WO1993010064A1

WO1993010064A1 - Cold temperature alkylation process

Info

Publication number: WO1993010064A1
Application number: PCT/US1991/008502
Authority: WO
Inventors: Ken Kranz
Original assignee: Stratco, Inc.
Priority date: 1991-11-20
Filing date: 1991-11-20
Publication date: 1993-05-27

Abstract

A cold temperature alkylation reaction process for the production of isoparaffins useful as a high octane fuel additive derived from the reaction of C3-C5 olefins and isobutane in the presence of sulfuric acid.

Description

COLD TEMPERATURE ALKYLATION PROCESS

Field of the Invention

This invention relates to the alkylation o isoparaffinic hydrocarbons. More particularly, the inventio concerns a one-step, cold temperature reaction process fo the alkylation of isoparaffins with olefins in the presenc of sulfuric acid to form branched chain paraffini hydrocarbons capable of boiling in the gasoline motor fue range and having enhanced anti-knock properties.

Background of the Invention

The alkylation of isobutane with light olefins suc as propylenes and butylenes in the presence of eithe sulfuric or hydrofluoric acid is an important refiner process for producing high octane blending stock fo gasoline. Hydrofluoric acid at room temperature an atmospheric pressure is a very corrosive gas. Recent desert tests of uncontrolled releases of hydrofluoric acid have proven that the dangers are much greater than previously thought. Sulfuric acid poses a much smaller risk to the population surrounding a refinery. Some refineries are considering converting from the hydrofluoric acid to sulfuric acid catalyst because of the safety issue. Current legislation limiting the use of lead as a gasoline octane booster has made the importance of selective and efficient alkylation even greater. Because of the increasing demand for higher octane gasoline blending stocks, refiners are constantly seeking ways to improve the operation of their alkylation units to increase their profitability. Because of the undesirable reactions occurring during the alkylation process, diluents called red oils or conjunct polymers dilute the acid catalysts reducing their efficiency as catalysts. Sulfuric acid is regenerated by burning the diluted or spent acid and recovering the S02, oxidizing it to S03, and reacting the S03 with water to produce sulfuric acid. The cost of regenerating sulfuric acid is approximately 30% of the operating cost of the alkylation process. A method of reducing this operating cost while maintaining the quality of the alkylate product would improve the operation an economics of the alkylation unit.

Summary of the Invention In accordance with the present invention, a improved process is used to react C₃-C₅ olefins with isobutan in the presence of sulfuric acid. The reactor is maintaine at preselected operating conditions to optimize the reactions taking place in a single reaction vessel to produce a high quality motor fuel alkylate and reduce dilution of the sulfuric acid catalyst. Specifically, the method includes the reaction of light olefins with isobutane in the presence of a sulfuric acid catalyst at temperatures lower than currently practiced to reduce undesirable side reactions which dilute the sulfuric acid catalyst while producing a high quality gasoline motor fuel alkylate containing predominately highly branched isoparaffinic hydrocarbons useful as a blending stock for gasoline. The conditions for the optimization are described more fully hereinafter.

Brief Description of the Drawings FIGURE 1 is a graph of the effect of temperature on acid consumption.

FIGURE 2 is a graph of the acid strength decline as a function of time at 28°F and 49°F reaction temperatures. FIGURE 3 is a graph of the effect of acid strength on product octane at 28°F and 49"F reaction temperatures.

FIGURE 4 is a graph of the effect of temperature on product RON.

FIGURE 5 is a graph of the effect of space velocity on product RON at 28° F.

Detailed Description of the Invention

In an embodiment of the present invention, a C₃-C₅ olefin containing hydrocarbon stream is contacted with sulfuric acid at temperatures below 38°F and preferably below

30°F in a hydrocarbon/acid emulsion in which the C₃-C₅ olefins, preferably C₄ olefins, are reacted with a mixture of low molecular weight isoparaffins preferably isobutane, i the presence of sulfuric acid to obtain the desired alkylat product.

