Znd. Eng. Chem. Res. 1990,29, 1025-1031
z can be as high as 0.95, but they are usually more expensive per unit area. They may still more than offset the cost by higher rates per unit area. Naturally, consideration must be given to the cake moisture content. If insufficient time is given to the dewatering of the cake in the filter cycle, this of course can be detrimental because of the excessive liquid content. Of particular interest is the influence of cake formation time. As can be seen from eq 10, as Of approaches zero, the filtration rate theoretically approaches infinity. Many attempts have been made to go to very short cycle times to increase productivity, but then cake discharge becomes a problem in a high percentage of the cases. Certain types of filters such as the continuous roller discharge drum filter can utilize short filter cycles as very thin cakes can be discharged by sticking them to the roller discharge for cutting off by a knife. For example, kaolin clay is filtered at a cycle time in as little as 15 or 20 s for 14-ft-diameter drums to maximize the rate. Undoubtedly, this principle of very short cake formation and cycle time will be very useful with some slurries and will promote increased productivity at reduced costs. However, a machine must be developed that can produce effective performance even though the total cycle time is 5 s or less. Considering the reduction of the moisture content on continuous filters, it has been shown to be a function of (t)d/ W), where dd is the dewatering time during the filter cycle, W is the cake weight in terms of weight of dry solids per unit area per cycle, and the factor e/, W is employed at constant pressure drop. Generally the moisture content as a function of (ed/ W ) , becomes asymptotic to some minimum value. Of course, pressure drop with vacuum is limited industrially to about 100 mmHg less than atmospheric pressure. However, on many materials it has been shown that each increase of 25 mmHg pressure drop reduces the moisture content between 0.2 and 0.35 percentage points of moisture. Thus,it would appear there are some definite limitations on the moisture content with continuous filters as long as vacuum is the driving force. Possibly other factors such as chemical additions (hydrocarbons have shown such
1025
potential in the past) or electrical phenomenon may offer potential for reduced moisture content. One real possibility is the combining of continuous filtration with mechanical compression. The latter is known to produce further moisture removal on compressible (squeezable) cakes with relatively little effectiveness on granular solids. In closing, the author has tried to show where possible new processes, machines, or certain techniques would yield further improvement in liquid-solid separation. Undoubtedly there will be areas not included by me that will produce increased results. The fact that we continuously must deal with finer solids, more difficult processes, and the necessity of reducing costs, makes effective creativity in the future in liquid-solid separation mandatory.
Nomenclature A = area of filtration D, = particle diameter d i = pore diameter g = force of gravity K = proportionality term normally called permeability K’ = proportionality constant L = length of pore n = exponent which must lie between 0.5 and 1.0 AP = pressure drop hp, = pressure drop during cake formation V = volume of filtrate Vf = volume of filtrate/hour during cake formation u = velocity of fluid ut = terminal velocity of a particle in a liquid u, = weight of dry solids fiiter cake per unit volume of filtrate z = fraction cake formation time of cycle time Greek Symbols a = resistance (reciprocal of permeability) t9 = time
0, = cake formation time, min/cycle
density of the liquid density of the particle = viscosity of the liquid
p =
pp = p
Received for review August 28,1989 Revised manuscript received December 11, 1989 Accepted December 14, 1989
Alkylation of Substituted Phenols with Olefins and Separation of Close Boiling Phenolic Substances via Alkylation/Dealkylation Basab Chaudhuri, Ajit A. Patwardhan, and M. M. Sharma* Department of Chemical Technology, University of Bombay, Matunga, Bombay 400 019, India
The alkylation of substituted phenols, such as, m-and p-cresols and 2,4-, 2,5-, and 2,6-xylenols, with olefins such as a-methylstyrene (AMS) and diisobutylene (DIB) in the presence of homogeneous catalyst p-toluenesulfonic acid (pTSA) and heterogeneous catalyst Amberlyst 15 was studied in the temperature range 60-160 O C . The relative rates of alkylation of various substituted phenols with AMS and DIB under different operating conditions and the ortho/para product distribution were determined. A separation strategy based on alkylation, separation of the alkylated products by dissociation extraction and their subsequent decomposition, is presented for the separation of industrially important close boiling isomeric/nonisomeric substituted phenols, such as, n-and p-cresols, 2,5- and 2,4-xylenols, and p-cresol/2,6-xylenol. A new method of refining technical grade 2,6-xylenol containing p-cresol impurity through the acid-catalyzed O-alkylation of p-cresol with isobutylene is presented. A variety of industrial mixtures consist of close boiling point isomeric/nonisomeric components. In most cases,
* Author to whom correspondence should be addressed.
