Reactor design tubular-flow

The pyrolysis experiments were conducted in an electrically heated, once-through tubular flow reactor, designed to simulate the time-temperature history experienced in commercial steam-cracking operations. Reactor effluent compositions were ascertained by gas chromatograph and mass spectrometer analyses. Material and hydrogen balances could always be effected, with typical closures of 98 2 wt %. 

Solution. Write the tubular flow reactor design equation and substitute the rate expression. 

An important effect in the design of a tubular flow reactor is the development of a radial temperature gradient in a highly exothermic reaction with wall cooling. The temperatures near the tube axis are... 

Schultz and Linden Ind. Eng. Chem. Process Design and Development, 1 (111), 1962] have studied the hydrogenolysis of low molecular weight paraffins in a tubular flow reactor. The kinetics of the propane reaction may be assumed to be first-order in propane in the regime of interest. From the data below determine the reaction rate constants at the indicated temperatures and the activation energy of the reaction. 

Plug-Flow Reactor Design of a Fall-Scale Tubular Reactor... 

Tubular Flow Reactor (PFR). After multiplying both sides of the tubular reactor design equation (1-10) by -1, we express the mole balance equation for species A in the reaction given by Equation (2-2) as... 

The design equation if the reaction is conducted in a tubular flow reactor is... 

The design equations for ideal tubular-flow reactors involve no new concepts but simply substitute a rate of reaction for a heat-transfer rate or mass-transfer-rate function. The increased complexity of reactor design in comparison with the design of equipment for the purely physical processes lies in the difficulty in evaluating the rate of reaction. This rate is dependent on more, and less clearly defined, variables than a heat- or mass-transfer coefficient. Accordingly, it has been more difficult to develop correlations of experimental rates, as well as theoretical means of predicting them. 

Reactors (both flow and batch) may also be insulated from the surroundings so that their operation approaches adiabatic conditions. If the heat of reaction is significant, there will be a change in temperature with time (batch reactor) or position (flow reactor). In the flow reactor this temperature variation will be limited to the direction of flow i.e., there will be no radial variation in a tubular-flow reactor. We shall see in Chap. 13 that the design procedures are considerably simpler for adiabatic operation. 

Interpretation of data to obtain a rate equation requires calculations which are the reverse of those for design. The two procedures differ because in the laboratory it may be feasible to operate at nearly constant temperature, and possibly nearly constant composition in the commercial unit it may be possible to approach isothermal conditions, but constant composition in a tubular-flow reactor is impossible. These differences simplify the analysis of laboratory results, as illustrated in Example 4-3. 

Comparison of Eqs. (4-2) and (4-5) shows that the form of the design equations for ideal batch and tubular-flow reactors are identical if the realtime variable in the batch reactor is considered as the residence time in the flow case. The important point is that the integral c/C/r is the same in both reactors. If this integral is evaluated for a given rate equation for an ideal batch reactor, the result is applicable for an ideal tubular-flow reactor this... 

A small pilot plant for the photochlorination of hydrocarbons consists of an ideal tubular-flow reactor which is irradiated, and a recycle system, as shown in the sketch. The HCl produced is separated at the top of the reactor, and the liquid stream is recycled. The CI2 is dissolved in the hydrocarbon (designated as RH3) before it enters the reactor. It is desired to predict what effect the type of reactor operation will have on the ratio [RH2Cl]/[RHCl2] in the product stream. Determine this ratio, as a function of total conversion of RH3, for two... 

Equation 7.5.16 is the dimensionless, differential energy balance equation for cyhndrical tubular flow reactors, relating the temperature, 0, to the extents of the independent reactions, Z s, and P/Pq as functions of space time t. To design a plug-flow reactor, we have to solve design equations (Eq. 7.1.1), the energy balance equation (Eq. 7.5.16), and the momentum balance (Eq. 7.5.12), simultaneously subject to specified initial conditions. 

Example 4-3 Producing 300 Million Pounds per lenr of Ethylene in a Plug-Flow Reactor Design of a Full-Scale Tubular Reactor... 

As in the situation for tank-type reactors, we need first to define the characteristic time quantities associated with the reactor design. The characteristic diffusion time, tj), is given in equation (8-207), and the extent-of-reaction time, is given in equation (8-208). The third time here is tp, the length of time an element of fluid remains in the reactor. This is reminiscent of the exit-age distribution function developed for homogeneous tubular-flow reactors, but the development of the theory for multiphase reactors has been different. " ... 

Tubular flow reactors are characterized by a continuous and decreasing concen-tiadon of reactants in the direction of flow. This is in contrast to the discontinuous characteristic of the CSTR reactor. Most of these units consist of one or several pipes or tubes in parallel. Either horizontal or vertical orientation is common. The reactants are charged continuously at one end, and the products are removed continuously at the other end. The unit almost always operates in a steady-state mode. This greatly simplifies design and predictive calculations. It is a unit that is amendable to automatic control and to experimental work. When heat transfer is required, a jacketed tube or a construction similar to that of a shell-and-tube heat exchanger is employed. In the latter case the reactants may be on either the tube or shell side. 

Solution. The differential design equation for a tubular flow reactor based on X = is as follows ... 

The latter is normally the preferred method employed in industry since it is the mass of catalyst present in the reactor that significantly impacts the reactor design. Since the rate expression is often more complex for a catalytic reaction than for a non-catalytic (homogeneous) reactor, the design equation may be difficult to solve analytically. Numerical solution of the reactor design equation is usually required when designing tubular flow reactors for catalytic reactions. 

A fixed bed (packed-bed) reactor may be physically viewed as a tubular flow reactor which is packed with solid catalyst particles. This type of heterogeneous reaction system is most frequently used to catalyze gaseous reactions. The design equation... 

ILLUSTRATIVE EXAMPLE 20,8 Refer to Illustrative Example 20.5. A similar situation exists with a tubular flow reactor. Rather than purchase a new reactor, what options are available to bring the reactor into compliance with the specified design conversion ... 

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