Lean-feed scheme

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 process scheme consists of contacting mixed-C4 feed with lean solvent in the extractive distillation column (1). The raffinate, butenes and butanes, which are not absorbed, flow overhead to the wash column... 

The process flow scheme is illustrated in Figure 15 [19]. The feed gas enters the absorber where it is contacted with lean solvent. The solvent selectively absorbs carbon monoxide and physically absorbs carbon dioxide, nitrogen, methane and a small amount of hydrogen. The carbon monoxide forms a complex with the solvent. The rich solvent leaves the bottom of the absorber and flows to a flash drum where the physically absorbed gases (carbon dioxide, nitrogen, methane and hydrogen) are flashed off. These gases are recycled to the absorber tower. The solution is then sent to the stripper where the carbon monoxide is released from the complex by... 

Figure 3 presents the schemes for successive precipitation fractionation (SPF) and successive solution fractionation (SSF). In both cases, by lowering the temperature (or adding nonsolvent) a homogeneous polymer solution (called feed phase F) splits into two coexisting phases, a polymer-lean sol phase I and a polymer-rich gel phase II, which are then separated. In SPF (Fig. 3a), the polymer is isolated from phase II as fraction FI. Phase I directly forms the feed phase for the next fractionation step, etc. 

Many modifications to the basic flow scheme have been proposed to reduce en gy consumption or equipment costs. For example, power recovery turbines are sometimes used on large, high-pressure plants to capture some of the raergy available when the pressure is reduced on the rich solution. A minor modiflcation aimed at reducing absorber column cost is the use of several lean amine feed points, hi an arrangement described by Polasek et aL (1990), most of the lean solution is fed near the midpoint of the absorber to remove the bulk of the add gas in the lower portion of the unit. Only a small stream of lean solution is needed for final clean-up of the gas in die top portion of the absorber, which can therefore be smaller in diameter. 

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