Lean oil

The butane-containing streams in petroleum refineries come from a variety of different process units consequently, varying amounts of butanes in mixtures containing other light alkanes and alkenes are obtained. The most common recovery techniques for these streams are lean oil absorption and fractionation. A typical scheme involves feeding the light hydrocarbon stream to an absorber-stripper where methane is separated from the other hydrocarbons. The heavier fraction is then debutanized, depropanized, and de-ethanized by distillation to produce C, C, and C2 streams, respectively. Most often the stream contains butylenes and other unsaturates which must be removed by additional separation techniques if pure butanes are desired. 

The bottoms, consisting of absorption oil, absorbed propane, and higher boiling hydrocarbons, are fed to the lean-oil fractionator. The LPG and the natural gas Hquids are removed as the overhead product from the absorption oil which is removed as a ketde-bottom product. 

The lean oil from the lean-oil fractionator passes through several heat exchangers and then through a refrigerator where the temperature is lowered to —37° C. Part of the lean oil is used as a reflux to the lower section of the rich-oil deethanizer. Most of the lean oil is presaturated ia the top section of the deethanizer, is cooled again to —37° C, and is returned to the top of the absorber, thus completing the oil cycle. 

The overhead product from the lean-oil fractionator, consisting of propane and heavier hydrocarbons, enters the depropanizer. The depropanizer overhead product is treated to remove sulfur and water to provide specification propane. The depropanizer bottoms, containing butane and higher boiling hydrocarbons, enters the debutanizer. Natural gasoHne is produced as a bottom product from the debutanizer. The debutanizer overhead product is mixed butanes, which are treated for removal of sulfur and water, then fed iato the butane spHtter. Isobutane is produced as an overhead product from the spHtter and / -butane is produced as a bottoms product. 

This procedure has been applied to the absorption of C5 and lighter hydrocarbon vapors into a lean oil, for example. 

FIG. 14-9 Graphical design method for multicomponent systems absorption of hiitane and heavier components in a soliite-free lean oil. 

When A is very much larger than unity and when N is large, the right-hand side or Eq. (14-55) becomes equal to unity. This signifies that the gas will leave the top of the tower in equiUbrium with the incoming oil, and when Xo = 0, it corresponds to complete absorption of the component in question. Thus, the least volatile components may be assumed to be at equiUbrium with the lean oil at the top of the tower. 

A hydrocarbon feed gas is to he treated in an existing foiir-theoretical-tray ahsorher to remove hiitane and heavier components. The recovery specification for the key component, hiitane, is 75 percent. The composition of the exit gas from the ahsorher and the required liqiiid-to-gas ratio are to he estimated. The feed-gas composition and the eqiiilihriiim K values for each component at the temperature of the (soliite-free) lean oil are presented in the following table ... 

Absorbers and strippers frequently operate with a liquid having essentially the same physical characteristics regardless of the pressure. An example of this is a gas absorber. The same lean oil is used if the tower is operating at 100 or l,000psi. This type of system is excluded from the CAFq limiting value. 

Compare to assumed total mols absorbed and reassume lean oil rate if necessary. 

Lean Oil. 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... 

Presaturators. A presaturator to provide lean oil/gas contact prior to feeding the lean oil into the tower can be a good way of getting more out of an older tower. Absorber tray efficiencies ran notoriously low. A presaturator that achieves equilibrium can provide the equivalent of a theoretical tray. This can easily equal 3-4 actual trays. Some modem canned computer distillation/absorp-tion programs provide a presaturator option. 

The calculations are handled similarly to a distillation column. The only new element introduced is qs, the fraction of the component in the lean oil. 

Sieve trays installed with holes behind false downcomer. Entrained liquid overhead. Limited lean oil flow. Vessel manufacturer error. 

Problem A lean oil still had unstable control and erratic operation. 

Conversion of Lean Oil Absorption Process to Extraction Process for Conditioning Natural Gas, U.S. Patent 4.696,688, Sep. 29, 1987. 

Enhancing the Profitability of Lean Oil Absorption Through Simulation, GasTIPS , Fail 1996, Volume 2, Number 3, p.38... 

Basically, a gas absorption tower is a unit in which the desirable light ends components are recovered from the gas feed by dissolving them in a liquid passing through the tower countercurrently to the gas. The liquid absorbent is called lean, oil, and it usually consists of a hydrocarbon fraction in the gasoline boiling range. After the absorption step, the liquid which now contains the desired constituents in solution is referred to as fat oil. A similarly descriptive nomenclature is applied to the gas, which is referred to as wet gas when it enters the tower and as dry gas when it leaves the absorber. 

The fat oil is fed to a splitter or stripping tower, where the absorbed tight constituents are separated from the oil by distillation. Usually the lean oil is the same material as the heavier part of the absorber feed, so that the bottoms from the stripper are split into lean oil, which is recycled to the absorber, and a stabilized gasoline product, which is passed on to subsequent processing operations. 

The Hj/400°FVT streams from each system are sent to separate flash drums where the bulk of the Cj and lighter material is removed. The virgin and cat cracker streams from the flash drums go to separate debutanizers while the Powerformer stream goes to an absorber-deethanizer followed by a debutanizer. The Q and lighter overhead streams from the virgin and cat cracker debutanizers are sent to this absorber- deethanizer for final deethanization. In the flow scheme shown this tower does not have a separate lean oil. It is called an absorber-deethanizer because the Powerformer stream serves in part to absorb the Cj and C4 components in the streams from the debutanizers. A separate lean oil stream is added in cases where higher Q and Q recoveries are justified. 

Instead of condensing out the ethylene at extremely low temperatures, as described above, the absorption process uses a very light, lean oil, such as Cj, at moderately low temperature to scrub the ethylene out of the gas. By stripping the fat oil, the ethylene and heavier cuts can be recovered as desired. 

Absorbent Flow Failure - For lean oil absorption generally, no relief requirement results from lean oil failure. However, in a unit where large quantities of inlet vapor may be removed in the absorber, loss of absorbent could cause a pressure rise to relief pressure, since the downstream system may not be adequate to handle the increased flow. In such cases, the effect of this additional vapor flow into downstream equipment must be analyzed. 

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. 

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. 

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. 

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 ... 

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. 

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