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KINETICS, CATALYSIS, AND REACTION ENGINEERING Formation and Separation of Sulfuric Acid/n-Heptane Dispersions: Applications to Alkylation Lyle F. Albright and Roger E. Eckert* School of Chemical Engineering, Purdue University, West Lafayette, Indiana 47907-1283
Experimental information has been obtained on the formation and separation of dispersions of the two immiscible liquids, concentrated sulfuric acid and n-heptane. These dispersions have physical properties similar to those used in reactors to alkylate isobutane with C3-C5 olefins for the production of high-quality, clean-burning gasolines. The alkylation reactions occur at or at least near the liquid-liquid interface. Larger interfacial areas promote more rapid alkylation reactions and generally result in higher-quality products. Once the alkylation reactions are completed, however, rapid separation of the dispersion is needed to prevent undesired side reactions of isoparaffins and especially trimethylpentanes (having high octane numbers). The rates of separation of the dispersions were found to depend significantly on the temperature, the volumetric ratio of the two liquid phases, the composition of the acid, and the level of agitation employed to produce the dispersion. Several dispersions of hydrocarbon droplets in water were also investigated. Introduction Isobutane is alkylated with C3-C5 olefins in large quantities to produce high-quality gasolines. The catalysts that have been employed for many years are either sulfuric acid or HF (hydrofluoric acid). In industrial units, the liquid hydrocarbon phase is dispersed as droplets in the continuous acid phase. The characteristics of the dispersions are known to only a limited extent even though the kinetics of the alkylation reaction and the quality of the alkylate product are strongly dependent on the nature of the dispersion. When sulfuric acid is employed as the catalyst, the main reactions occur at, or at least close to, the interface between the two immiscible liquids.1,2 Vigorous agitation is required in commercial reactors to obtain acidcontinuous dispersions, i.e., to dispers hydrocarbon droplets in the sulfuric acid phase. The interfacial areas between the two liquids depend on the number, size, and shape of the dispersed droplets. The level of agitation, acid/hydrocarbon ratio, physical properties of the two liquid phases, and design and operation of the reactor affect the interfacial areas. Increased agitation results in smaller and more numerous hydrocarbon droplets; hence, the interfacial area is increased. Li et al.3 found that increased levels of agitation resulted in better-quality alkylates. In one comparison in a laboratory continuous-flow reactor, the research octane numbers (RONs) increased with more vigorous agitation from 87 to 98.7. Simultaneously, the optimum residence times decreased to the 2-5 min range. In commercial reactors, residence times are often in the 15-30 min range, which results in significant decomposition reactions, as discussed later. No known experimental information is available on the interfacial areas present in commercial alkylation
reactors using either sulfuric acid or HF as the catalyst. In a laboratory unit, am Ende et al.4 found that the interfacial area of sulfuric acid/2,2,4-trimethylpentane dispersions is highly dependent on the composition of the acid, acid/hydrocarbon ratio, and temperature. For a commercial-sized reactor provided with an agitator, a computer model, VisiMix 2000,5 predicts similar findings (Albright and am Ende).6 Once alkylation is completed in the alkylation reactor, the dispersion is separated by gravity in decanters to obtain an acid phase and a hydrocarbon phase. Several investigators7-9 have found that degradation reactions occur when certain isoparaffins, and especially trimethylpentanes, come into contact with sulfuric acid. Similar reactions likely occur when HF is used. Such degradations result in reduced-quality alkylates and simultaneously in a small loss of alkylate quantity. Appreciable degradation reactions occur in times as short as 10 min. Rapid separation of the acid/hydrocarbon dispersions is obviously desired as soon as the alkylation reactions are completed. There is little quantitative information on the rates of separation in decanters of the acid/ hydrocarbon dispersions. Undoubtedly, the rates depend significantly on the size and number of dispersed droplets, as well as the physical properties of the liquids. In the present investigation, dispersions of sulfuric acid and n-heptane were separated by decanting at operating conditions similar to those used in commercial alkylation units. Several operating variables were investigated to determine their effects on the formation and separation of the two phases. Primarily acidcontinuous dispersions were investigated, but limited information was also obtained for hydrocarbon-continuous dispersions. Several dispersions of n-heptane droplets in water were also investigated.
