409 S University St. SLC, Utah 84102 December 8, 2005

Professor Terry A. Ring Department of Chemical Engineering University of Utah SLC, Utah 84112

Dear Dr. Ring:

The members of Group B (N.D. Brown, M.R. Siddoway, and myself) were asked to explore the possibility of removing a 10-25% impurity of cumene from an isooctane stream by vapor-phase catalytic decomposition to benzene and propylene during the period of November 15 to December 6, 2005. In order to determine the practicability of removal our objectives were to characterize the order of the reaction and the rate constant as a function of temperature. Furthermore, we were asked to look at the catalyst deactivation kinetics, specifically the amount of time needed to reach line out as well as the reaction order and rate constant at line out conditions. The model carrier gas flowed through a small-scale fixed bed reactor with low-grade SiO2-Al2O3 catalyst to decompose a <5% impurity. Due to the lack of reproducible data the results were inconclusive and most of the objectives could not be determined. However, we conclude that the removal of cumene from an isooctane stream by decomposition to propylene and benzene is not possible below 60% of the initial cumene present since our data showed that the extent of the reaction was at a maximum of 0.4±0.1 for line out conditions. A précis of the experiment and recommendations follow:

Located in room 3520 C of the Merrill Engineering Building (MEB), a glass tube fitted within a heating jacket modeled the small-scale fixed bed reactor. The jacket was manufactured by Lindberg, and a Honeywell controller modified the reactor temperature. The catalyst was ground with a mortar and pestle and sifted with trays to obtain a 351- 800 um particle size. The prepared SiO2-Al2O3 catalyst was added to the tube and positioned in the center by a glass-wool plug supported by a metal spring. The helium used to model the isooctane was controlled using a Sierra mass flow controller. The amount of cumene present in the stream was obtained by bubbling helium through liquid cumene kept at a set temperature controlled by a Forma Scientific model # 2095 water bath. The reactor was a continuously flowing system. The samples were taken from a septum located after the cumene exited the reactor. Composition analysis of the product stream was completed using a HP 5890A gas chromatography instrument (GC). An HP- 5 Cross linked 5% Ph ME Silicone 25 m long column was installed on the GC with a flame ionization detector and analysis was completed using ChromPerfect Software. The GC was located in MEB room 3215. Gas tight syringes (2 mL) were used to sample the product streams. Both the reactor and the GC were shut down after use and restarted each day. The assumption was made that the helium stream becomes saturated with cumene at the vapor pressure correlating to the bath temperature. This being the case the maximum concentration of cumene was obtained at a set point of 60oC and using the Antoine constants of cumene a concentration of 0.0015±0.0001 mol/L was calculated. The settings on the GC eluted the benzene and cumene at 2 minutes and 4.95 minutes. The procedure was as follows: a) The flow rate was varied from a minimum flow rate of 2 SLPM to a maximum flow rate of 4 SLPM. This allowed a varied range of residence time in the reactor. b) The reactor temperature was varied from 335oC to 450oC. This range allowed the rate constant to be found as a function of temperature. c) Line out conditions were achieved at a reactor temperature of 450oC and a maximum flow rate of 4 SLPM. These conditions were held constant while samples were taken. d) Samples were analyzed on the GC using a 0.2 mL sample size. Calibration of cumene and benzene was completed with equivalent volumes in sample sizes of 0.1-0.4 uL diluted in acetone.

Because the carrier gas of the experiment and the GC carrier gas were the same, we were unable to compare the initial amount of cumene in the feed stream to the amount reacted using GC analysis. This required analysis of the conversion in the product stream to rely solely on the respective areas of the products. A fundamental principle of gas chromatography is that the burn ratio of chemicals should be the same. The burn ratio is the ratio of the areas of equal amounts of chemicals on a GC experiment. Calibration of the GC and an average of four measurements showed that the burn ratio of cumene to benzene was 1:1 with a relative error of 30% as shown in Table 1. From this we were able to conclude that the area of benzene present in the product stream sample would be equivalent to the amount of cumene reacted to form the detected area. In the reaction, cumene is assumed to decompose to propylene and benzene in equal amounts, therefore the areas of the products should have a consistent burn ratio. In taking an average of the burn ratio of benzene to propylene for the thirty four experiments the burn ratio was found to be 0.8:1 with an error of 80%. From this high deviation in burn ratio we suspected that something was erring in the amounts of propylene and benzene collected.