In this embodiment, the olefin-containin hydrocarbon stream and additional isobutane stream ar contacted with sulfuric acid in the temperature rang indicated at an acid-to-olefin molar ratio of from about 10 to about 25 to produce the desired alkylate reaction product.

The olefins which can be employed in the practice of the invention include any olefin having from 3 to 8 carbon atoms with the C₄ olefins preferred and 1-butene and 2- butenes being particularly preferred. The branched paraffinic hydrocarbons typically have from 4 to 8 carbon atoms with isobutane preferred. Typically, the branched paraffin is obtained by the distillation and/or cracking of petroleum oils and of other hydrocarbonaceous oils which are recovered as side streams or fuel gas streams. Preferably, the isoparaffinic hydrocarbons have been segregated as by fractionation. In a conventional sulfuric acid alkylation process, olefins are reacted with isobutane at 5-20"C to form highly branched C5+ hydrocarbons, primarily dimethylpentanes and trimethylpentanes. Typical overall alkylation reactions for propylene and 2-butene are:

CH3 CH3 CH3

I I I

CH2 = CH - CH3 + CH3 - CH - CH3 → CH3 - CH - CH - CH2 - CH3

Propylene + Isobutane → 2,3 Dimethylpentane

CH3 CH3 CH3 I I I

CH3 - CH = CH - CH3 + CH3 - CH - CH3 → CH3 - C - CH2 - CH - CH3

I

CH3 2-Butene + Isobutane → 2,2,4 Trimethylpentane

In the conventional process and that contained in this invention, the olefins are combined with an excess amount of isobutane and are fed to a reactor containing an emulsion of sulfuric acid and hydrocarbons. The acid and hydrocarbon phases of the emulsified mixture leaving t reactor are separated typically by gravity settling. T acid is recycled back to the reaction zone, and t hydrocarbon reactor effluent is separated by distillation stripping into the respective product streams. The isobuta in the reactor effluent stream is recovered and recycled the reaction zone. Typical prior art processes are describ in U.S. Patent Nos. 3,000,994 and 3,227,774, and Canadia Patent No. 446,901. Ideally, the sulfuric acid, acting as a catalyst serves to promote the alkylation reaction without bein consumed. In actuality, competing side reactions occur an the presence of certain feed impurities consume or dilute th acid. In order to maintain the acid strength in thes conventional single stage alkylation units above about 88-9 wt% H₂S0₄, fresh sulfuric acid of about 98 to 99 wt% H₂S0 must be added to the reaction zone when spent acid is with drawn. Some refineries continuously withdraw used acid an add fresh acid as make-up; other refineries withdraw and ad the acids intermittently. The spent sulfuric acid i regenerated by a facility at the refinery site or shipped t a regeneration facility where it is burned, the S02 vapor oxidized to S03, and then combined with water to produc fresh 98 to 99 wt% sulfuric acid. With conventional sulfuric acid alkylation, th reaction zone conditions that promote the alkylatio reactions and minimize side reactions are:

1) improved mixing;

2) decreased temperatures when operating in the range of about 5°C to 25"C which is the range of current commercial practice;

3) higher isobutane concentration;

4) larger acid volume in reactor (lower olefin space velocity) ; 5) lower feed contaminant concentrations; and

6) lower residence time of acid in the in acid settler. In addition, the composition of the sulfuric acid will affec the octane number (or quality) of the product and aci consumption. Maximum octane numbers are obtained at aci strengths of 93 to 94 wt% H₂S0₄ with water concentrations o 5 0.5 to 1 wt%¹.

The preferred feed is C₄ olefins with n-butene being especially preferred to produce high quality alkylate and low acid consumption when the alkylation process of th present invention is operated under the described operatin 10 conditions. The acid consumption is much lower and th alkylation products produced have higher octane numbers tha the same olefins reacted at higher temperatures.

