Article pubs.acs.org/IECR
Novel Reactive Distillation Process for Phenol Production with a Dry Cation Exchange Resin as the Catalyst Jianchu Ye, Jun Li, Yong Sha,* Yale Xu, and Daowei Zhou Department of Chemical Engineering and Biochemical Engineering, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, China S Supporting Information *
ABSTRACT: A novel reactive distillation (RD) process for phenol production with high energy efficiency was developed, in which concentrating cumene hydroperoxide (CHP) was integrated with its decomposition in the RD column using a cation exchange resin as the catalyst. Results from the kinetic experiments showed that the dry resin (AMBERLYST 35DRY) is superior to the wet one (AMBERLYST 35WET) because of the negative influence of the high water content in the latter. Meanwhile, the intrinsic kinetics of CHP decomposition catalyzed by the dry resin was obtained by regression analysis with consideration of diffusion resistances of the catalyst. A lab-scale RD column was designed by some calculations with the obtained kinetics, and the corresponding experiments were carried out to study the technical feasibility and the performance in terms of energy efficiency. Besides the high conversion and selectivity of the new RD process, results also showed obvious significance of utilizing the decomposition heat for the distillation separation.
1. INTRODUCTION The production of phenol as an important chemical product has already reached million tons per year. Over 90% of phenol is produced based on the oxidation of cumene.1 By this route, the oxidation products containing cumene hydroperoxide (CHP) are first concentrated, and then the concentrated CHP is fed into a series of decomposition reactors operated at 60−120 °C,2,3 where phenol is produced as well as acetone. Finally, phenol can be separated from the mixture by distillation. For the decomposition, concentrated sulfuric acid is used as the catalyst in the industrial process because it offers high yield of phenol. But the homogeneous catalyst is corrosive and presents many operational problems such as separation difficulties.1−4 Recently, metal oxides, zeolites, resins, bentonites, and heteropolyacids possessing the Keggin structure have received much attention as heterogeneous catalysts for phenol production because of their simple preparation, strong acidity, and high selectivity.5−10 Especially, as a kind of large-scale commercial product, the cation exchange resin has been applied in many industrial reactors.11−13 So it is also used as the catalyst in this work. The reaction distillation (RD) process with solid acid catalysts was applied to phenol production by Doron et al. to improve the process integration and use the decomposition heat for separation.5 In their study, the concentrated CHP was fed into the RD column with a metal oxide as the catalyst to produce phenol that was discharged from the bottom of the column; acetone was simultaneously recovered as the distillate by utilizing the decomposition heat. However, in order to completely remove the decomposition heat, extra acetone had to be introduced from a side feed or directly mixed with the column feed. This led to large equipment size and low utilization of decomposition heat. Moreover, to achieve desired reaction temperature, part of the adding acetone should be discharged, suggesting a low separation degree of the column. © 2014 American Chemical Society
It is worth noting that a larger amount of energy is consumed from the concentrating of CHP in comparison with the recovery of acetone in phenol production. In the effluent of an oxidization reactor, the mass ratio of cumene to CHP is generally in the range of 3−4.1 The energy consumed for recovering cumene is at least 1.605 MJ in producing 1 kg of phenol. And yet the energy required for recovery of acetone is only 0.274 MJ, which is much less than the CHP decomposition heat of 2.570 MJ with respect to production of 1 kg of phenol. Accordingly, a novel RD process could be possible that has better utilization of the reaction heat if the concentrating unit for CHP can be integrated into the RD column. This means that the nonconcentrated CHP could be fed directly into the RD column, and both cumene and acetone would be recovered from the top of the column with the decomposition heat. Obviously, the decomposition conditions of CHP in the novel RD column are different from those in the conventional reactor or the traditional RD column. Especially, the significant rise of the cumene content in the feed of the RD column dramatically affects the conditions. This change may further impact the activity of the resin catalyst, which is similar to the synthesis of cumene by reacting benzene with propylene in a RD column.14 Therefore, investigation on the catalyst with respect to the decomposition conditions of CHP in the RD column is necessary. Before performance study of the novel RD process, the type of resin was first studied in this work. Then a kinetic model was developed to perfectly predict reaction rates in the RD column. Finally, a novel lab-scale RD column was designed with the Received: Revised: Accepted: Published: 12614
January 15, 2014 May 29, 2014 June 4, 2014 June 4, 2014 dx.doi.org/10.1021/ie500165m | Ind. Eng. Chem. Res. 2014, 53, 12614−12621
Industrial & Engineering Chemistry Research
Article
obtained kinetics, and corresponding experiments were carried out to confirm the feasibility of the RD process and its performance in energy efficiency.