the separation of such mixtures cannot be economically realized by the conventional methods of separation such as those based on distillation and modified distillation processes, crystallization, etc. In such cases, the strategy
0888-5885/90/26291025$02.50/0 0 1990 American Chemical Society
1026 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990
of selective reactions may prove to be useful. The alkylation of phenol with a variety of olefins such as propylene, l-butene, isobutylene, a-methylstyrene (AMS), and diisobutylene (DIB)in the presence of acid catalystshas been studied by Chaudhuri and Sharma (1990). It was found that the alkylation of phenol with AMS in the presence of cation-exchange resin Amberlyst 15 gave a product consisting of 97% p-cumylphenol and 3% o-cumylphenol. The high ratio of para- to ortho-alkylated products in the alkylation of phenol with AMS was due to the slower rate of ortho alkylation with AMS. The alkylation of phenol with DIB also gave a para-to-ortho ratio of 955. The high ratio of para- to ortho-alkylated products in the alkylation of phenol with AMS and DIB gave us a clue to how to separate a mixture of close boiling isomeric/nonisomeric phenolic substances, such as m- and p-cresols, 2,5- and 2,4-xylenols, 2,6-xylenol and p-cresol, etc., with AMs and DIB in the presence of homogeneous and heterogeneous catalysts. It was thought that the alkylation of cresols and/or xylenols with AMS in the presence of acid catalyst would give para alkylation more readily than ortho alkylation. The alkylated product mixture would thus be richer in the para-alkylated product, which could be separated and subsequently cracked to recover cresol and/or xylenol. This would then provide a new strategy for the separation of close boiling isomeric cresols and xylenols. It was also thought that, in the reaction of p-cresol with isobutylene in the presence of ion-exchange resin, the O-alkylation could be promoted, by manipulating the temperature, to give predominantly 4-methylphenyl tert-butyl ether. This could then be exploited in a novel way to remove p-cresol impurity from technical grade 2,6-xylenol. This work was undertaken to examine the above aspects and to develop new separation strategies. Various techniques such as fractional crystallization and dissociation extraction were considered for the separation of ortho- and para-alkylated products of cresols and/or xylenols. The idea of differential cracking was also thought to be useful in getting additional enrichment of one of the components of cresol/xylenol mixtures.
Previous Studies The separation of m- and p-cresols has been a challenging problem and has attracted the attention of many workers in the past 4 decades. A number of methods have been proposed for separating m-and p-cresols. Physical methods, such as distillation or even azeotropic distillation, were unsuccessful owing to the similarity in properties of these isomers (Othmer et al., 1949). A potentially useful method depends on the preferential adduct formation with substances such as oxalic acid (Engel, 1937), phenol (Ludewig and Wilke, 1965), benzylamine (Fleischer and Meier, 1966), urea (Santhanam, 1970; Orlova et al., 1975), benzidine (Savitt and Othmer, 1952), 2,6-lutidine (Cislak and Otto, 1948), and piperazine (Gaikar and Sharma, 1987). Chemical methods based on sulfonation-desulfonation or dialkylation of a mixture of m-and p-cresols with isobutylene followed by fractional distillation of butylated products and then debutylation of the butylated cresols are other methods of separation (Stevens, 1943). The method involving dialkylation requires strong sulfuric acid as the catalyst, and its separation from the product mixture poses problems. It is also easier to handle liquid-liquid reactions compared to gasliquid reactions. It was thus thought worthwile to investigate the separation of m- and p-cresols through monoalkylation with AMS either in the presence of a trace quantity of pTSA or with an ion-exchange resin catalyst,
separation of the alkylated products, and subsequent dealkylation. 2,4-Xylenol and 2,5-xylenol (boiling points at 760 mm = 211.5 "C) present together in the low-boiling fraction of xylenols obtained from, e.g., coal tar have been separated by tert-butylation followed by distillation of the tert-butylated products and subsequent decomposition. The boiling points of 6-tert-butyl-2,4-xylenol and 4-tert-butyl-2,5-xylenol are, respectively, 131and 151 "C (at 20 mm Hg), and they can be easily separated by fractional distillation (Stevens, 1943). In this work, the separation of 2,4- and 2,5-xylenols through alkylation with AMS and subsequent separation of the alkylated products by dissociation extraction or fractional crystallization have been tried. Various methods of the separation of 2,6-xylenol and p-cresol from their mixture have been tried (Gaikar and Sharma, 1985,1987; Ciernik and Spousta, 1986);successful methods are based on dissociation extraction and dissociation extractive crystallization, and a separation factor as high as 562 has been realized in the case of dissociation extractive crystallization (Gaikar and Sharma, 1987). Extractive separation with hydrotropes was also successfully tried for the separation of 2,6-xylenol and p-cresol (Gaikar and Sharma, 1986). The separation of p-cresol and 2,6-xylenol through selective alkylation with AMS and the separation of the alkylated products have been tried in this work. An attempt has also been made for the removal of p-cresol from the p-cresol/2,6-xylenol mixture by selective O-alkylation of p-cresol with isobutylene at lower temperature.
