Stratco reactors

Here, a control law for chemical reactors had been proposed. The controller was designed from compensation/estimation of the heat reaction in exothermic reactor. In particular, the paper is focused on the isoparafhn/olefin alkylation in STRATCO reactors. It should be noted that control design from heat compensation leads to controllers with same structure than nonlinear feedback. This fact can allow to exploit formal mathematical tools from nonlinear control theory. Moreover, the estimation scheme yields in a linear controller. Thus, the interpretation for heat compensation/estimation is simple in the context of process control. 

Stratco Reactors. These reactors designed by Stratco, Inc. produce about 34% of the alkylate produced worldwide (P. Pyror, personal communication). In 2003, Stratco was purchased by E.I. du Pont de Nemours, Inc. A reactor, often called a Contactor, is a horizontal cylindrical vessel, as shown in Fig. 1, with the following features. An impeller at one end recirculates the acid/hydrocarbon dispersion many times (on average) over a U-tube bundle to regulate the temperature of the dispersion at about 5-10°C. A cylindrical circulation shell located in the reactor provides an annular space so that the dispersion flows from one end of the reactor to the other, where it makes a 180° turn and flows back over the tube bundle. 

The kinetics of the coalescence-fragmentation sequence in cascade reactors is unknown, but, obviously, droplets with very different compositions are present in a reaction zone. Hence, alkylates of different compositions are being produced in different droplets in each zone. In addition, part of the hydrocarbons in the droplets is vaporized to maintain the desired temperatures. Most, but not all, of the heats of reaction and of the energy supplied by the agitators are removed. A small temperature rise occurs from one zone to the next as indicated by Ackerman and co-workers (8). In one case, the temperature increased from 1.2°C in the first zone to 6.4°C in the 10th zone. These temperatures are lower than those normally used in Stratco reactors. The average quality of the alkylates in this... 

For example, on one unit with a Stratco reactor, the impeller turbine (i.e., the mixer) was shut down for several hours. Nothing dramatic hap-... 

Fig. 11. Stratco Contactor reactor used in sulfuric acid-catalyzed alkylation (12).

M. Vichalaik, J.R. Hooper, C.L. Yaws, and R.W. Pike. Alkylation of mixed olefin with isobutane is a STRATCO chemical reactor. In 5th World Congress of Chem. Eng., pages 238-243, San Diego, Cal., July 1996. 

HF alkylation reactors have undergone more drastic changes than H2S04 reactors. In the middle 1950 s a Stratco contactor similar to that for H2S04 alkylation was used. Later a vertical reactor was developed in which the reactants were bubbled up through liquid HF. Cooling was... 

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. 

The alkylation reactors used in the pilot unit operation and also the commercial operation were Stratco Contactors. However, the writer knows of no reason, and no evidence is available, to indicate that any other type reactor, such as the cascade and pump and time tank, would not be satisfactory. It is believed that they would give at least as good results with SARP as without SARP for the same operating conditions and olefin feed stock, and of course with a much lower acid consumption. 

In addition to temperature variations in the reactor, there are variations throughout the reactor in the composition and size of dispersed droplets, and also different acid/hydrocarbon ratios. Stratco has rather... 

A comparison of Stratco and Cascade reactors indicates the following differences ... 

Stratco unit with the single mixer on one end is approximated by a single mixed tank, as shown in the upper part of the figure. However, the Kellogg cascade unit has a series of compartments with mixers and olefin is sparged into each compartment to keep the concentration low so that it reacts with the isobutane rather than polymerizing. The tank-in-series model may be used to model this type of unit and this is shown in the lower part of the figure. A mass balance can be made for a stirred tank reactor readily because the composition is the same everywhere in the vessel. 

Parameters Tubular turbulent reactor Mixing reactor, Russian company Lenniihimmash Mixing reactor, US company Stratco Mixing reactor, US company Amoco... 

Fig. 1. Alkylation reactor (or Contactor) in the Stratco process of (A) Tube bundle (B) shell (C) circulation tube (D) impeller (E) hydraulic head and (f) difiCuser vanes.

Other variations in the different zones of cascade reactors, which have not been reported, include residence time (space velocity), acid/hydrocarbon ratio, and character of the dispersions. The decreases in RON for alkylates in the latter zones are also partly due to degradation reactions. It has been shown (16,17) that TMPs degrade in the presence of sulfuric acid. It has also been indicated (14) that both the quality and quantity of allgrlates are reduced by such degradations the quality was estimated to reduce by perhaps 0.11 RON in cascade units. In Stratco units, the decrease was perhaps 0.07 RON this lower value is because of lower residence times. Degradation reactions of TMPs are also thought to be more pronounced in cascade reactors because of increased amounts of mixing of TMPs with olefins. 

The acid/hydrocarbon dispersion leaving the cascade reactor flows to the decanter (or settler) as shown in Figure 2. After the acid separates, it is recycled to the reactor. Not shown in Figure 2 are lines to discard a small fraction of the acid leaving the decanter and to provide fresh (or make-up) acid. In both cascade and Stratco units, the acid is recycled on the average numerous times before it is rejected. 

The costs of units using the Stratco or cascade reactors can be compared. Costs to compress the light gases (mainly isobutane) that are vaporized are significantly less for cascade reactors. Less isobutane needs to be compressed and in addition the pressure increases are less. The Stratco process, however, has the advantage that the cost of separation of the unreacted isobutane from the alkylate, by either distillation or stripping, is low. In the Stratco process, less isobutane is, of course, present in the liquid stream to be separated. 

When two or more reactors are used in a refinery, each reactor can be operated with acids of different strengths. If three reactors were used with the acid flowing in series through them, the acidity levels would often be approximately 96, 93, and 90% in the three reactors. If, however, parallel acid flows were employed in the first two reactors and the exit acids from these two reactors were used as the feed acid to the third reactor, benefits would often occur relative to acid consumption and alkylate quality. Stratco Contactors because of their smaller production capacities are easier to adapt to such techniques than cascade reactors, which tend to have larger capacities. 

Many alkylation units use auto refrigeration to cool their reactors. Effluent refrigeration commonly used on Stratco units also falls within this classification. To maximize isobutane purity in the refrigerant recycle stream, circulating refrigerant is depropanized. 

Pryor, P., Peterson, R., Gody, T, and Lin, Y. (2002) STRATCO Contactor reactor performance upgrade - alkylation tube insert technology. Available at http //www2.dupont.com/ Clean Technologies/es MX/assets/down-loads/2002 STRATCO NPRA AM Paper. pdf. 

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