Flow isobutane alkylation

When usiag HF TaF ia a flow system for alkylation of excess ethane with ethylene (ia a 9 1 molar ratio), only / -butane was obtained isobutane was not detectable even by gas chromatography (72). Only direct O -alkylation can account for these results. If the ethyl cation alkylated ethylene, the reaction would proceed through butyl cations, inevitably lea ding also to the formation of isobutane (through /-butyl cation). 

A simplified flow diagram of a modern H2SO4 alkylation unit is shown in Eigure 1. Excess isobutane is suppHed as recycle to the reactor section to suppress polymerization and other undesirable side reactions. The isobutane is suppHed both by fractionation and by the return of flashed reactor effluent from the refrigeration cycle. 

Figure 4-13. Liquid-liquid heterogeneous tubular flow reaotor (e.g., alkylation of olefins and Isobutane). (Source J. M. Smith, Chemloal Engineering KInetlos, 3rd ed., McGraw-Hill, Inc., 1981.)...

Figure 11.4-2 shows process flows for an HF alkylation unit. The three sections are 1) reaction, 2). settling and 3) fractionation. In the reaction section isobutane feed is mixed with the olefin feed (usually propylene and butylene) in approximately a 10 or 15 to 1 ratio. In the presence of the HF acid catalyst the olefins react to form alkylate for gasoline blending. The exothermic reaction requires water cooling. The hydrocarbon/HF mixture goes to the settling... 

The primary process variables affecting the economics of sulfuric acid alkylation are the reaction temperature, isobutane recycle rate, reactor space velocity, and spent acid strength. To control fresh acid makeup, spent acid could be monitored by continuously measuring its density, the flow rate, and its temperature. This can reduce the acid usage in alkyla-tion units. 

The alkylation unit in a petroleum refinery is situated downstream of the fluid catalytic cracking (FCC) units. The C4 cut from the FCC unit contains linear butenes, isobutylene, n-butane, and isobutane. In some refineries, isobutylene is converted with methanol into MTBE. A typical modern refinery flow scheme showing the position of the alkylation together with an acid regeneration unit is displayed in Fig. 1. 

At the heart of the UOP HF alkylation unit is a vertical reactor-heat exchanger, shown in Fig. 14. The isobutane-alkene mixture enters the shell of the reactor through several nozzles, and HF enters at the bottom of the reactor. The reaction heat is removed by cooling water, which flows through cooling coils inside the reactor. After phase separation in the settler, the acid is recycled to the reactor. The hydrocarbon phase together with a slipstream of used acid and makeup isobutane is sent to the isostripper , where the alkylate product, n-butane, and isobutane are separated. The isobutane is recycled to the reactor. During normal... 

The use of acidic chloroaluminates as alternative liquid acid catalysts for the alkylation of light olefins with isobutane, for the production of high octane number gasoline blending components, is also a challenge. This reaction has been performed in a continuous flow pilot plant operation at IFP [44] in a reactor vessel similar to that used for dimerization. The feed, a mixture of olefin and isobutane, is pumped continuously into the well stirred reactor containing the ionic liquid catalyst. In the case of ethene, which is less reactive than butene, [pyridinium]Cl/AlCl3 (1 2 molar ratio) ionic liquid proved to be the best candidate (Table 5.3-4). 

In the ethane-ethylene reaction in a flow system with short contact time, exclusive formation of n-butane takes place (longer exposure to the acid could result in isomerization). This indicates that a mechanism involving a trivalent butyl cation depicted in Eqs. (5.1)—(5.5) for conventional acid-catalyzed alkylations cannot be operative here. If a trivalent butyl cation were involved, the product would have included, if not exclusively, isobutane, since the 1- and 2-butyl cations would preferentially isomerize to the rm-butyl cation and thus yield isobutane [Eq. (5.9)]. It also follows that the mechanism cannot involve addition of ethyl cation to ethylene. Ethylene gives the ethyl cation on protonation, but because it is depleted in the excess superacid, no excess ethylene is available and the ethyl cation will consequently attack ethane via a pentacoordinated (three-center, two-electron) carbocation [Eq. (5.10)] ... 

Propane as a degradation product of polyethylene (a byproduct in the reaction) was ruled out because ethylene alone under the same conditions does not give any propane. Under similar conditions but under hydrogen pressure, polyethylene reacts quantitatively to form C3 to C6 alkanes, 85% of which are isobutane and isopentane. These results further substantiate the direct alkane alkylation reaction and the intermediacy of the pentacoordinate carbonium ion. Siskin also found that when ethylene was allowed to react with ethane in a flow system, n-butane was obtained as the sole product, indicating that the ethyl cation is alkylating the primary C—H bond through a five-coordinate carbonium ion [Eq. (5.66)]. 

