NAPHTHALENE VIA HYDRODEALKYLATION Comparison of Pilot and Commercial Plant Data with an Analog Computer Model C D
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R
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A N D E RSS0 N
E, LA M B
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Sun Oil Go., .Warms Hook, Pa.
University of Delaware, Newark, Del.
Production of naphthalene via hydrodealkylation in an adiabatic tubular reactor was simulated using an analog computer. A mathematical model was developed based on available kinetic data and on pilot plant results in which commercial plant charge stocks were used. Fifteen separate reactions were included in the final model to describe the hydrodealkylation of commercial charge stock. Coefficients in some of the kinetic equations were established by parameter estimation techniques using an analog computer. The model was used to predict temperature distributions and reactor effluent concentrations for a commercial plant in which unreacted product was recycled to the feed. The effects of reactor inlet temperature, hydrogen purity, and gas-oil ratio on plant operation were studied. Predictions of plant performance obtained through use of the model were in accord with commercial plant operating experience.
N RECENT YEARS,
increasing efforts have been directed toward
I analysis of process behavior for purposes of simulation and optimization. Despite notable successes in this area, formidable difficulties often arise in the application of theoretical techniques to practical process situations. These difficulties may result from incomplete knowledge of fluid mechanics and reaction kinetics, or, in some cases, from uncertainty of ratecontrolling steps. Establishment of reaction kinetics is particularly difficult in petrochemical processes because of the presence of large numbers of compounds which may undergo a variety of simultaneoiis and consecutive reactions. T o overcome these difficulties, it is important that process analysis together with process simulation be initiated in the early stages of process development. Analysis and simulation begun a t the bench scale level can lead to better pilot plant design and can indicate the most significant experiments to be performed with bench and pilot scale equipment. T h e interplay of analysis, simulation, and experiment throughout the development of a process can often result in significant economies in the design and operation of a commercial plant. A case history of the application of analysis and simulation techniques to the development of the hydrodealkylation reactor used in Sun OiP Co.'s naphthalene process is described in this paper. Although the analysis is limited to steady-state con-
siderations, results have proved useful not only in design and operation of the reactor but also in specification of control devices and safety features. Process Description
The salient features of Sun Oil Co.'s process for the production of naphthalene from catalytic gas oil have been described ( 7 ) . T h e principal components of the process are shown in Figure 1, and can be grouped into three categories: feed preparation section, conversion section, and naphthalene purification section. In the feed preparation section catalytic gas oil is distilled to produce a 440' to 515' F. true-boiling-point cut. An aromatic concentrate containing essentially all of the monomethyl- and dimethyl-substituted naphthalenes present in the gas oil is extracted from this cut. The extract is catalytically desulfurized before being used as feed to the conversion section of the process. T h e conversion section contains the tubular hydrodealkylation reactor shown in Figure 2. Feed to the reactor consists of extract from the feed preparation section of the process mixed with the hydrogen-rich gas and a reactor recycle stream containing unconverted methylnaphthalenes. A series of reactions takes place within the reactor, which result in dealkylation of alkyl-substituted dicyclic aromatics to produce naphthalene. Monocyclic aromatics in the reactor are simul-
HIGH CETANE DIESEL FUEL
GAS COMPONENT H2 CH4 -TO
ACID GAS SULFUR RECOVERY
--
MOLE 'A 35-80 20-60
FRESH FEED COMPONENT ALKYLNAPHTHALENES
LIQUID PRODUCT
40-80 ADIABATIC
NON-NAPHTHALENES:
NAPHTHALENE 30-70
ALKYLBENZENES HYDROAROMATICS INDENES SATURATES
H E A V ~FUEL
Figure 1.
Principal components of naphthalene process
NAPHTHALENE ALKYL NAPHTHALENE
Figure 2.
0-25 75-100
Hydrodealkylation reactor VOL. 3
NO. 2
APRIL 1964
177
taneously dealkylated to yield primarily benzene, toluene, and xylene. Adiabatic operation results in significant temperature rise through the reactor. The reactor effluent is sent to the purification section of the process. Here in a series of distillations the reactor effluent is separated into high purity naphthalene, a recycle stream containing unconverted methylnaphthalenes, high octane gasoline, and a small amount of heavy fuel. The naphthalene cut is clay-treated and then rerun to produce a product containing 99+% naphthalene. Model Requirements and Background Information
The process analysis described here is restricted to the hydrodealkylation reactor. An objective of the analysis was prediction of the variation in concentration through the tubular reactor for each important chemical species together with the temperature variation through the reactor. I t was necessary to do this over a range of the following variables: 1. 2. 3. 4. 5. 6.
Naphthalene formers in charge Hydrogen-oil feed ratio Hydrogen feed stream purity Gas-oil feed ratio Reactor inlet temperature Reactor residence time
40-80 wt. yo 7-1 5 moles/mole 35-80 mole yo H2 13-25 moles/mole 1100-1250' F. 5-30 sec.