10.1021/ie010238b CCC: $20.00 © 2001 American Chemical Society Published on Web 08/11/2001
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Experimental Details The mixing vessel employed in this investigation was the bottom end of a glass graduate cylinder of 1000-mL capacity. A section of about 12.5-cm length, or 130-mL capacity, was cut from the cylinder. It was then fitted with four baffles spaced 90° apart. The baffles were connected to aluminum circular supports, one on the top of the baffles and the other at their bottom. The baffles and supports were fit snugly and did not rotate in the mixing vessel even at high rates of agitation. An agitator shaft was positioned at the axis of the mixing vessel. Two impellers were attached to the agitator shaft. An electric motor was used to rotate the shaft and impellers up to 2500 revolutions per minute (rpm). A Variac was employed to control the rotation speed, as measured by a stroboscope. Either a rubber or a Teflon stopper was used at the top of the mixing vessel. The agitator shaft entered through a hole in the middle of the stopper. This hole had a diameter slightly larger than that of the shaft. The mixing vessel was immersed (except for the top 1.0 cm) in an agitated water bath. The bath temperature was controlled to within (0.5 °C in the 1-20 °C range by the addition of ice as needed. The front face of the bath was constructed of plate glass. An electric light immersed in the bath permitted good visibility of the dispersion in the mixing vessel, so that large n-heptane droplets, entrained air bubbles, and vortices could be observed when they occurred. The locations of the interfaces between the gas and the top level of liquids (either n-heptane or the dispersion), between n-heptane and the dispersion, and between the dispersion and the acid (or water) phase were also determined using the volumetric markings on the outer walls of the mixing vessel. Two acids were employed: first, a fresh acid of 96.5% acid and the remainder water, and second, a used acid containing 90% acid, 7.0% conjunct polymers, and 3.0% water. This used acid was obtained from a commercial alkylation reactor. To minimize degradation (including oxidation) reactions, this acid was stored prior to use at about -15 °C. Hence, its composition was essentially identical to that of the acid in the reactor from which it was removed. A commercial grade of n-heptane was employed for most runs for three reasons. First, nparaffins, including n-heptane (density ) 0.68), are essentially inert to concentrated sulfuric acid as they have only primary and secondary C-H bonds. Isoparaffins containing tertiary C-H bonds are, however, much more reactive. Second, n-heptane has a rather low volatility at the temperatures investigated. Finally, the physical properties of n-heptane are relatively similar to those of the hydrocarbon mixtures in alkylation units. Formation and Separation of Acid-Continuous Dispersions To start a dispersion experiment, acid and liquid n-heptane were added to the mixing vessel to obtain the desired volumetric ratio in the 20:80 to 75:25 range. The apparent volumes of each liquid were recorded by noting the marks on the outer glass walls of the mixing vessel. The apparent volumes were about 10% greater than the true volumes because of the presence of the impellers, impeller shaft, baffles, etc., present in the mixing vessel. An almost straight line resulted when the apparent volumes were plotted on a graph versus the true
volumes. Once the vessel was loaded, the agitator motor was started. The method of increasing agitation to the desired rate and the ratio of the acid and hydrocarbon phases affected the type of dispersions formed. At ratios of about 50:50 and higher, acid-continuous dispersions were produced in all cases investigated. At lower levels of agitation, however, only part of the hydrocarbon liquid was dispersed. With ratios in the 20:80 to 40:60 range, when the level of agitation was increased rapidly to a level to obtain complete dispersions, hydrocarboncontinuous dispersions resulted. Relatively large acid droplets were generally formed. When at least one impeller was positioned in the acid phase, acid-continuous dispersions were obtained in the 20:80 to 40:60 range with the following startup procedure: The impeller was started at