Experimental Section Experiments were carried out in a 0.05-m4.d. fully baffled mechanically agitated reactor. A six-bladed glass-disk turbine impeller was used for agitation. Some crucial experiments were also carried out in a 0.10-mid. fully baffled mechanically agitated reactor. AMS was obtained from Herdillia Chemicals Ltd. and was typically 95% pure; contaminants were close-boiling tert-butylbenzene and cumene. DIB was of Fluka grade. The phenolic substances, m-cresol, p-cresol, 2,5-xylenol, and 2,4-xylenol, were of Fluka grade. 2,6-Xylenol was obtained from a reputed European manufacturer. The cation-exchange resin catalyst Amberlyst 15 was obtained from Rohm and Haas Co., Philadelphia, PA. p-Toluenesulfonic acid was obtained from Sisco Research Laboratories. The cation-exchange resin catalyst was initially washed with deionized water and acetone to remove any impurity present on the external surface of the catalyst and then was dehydrated under vacuum (1-2 mmHg pressure) at a temperature of 100 "C ( f 2 "C) for 2-4 h to remove traces of moisture adsorbed on the catalyst surface. All the experiments were carried out at the desired temperature by placing the reactor in a constant-temperature bath. The analysis of the reaction mixture consisting of cumene (solvent), unconverted olefin and olefin oligomers, substituted phenols, and the alkylated products was done on Perkin-Elmer 8500 gas chromatograph. A stainlesti steel column, OV-17 (10%) on Chromosorb WHP, was used for analysis, the length of which was 3.5 m. The initial oven temperature was kept at 100 "C and then was increased to 300 "C by programming at the rate of 0.25 "C/s. Some crucial analyses were done in a 4-m-long, 10% NPGA on Chromosorb WHP glass column. A mixture of m- and p-cresols was analyzed in a 3.5-m-long stainless steel Bentone-DIDP (10%) column at an oven temperature of 140 "C. The Bisphenol A oligomers were analyzed on HPLC. A C,, (3-pm)column was used for HPLC analysis;
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 1027
K z are the second-order rate constants of reactions 1 and
Table I. Relative Rates of Alkylation of Various Substituted Phenols with AMS and DIB at 60 O C in the Presence of Homogeneous and Heterogeneous Catalysts" reactivity ratio catalyst substituted phenols olefin 4.51 pTSA m-/p-cresol AMS 4.81 Amberlyst 15 m-/p-cresol AMS 3.81 m-/p-cresol DIB DTSA 5.6:l m-/p-cresol DIB Amberlyst 15 4.91 pTSA 2,5-/2,4-xylenol AMS 4.91 Amberlyst 15 2,5-/2,4-xylenol AMS 1:l pTSA 2,6-xylenol/p-cresol AMS Amberlyst 15 13:l 2,6-xylenol/p-cresol AMS 1:lO pTSA 2,6-xylenol/p-cresol DIB 2.3:l Amberlyst 15 2,6-xylenol/p-cresol DIB a Reaction
2.
It has been described already in a separate communication that the dimerization of AMS takes place simultaneously with alkylation; the dimer then gradually decomposes to give the alkylated products. The formation of AMS dimers and their subsequent decomposition and reaction with cresols (and/or xylenols) tends to decrease the rates of alkylation of cresols, but it does not alter the selectivity with respect to PCMC or OCPC. The rates of reaction of MC and PC are, respectively, -d[MC] / dt = K1[MC] [AMs] (3) and
temperature = 60-70 "C.
-d[PC]/dt = K,[PC][AMS]
the eluent (methanol + water) flow rate was maintained at 0.9 mL/min. In all experiments, the solvent, the substituted phenols, and the cation-exchange resin were taken into the reactor and heated while stirring to the reaction temperature. The olefin was then added to the reactor at the reaction temperature; the moment of addition of the olefin into the reactor was taken as the starting time of the reaction. The same catalyst (cation-exchangeresin) was used repeatedly and the activity of the catalyst was checked from time to time by carrying out the alkylation at a specified temperature and matching the conversion level (of the olefin). I t is necessary to exercise caution in using fresh catalyst, as leaching of "free acid" is frequently encountered in initial experiments, and this can give disguised results. Results and Discussion Separation of m -and p-Cresols. The alkylation of an equimolar mixture of m- and p-cresols was carried out with both homogeneous pTSA catalyst and heterogeneous ion-exchange resin Amberlyst 15 in the temperature range 60-100 "C. In all the experiments, cumene was used as the solvent, AMS (33-50 mol % of the total cresols) was added quanta by quanta into the cumene-cresol-catalyst slurry over a period of 2-3 h, and the reaction was continued for 3-4 h after complete addition of AMs. The Amberlyst 15 loading of 2% (w/w) was used in the reaction. For pTSA, the catalyst loading was much less, being only 0.20% (w/w). In a typical experiment, 30 g of cumene, 30 g of cresols (15 g of m-cresol and 15 g of p-cresol), and 1.42 g of Amberlyst 15 were taken into the reactor and M s was then brought to the reaction temperature; 11g of A added quanta by quanta into the reactor. The relative rates in the alkylation of m- and p-cresols, 2,5- and 2,4-xylenols, and 2,g-xylenol and p-cresol with AMS and DIB at 60 "C are summarized in Table I. The relative rates (reactivity ratio) were calculated by knowing the initial rates of formation of the alkylated products. The percentage of para- to ortho-alkylated products may be predicted by the following treatment. Prediction of Product Distribution. The alkylation reactions of m- and p-cresols with AMS are represented as follows:
+ AMS 5 PCMC PC + AMS -!% OCPC
MC
(4)
From eqs 3 and 4, (5)
At t = 0, [MC] = [MC], and [PC] = [PC],. The solution of eq 5 gives us
i.e., i.e.,
From eqs 1 and 2, -d[AMS]/dt = Ki[MC][AMS] + Kz[PC][AMS]
(7)
Combining eqs 3 and 7 gives 4MCl --
d[AMS]
Kl[MCI [AMs1
- Kl[MC][AMS] + Kz[PC][AMS]
i.e.,
Substituting the value of [PC]/[MC] from eq 6 into eq 8, we get dlMCl 1
Integrating the above equation with the initial condition t = 0, [AMS] = [AMs],, [MC] = [MC], we get
(1)
(2) where MC, PC, AMs, PCMC, and OCPC stand for mcresol, p-cresol, a-methylstyrene, p-cumyl-m-cresol (ortho product is formed to a very small extent, 1-2% in the case of MC), and o-cumyl-p-cresol, respectively; and K 1and
i.e.,
[MCIK2IK1)= [AMs], - [AMS] (9)
1028 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 Table 11. Reactivity Ratio of m-Cresol/p-Cresol and 2,5-Xyleno1/2,4-Xylenol with AMS at Different Temperatures' temp, "C substituted phenols reactivity ratio m-lp-cresol 4.5:l 60 3.0:l 80 m-/p-cresol 100 m-lp-cresol 1.2:l 60 2,5-/ 2,4-xylenol 4.9:l 110 2,5-12,4-xylenol 12 140 2,5-f 2,4-xylenol 1:8b 160 2,5-/2,4-xylenol 1:lOb "Catalyst = pTSA. bThese are not initial reactivity ratios.
