where B, P, I and D represent benzene, propylene, cumene and dialkylbenzene, respectively, and the first two are alkylation reaction, the latter transalkylation reaction.
The kinetic data of alkylation of benzene with propylene was determined in a continuous flow system (Fig. 15). The chemical materials, benzene and propylene, are all supplied by Yansan Petrochemical Corporation with over 99% purity.
The experimental system comprised three parts, a feed blending station for preparing the reaction mixture with different composition, an assembly of fixed-bed laboratory reactor-electric oven with a multi-channel temperature controller, and an off-line gas chromatogram (GC) and gas chromatogram-mass spectrometer (GC-MS).
The feed blending station consisted of two metering pumps for driving benzene and propylene, respectively, a nitrogen tube for sweeping and a mixer. The metering pumps were calibrated in advance. Feed composition was calculated based on the reading of the metering pumps and checked by gas chromatograph analysis.
The main parts of micro reactor-electric oven assembly are a fixed-bed micro-reactor of 8 mm i.d, 300 mm long and an electric oven of 800 W power. The temperature of reaction section was controlled with a temperature programmable controller and measured with a micro-thermal couple inserted in its center through a small jacket tube. The reaction section of the reactor was charged with 0.2 g catalyst of 80-100 mesh and some amount of quartz chips was loaded in both sides of the reaction section. Suitable particle size of the catalyst was determined by a preliminary experiment so as to eliminate the influence of internal diffusion. A back-pressure regulator was equipped downstream in the micro-reactor. The composition analysis system for feed and product consisted of an off-line gas chromatograph, SP 3420, equipped with a FID detector and an OV-lOl capillary column (60 m long and 0.32 mm o.d.). The experiments were made under the following condition: the temperature 160 -220 °C, the pressure 3.0 MPa, mole ratio of benzene to propylene 4 - 9, and weight hourly space velocity (WHSV) above 600 h"1.
benzene propylene 1
Fig. 15. The schematic diagram of fixed-bed experimental setup. 1,2 - benzene and propylene metering pump; 3 - adjusting valve; 4 - mixer; 5 - catalyst bed; 6 - temperature controller; 7 -thermocouples; 8 - pressure adjusting valve; 9 - metering vessel.
The rate equations for alkylation of benzene with propylene were obtained after correlating the experiment data by the Marquardt method and expressed as follows:
where R is gas constant (8.314 kJ kmol1 K"1), T is temperature (K), and C is mole concentration (kmol m" ).
2,2.2. The improved SCD process
The original SCD process, in which alkylation reaction takes place in SCD column, but transalkylation reaction in another fixed-bed reactor, is shown in Fig. 16, In this work an improved SCD process, in which alkylation and transalkylation reactions take place simultaneously in a SCD column, is proposed and illustrated in Fig. 17.
In the improved SCD process, distillation and reaction always take place at the same time in the SCD column. But the upper section of this column is used Tor alkylation reaction and the lower section for transalkylation reaction. Transalkylation of DIPB having a high boiling point is more concentratcd in the lower section than in the upper section of the column in terms of vapor-liquid equilibrium (VLB), 'lhe solid-liquid mixture leaving from the bottom is treated by a gas-liquid separator (i.e. a static sedimentary tank). The separated catalyst is recycled, and a pari of them is regenerated from time to time. The separated liquid is the mixture of benzene, cumene and DIPB, which enter a benzene column (column2 ) and a cumene column (column 3 ) in series to remove unconverted benzene and side product DIPB. The ultimate product, cumene, is obtained from the top of column 3, and DIPB at the bottom of this column then flows into the lower section of column 1 for carrying out transalkylation reaction. However, in the original SCD process, only alkylation reaction is carried out in the SCD column. DIPB leaving from column 3 doesn't flow into column 1, but enters a fixed-bed reactor where only transalkylation reaction takes place. As a result, by using the improved SCD process, the equipment investment can be decreased and the process is simplified.
It is evident that the SCD column is the key to the SCD process and should be paid more attention in the simulation of the SCD process. It is interesting to explore whether it is possible to integrate alkylation with transalkylation into a single SCD column by means of the simulation. In this study the equilibrium stage (EQ) model was established to simulate the SCD column. The equations that model equilibrium stages are known as the MESHR equations, into which the reaction terms including reaction rate equations and reaction heat equations are incorporated. The UNIFAC model is used for description of liquid phase nonideality, while the Virial equation of state is used for the vapor phase. The extended Antoine equation is used for calculation of the vapor pressure. The vapor enthalpy of every component is calculated by the modified Peng-Robinson (MPR) equation, and the sum of every vapor enthalpy multiplied by every mole fraction is the total vapor enthalpy. The liquid phase enthalpy is deduced from vapor phase enthalpy and evaporation heat. Thermodynamic data for this reaction system are taken from the sources [81, 84].
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