2. EXPERIMENTAL SECTION 2.1. Material and Analysis Methods. All chemicals used in experiments were purchased from Sinopharm Chemical Reagent Co., Ltd. except DMBA (α,α-dimethylbenzyl alcohol) which was obtained from Adamas-beta. Two types (wet and dry) of cation exchange resins used as the catalysts, namely, AMBERLYST 35WET and AMBERLYST 35DRY, were bought from Dow. The main parameters and properties of the resins are shown in Table S1 in Supporting Information. The reaction products were analyzed by GC (model GC9160) with a FID detector and a capillary column (OV101 with a length of 60 m). The injector and detector temperatures were respectively set to 210 and 250 °C. The oven temperature was set to 80 °C first and then increased at a fixed rate of 10 °C/ min to 180 °C for 2.5 min. An internal standard method using isobutanol as the standard was adopted. And yet the CHP content is detected by iodometry because of the thermal decomposition of CHP under the detection conditions of GC analysis.15,16 The analysis on the chemically pure CHP showed that the content of CHP, DMBA, and cumene is 84.5, 8.0, and 4.5 wt %, respectively. 2.2. Setup and Procedure. The experiments for the acidcatalyzed decomposition of CHP were carried out in a 250 mL four-necked flask. A condenser was installed for the reflux of used acetone because of its low boiling point (329.44 K at 1 atm). The reaction mixture was agitated at 200 rpm to eliminate the influence of the external diffusion resistance of the catalyst as much as possible. In addition, the real-time reaction temperature was monitored by a thermometer plugged into the reactor. In a typical experiment, the reactor was heated in advance with a thermostatic water bath until thermal equilibrium; after the reaction commenced, 0.5 mL of the reaction mixture was sampled and analyzed every 5 min with a syringe. Figure 1 shows the flowchart of the lab-scale continuous RD process for the production of phenol. It comprises a column, two coolers, a relay for refluxing distillate, a reboiler, top and bottom product collectors, a refrigerator, a U tube manometer, and a vacuum pump. The column (700 mm × 30 mm i.d.) was insulated to minimize the heat loss to the surrounding. The reboiler has a capacity of 500 mL, while the U tube manometer was used to measure the pressure drop inside the column. The column consists of three sections: rectifying, stripping, and reactive sections. The two separation sections in the column were packed with random Dixon rings (3 mm × 3 mm). The small column diameter (30 mm) suggested impracticable installation of structured catalytic packing such as MULTIPAK-I.17 Therefore, the catalysts (cation exchange resin particles) were encapsulated with a “tea-bag” catalytic packing configuration because of their small particle size.18,19 The catalyst bags, 30 mm × 10 mm × 2 mm, shown in Figure 2a, were made and inserted into the reactive section of the column; the gaps between the catalyst bags in the column were packed with the Dixon rings. To prevent the catalyst bag from damage, each bag was wrapped with a wire. This also facilitated fixing the bag appropriately in column with a suitable slope (see Figure 2b), which is helpful to decrease the external diffusion resistances of the catalyst bag even though it could cause a slightly higher pressure drop. The discussions on these column
Figure 1. Flowchart for the lab-scale RD column.
sections will be presented in detail in section 4.1 for the column design. We started the RD experiment with a shift from the batch process to the continuous process (state 1 to state 2 as shown in Table 1). Clearly, there was no CHP in state 1, suggesting that the column in this state was not a RD column. As a conventional column, its feed can be put into the reboiler and then the column runs under total reflux by the batch operation until steady state. Because of the relationship between feed compositions of the two states, the concentration profile of the column in the steady state of state 1 is similar to that of state 2. It suggests that the steady state of state 2 can be reached in a short time by the shift. Other operating parameters of the two operational states such as vacuum, reboiler duty, etc. will be further determined by calculation in subsequent section 4.1. To monitor the shift process, three temperature sensors were positioned in the cooler, the middle of the column, and the reboiler (Figure 1). The sensor in the cooler monitored the 12615
dx.doi.org/10.1021/ie500165m | Ind. Eng. Chem. Res. 2014, 53, 12614−12621
Industrial & Engineering Chemistry Research
Article
inlet and the reactive section is vital in the state shift, so the middle temperature sensor was installed in this section.