Equation 9 can be solved by trial and error to find [MC], Le., unreacted m-cresol present in the reactor by knowing the total quantity of AMS reacted. Equation 6 will give the value of unreacted p-cresol, [PC]. Thus, by knowing the reactivity ratio from the initial reaction rates of m- and p-cresols with AMS, the product distribution of PCMC and OCPC can be predicted by eqs 6 and 9. The initial reaction rates are taken as the rates of alkylation of m- and p-cresol with AMS just after the start of the reaction, Le., after 300-600 s where the level of conversion of AMS is below 15%. Thus, the ratio of PCMC to OCPC is equal to ([MClo - [MCl)/([PClo - [PCI). The reactivity ratio of m- and p-cresols with AMS in the presence of Amberlyst 15 a t 60 "C was found to be 4.8:l (Table I). At higher temperatures, the reactivity ratio was found to decrease (Table 11). Also with an increased quantity of pTSA catalyst (0.5%w/w), the para-alkylated product was found to give dialkylated product on prolonged stirring a t the reaction temperature; decomposition of the para-alkylated product to give m-cresol back was also observed. The concentration of homogeneous pTSA catalyst in the alkylation of cresols with A M s should, thus, be correctly adjusted so as to get adequate reaction rates without any decomposition or dialkylation of the paraalkylated product. The maximum quantity of cresols that could be alkylated with AMS in one batch was 50-55% of the total cresols charged in the reactor. Even in the presence of excess AMS, the reaction did not proceed beyond a total cresol conversion of 55%. The temperature was found to have an insignificant effect on the conversion of cresols. With Amberlyst 15, a substantial quantity of phenylindan was formed at higher temperature (100 "C). In the presence of pTSA, dialkylation of para-alkylated product was promoted a t higher temperatures. There was almost no difference in the relative rates of reaction of m- and p-cresols with AMS for homogeneous and heterogeneous catalysts (Table I). This clearly points out that the polymeric support does not have any effect on the selectivity to PCMC or OCPC. When 33 mol % of AMs completely reacted with cresols, the ratio of [PCMC] to [OCPC] in the product was found to be 77:23; the calculated product distribution was in the ratio of 78:22. The experimentally determined ratio of [PCMC] to [OCPC] thus matched well with the predicted value. Several strategies were examined for the separation of the para-alkylated product, PCMC, from the ortho-alkylated product, OCPC. In OCPC, the bulky cumyl group a t the ortho position is supposed to increase its solubility in the organic phase. It was, thus, thought that PCMC could be separated from OCPC by crystallization from an organic solvent such as n-heptane. This strategy, however, did not work well for the separation of PCMC from OCPC, although, by the same strategy, it was possible to separate p-cumyl-2,5-xylenolfrom o-cumyl-2,4-xylenol (this will be
CHl
Figure 1. Acid-catalyzed decomposition of p-cumylphenol.