a relatively low rate of rotation, and a small amount of n-heptane became dispersed in the acid phase. The agitator speed was then slowly increased, causing more n-heptane to be dispersed as droplets in the acid. Eventually, at a sufficiently high level of agitation, all of the n-heptane was dispersed. This dispersion had a overall uniform appearance and seemed to be relatively stable. When the agitator speed was further increased by several hundred rpm’s, the acid-continuous dispersion with the 20:80 ratio suddenly converted to a hydrocarbon-continuous dispersion. When agitation was stopped, the acid-continuous dispersions generally separated in about 2-120 min, depending on the specific dispersion. Figure 1 indicates how a 50:50 dispersion separated as a function of time at 1, 10, and 20 °C. The locations of the interfaces between the n-heptane phase (upper phase), the dispersion (middle portion), and the acid phase (lower phase portion) are shown for each of the three temperatures. At complete separation, the two interfaces coincided, and the dispersion disappeared. The following observations were made during separations of acid-continuous dispersions: (1) Within about 10-20 s after the agitation was stopped, an acid layer relatively free of dispersed n-heptane droplets was noted on the bottom of the mixing vessel. In addition, a relatively clear hydrocarbon (n-heptane) phase began forming on the upper level of the liquids after about 40 s. The exact time at which initial separation of a phase occurred was not easy to observe as the initial rates of separation were low. During this time period, some hydrocarbon droplets in the dispersions obviously coalesce. As the droplets rise, acid flows down and around them. (2) Following the startup period, the rates of separation of both the acid and n-heptane phases generally increased significantly, as shown in Figure 1. The increased rates are explained as follows: Some hydrocarbon droplets coalesce, and the larger droplets rise more rapidly in the dispersion as compared to the smaller droplets. The rate of separation of the acid phase was always relatively higher during the initial phase of separation as compared to that of the hydrocarbon phase. Hence, the ratio of the acid to n-heptane in the remaining dispersions decreased significantly with time; the volume of each phase in the dispersion can be determined using Figure 1. These volumes are directly proportional to the vertical distances shown in Figure 1 between each interface and the position of the interface upon complete separation.
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Figure 1. Separations of 50:50 dispersions of n-heptane and fresh sulfuric acid at 1, 10, and 20 °C; 1350 rpm; 110 mL total volume.
(3) During later phases of separation, relatively large n-heptane droplets often accumulated near the upper level of the acid/hydrocarbon dispersion. The thickness of the acid layer between these droplets decreased; hence, the rate at which the acid flowed downward between the droplets decreased. In some cases, as will be discussed later, a relatively stable froth consisting of hydrocarbon droplets surrounded by a thin layer of acid formed. These froths, if undisturbed, were relatively stable, sometimes existing for 30 min or longer before breaking or collapsing. Separation results such as those shown in Figure 1 were used to determine the time of agitation required to obtain equilibrium dispersions, defined as dispersions for which additional agitation has no effect on the times of separation. After all of the n-heptane was dispersed in the acid phase, another 30 s of agitation generally resulted in equilibrium dispersions, i.e., the times of separation were then identical to within (5 s. In the present investigation, about 300 s of agitation was always used to ensure that equilibrium dispersions were obtained. The times to obtain equilibrium dispersions undoubtedly vary with different mixing vessels, agitators, baffles, etc. The single-phase layers separated slowly at the start, and then, gradually, the rate of separation increased. Subsequently, the rate decreased, and finally, complete separation was reached slowly. Therefore, cumulative normal distribution models, which follow this behavior, were used to describe amount of each single-phase layer with time.