T h e gas stream consisted of a mixture of hydrogen and methane, whereas the oil stream consisted of substituted aromatics which had been extracted in the feed preparation section of the process. Optimization of both reactor design and reactor operating conditions is greatly aided by prediction of the effects of these variables on concentration and temperature distributions through the reactor. A second related objective consisted of calculating the sensitivity of reactor effluent temperature and concentration to changes in reactor inlet conditions. Knowledge of this sensitivity is used in control system design and in specifying conditions which must be met to ensure safe plant operation. Development of a mathematical model to satisfy these objectives was complicated by two factors. First, a large number of compounds are present in the reactor feed stream and a variety of simultaneous and consecutive reactions takes place in the reactor. Simplifications involving grouping together of similar compounds are essential to obtain a useful model. Second, the kinetic rate constants and heats of reaction were not known for many of the reactions which take place in the reactor. I t was therefore necessary to develop a model in which these parameters were adjustable. Experimental pilot plant information was used to determine how complex the mathematical model need be-Le., how many chemical reactions must be included-and to estimate the reaction rate constants and heats of reaction. The pilot plant reactor consisted of a tube 2 inches in diameter and 36 inches long. Adiabatic operation was obtained by holding radial temperature gradients through the tube wall close to zero by means of a series of electrical windings. Composition of both reactor feed and product streams was determined by distillation to obtain narrow boiling fractions followed by vapor phase chromatography and low ionizing voltage mass spectroscopy. Temperature distributions through the reactor were obtained using a thermocouple which could be moved along the longitudinal axis of the reactor. Design throughput of the pilot scale reactor was 500 ml. per hour of liquid charge a t 20 to 1 gas-oil mole ratio. Temperature and composition of feed to the reactor were varied, as were gas-oil ratio, hydrogen purity, and reactor residence time. The requirements which any mathematical model of a process must satisfy are usually the result of compromise between rigor and model complexity or availability of basic 178
l&EC PROCESS DESIGN A N D DEVELOPMENT
process information. Requirements of the naphthalene reactor model are summarized as : Prediction of pilot plant temperature profiles to within =tl standard deviation of the measured profile. In most cases this is approximately 1 5 ' F. Prediction of concentration of naphthalene and methylnaphthalenes in the pilot plant reactor effluent to within f15% of their measured values. Information upon which the model was based came both from literature sources and pilot plant operation. Much work has been reported on the thermal hydrodemethylation of methylnaphthalenes and methylbenzenes (2, 3, 5, 8 ) . However, kinetic information for other hydrodealkylation and cracking reactions which take place in the reactor is not available in the literature. In early attempts to develop a satisfactory model it was assumed that the kinetics of all reactions which do not lead to production of naphthalene could be ignored, and that the thermal effects of these reactions could be approximated by assuming their rate of heat production to be proportional to that of the naphthalene-producing reactions. Results of analog computer simulation based on this simple model showed that the model was incapable of adequately describing pilot plant operation. Inclusion of the kinetics of nonnaphthaleneproducing reactions was required in the development of a more sophisticated model to satisfy the requirements. Model Development
The mathematical model of the hydrodealkylation reactor was based on the following assumptions: The reactor was assumed to be adiabatic. Plug flow with no axial dispersion of heat or mass was assumed throughout the reactor. Physical properties of the reacting mixture, such as density and heat capacity, were assumed constant throughout the reactor. I t was assumed that first-order kinetic expressions could be used to describe the rates of dealkylation reactions which take place in the reactor. This was possible because of the large excess of hydrogen present in the reactor. The rate of hydrodemethylation of monomethylnaphthalenes has been described ( 3 ) by the equation:
Although hydrogen is consumed during the course of reaction, the change in hydrogen concentration is sufficiently small to permit simplification of the rate expressions to the following form : ri =
k,Zi
(2)
Under no conditions was the change in hydrogen concentration sufficiently great to cause errors of more than 1OYo in any of the first-order rate constants. Silsby and Sawyer (8)have calculated the reaction rate constant in Equation 2 from toluene demethylation data obtained a t various concentrations. The rate constant was found to have essentially the same value for all hydrogen-toluene ratios greater than 4. I t was assumed that the set of reactions which take place within the reactor can be represented by the reaction sequence shown in Figure 3. These reactions are divided into three groups. The first group consists of compounds which when dealkylated yield naphthalene. In Figure 3 some of the reactants in this group are lumped together and represented by a single letter. For example, Crepresents the mole fraction of the sum of 1,4-, 1,5-, and 1,8-dimethylnaphthalene plus some other naphthalene-producing compounds. It was assumed that these compounds dealkylate a t the same rate to produce 1-methylnaphthalene and similar compounds represented by A , which upon further dealkylation yield naphthalene, shown by N . Similarly, 2,3-, 2,6-, and 2,7-dimethylnaphthalene plus
dZ' - = kgZ
de
Figure 3.
Kinetic model
some similar compounds are represented by D. These compounds were assumed to dealkylate a t the same rate to produce 2-methylnaphthalene and similar compounds, indicated by B. 1,2-, 1,3-, 1,6-, and 1,:-dimethylnaphthalene and similar compounds are represenied by E, and ethylnaphthalenes with similar compounds are represented by F. The second group of reactions involves compounds which dealkylate to yield compounds other than naphthalene. These include monocyclic aromatics such as alkyl indans, indenes, and benzenes. As in the case of the first group of reactions it was assumed that all compounds represented by the same letter react a t the same' rate. T h e cracking reactions, Z + L, J -c L , and X + X',constitute the third group of reactions shown in Figure 3. T h e first reaction represents cracking of alkyl hydroaromatics to form alkyl benzenes. The second reaction shows cracking of alkyl indenes to produce alkyl benzenes. In the last reaction small amounts of saiiirated hydrocarbons crack to produce methane and ethane. I t was assumed that the rates of all cracking reactions can be represented by first-order rate expressions. I t is known that the second and third groups of reactions produce >70yo of the heat evolved in the reactor, although the reacting compounds in these groups comprise