As previously described, the desired result of th alkylation of isobutane and light olefins is to produce 15 hydrocarbon product with a high octane number while maintain ing or reducing sulfuric acid consumption or dilution wit conjunct polymers which can occur as the result o undesirable side reactions. Consumption of the sulfuric aci in commercial operations is a large cost (approximately 30% 20 of the overall process costs. As far as product quality i concerned, an improvement of one-half octane number i significant. Higher quality alkylate products allo refineries to add less hydrocarbons such as reformate, whic contain benzene, or permit reducing the addition of ethano 25 to the gasoline pool in order to maintain the octane rating. At this time, it appears that the alkylation process of th present invention described herein can significantly improv the product quality and reduce sulfuric acid consumption.

The following experiments were conducted t 30 evaluate the use of temperatures colder than presently use in single-stage commercial alkylation schemes.

Bench scale equipment was used for this study. This includes a 450 ml stirred reactor. The mixer speed fo this reactor is maintained at about 1100 rpm.

35 ¹Albright, L.F., Houle, L. , Su tka, A.M., and Eckert, R.E., "Alkylation of isobutane with Butenes: Effect of Sulfuric Acid Compositions", Ind. Enq. Chem. Process Des.ll, 446 (1972) . A five gallon water cylinder was pressured approximately 120 psi with nitrogen. The water flows fro the water cylinder through a flowmeter and into the botto of a cylinder of mixed isobutane/olefin feed. Th hydrocarbon feed was fed through a molecular sieve to remov water from the feed, and then into the reactor. Effluent wa drawn off the bottom of the reactor and then directed to settler. Acid is drawn from the bottom of the settler an recirculated back to the reactor. The hydrocarbon produc flows from the top of the settler through a caustic bed, an into the product cylinder.

A large batch of synthetic used sulfuric acid wa prepared by first spiking fresh acid with oleum to raise th acidity to 98.5-99% H₂S0₄. Butene-1 was then bubbled throug the acid until the acidity is reduced to approximately 97.5 H₂S0₄. Then 2-butenes are bubbled through the acid until a acidity of 94.5-95% H₂S0₄ is obtained.

The system described was then charged with 500 m of synthetic used acid prepared as directed. An average aci to hydrocarbon volume ratio in the reactor of 45 to 65% was maintained throughout the runs. Usually the acid to hydrocarbon ratio was maintained at 50-55% (v/v) . The settler temperature was monitored in the middle of the settler and where the acid exits. The temperature of the middle of the settler was kept at or below the reactor temperature while the temperature of the acid exiting the settler is always about 5 degrees warmer.

A sample of acid and product was analyzed about every 1.5 hours. The acid was sampled by purging 5 ml of acid through a sampling valve and collecting it in a centrifuge tube. The acid sample was then centrifuged for 15 minutes and about 0.5g (weighed to +/-0.1mg) was titrated to the methyl red end point with standardized NaOH. The alkylate samples were analyzed by a standard gas chromatograph (G.C.) procedure. The G.C. was equipped with a 50 m capillary column which could separate up to C14 hydrocarbons. Using the previously described reaction scheme, th following results were observed.

It was found that by maintaining the temperatur of the reactor within a predetermined temperature range, th acid consumption was greatly reduced. This optimu temperature range was found to be between about 20 and 30°

(-6°C and -1°C) . The quality of the alkylate product wa also improved by operating at lower than normal temperatures

Figure 1 shows the optimum temperature of the reactor to b in the mid 20°F range. Increased acid consumption below certain temperature is an unexpected result. It is generall assumed in the industry and in the art that as th temperature is lowered, the acid consumption will continu to decrease. It is not completely understood why the aci consumption started to increase when the reactor temperatur was less than 24°F.

To more completely test the benefit of the optimu temperature range, a set of two extended laborator experiments were conducted. The flow rate of the reactant was doubled in order to operate under more severe condition and to reduce the time of the experiments. These extende experiments show the performance of an alkylation process o the present invention throughout the acidity range used i the industry. The feed composition for these two extende laboratory experiments was 0.30 wt% propane, 86.14 wt isobutane, 1.93 wt% n-butane, 3.92 wt% 1-butene, 4.07 wt% t 2-butene, and 3.60 wt% C-2-butene.

The graphs in Figures 2 and 3, respectively, ar from the extended experiments which compares a conventional alkylation experiment conducted at a typical temperatur (49°F) and a cold temperature experiment (28°F) . The grap in Figure 2 indicates acid consumption and the graph i Figure 3 shows the product quality.

