is an economic study. A light lean oil sustains relatively high lean oil loss, but has the advantage of high mols/gal compared to a heavier lean oil. The availability of a suitable material has a large influence on the choice. A lean oil 3 carbon numbers heavier than the lightest component absorbed is close to optimum for some applications. In natural gas plant operations, however, the author generally sees a lean oil heavier by about 10-14 carbon numbers. 

The Azerbaijan Gas Processing Plant, located at Karadagh, is a multitrain extraction unit based on the lean oil technology. Of the six extraction units, a number are now in disuse. The fractionation unit facility is currently employed on the batch run basis to produce a stable condensate product and a butane rich LPG. 

Lean oil absorption oil from which gasoline fractions have been removed oil leaving the stripper in a natural-gasoline plant. 

As the fluids pass from the absorber to the ROF, the temperature rises from ca -35 C to about 200°C (boiling point of kerosene). This temperature difference requires extensive use of heat exchange equipment between the unit operations within the plant. Furthermore, the pressure progressively falls from about 100 atm in the absorber to about 50 atm in the ROD to less than 10 atm. in the ROF. This requires the lean-oil stream to be pumped against this pressure drop from 10 to 100 atm. 

As a first step, I introduced myself to the chief operator and explained the purpose of my visit. Having received permission to run my test, I switched all instruments on the gas-plant control panel finm automatic over to local/manual. In sequence, I then increased the lean oil flow to the absorber, the debutanizer reflux rate, and the hot-oil flow to the debutanizer reboiler. 

To regain control of the gas plant, I cut reflux to the debutanizer and lean-oil flow to the absorber. I was now back where I started. The thought of impending of failure loomed. 

Within the Esso gas facility, there was a unit dedicated to removing ethane, propane, and other light hydrocarbons from natural gas via absorption into lean oil. This oil, now made rich by the addition of these hydrocarbons, was later distilled to remove the hydrocarbons and recycled back into the system. This distillation process involved the use of a reboiler and a fractionation column. The cold (tube) side of the reboiler contained the rich oil and the warm (shell) side contained lean oil. On September 25, 1998, a process upset caused the lean oil pump to stop, disrupting circulation of the warm lean oil through the reboiler for several hours. As a result, the overall temperature of the reboiler dropped to the temperature of the rich oil, which was -54 °F (-47.8 °C). Ice formed on the outside of the reboiler while plant personnel worked to bring the upset back to normal operating conditions. 

The design of most natural gas processing plants is based on the recovery of a certain component, usually either propane or ethane. Since both the absorber exit gas and the ROD overhead gas contain the base component as the heavy key, the design material balance of those two systems must be interdependent. For example, in setting up the preliminary material balance for a gas plant based on 70 percent propane recovery, the absorber must overabsorb so that there is enough extra propane in the ROD feed to permit a reasonably low lean oil rate to the latter tower. The optimum absorption will result in the lowest total lean oil rate to the absorber and the ROD combined. As a rule-of-thumb, the absorption section of the ROD will have to absorb a minimum of 95 percent of the heavy key vapor feed, including the internal vapor. 

The temperature level chosen for the lean oil will be determined by several factors. Obviously, the lower the temperature, the lower the oil rate and the smaller the re-boilers and lean oil chillers. But, on the debit side, lower temperatures increase refrigeration costs, require somewhat more expensive materials of construction and more insulation and suffer more severe startup and operating problems. The lean oil temperature will dictate the overall economics of a given plant design once the key component recovery has been set. This is a detailed subject outside the scope of this work but is mentioned here so that it is well understood that lean oil temperature cannot be determined arbitrarily. 

Lean oil from the still is pumped through a heat exchanger and cooler and back to the absorber. Vapors from the still are cooled with partial condensation by indirect heat exchange with the rich oil. The condensate is passed to a separator, which removes water, and the liquid hydrocarbons are returned to the still as reflux. The partially cooled vapors are then passed to the benzol condenser where relatively complete condensation is obtained by the use of cooling water. In this particular plant, the condensate and noncondensed vapors are then passed through a vacuum pump to a separator. From this, noncondensed gases are returned to the fuel gas main, water condensate is removed for disposal, and the hydrocarbon layer is transferred to the crude-benzol storage tanks. 

Adsorption. Adsorption processes have been used to recover hydrocarbons that are heavier than ethane from natural gas. Although the adsorption process has appHcations for the recovery of pentane and heavier hydrocarbons from lean gas, the percentage recovery of LPG components in these plants usually is low compared to the normal recovery of LPG in modem turboexpander or oil-absorption plants. 

Cleary, M. P., F. C. Phillips, and R. A. Morton. Genotype and diet effects in lean and obese Zucker rats fed either safflower or coconut oil diets. Proc Soc Exp Biol Med 1999 220(3) 153-161. Lai, J.J., C. V. Sreeranjit Kumar, M. V. Suresh, M. Indira, and P. L. Vija-yammal. Effect of in utero exposure of Toddy (coconut palm wine) on liver function and lipid metabolism in rat fetuses. Plant Foods Hum Nutr 1998 52(3) 209-219. 

Acetone also has some desirable attributes as an extraction solvent (Wan Wakelyn, 1997). Vegetable oil is highly soluble in dry acetone but only sparingly when water is added. Thus, water can be added to the miscella to phase separate the oil by gravity settling, and then the oil-lean phase can be distilled to remove the water and restore solvating capacity. Reportedly, acetone was used for a short period in one soybean plant operating in Europe. 

A copolymerization plant uses benzene solvent. Benzene must be recovered from its gaseous waste stream. Two lean streams in the process, an additive stream and a catalytic solution, are potential process MSAs. Organic oil, which can be regenerated using flash separation, is the external MSA. The stream data are shown below ... 

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