discussed later in the text). PCMC was separated from OCPC by dissociation extraction. Thus, a 77:23 mixture of PCMC to OCPC was taken in cumene (50%w/w), and 20% aqueous NaOH solution equivalent to the stoichiometric quantity of PCMC was added to the mixture. A separation factor as high as 120 was realized, and the purity of PCMC was around 95%. Thus, dissociation extraction was found to be a very effective technique for separation of PCMC from OCPC. An inert solvent like n-heptane instead of cumene was also tried; the solubility of the alkylated cresols in n-heptane was, however, low. A separation factor of 200 was realized from a 5% solution of PCMC and OCPC (77:23)in n-heptane. Inert solvents like n-heptane or cyclohexane have been used for the separation of 2,6-xylenol/p-cresol,and separation factors as high as 25-30 have been reported (Gaikar and Sharma, 1985). Cracking (Decomposition) of the Alkylated Products of m -and p-Cresols. The cracking of the alkylated products such as PCMC or OCPC should give back the cresol isomers. The optimum cracking conditions were found by cracking p-cumylphenol (PCP) and Bisphenol A. Since the alkylation of phenol/substituted phenols with AMS gave unsaturated dimers of AMS in addition to the alkylated products, it was necessary to decompose unsaturated dimers of AMS and examine the product distribution. Cracking under different conditions such as thermal cracking, acid-catalyzed cracking, and base-catalyzed cracking was tried. The idea of differential cracking was also exploited to achieve additional enrichment with respect to one of the cracked products in the final step of the separation. All the cracking experiments were carried out in a 0.1-L round-bottom flask and at atmospheric pressure; a 0.1-L heating mantle was used for heating, and the heat input was adjusted carefully. Higher heat input led to a substantial quantity of byproduct formation. For instance, the thermal cracking of the unsaturated dimers of AMS at 260-280 "C (67% 1-pentene and 33% 2-pentene) gave a mixture of 58% AMS and 42% cumene (+tar). The formation of cumene by hydrogen transfer can possibly be completely suppressed by addition of a small quantity of acid (pTSA, 0.5% w/w) in the dimer, and the purity of AMS obtained on cracking was 99.8%. The unsaturated dimers of AMS formed in the alkylation reaction could thus be easily cracked to give very pure AMs, free of any tert-butylbenzene or cumene, which could then be used for alkylating phenol/substituted phenol mixtures. The acid-catalyzed cracking of PCP proceeds according to the mechanism given in Figure 1. Thermal cracking of PCP was found to be slow and produced a substantial quantity of cumene. The formation of cumene could be suppressed to less than 1% by the use of 1%w/w of homogeneous pTSA catalyst. It was thought that the presence of base such as NaOH may also catalyze the decomposition of PCP: dissolved
Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 1029
I
Figure 2. Base-catalyzed cracking of Bisphenol A. Table 111. Product Distribution on Cracking of an 8020 Mixture of p-Cumyl-m-Cresol and o-Cumyl-p-Cresol at 120 OC in the Presence of 3% (w/w)Amberlyst 15 time, min % dec. (cracking) ratio of m-/p-cresol 30 50 86:14 60 62.3 85:15 120 81.7 83.8:16.2 91.7 80.5:19.5 300
alkali would interact with the phenolic hydrogen to produce phenolate anion and catalyze the decomposition rate. For the base-catalyzed cracking of PCP, 0.5-1% (w/w) NaOH was used as the catalyst. It was observed that the acid-catalyzed cracking took place at a much faster rate than the basecatalyzed cracking. It has been reported that PCP is stable in the presence of basic catalysts to 300 "C (Schnell and Krimm, 1963). Preliminary experiments on the thermal and base-catalyzed cracking of Bisphenol A were done. Thermal cracking produced p-isopropylphenol as a byproduct, and ita concentration was almost equal to the p-isopropenylphenol produced. The base-catalyzed cracking of Bisphenol A suppressed the formation of p-isopropylphenol but gave a considerable amount of p-isopropenylphenol dimers and trimers, which were not characterized. The base-catalyzed cracking of Bisphenol A is likely to proceed by the mechanism given in Figure 2. On the basis of the experiments mentioned above, the cracking of PCMC and OCPC was carried out under mildly acidic conditions at atmospheric pressure. It was found that a 955 mixture of PCMC and OCPC, on cracking, gave a product containing 98% m-cresol and 2% p-cresol at 50% conversion (cracking) level. PCMC thus underwent cracking at a much faster rate than OCPC under mildly acidic conditions. The cracking experiments were also carried out with acidic resin Amberlyst 15 in the temperature range 110-120 "C;cumene was used as diluent in all the experiments. The catalyst (Amberlyst 15) loading was between 2% and 3% (w/w). The results are given in Table 111. A 80:20 mixture of PCMC to OCPC at 50% conversion (cracking) level gave a product containing 86% m-cresol and 14% p-cresol; no AMS could be obtained by this procedure as A M s produced on decomposition of the alkylated products dimerized rapidly in the presence of Amberlyst 15 to give