3. CATALYST AND KINETICS 3.1. Effect of the Process Change on Resin Activity. According to Huang et al.,7 wet resins had good catalytic activity for acid-catalyzed decomposition of CHP carried out in a three-phase circulating fluidized bed reactors. Their results showed the optimal volumetric ratio of CHP to acetone was as high as 1:2 to 1:3 in the reactor feed. But in our novel RD column, the significant rise of cumene content in the column feed and the low boiling point of acetone caused higher cumene content and lower acetone content in the liquid phase. The mechanism that formation of phenol by CHP decomposition goes through an intermediate dehydration step suggests that the use of wet resins with high water content may decrease the rate of the dehydration step when the acetone content in the system is low.9 The reason is that acetone can effectively remove water from wet resin particles because of the miscibility between water and acetone. However, the presence of cumene limits this effect because of the low solubility of water in cumene. Therefore, this obvious difference of the liquid composition in the novel RD column may significantly impact the catalytic activity of the cation exchange resin. In order to study the detailed influence of water content and select suitable resin for the novel RD process, the experiments under the conditions listed in Table 2 were carried out. The Table 2. Reaction Conditions of Kinetic Experiments with Wet and Dry Resins experiment resin type
Table 1. Feed Information for the Lab-Scale RD Experiment parameter
state 1
state 2
batch 300 mL once 298.1
continuous 10 298.1
N/A 13.62 8.40 77.98 N/A
21.41 N/A N/A 75.80 2.03
2 35WET
temp, K 339.6 1.757 catalyst content w, kg/m3 reactant composition, wt % CHP 20.41 cumene 55.47 acetone 21.48 DMBA 1.63
Figure 2. Catalyst bags in the lab-scale RD column: (a) picture and (b) arrangement.
operation feed flow rate, mL/min feed temp, K feed composition, wt % CHP phenol acetone cumene DMBA
1 35WET
3
4 35DRY
338.2 20.473
dehydrated 35WETa 339.4 1.740
19.61 77.83 N/A 1.57
19.81 77.60 N/A 1.59
19.81 77.60 N/A 1.59
339.5 1.750
a
35WET resin is soaked in acetone for 1.5 h to dehydrate before reaction.
existence of low content of DMBA in the experiments was unavoidable because of the chemically pure CHP as the reactant. Although DMBA reacts with CHP to form DCP and further produce phenol and acetone, these reactions could be fast only under higher temperature and longer reaction time.2,3 Therefore, the effects of its existence on experimental results are inconclusive. The temperature of the reactor was set at 338.15 K. However, the large decomposition heat led to the increase of the reactor temperature, so the temperatures presented in Table 2 are the average values. The reaction conversions are shown in Figure 3. The decomposition rate of CHP can be described by eq 1, with the slope of the plot of −ln(1 − xCHP)/ wcat against t providing the apparent kinetic constant kp,app.
temperature of the vapor phase at the top of the column instead of the reflux. A lower feed temperature (298.1 K) than that inside column was used for the continuous process (state 2 in Table 1) to ensure safety because of the large decomposition heat and volatility of CHP. This low temperature could cause a decrease of the liquid temperature near the feed inlet in the start period (from state 1 to state 2). However, when the condensed fluid flowed into the reactive section, it could be heated again by the decomposition heat. This suggests that the temperature change of the column section between the feed
rCHP = 12616
−dcCHP = k p,appwcatcCHP dt
(1)
dx.doi.org/10.1021/ie500165m | Ind. Eng. Chem. Res. 2014, 53, 12614−12621
Industrial & Engineering Chemistry Research
Article
Figure 3. Effect of the catalyst on conversion of CHP. Figure 4. Effect of the reaction temperature on the CHP conversion when using dry resin as catalyst.
Comparison of experiment 1 (Exp.1) and experiment 2 (Exp.2) in Table 2 showed that the wet resin had catalytic activity only when acetone was present. In fact, acetone content had significant influence on the results of the wet resin. For instance, the apparent kinetic constant was (9.783 ± 0.033) × 10−5 m3/(kg cat.·s) under the reaction conditions of Exp.1 in comparison with (1.325 ± 0.023) × 10−6 m3/(kg cat.·s) of Exp.2. The results further revealed that the wet resin was inappropriate for the novel RD process although it was applicable to traditional processes. Subsequently, experiment 3 (Exp.3) was conducted to further confirm that the dehydration effect of acetone can improve the performance of the wet resin. For this case, the wet resin was initially soaked in acetone for 1.5 h to dehydrate before the reaction. The conversion from Exp.3 was similar to that from Exp.1 even though no acetone was used in Exp.3 (Figure 3), again revealing the unavailability of the wet resin without acetone. Consequently, the dry resin (water