Further evidence for this suspicion is shown in repeated experiments that show an error of greater than 20% in conversion as shown in Table 2. This error was consistent among all repeated experiments averaging to >50% relative error. Assuming that the reactor is converting the same amount of cumene to benzene and propylene the error must be attributed to the sampling. Variation in obtaining the sample is attributed to the small measurement samples taken. At 0.20±0.05 mL the accuracy in the samples due to small measurement is very low. Taking a larger sample size would attenuate the error due to its relativity. Further error in sampling the product stream could be caused by the residual amounts of components left in the needle. Flushing the needle with the stream before obtaining the new sample may not have purged the needle of the residual components, which might have condensed. These remaining concentrations of propylene, benzene or cumene left in the needle may have subsequently been injected into the GC thereby causing the areas to be non-representative of the sample taken. The cause in the condensation could be due to the time-delay from taking the sample to injecting the sample. At the point where samples were taken, the stream was cooled to ambient temperature due to the surroundings. Because of this, I do not expect condensation to be large. However, because liquid and gas samples are detected with different sensitivity it can be logically concluded that the conversion would be detected as widely different for two similar runs dependent on the amount of condensation or residual components in the needle. The enormous error and irreproducible results are attributed to this suspicion. Consequently, most of the results are inconclusive in the determination of the objectives.

The principle findings of our experiment are such that the maximum conversion of cumene is estimated to be no greater than 0.4±0.1 (taken at close to line out conditions). This value is found by averaging the line out conditions. As shown in Figure 1, the line out conditions can be approached at 450oC and a flow rate of 4 SPLM for more than two hours. The measurements show a slight downward trend, yet the noise in measurement implies grouping of conversion amounts instead of a trend. This analysis leads to the reasoning that the removal of cumene from an impure stream is unpractical. At that conversion, the stream will retain at least 60% of the original cumene impurity after reaction with the catalyst. This conclusion would be supported by characterizing the reaction kinetics. Investigation of the data showed that to get a reasonable standard deviation with the amount of inaccuracy as mentioned, at least five samples of the product stream would be needed at each variation of residence times and reactor temperatures. The maximum number of runs completed in three hours is approximately 20 samples, therefore in order to obtain reproducible results within a 10% error the time allotted for the four objectives would need to be increased significantly. If the decomposition of cumene is assumed a first order reaction the data should show a linear trend. Determination of the deactivation kinetics of the catalyst requires the order of the reaction to be known and an initial guess of the order of the deactivation kinetics. According to Fogler (2002)1, the first order decay kinetics and the first order reaction are can be used to find the decay order of the reaction as shown in Equation 1.

 kd t  ℓn(0 /Wk)  ℓn(ℓn(Cao / Ca )) . (1) Plotting the residence time versus natural logarithm of the natural logarithm of the ratio of initial concentration to final concentration should show a linear trend if the decay and reaction are, in fact, found to be first order. However, in order to use the equations, a reasonable standard deviation would need to be used to have any confidence in the reaction order and kinetic rate constants obtained.

The catalytic reactor is assumed to be performing adequately and consistently. It is also reasoned that the GC is running with a 30% relative error. The following problems in the method of sampling the product stream should be addressed before this lab can provide conclusive results: a) The time delay between taking the samples of the data and injection into the GC would need to be decreased. This would ensure that condensation does not occur and that the composition of the product

1 Fogler, H. Scott. Elements of Chemical Reaction Engineering. Prentice-Hall, Inc., New Jersey (2002) stream injected is similar to what it was when sampled. Decreasing the time delay could be achieved by moving the analysis instrument closer to the reaction process, or decreasing elution time on the instrument. b) The method of sampling would need to be calibrated to account for error. Inline injection would ensure that the sample is injected uniformly for all experiments. Another recommendation would be to provide a chemical or inert air stream to be used in completely purging the needle of any residual components before taking subsequent samples. c) Adequate time is given to complete all objectives. With the necessity of taking five samples due to the deviation, only two of the four objectives could have been met. A lower deviation in measurement would require at least three samples due to the standard 30% error in the instrument.

If these recommendations can be implemented the cumene reaction kinetics can be adequately characterized and the objectives met. This would allow supporting evidence to be obtained for whether or not the removal of cumene impurity from an isooctane stream is possible.

Sincerely,

Kara Stowers

Table 1. Calibrated burn ratio of Cumene:Benzene As shown the Cumene to Benzene has a 1:1 burn ratio. From this data we see that for equal amounts of cumene and benzene, equal areas will be read by the GC. This allows a basis for conversion. Cumene Benzene Ratio Calibration Peak Area Peak Area Burn Ratio Average Deviation 1 447538 619426 0.723 2 370375 349760 1.059 1.0 0.3 3 527309 501891 1.051 4 1185373 882940 1.343

Table 2. Repeated Experiments with very high error and deviation Experiments were completed at a reactor temperature of 400oC and a water bath temperature of 40oC. Similar experiments show a relative error of 80% for a flow rate of 2 SPLM and a relative error of 20% for a flow rate of 4 SPLM. Individual Peak deviation was calculated assuming a 20% error in GC determination of the peak area. Flow Cumene Benzene Peak Deviation Rate(SPLM) Peak Area Peak Area Conversion Deviation between runs 2 44543 12563 0.22 0.05 0.38 2 39550 128340 0.76 0.05 4 192156 322388 0.63 0.07 0.12 4 69497 56878 0.45 0.07

Figure 1. Line out conditions at a temperature of 450oC and flow rate of 4 SPLM Line out was approached as shown in the figure. The general trend was decreasing but as shown the deviation in measurement showed huge variance over time.

Approaching Line Out

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