1 hH ) ht - (ht - hf) σHx2π hA ) hf
1 σAx2π
∫0 e t
-(t-µH)2/2σH2
∫0t e-(t-µ ) /2σ A
2
2
A
dt
dt
(1)
(2)
Cumulative normal distribution equations have two
Table 1. Separation Time in Seconds for 50:50 Dispersions of n-Heptane in Sulfuric Acida acid temperature (°C) viscosity of acid (cP) density of acid (g/mL) (s) σA (s)
µA
fresh
fresh
fresh
fresh
used
1 46 1.86
10 33 1.85
20 22 1.84
20 22 1.84
20 28 1.72
78 33
76 36
182 115
104 33 0.75 >150
103 33 0.74 150
247 93 0.74 380
acid interface eq 2 155 102 61 42
n-heptane interface eq 1 µH (s) 205 135 σH (s) 69 46 ratio µA/µH 0.76 0.76 complete separation time (s) 290 185
a 110 mL of acid and n-heptane; 1350 rpm. Note: 110 mL of actual volume equals 121 mL of apparent volume.
constants, µ and σ. µ values are the times at which onehalf of hydrocarbon phase (µH) and one-half of acid phase (µA) have separated from the starting dispersion. σ values are the standard deviations of the height vs time curves. One unit of σ is the time required for an interface to move from 16 to 50% separation or from 50 to 84% separation. For all 10 nonlinear regressions performed on the hydrocarbon and acid layer formation reported in Table 1, the multiple correlation coefficients (R2) are from 99 to >99.9%. This represents the percentage of the total variation about the average response explained by the regression. Temperature Results Comparative runs indicated the effect of temperature on the separation of dispersions. One comparison is reported in Figure 1 and Table 1. This specific test was performed using fresh (96.5%) acid, a 50:50 ratio of acid to n-heptane, and an agitation rate of 1350 rpm. The two times (µ) required to obtain 50% separations of the acid and n-heptane phases are shown in Table 1. A duplicated run at 20 °C shows close reproductibility. The ratios of µΑ/µΗ for a given run were in the range of 0.74-
Ind. Eng. Chem. Res., Vol. 40, No. 19, 2001 4035 Table 2. Total Times for Separation of Acid/n-Heptane Dispersions with Ratios of 70:30 to 30:70 series acid temperature (°C) viscosity of acid (cP) density of acid (g/mL) total liquid (mL) agitation (rpm)
1 fresh 20 22 1.84 110 2000
2 fresh 18 24 1.84 110 1200-1300
acid/HC ratio 70:30n 60:40 50:50 40:60 30:70
3 fresh 10 33 1.85 100 1100-1200
4 used 10 40 1.73 100 1270-1320
5 used 10 40 1.73 120 1100-1200
190
520
190
490
580 600 680 640 600
separation time (s) 150 140 >230 (froth)
153 170
0.76. The total times of separation for these runs increased from about 150 to 290 s as the temperature decreased from 20 to 1 °C, i.e., there was almost a 100% increase. Other temperature comparisons were made at different ratios (70:30, 60:40, and 40:60), and still others employed a used acid from a commercial alkylation unit. Lower temperatures in all cases resulted in relatively similarly increased times of separation. The results of am Ende et al.4 help explain the above results. In the range of 0 to 15 °C, the largest hydrocarbon droplets dispersed in the acid occurred at about 15 °C. In the temperature range of 0-20 °C, the acid viscosities varied by a factor of about 2, with the highest viscosity at 0 °C. The acid viscosity is obviously a major factor affecting the rate at which hydrocarbon droplets rise in the acid phase. Temperature also changes the interfacial tension appreciably. Results for Used and Fresh Acids Several comparisons were made between the separation results for dispersions containing either fresh or used acid. Table 1 shows one comparison at 20 °C with dispersions containing a 50:50 ratio of acid to n-heptane and employing an agitation rate of 1350 rpm. The dispersion containing used acid separated in 380 s, i.e., about 2.5 times longer than the time needed for the dispersion containing fresh acid. Other comparisons made at other ratios of acid to n-heptane and at 10 °C indicated relatively similar differences of separation times. am Ende et al.4 found that the diameters of hydrocarbon droplets in dispersions