The colder reaction temperature of the reactor di result in a significant decrease in acid consumption. This is shown in the graph in Figure 2 entitled H₂S0₄ use stud comparing acidity vs. time. Constant acid consumption woul result in a gradual downward curve due to a decreasin inventory of acid in the system. The slope of the curve i proportional to the acid consumption in that the steeper th curve, the greater the acid consumption.

Acid consumption values are based on a 98.5%-90 spending range (98.5% fresh acid fed in, and 90% aci withdrawn) . Average acid consumption values do not includ that during the first 2 hours. The 28°F run averaged 0.19 #/gallon alkylate produced while the 49"F run averaged 0.32 #/gallon alkylate produced which is about a 40% reduction i sulfuric acid consumption. Many commercial alkylation unit average 0.4-0.6 #/gallon acid consumption.

The 28°F process of the present invention gav about a half an octane number higher quality alkylate tha the 49°F run. Lowering the reaction temperature from 49°F to 28°F resulted in isoparaffins containing less lights and more heavies being produced with the same amount of C8*s. The octane increase of the lower temperature appears to be from a decrease of 2,2,4-TMP and an increase of higher octane 2,3,4- and 2,3,3-TMP isomers. Figure 4 shows the effect of reactor temperature on product quality. It shows that as reactor temperature decreases, the octane generally increases. The largest increase in octane seems to occur as the temperature decreases from 50 to 35°F. Figure 5 demonstrates the relationship of space velocity on octane of the alkylate. Space velocity is a measure of the throughput or residence time in the reactor. As previously mentioned, the extended laboratory runs were conducted at a space velocity of 0.6 to double the recommended production rate. Even at this high rate, the cold process of the present invention produced a high quality product with low acid consumption or dilution of the H₂S0₄. It can be seen that by using the process described herein, a refinery could increase the production rate of an alkylation unit and maintain or even improve the quality of alkylate product and consumption of the H₂S0₄ acid catalyst. A factor to consider in evaluating the overal performance of any process is the extra production cost associated with running an alkylation process at colder tha "normal" or previously employed temperatures. A savings i acid consumption at lower temperatures will help offset th increased refrigeration costs. If the acid consumption i lowered by about 0.15 lbs. per gallon, the extr refrigeration costs can be offset based on curren electricity costs and acid regeneration costs. In summary it appears that the process of the present invention i economically feasible in this regard. The process operate very well in the acidity range currently used in th industry.

At the preferred conditions of the alkylatio process of the present invention, the isooctanes (iC₈H₁₈) produced are a mixture that are predominantl trimethylpentanes which as a mixture have research octan numbers (RON) values of about 102 to 103. Relatively smal amounts of light ends (C₅-C₇ isoparaffins) , dimethylhexame (DMH's) and heavy ends (C₉ and heavier isoparaffins) ar produced as by-products.

The present invention has been described herein i its preferred embodiments. It will be appreciated, however, that many alternative materials and methods can be employe within the teachings contained herein to produce some or all of the advantages shown and that these alternatives are encompassed within the scope of the claims as limited onl by the application of pertinent prior art.

Claims

CLAIMSI Claim;

1. A method for alkylation of isoparaffinic hydrocarbons with olefinic hydrocarbons in the production of high quality motor fuel hydrocarbons comprising: a) contacting in a reaction vessel C₃-C₅ olefins and C₄-C₈ isoparaffins with sulfuric acid having a concentration of from about 86% to about 99.5% b) maintaining said reaction vessel at a temperature of about -20°C to about 20°C; and c) subsequently removing alkylate reaction product from said first reaction vessel.

2. The temperature of the reactants in said reaction vessel in claim 1 varies from about 0°C to about -6°C.

3. The method of claim 1 wherein the olefinic hydrocarbons used are selected from the group consisting of 1-butene, cis-2-butene, and trans-2-butene and isobutylene and the temperature in the reaction vessel is about -5°C.

4. The method of claim 1 wherein the isoparaffin used is isobutane.