Deisobutanizer operation

Complete simulation models have been formulated for cascade and Stratco sulfuric acid alkylation units and studies have confirmed the accuracy of the models. Application studies include cases in which model usage Identified profitable unit modifications, determined optimal unit capacity and optimal distillation tower operation, and compared the performance of cascade and Stratco units. "Isostripper" deisobutanizer operation was determined to be relatively unprofitable for sulfuric acid alkylation units and acid consumption on a modified cascade unit was found to be 36% below that expected for a Stratco unit. The examples presented suggest the broad applicability of the simulation models for improving alkylation unit operation. Use of the models not only pinpoints areas where significant improvements are possible, but also quantifies incentives needed to get them implemented quickly. 

Figure 7. Effect of deisobutanizer operating variables on unit profitability...

Computer simulation models have been formulated for cascade and Stratco sulfuric acid alkylation units. These complete models incorporate mathematical descriptions of all the interacting parts of the units, including reactors, distillation columns, compressors, condensers, and heat exchangers. Examples illus-strate diverse model applications. These Include identifying profitable unit modifications, comparing cascade to Stratco performance, evaluating optimal unit capacity and determining optimal deisobutanizer operation. 

Feed to the debutanizer is presumably bottoms from a deisobutanizer operating at a pressure of perhaps lOOpsia or more. Results of an adiabatic flash of this feed to SOpsia are given by Bachelor as follows. 

Plots of A T versus (mol fraction 1C4) showed that it was affected very little by disturbances, but they did a show a maximum at about Xb = 0.02. Sincx most deisobutanizers operate in a rang oixs = 0.03 — 0.10, this might well be useful for control. 

A section drawn through Figure 7 at a constant isobutane recycle rate is shown in Figure 8, for which the abscissa is now reflux ratio (reflux/isobutane recycle). The optimum corresponds to the value of reboiler duty giving the maximum profit for the specified recycle rate. As the reboiler duty (or reflux rate) decreases, the profit drops sharply. No reflux corresponds to the operation of the deisobutanizer as an "isostripper". Figure ff shows that "isostripper" operation significantly reduces unit profitability. 

To control an operating plant for minimum acid make-up, the Isobutane concentration in the reactor must be maintained at the maximum possible level. This means operating both the depropanizer and the deisobutanizer under optimum conditions. The operator should adjust tower feed rates and operating conditions, always using the isobutane concentration in either the total effluent hydrocarbon or the net effluent hydrocarbon from the reactor as his primary reference. Isobutane-to-olefin ratio, deisobutanizer overhead purity, depropanizer recycle purity, and refrigerant recycle purity are significant only as they relate to reaction zone isobutane concentration. 

In the case of n-butane the operator must find the optimum mode of operation of the deisobutanizer, or isostripper column. 

A plant was operated by Cities Service for several years. Some years ago the entire refinery where the SARP unit was located was discontinued. As was the case with the pilot unit, the SARP unit was easy to operate, and in general a steadier operation was obtained. Upsets and poor operation of units not directly a part of the recovery section, such as the feed splitter, butane isomerization unit, and deisobutanizer were not as serious when SARP was in operation. The general feeling was that an octane increase was obtained, as well as a demonstrated two thirds to three fourths reduction in acid consumption. Sufficiently precise runs under stable conditions were not made in the pilot unit or in either of two commercial plants to be certain that an increase in octane was obtained. 

A 350-ton deisobutanizer distillation column, 212 feet high, was raised into position in one piece at the El Segundo refinery of Standard Oil Co. of California, Western Operations, Inc. The lift was one of the heaviest ever accomplished in the U.S. with a load of this type. Macco Refinery and Chemical Division, California, was the prime contractor for construction. [Petroleum Refiner, 37, No. 2, 184 (1958)]. Column shown was designed by one of the authors. 

In the following discussion of design-parameters, it should be evident that not all parameters are of equal importance in all operations. Pressure drop, for example, is of central importance in vacuum crude stills-but of little import in the liquid-liquid extraction of penicillin from fermentation mashes stage efficiency is not important in the design of a whiskey distilling column, which requires few stages, but it can be critical for a deisobutanizer, which may require over 100 stages. 

Stanton and Bremer (378) demonstrated the superiority of the response of the analyzer/temperature control system over both a temperature control and a direct analyzer control in a 72-tray deisobutanizer. A temperature control alone produced a product purity offset due to variations in nonkeys an analyzer-only control had a long lineout time and was sensitive to feed flow changes. An analyzer/temperature control gave a fast response and eliminated all these ill effects. Other favorable experiences with analyzer/temperature control have also been reported (25, 89, 203, 287, 379). Two references (25, 379) contain in-depth descriptions of tuning considerations and of other accessories that can improve system operation, particularly if the analyzer control is performed through a computer system. 

Operating costs will be lower since the deisobutanizer... 

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