the saturated dimer, 1,1,3-trimethyl-3-phenylindan. The faster rate of cracking of PCMC could thus be exploited to get a product enriched with respect to m-cresol. The effect of temperature on the decomposition rates of paraand ortho-alkylated products has been discussed elsewhere in the text. The reaction of m- and p-cresols with DIB in the presence of homogeneous pTSA catalyst and heterogeneous Amberlyst 15 was considered for the separation of cresols. With the homogeneous catalyst, the reactivity ratio of m- and p-cresols with DIB at 60 "C was 3.77:1, whereas with Amberlyst 15 the ratio was 5.6:l. Dry acidactivated clay, Filtrol 24, was found to give a higher reactivity ratio of 10-12:l in the alkylation of m- and p cresols with DIB at temperatures around 60-70 "C. The catalyst, however, gave a very large quantity of tetraisobutylene along with the alkylated octylcresols. The cracking characteristics of tert-odylcresols were also found to be poorer, and hence, this strategy was not considered for separation. Separation of 2,5- and 2,l-Xylenols. The reactivity ratios of 2,5- and 2,4-xylenols with AMS at different temperatures are given in Table 11. The reaction did not proceed well in the presence of Amberlyst 15, and the dimerization of AMS was the predominant reaction. A small quantity of pTSA (0.20% w/w loading), however, catalyzed the alkylation. The rates of reaction of AMS with xylenols were found to be lower than the rates with cresols. The decrease in rates was probably due to the decrease in the reactivity of xylenols because of the presence of two methyl groups in the benzene ring. It has been reported that successive introduction of the alkyl groups in the aromatic ring of phenol reduces its reactivity toward electrophiles; the cyclohexylation of cresol isomers was thus found to proceed at slower rates compared to the cyclohexylation of phenol (Babin et al., 1985). The separation of p-cumyl-2,5-xylenol (PCX) and ocumyl-2,4-xylenol(OCX) was achieved by crystallization from heptane solution. In the first crop of crystals, the concentration of PCX was more than 99%, and when 50% of the alkylated xylenols crystallized out, the concentration of PCX in the solid dropped to 91%. The separation of PCX and OCX was also realized by dissociation extraction. Aqueous NaOH solution of two different strengths was used for extraction; the separation factor was found to be independent of the alkali concentration. With a 40% aqueous NaOH solution, the Na salt of PCX came out of the solution as a solid product; the separation factor from a 50% cumene solution was 130. This strategy of dissociation extractive crystallization can be successfully utilized for the separation of PCX and OCX. With 20% aqueous NaOH solution, however, no separate solid phase appeared, but a separate viscous liquid phase was obtained, which was subsequently separated and acidified to get back the alkylated xylenols. The effect of temperature on the reactivity ratio of 2,5and 2,4-xylenols with AMS and, hence, the selectivity of PCX and OCX were found to be interesting. The results are summarized in Table 11. It was found that para alkylation of phenol and/or substituted phenols with AMS is characterized by low activation energy. A reaction of high activation energy is favored at higher temperatures. Ortho alkylation of phenol and/or substituted phenols with AMS in the presence of acid catalyst was found to be slow. It was, thus, thought that the selectivity with respect to OCX would increase at elevated temperatures. Experiments were conducted in the temperature range 110-160 "C to examine the effect of temperature on the selectivity
1030 Ind. Eng. Chem. Res., Vol. 29, No. 6, 1990 1OOC
01
0
I
I
30
60
I
1
90 120 TIME ( M I N 1
I
1
150
110
I
Figure 3. Selectivity variation of o-cumyl-2,4-xylenol and p-cumyl-2,5-xylenol with time at 160 "C.
of PCX and OCX in the reaction of 2,5- and 2,4-xylenols with AMS. At 110 OC, the ratio of PCX to OCX in the reaction mixture was 33:66 (1:2). When the reaction mixture was heated to 160 "C and stirred at that temperature for 3 h, the selectivity of PCX to OCX dropped to 9:91 (1:lO). Starting a t 160 "C, the selectivity of PCX and OCX in the alkylation of 2,5- and 2,4-xylenols with AMS with 0.2% (w/w) pTSA catalyst was found to vary with time according to Figure 3. PCX was found to decompose at elevated temperature in the presence of a small amount of pTSA catalyst; OCX was, however, much more stable. This difference in the decomposition rates of paraand ortho-alkylated products was exploited in a novel way in the separation of m- and p-cresols and 23- and 2,4xylenols. A 955 mixture of PCX and OCX, on cracking, gave a product containing 99% 2,5-xylenol and 1% 2,4xylenol a t 50% decomposition level. The alkylation of mixture of 2,5- and 2,4-xylenols with AMS was also conducted with 5% (w/w) Nafion NR50 catalyst at 160 "C. AMS was added quanta by quanta into the cumene-xylenol-catalyst slurry. The main product in the reaction was the saturated dimer of AMS, 1,1,3-trimethyl-3-phenylindan. The ratio of PCX to OCX was almost 1:1, which proves that the higher acidity of the catalyst will promote the decomposition of PCX and OCX to almost the same extent, and no advantage of differential cracking can be obtained with the superacidic Nafion NR50 catalyst. Based on the information derived from the effect of temperature on the selectivity to PCX and OCX, a separation strategy was devised for the separation of 2,5- and 2,4-xylenols; the xylenols are first reacted with AMS at elevated