Tubular Reactors in Isolation

It is useful to initially examine the tubular reactor as an isolated unit so that some insight can be gained about the effects of various design and operating parameters on its inherent behavior. The equations describing the steady-state operation of a tubular reactor are presented and illustrated for a specific numerical example. Both adiabatic and nonadiabatic tubular reactors are considered. 

The flow patterns, composition profiles, and temperature profiles in a real tubular reactor can often be quite complex. Temperature and composition gradients can exist in both the axial and radial dimensions. Flow can be laminar or turbulent. Axial diffusion and conduction can occur. All of these potential complexities are eliminated when the plug flow assumption is made. A plug flow tubular reactor (PFR) assumes that the process fluid moves with a uniform velocity profile over the entire cross-sectional area of the reactor and no radial gradients exist. This assumption is fairly reasonable for adiabatic reactors. But for nonadiabatic reactors, radial temperature gradients are inherent features. If tube diameters are kept small, the plug flow assumption in more correct. Nevertheless the PFR can be used for many systems, and this idealized tubular reactor will be assumed in the examples considered in this book. We also assume that there is no axial conduction or diffusion. 

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Now that some insight has been gained in looking at adiabatic and cooled tubular reactors in isolation, we are ready to explore an entire process that contains reactors, compressors, separators, heat exchangers, makeup feedstreams, a recycle stream, and a product stream. This material is taken from the paper by Jaisathapom and the author.1... 

Tubular reactors in series in heating box with external isolation valves... 

This chapter presents a comparison of the steady-state economics of four alternative tubular reactor systems. The entire process will be considered, not just the reactor in isolation, because the optimum economic steady-state design can be determined only for the entire plant. The type of recycle, the phase of the reaction, and the heat transfer configuration all affect the optimum design. 

The last example is a gas-phase process with a tubular reactor, gas recycle compressor, feed-effluent heat exchanger, condenser and separator. The steady-state design of this process leads to an uncontrollable system if the reactions are highly temperature sensitive. We demonstrate that changing the design produces a much more easily controlled process. We consider a complete plant, not just the reactor in isolation. 

The dynamics and control of a number of tubular reactor systems have been studied in this chapter. Both adiabatic and cooled tubular reactors have been explored in both isolation and a plantwide environment. Ideal systems have been studied using Matlab programs. Real chemical systems have been studied using Aspen Dynamics. 

Dimethylketene (15.2 g) enclosed in a tubular reactor was placed in an acetone Dewar flask at — 30°C and treated with 38 ml of carbon tetrachloride and 2.5 ml of 0.86M aluminium tribromide solution. Thereafter the mixture was stirred for 5 hours at 30°C and at ambient temperamre for 19 hours. The reaction was quenched by the addition of 20 ml of methanol, and the polymer was isolated after precipitation in 200 ml of methanol containing 4 ml of hydrochloric acid. 

Cumene Cracking Experiments. The cumene cracking studies were done in a manner similar to that reported by Richardson (15), The faujasites were compressed into pills which were cracked, and a 28-35 mesh fraction was isolated. A 300-mg portion was first calcined at an elevated temperature. This generally involved a 16-hour treatment in air at 538°C. Other calcination conditions are as noted. The catalyst was then transferred under nitrogen to a nominal 1/4-inch stainless steel, 20-gage tubular reactor which was placed in a constant-temperature fluidized sandbath. After equilibration with helium carrier gas, the cumene was introduced to the reactor by permitting the carrier gas to... 

The nonlinearity of chemical processes received considerable attention in the chemical engineering literature. A large number of articles deal with stand-alone chemical reactors, as for example continuously stirred tank reactor (CSTR), tubular reactor with axial dispersion, and packed-bed reactor. The steady state and dynamic behaviour of these systems includes state multiplicity, isolated solutions, instability, sustained oscillations, and exotic phenomena as strange attractors and chaos. In all cases, the main source of nonlinearity is the positive feedback due to the recycle of heat, coupled with the dependence of the reaction rate versus temperature. 

In a packed catalytic tubular reactor, reactants are converted to products via chemical reaction on the internal surface of the catalyst. This problem was described in detail for a single isolated pellet. At this stage of the design, one seeks an expression for the volume-averaged rate of reactant consumption within... 

Separation of variables provides the analytical solution to this first-order ODE given by (30-60). When the external resistance to mass transfer is significant, the following result allows one to predict reactant conversion in the exit stream as a function of important design parameters based on isolated pellets as well as the entire packed catalytic tubular reactor ... 

Azides are a very useful intermediate in organic synthesis however, the hazards associated with their preparation have limited their production and use. Kopach et al. demonstrated the use of a continuous flow reactor suitable for the synthesis of l-(azidomethyl)-3,5-bistrifluoromethyl) benzene 1 from the benzyl chloride derivative 2 (Scheme 6.1), as a means of circumventing the hazards associated with the buildup of hydrazoic acid in the head-space of batch reaction vessels. Using a stainless steel tubular reactor, the authors investigated the reaction at a series of temperatures. The authors identified 90°C as the optimal temperature at a residence time of 20 minutes, affording azide 1 in 97% conversion. Operating the reactor continuously for 2.8 hours enabled them to produce 25 g of product in 94% isolated yield. 

Many of these difficulties can be overcome by choosing an appropriate configuration of the photoreactor system. One such a system is the mechanically agitated cylindrical reactor with parabolic reflector. In this type of reactor, the reaction system is isolated from the radiation source (which could also simplify the solution of the well-known problem of wall deposits, generally more severe at the radiation entrance wall). The reactor system uses a cylindrical reactor irradiated from the bottom by a tubular source located at the focal axis of a cylindrical reflector of parabolic cross-section (Fig. 40). Since the cylindrical reactor may be a perfectly stirred tank reactor, this device is especially required. This type of reactor is applicable for both laboratory-and commercial-scale work and can be used in batch, semibatch, or continuous operations. Problems of corrosion and sealing can be easily handled in this system. 

Except for the reactor zones, autoclave and tubular processes are very similar (3, 4). Peripherals in both cases are designed pre-reactor to ramp pressures and temperatures to very high levels and post-reactor to reduce temperatures and lower pressures to near ambient conditions to enable product isolation. Simplified process flow diagrams for the autoclave and tubular processes are shown in Figures 7.1 and 7.2, respectively. 

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