Isobutane Concentration

Isobutane concentration is generally expressed in terms of the isobutane-to-olefin ratio (I/O). This ratio is the most important process variable to control in terms of refinery alkylation productivity, yield, and quality of alkylate, as well as the add 

Unfortunately, the advantages of very high I/O ratios come at a certain price a low olefin concentration lead to low space-time velocity in the process. Moreover, at high I/O ratios, more isobutane has to be separated from the produd and its recycling requires larger volumes of equipment and higher energy consumption. 

---

Because of its large reactor volume, the auto-refrigerated process can operate at very low alkene space velocities of about 0.1 h-1 LHSV (WHSV ca. 0.03 h 1). This design helps in increasing the octane number of the product and lowering acid consumption. The reaction temperature is maintained at about 278 K to minimize side reactions. Spent acid is withdrawn as 90-92 wt% acid. The isobutane concentration in the hydrocarbon phase is kept between 50 and 70 vol%. 

As in sulfuric acid alkylation, the important reaction operating variables are isobutane concentration, catalyst purity, acid-hydrocarbon ratio, and reaction temperature. Normally these variables are maintained at the same level as in sulfuric acid alkylation except for temperature. Most commercial plants operate from 75° to 100° F. This higher operating temperature often allows cooling water rather than the usual vaporizing refrigerants to be used. 

Figure 1. Changes in the 18q16o/18o2 ratio and in the isobutane concentration in gas phase over Ti02 as a function of illumination time...

To build a kinetic model for isobutane conversion, we must first specify the adsorbed C species to be included. Under the conditions employed in our kinetic studies (i.e., temperatures between 523 and 573 K, total pressure of 1 atm, isobutylene feed contents near 50-400 ppm, and isobutane concentrations of 20 and 80%), the primary olefins observed in the reactor effluent are isobutylene, 1-butene, cis- and trans-2-butene, propene, and smaller amounts of isopentenes. Accordingly, the following adsorbed species... 

Because the cracking product slate is quite different between normal and isobutane, it is important to take cognisance of the isobutane concentration in a butane stream used for cracking. The difference is illustrated in Table 8.6. 

Response f (olefin feed composition, temperature, space velocity. Isobutane concentration, acid strength, mixing conditions, etc.)... 

Figure 4. Effect of isobutane concentration on research octane number...

Increasing recycle rate at constant reboiler duty initially leads to improved performance because of higher reactor isobutane concentrations. Further increases in recycle rate will eventually have a negative effect on performance because of decreased isobutane concentration in the recycle as well as other effects. 

Increasing reboiler duty at constant recycle leads to increased recycle purity and a higher reactor isobutane concentration. As a result, performance improves. Further increases In reboiler duty will continue to improve octane and decrease acid consumption. Eventually,the value of these improvements will not be sufficient to pay for the increased reboiler energy usage. 

The effect of reduced reflux rate on Motor octane is shown in Figure 9. As "isostripper" operation is approached, octane drops markedly because of sharply reduced reactor isobutane concentration. 

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. 

The correlations published by Putney (7) show the effects of Isobutane concentration on octane number, and octane number on acid consumption. These are for butylene feedstocks. By combining the data it is possible to construct the curve shown in Figure 4 illustrating the influence of isobutane concentration on acid make-up requirement, with all other variables constant. The potential acid savings provide an obvious incentive for optimizing fractionation operation in both butylene and mixed feed operations. ... 

For refluxed columns this involves determining the best feed point location, and the best division between reflux and overhead recycle to the reactor section. In some cases it has been found that isobutane concentration in the reactor could be increased by moving the feed point up, reducing the reflux and increasing the recycle to the maximum that the reactor loop could handle. 

A large excess of isobutane is used, e.g., as much as 60-80% by volume of the hydrocarbons in the alkylation reaction mixture. Consequently, a large quantity of isobutane must be recovered and recycled. It is also available for SARP. The isobutane concentration is one of the most important variables, as far as quality of product and acid consumption are concerned. A high concentration of isobutane tends to promote alkylation and... 

Ideal reaction conditions of high Isobutane concentration, and isobutane diffusion into the acid phase at a rate greater than the olefin diffusion rate, result in high quality alkylate product. Ester formation, exclusive of those formed from feed contaminants,... 

Crystallization is performed in if obutane liquid to remove heat of fusion, but more Inportant - to maintain a high Isobutane concentration, so that-as the ester and water concentration is increased, Isobutane will continue to react with the mono ester. 

Reznikov, a. N. 1969. On the geochemical significance of the ratio of n-butane and isobutane concentrations in petroleum gases. Geol. Neft. Gaza, 4, 43 7. 

The fundamental parameters of alkylation are isobutane concentration, acid strength and volume in the reactor, temperature, and mixing intensity. The engineer troubleshooting an alky problem should turn first to these fundamental precepts for help. 

The process variables temperature, acid strength, isobutane concentration, and mixing have to be carefully optimized in refinery alkylation to obtain high fuel quality. The optimum parameters differ for the H2SO4- and HF-catalyzed processes. 

---