Lean oil requirement

Absorption Lean Oil Requirement for Fixed Component Recovery in Fixed Tower [18]... 

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 ... 

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. 

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 gas stream shown in Table 8-10 is fed to an isothermal absorber operating at 90°F and 75 psia. 90% of the n-butane is to be removed by contact with a lean oil stream consisting of 98.7 mol% non-volatile oil and the light components shown in Column 2 of Table 8-10. Estimate the composition of the product streams and the required number of theoretical stages if an inlet rate of 1.8 times the minimum is used. 

A second approach to the same result involves the same requirements as for a balanced heat design the heat of absorption of the actual components absorbed must equal the sum of the heat added to the lean oil and to the lean... 

For a new design a study should be made of number of trays against required lean oil for a given absorption. 

The SR method requires that the total molal flow rate, composition, location, and thermal condition of all feeds be specified, including the lean oil, L , to the top stage and the rich gas, + lf to the bottom stage. The steps of the algorithm are as follows ... 

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. 

It is desired lo recover 75% of the propane (called the key component) from a gas stream of the composiliou givan in Table 6.2-1 using oil absorption. The absorber is equivalent to six theoretical trays nad operatra at 1000 psig (7 X ICh kPa). Il is assumed that the entering lean oil is stripped completely of rich gas components and that the absorber operates at a constant temperature of I04 F (40°C). What oil cjioslalion rate is required, and what will be the composiliou of the residue gas leaving the absorber ... 

Gas processing Lean oil still Column did not achieve required separation because of insufficient refliK induced ly an undersized reflux orifice plate. Problem was difficult to diagnose because of unusual behavior of top temperature. In some multicomponent distillations, normal top and bottom temperature can be observed even if adequate separation is not achieved. 

In order to investigate the effect of the LIV ratio on the separation, the lean oil rate is varied from 25 to 5,000 mol/hr at a fixed rich gas feed rate of 1,000 mol/hr. Table 8.4 Usts the recoveries in the rich oil and concentrations in the lean gas of several components. The recovery is the percentage of a component in the rich gas feed that is recovered in the rich oil. As implied by the definition of key components in absorbers, the key component is that which has the lowest recovery above 50%. At low lean oil rates, only the heavier components recoveries are over 50%. As the lean oil rate is increased, the recovery of all components goes up and the key component changes. Table 8.4 indicates that at a lean oil rate of 25 mol/hr the key component is pentane. At 100 mol/hr the key component is butane. At 300 and 600 mol/hr it is propane, and so on. The required lean oil rate is thus determined in part by where the separation of the rich gas should take place. A closer look at the effect of the liquid solvent feed rate (or stripping gas rate) on the column performance is deferred to Section 8.4. 

If one variable is specified, the lean oil rate and the column performance are determined. For instance, if the mole fraction of methane in the overhead is specified at 0.9, the lean oil rate required to meet this specification is 440 mol/hr. At this lean oil rate, the mole fraction of ethane in the overhead is 0.06, the recovery of methane in the liquid is 8%, that of ethane 35%, propane 84%, n-butane 100%, and so on (Figure 8.4). 

The function of the reboiied absorber is to make a separation between two components recovering the desired amount of the heavier as part of the bottoms liquid and rejecting the lighter of the two to the overhead gas stream. As in distillation, these two components are called the heavy key and the light key, respectively. Note that all components present which are heavier than the heavy key will be recovered in the bottoms liquid. Liquid to the top tray of the tower is provided by a cool, relatively heavy oil. This is used to wash back heavy components in the upflow-ing gas stream by the mechanism of absorption rather than by condensing and refluxing a part of the column overhead stream as in a distillation operation. The lean oil is usually either a special oil heavier than any of the components in the tower feed, or it will correspond to the composition of the heaviest product stream made from the feed. The only requirement, however, is that the lean oil must be heavier than the liglit key. 

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. 

The calculation procedures discussed in this section are based on estimating the total vapor feed to the absorption section. This is followed by conventional calculations to determine lean oil and tray requirements for the desired separation. The final step calculates the temperature profile across the absorption section and estimates intercooler requirements. 

The question of trays versus lean oil rate is a classical illustration of economic analysis. However, in the case of reboiled absorbers, the solution is seldom clear-cut since a savings in trays and tower height requires an increased lean oil rate. This in turn increases the refrigeration requirements and necessitates greater capital investment and operating costs. For the most part, the number of trays is preset, allowing lean oil rates to vary as a function of key component recovery. A suggested value for the number of trays is 10 theoretical trays, which will usually result in approximately 20 real trays. For low recoveries, this may result in lean oil rates which are too low for satisfactory tray operation. In this case, the number of trays should be reduced to provide an adequate oil rate. 

At the same assumed temperature, the lean oil rate required to form a bubble point liquid with the estimated top tray absorbate is calculated. This rate is then used to calculate the average L/V and the key component absorption factor. 

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