containing fresh acid are about twice those in used acid. Furthermore, the used acid has a much higher viscosity as compared to the fresh acid. The slower separation of dispersions containing used acid is undoubtedly caused by a combination of the more viscous acid and the smaller droplets. Density differences between the acid and hydrocarbon droplets is the driving force for the separation. For fresh acids, there is approximately a 10% greater driving force as the density of used acid is about 0.1 g/mL lower. Results with Different Acid/n-Heptane Ratios For acid-continuous dispersions, the separation results for five series of runs were obtained with acid/nheptane ratios that varied from 70:30 to 30:70 (Table 2). With fresh acid, the acid-continuous dispersions at a 30:70 ratio were unique for two reasons: acidcontinuous froths formed having acid/hydrocarbon ratios of about 2:98 or even 1:99, and the times of separation were longest as compared to those for the other ratios investigated. These froths persisted for relatively long
>340 (froth)
440
times. Rather large and stable hydrocarbon droplets accumulated in these froths located above the separated acid phase. The thin acid layer between the droplets appears to stabilize the droplets and to minimize droplet coalescing. In addition, this thin acid layer drains slowly into the acid layer below. Gentle stirring or shaking of the vessel promoted separation of the froth. For at least one dispersion with a 70:30 ratio, the rate of separation was lower than that of a 50:50 dispersion. Judging from the findings of am Ende et al.,4 70:30 dispersions have smaller droplets. Table 2 summarizes the separation results for series 2 and 3 made using fresh acid. The trends agree well qualitatively with those of series 1. For 30:70 dispersions prepared with used acid (see series 4 and 5 of Table 2), the separation patterns differ from those of dispersions prepared with fresh acid. With used acid, no froth was noted in either run toward the end of the separation. Furthermore, the times of separation for 30:70 dispersions, although long, were shorter than those for dispersions with higher acid/n-heptane ratios. For series 4, the 70:30 dispersions separated slowest, but for series 5, the 50:50 dispersions separated slowest. The conjunct polymers in the used acid increase the viscosity of the acid significantly. The conjunct polymers also have surfactant properties and obviously accumulate at the interface between the acid and hydrocarbon phases. The failure to observe froths with dispersions containing used acids might be significant. The results shown in Table 2 indicate that acid/ hydrocarbon ratios are important relative to the separation of dispersions. am Ende et al.4 and Albright and am Ende6 have shown that these ratios are important relative to the interfacial areas in the dispersion of the alkylation reactor. Agitation Results for Acid/n-Heptane Dispersions Several runs were made in which different rates of agitation (expressed in rpm’s) were employed. At lower levels of agitation, only part of the n-heptane was dispersed in the acid phase. As the agitator rotation rate increased from 560 to 950 rpm in one example, the volume of n-heptane dispersed in 50 mL of used acid increased from about 33 to 50 mL (which is complete dispersion of n-heptane). When the agitator speed was incrementally reduced to 860 rpm, however, about 1 h was required to reach equilibrium; during this time, approximately 4 mL of n-heptane phase separated from the dispersion. Once all of the n-heptane was dispersed in the acid phase, several tests were made to determine the effects of still higher levels of agitation on the dispersions. An
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Figure 2. Effect of total liquid volume of fresh sulfuric acid and n-heptane in acid-continuous dispersions with sufficient agitation to obtain complete dispersions; 40:60 ratio; 1600-1800 rpm; 10 °C.