Silica-supported triflic acid catalysts were prepared by various methods (treatment of silica with triflic acid at 150°C or adsorption of the acid from solutions in trifluoroacetic acid or Freon-113) and tested in the isobutane-1-butene alkylation.161 All catalysts showed high and stable activity (near-complete conversion at room temperature in a continuous flow reactor at 22 bar) and high selectivity to form saturated C8 isomers (up to 99%) and isomeric trimethylpentanes (up to 86%). Selectivities to saturated C8 isomers, however, decreased considerable with time-on-stream (79% and 80% after 24 h). 

Both the contact efficiency of isobutane and olefin in the zones and the mixing efficiency of acid and hydrocarbon in these reactors are very important. The flow must be such that no isobutane recycle will bypass the reaction zone and no hydrocarbon will bypass the acid-hydrocarbon mixing zone. With this type of unit, a mixer driven by a 40-hp. motor is usually used in each of the zones. A reactor containing five such mixers can normally make 1500 bbl. per day of alkylate. The seven-zone reactors can make 3400 bbl. per day of alkylate. 

The input-output structure of the flowsheet is presented in Figure 9.1. Butene (feed rate FA,0) and isobutane (feed rate FB-0) are the raw materials. The butene feed is impure with quite large amounts of propane (FI 0). The main product is the alkylate C8Hi8, at the rate FP. The selectivity of the process is not 100%, therefore heavy products are formed at the rate FR. The inert fed into the process must also leave the plant, the flow including light byproducts that are formed in secondary reactions. Often, significant quantities of n-butane are mixed with the isobutane fresh feed. For this case, development of the flowsheet and the design of the main units is left as an exercise for the reader. 

The first diagram of Figure 9.5 shows that the flow rate of the reactor-outlet stream is almost insensitive to variations of the recycle rate Fj. A similar picture is obtained when variations of the feed rate F0 are considered (not shown). The second diagram shows that, as expected, increasing the excess of isobutane by larger recycle has a beneficial effect on selectivity, and more alkylate is obtained. 

ANHYD1—reaction of acetone and acetic acid to form acetic anhydride BUTAL1—alkylation reaction of butene-1 and isobutane to produce iso-octane Select the new plant design mode and the look at existing flow sheet option, and follow the synthesis steps for one of these processes. Obtain the current flow sheet as output at each level of the synthesis procedure. Also list the heuristics used at each step. 

A condensed version of the ERDL alkylation pilot plant flow plan is shown in Figure 4. Olefin and isobutane feed streams are separately pumped to the unit from large feed storage vessels with Lapp Pulsafeeder diaphragm pumps and metered with turbine flow meters. The streams are then combined, caustic scrubbed, water washed and dried with molecular sieves before being sent to the reactor. The combined feed stream is then injected into the acid-hydrocarbon emulsion in the stirred reactor vessel. In order to maintain a constant temperature environment both reactor and settler are coolant jacketed. 

In making all operating and design decisions. It Is Important to keep in mind the definition of the true reaction zone. Fundamentally, this Is the Interfaclal area between the immiscible hydrocarbon and acid catalyst liquid phases in the reactor. Reactants and products flow across this boundary. The olefins In the feed stream react Instantaneously with the sulfuric acid catalyst and combine with the relatively small amount of isobutane present In solution In the acid catalyst to form alkylate. Alkylate passes out through the Interfaclal surface reaction boundary into the hydrocarbon phase while Isobutane passes in to resaturate the catalyst. To suppress undesirable polymerization and other reactions It Is necessary to ... 

A portion of this chilled Isobutane liquid flows upward to chill the downflowing acid, remove the heat of fusion and convey the alkylate back to the alkylation reactor. The remainder conveys the acid crystals plus ester concentrate mixture to the centrifugal... 

Similarly, Siskin " found that when ethylene was allowed to react with ethane in a flow system, only n-butane was obtained. This was explained by the direct alkylation of ethane by ethyl cation through a pentacoordinated carbonium ion (equation 127). The absence of a reaction between ethyl cation and ethylene was explained by the fact that no rearranged alkylated product (isobutane) was observed. 

Table 12.4 Alkylation of isobutane with individual C3-C4 olefins (96wt% H2SO4 at 7°C well-mixed flow reactor)" [6],...

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