temperatures to produce alkylated xylenols richer in the ortho isomer, which can be subsequently separated from the para isomer by a physical method such as fractional distillation or dissociation extraction. It was observed that during the alkylation of 2,5- and 2,4-xylenolswith AMs a t elevated temperatures (refluxing condition) the decomposition of AMS dimers to give AMS back took place, and an equilibrium between AMS and products (AMS dimers + alkylated xylenols) was established, complete conversion of AMs and/or xylenols to give alkylated xylenols could thus not be achieved even on prolonged stirring at elevated temperatures with pTSA as catalyst. The reactivity ratio of 2,4- and 2,5-xylenols with DIB in the presence of pTSA catalyst under refluxing condition was found to be 4.36:l. Here also ortho alkylation was found to be predominant at higher temperatures. The alkylation reactions of xylenols with DIB was found
to be slower than those with AMS and required considerably larger amount of pTSA as the catalyst. The slower M s were perhaps rates of reaction with DIB compared to A due to the weaker electrophile generated from DIB in the presence of acid catalyst. Separation of 2,6-Xylenol and p-Cresol. The alkylation of an equimolar mixture of 2,6-xylenol and p cresol with olefins such as AMS and DIB was carried out in the presence of homogeneous (pTSA) and heterogeneous (Amberlyst 15) catalysts a t 60 "C. The reactivity ratios were measured and are reported in Table I. It is interesting to note that, in the presence of Amberlyst 15,2,6xylenol reacted with AMS a t a much faster rate than p cresol, whereas with pTSA they reacted at almost the same rate. This difference in the reactivity ratio of 2,6-xylenol and p-cresol from heterogeneous to homogeneous catalyst brings out the utility of supported catalyst in altering the positional selectivity. The results of the reactions of 2,6xylenol and p-cresol with DIB and AMS in the presence of homogeneous catalyst are also interesting. With pTSA, p-cresol was found to react with DIB at a rate much faster than 2,6-xylenol;the reactivity ratio was 101. With AMS, however, the reactivity ratio was around 1:l. This decrease in the reactivity ratio in the reaction of p-cresol and 2,6xylenol with DIB and AMS, respectively, can probably be explained by examining the nature of the electrophile generated from DIB and AMs. The electrophile generated from DIB is weaker than that generated from AMs. Ortho alkylation is favored with weak electrophiles. In the reaction of 2,6-xylenol and p-cresol with DIB, ortho alkylation of p-cresol thus occurred much more readily than para alkylation of 2,6-xylenol. The steric effect in 2,6-xylenol weakened the conjugation between the hydroxyl group and the ring and decreased its reactivity (Gurvich et al., 1985). p-Cumyl-2,6-xylenol and o-cumyl-p-cresol could be separated by crystallization from heptane. Dissociation extraction from cumene (solvent) gave only a low separation factor of 5.8. This low separation factor is due to the inaccessibility of the OH group in both p-cumyl-2,6-xylenol and o-cumyl-p-cresol, sterically hindered phenols. Removal of p -Cresol Impurity from %,6-Xylenol. The removal of a small amount of p-cresol impurity from a large excess of 2,6-xylenol is an important industrial problem. Various strategies, such as selective reaction of p-cresol with HCHO (Ciernik et al., 1988), dissociation extraction, etc., were considered for the removal of p-cresol impurity from 2,6-xylenol. It was observed that the rate of C-alkylation of p-cresol with isobutylene to give tertbutyl-p-cresols in the presence of Amberlyst 15 a t temperatures around 5-10 "C was very low; a t these temperatures, however, O-alkylation was found to proceed smoothly to give 4-methylphenyl tert-butyl ether. The O-alkylation of 2,6-xylenol with isobutylene in the presence of Amberlyst 15 was similarly tried; the reaction, however, did not proceed at all. This is undoubtedly due to steric hindrance; the OH group in 2,6-xylenol is protected from attack by the tert-butyl carbocation because of the presence of the two ortho methyl groups. On the basis of the above results, the O-alkylation/etherification reaction was exploited in a novel way to remove p-cresol impurity from 2,6-xylenol. Both pTSA and Amberlyst 15 were found to work well in the reaction. Thus, a mixture of 98% 2,6xylenol and 2% p-cresol (w/w) was dissolved in toluene; the concentration of substituted phenols in toluene was 50% (w/w). A catalyst (Amberlyst 15) loading of 5% (w/w) was used, and isobutylene was passed through the solution maintained a t 8 "C ( f 2 "C). Higher catalyst loading (10% w/w) gave considerable C-alkylation of
Znd. Eng. Chem. Res. 1990,29,1031-1042
2,g-xylenol and rearrangement of 4-methylphenyl tertbutyl ether to 2-tert-butyl-p-cresol. It is possible to selectively remove p-cresol by low-temperature O-alkylation with isobutylene, and the 98:2 mixture of 2,6-xylenol to p-cresol could be refined to 99.802 (w/w). The C-alkylation of 2,6-xylenol took place to a very small extent of about 5%. 2,6-Xylenol of purity higher than 99.8% can be recovered from the xylenol/ether mixture by distillation. The yield of 2,6-xylenol on separation was about 95% as a small amount of 2,6-xylenol was C-alkylated during the process. This method is expected to have wider utility.