increase of about 200 rpm for the agitator(s), often resulted in separation times that were longer by 1020%. Further increases in agitation rate generally resulted in vortexing and air entrainment. The separation results then became quite erratic; air bubbles often attached themselves to the hydrocarbon droplets and promoted faster separations. In an attempt to minimize vortexing, the equipment was modified in regard to the baffles, agitators, spacing of agitators, and volume of gas in the top of the mixer vessel, as well as installation of solid metal disks on the impeller shaft. These modifications were not fully successful in eliminating vortex formation at high agitation rates. Complete elimination of the gas phase in the vessel might be required, but a modified mixing vessel would be needed for this change to be made. Loading Level of Mixing Vessel Three different dispersions were prepared with total liquid volumes of 100, 110, and 120 mL. Hence, the total liquid levels varied in these dispersions. Each dispersion had a fresh acid-to-n-heptane volumetric ratio of 40:60 and was controlled at 10 °C. In each case, the agitation provided was just sufficient to completely disperse the n-heptane in the acid phases. An agitation level of about 1600 rpm was needed for the volume of 100 mL, 1700 rpm for 110 mL, and 1800 rpm for 120 mL. Figure 2 indicates that the rates of hydrocarbon separation were essentially identical for each of the three runs during the first 210 s. A similar occurrence was also noted for the rates of acid separation. Of course, the amounts of both acid and n-heptane that separated varied for the three dispersions. These results suggest that the size and number of droplets per given volume were essentially identical in the three dispersions just before the agitation was stopped. Results with n-Decane and Surfactant Several experiments were conducted in which ndecane was substituted for n-heptane. It was dispersed in fresh sulfuric acid. Separation times for such dispersions were essentially identical to those obtained using
n-heptane. However, when small amounts (1.4-3.7 × 10-3 g/mL) of n-dodecylamine were dissolved in the fresh acid, the times of separation for the resulting dispersions were increased substantially, often by at least 100%. Hydrocarbon-Continuous Dispersions Several n-heptane-continuous dispersions were produced having acid/n-heptane volumetric ratios in the 20: 80 to 40:60 range. These dispersions separated much more rapidly, often in 10-15 s, than the acid-continuous dispersions. The rapid separation undoubtedly results from at least the following two factors. First, the dispersed acid droplets appeared to be significantly larger than the hydrocarbon droplets in acid-continuous dispersions. Second, the acid droplets were settling through the relatively low-viscosity hydrocarbon liquid, whereas the hydrocarbon droplets were rising in the high-viscosity acid phase. Results for Water/Heptane Dispersions Several dispersions of n-heptane droplets in water were investigated. Figure 3 shows the results for separation of one specific dispersion, which had an approximately 50:50 mixture of phases and was formed using an agitation rate of 1400 rpm at 10 °C. Complete separation occurred in slightly greater than 250 s; such a time is similar to those for separations of numerous acid/hydrocarbon dispersions investigated here. Yet, the viscosities of the sulfuric acid used are much greater than those of water. Hence, assuming that the sizes of the droplets were identical, which was probably not true, the resistances for hydrocarbon droplets rising through the water should be much lower. On the other hand, rising droplets in water have lower driving forces because of the lower density difference between the two phases. The shapes of the separation curves, shown in Figure 3, differ greatly from those shown in Figure 1. There was little, or possibly even no, induction period for the separation of water from the water/hydrocarbon dispersion. The highest rates of water separation occurred
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Figure 3. Separation of 50:50 dispersion of water and n-heptane at 10 °C; 1400 rpm; 100 mL total volume.
within approximately the first 10 s after the agitator was stopped, i.e., these separations occurred essentially as soon as the swirl of the dispersion ceased after agitation was stopped. For the acid/hydrocarbon dispersions, the rates of acid separation, as shown in Figure 1, increased during the first 75-100 s before reaching a maximum. As shown in Figure 3, 50% of the water separated in about 35 s. These results, together with the results with different levels of agitation, indicate the following: At 1200, 1400, and 1600 rpm, 50% of the water separated in 10, 14, and 20%, respectively, of the times required for complete separation. As all were 50:50 dispersions, when 50% of the water had separated, the hydrocarbon droplets had moved upward and out of the bottom 25% of the initial dispersion in 10-20% of the total separation time. Hence, 80-90% of the time was required to separate the remaining 50% of the water, i.e., the hydrocarbon droplets moved upward during this time period at much reduced rates. Such a phenomenon was observed even though some agglomeration of droplets occurred. As indicated by Figures 1 and 3, the rates of hydrocarbon separation from the dispersions were qualitatively similar regardless of the heavy phase. The rates of separation increased initially for an appreciable time before passing through maxima. As complete separation was approached, the rates decreased toward zero. Separation data were obtained for three additional dispersions of water and n-heptane at 10 °C. The three dispersions had (a) a 50:50 ratio with a total volume of 100 mL, (b) a 50:50 ratio with a total volume of 140 mL, and (c) a 70:30 ratio with a total volume of 100 mL. The findings are summarized as follows: (1) The 100-mL dispersion with the 50:50 ratio, based on visual observations, appeared to be uniform from the upper to lower levels. In the 140-mL dispersion, much larger hydrocarbon droplets occurred in most cases in the top portion of the dispersion, which was above the impellers. As the level of agitation increased, the size of droplets throughout the dispersion became more uniform and relatively small. With the same agitation rates, the 140-mL dispersion separated within about 25-100% less time in the 1000-1600 rpm range of
agitation. Above about 1800 rpm, air entrainment became pronounced. (2) Comparing the separation times of the two dispersions with a total volume of 100 mL, the 50:50 dispersion generally separated more slowly, by perhaps 1015%, than the 70:30 dispersion. Uniform dispersions resulted in both cases. (3) As the agitation levels increased, generally above 1600 rpm, air entrainment started. In the range from about 1000 to 1400 rpm, the times for complete separation of the dispersions increased by about 80-150% as the agitation levels increased. Discussion of Results Although hydrocarbon droplets are dispersed in sulfuric acid in alkylation units in many refineries, little quantitative information has apparently been reported in the past concerning the formation or separation of these dispersions. In the present investigation, dispersions containing used acid often separated in about 420-600 s (7-10 min). Jerigan et al.10 reported what is probably a similar combination of acid and hydrocarbons leaving an alkylation reactor as separating in about 20 min. However, their decanter was probably much larger, which would account for the longer separation times. In addition, Jerigan et al. investigated different volumetric ratios of the two liquids. They found that the characteristics of the dispersions formed were different but gave no details. Apparently, they never investigated hydrocarbon-continuous dispersions. Rapid separation of the dispersions leaving a reactor is highly desired to maintain a high octane number. Undesired side reactions of isoparaffins, particularly of trimethylpentanes having high octane numbers, occur until separation is complete. In the present investigation, considerable quantitative data were obtained for the first known time indicating the importance of the operating variables temperature, volumetric ratios of acid to hydrocarbon, composition of the sulfuric acid, liquid level in the mixing unit (or reactor), and agitation. The design and size of the equipment employed are, of course, also important variables. For example, the agitator, baffles, etc., in the mixing vessel used in this
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investigation acted to some extent to impede the upward motion of the dispersed hydrocarbon droplets. The hydrocarbon droplets sometimes became trapped under the agitator blades, and droplets also sometimes adhered to the solid surfaces in the vessel, i.e., wall effects occurred. The results obtained here must be applied with caution to commercial units. First, the uniformity and level of agitation in the various dispersions likely differs from values found in dispersions in commercial units. Higher agitation levels were probably sometimes tested in the present investigation than are used in commercial reactors. Second, the commercial units are much larger in size. Nevertheless, it seems safe to conclude that many commercial decanters do not provide the desired rapid separations of dispersions once alkylation has been completed. Modified or different separation equipment might be cost-effective. The separation data reported in the present investigation were obtained by visual observations only. The results of Buckler11 suggest that small amounts of hydrocarbons were probably present in the acid phase as very minute droplets. He found that centrifuging is often needed to separate these hydrocarbon droplets, as indicated by acidity measurements before and after centrifuging. In addition, some evidence has been reported in the past that tiny droplets of acid are sometimes present in the separated hydrocarbon phase, even though complete separation appears to have occurred. Despite these separation problems, the present results likely mimic the quality of separations found in current commercial units. Relatively stable froths have been reported to occur on occasions in commercial decanters. Judging from the results of this investigation, these froths can have ratios of acid to hydrocarbons as low as 2:98. Such froths are highly undesired for several reasons, and additional research is needed to determine how to prevent their formation. Acid composition was found in the present investigation to be of importance. A method of producing acid-continuous dispersions at acid/hydrocarbon ratios in the range of 20:80 to 50:50 was developed in the present investigation. These results along with those of Mosby and Albright12 suggest that acid-continuous dispersions can be produced in flow reactors at