Conclusions The separation of close boiling point isomeric phenols, such as m- and p-cresols and 2,5- and 2,4-xylenols, was accomplished through a novel strategy of alkylation/dealkylation. From a 5050 mixture of m- and p-cresols, 98% pure m-cresol was recovered; from a 50:50 mixture of 2,5and 2,4-xylenols, 99% pure 2,5-xylenol was obtained. Higher purity materials can be obtained. The product distribution in alkylation of an equimolar mixture of 2,6-xylenol and p-cresol was greatly influenced by the choice of the catalyst. A technical grade 2,6-xylenol containing 98% 2,6-xylenol and 2% p-cresol was refined to give 2,6-xylenol of purity >99.8% by the selective O-alkylation of p-cresol with isobutylene in the presence of acid catalysts. Acknowledgment B.C.and A.A.P. are thankful to the University Grants Commission, New Delhi, for the award of Senior Research Fellowship. Nomenclature [AMS] = concentration of a-methylstyrene, kmol/m3 [AM& = initial concentration of a-methylstyrene, kmoi/m3 K1 = second-order rate constant in eq 1, m3/(kmol-s) Kz = second-order rate constant in eq 2, m3/(kmol.s) [MC] = concentration of m-cresol, kmol/m3 [MC], = initial concentration of m-cresol, kmol/m3 [OCPC] = concentration of o-cumyl-p-cresol, kmol/m3 [PC] = concentration of p-cresol, kmol/m3 [PC], = initial concentration of p-cresol, kmol/m3 [PCMC] = concentration of p-cumyl-m-cresol, kmol/m3
1031
Registry No. PCP, 599-64-4; AMs,98-83-9; DIB,25167-70-8; pTSA, 104-15-4; Amberlyst 15,9037-24-5;isobutylene, 115-11-7; biphenol A, 80-05-7; m-cresol, 108-39-4; p-cresol, 106-44-5; 2,5xylenol, 95-87-4; 2,4-xylenol,105-67-9; 2,6-xylenol, 576-26-1; p cumyl-m-cresol,27421-06-3; o-cumyl-p-cresol,2675-76-5.
Literature Cited Babin, E. P.; Dzhafarova, N. A.; Fanaliev, Y. M.; Allakhverdiev, M. A. Certain kinetic relationship of cycloalkylation of phenol and ita homologs. Zh. Prikl. Khim. 1985,58 (2), 424-427. Chaudhuri, B.; Sharma, M. M. Paper submitted for publication, 1990. Ciernik, J.; Spousta, E. Czech. Patent 230,842, 1986; Chem. Abstr. 1986,105, 152704. Ciernik, J.; et al. Czech. Patent 240,893, 1988; Chem. Abstr. 1988, 109, 210689. Cislak, F. E.; Otto, M. M. US. Patent 2,432,062,1948; Chem. Abstr. 1948,42, 1967. Engel, K. H. U.S. Patent 2,095,801, 1937; Chem. Abstr. 1937, 31, 88949. Fleischer, J.; Meier, E. Ger. Patent 1,215,726, 1966; Chem. Abstr. 1967,65, 5401. Gaikar, V. G.; Sharma, M. M. Dissociation extraction: prediction of separation factor and selection of solvent. Solvent Extr. Ion Exch. 1985,3,679-696. Gaikar, V. G.; Sharma, M. M. Extractive separation with hydrotropes. Solvent Extr. Ion Exch. 1986, 4, 839-846. Gaikar, V. G.; Sharma, M. M. Dissociation extractive crystallization. Ind. Eng. Chem. Res. 1987,26, 1045-1048. Gurvich, Ya. A.; et al. Zh. Org. Khim. 1985,21,411 (Russian); Chem. Abstr. 1985, 103, 36885. Ludewig, R.; Wilke, H. U.S. Patent 3,193,585, 1965; Chem. Abstr. 1965,63,6262. Orlova, 0. S.; et al. Khim. Prom. SSSR 1975, 1 , 20; Chem. Abstr. 1975,82, 124959. Othmer, D. F.; et al. Composition of vapors from boiling binary solutions. Ind. Eng. Chem. 1949, 41, 572-574. Santhanam,C. J. Br. Patent 1,191,631,1970;Chem. Abstr. 1970,73, 14466. Savitt, S. A.; Othmer, D. F. Separation of m- and p-cresols from their mixtures. Ind. Eng. Chem. 1952,44, 2428-2431. Schnell, D.; Krimm, H. Formation and cleavage of dihydroxydiaryl methane derivatives. Angew. Chem., Int. Ed. Engl. 1963, 2, 373-379. Stevens, D. R. Separation of individual cresols and xylenols from their mixtures. Ind. Eng. Chem. 1943, 35, 655-660.
Received for review August 4, 1989 Revised manuscript received December 4, 1989 Accepted January 4, 1990
Fundamental Model for the Prediction of Sieve Tray Efficiency Miguel Prado and James R. Fair* Department of Chemical Engineering, The University of Texas at Austin, Austin, Texas 78712
Fundamental considerations of sieve tray hydraulics, such as hole activity (jet and bubble formation), bubble sizes and rise velocities, and average void fraction have been combined with diffusional mechanisms t o develop a model for predicting mass transfer on an active sieve tray. Experiments were conducted to support model development. A computer-based data acquisition circuitry was designed and built to monitor digitally and simultaneously eight electroresistivity probes, independently located in separate holes of a sieve tray of an air-water simulator. With this monitoring system, it was possible to determine bubble size formation distributions and, importantly, the fraction of active holes that were either jetting or bubbling under a given set of hydrodynamic conditions. In addition, concurrent gas- and liquid-phase resistant mass-transfer efficiencies were measured. The crossflow sieve tray is a common type of device used for vapor-liquid contacting in distillation columns. Its purpose is to promote intimate contacting of the phases and thus to encourage rapid transport of material between 0888-5885/90/2629-lO31$02.50/0
the phases. The transport process is visualized as diffusional movement of molecules to and across the phase boundary, and therefore, the rate of transport is considered to be directly proportional to the extent of the interfacial 0 1990 American Chemical Society
