Tubular Plug Flow Reactor

Species C is the desired product. We want to design an isothermal, isobaric tubular reactor (plug-flow reactor) to be operated at 489°C and 5 atm A stieam of reactant A at a rate of 1 mol/s is available in the plant. 

The reactor in which chemical reactions lake place is fhe mosl imporlanl piece of equipmenl in each chemical planl. A variety of reactors are used in induslry, bul all of Ihem can be assigned to cerlain basic types or a combination of fhese ideal reactors [53] (1) bafch slirred-lank reactor, (2) continuous slirred-lank reactor, and (3) lubular reactor. The ideal slirred-lank bafch reactor is characterized by complete mixing, while in the ideal tubular reactor, plug flow is assumed. In contrast to the stirred-tank batch reactor with well-defined residence time, the continuous stirred-tank reactor has a very broad residence-time distribution. In... 

In this system of coupled differential equations, the mass balance corresponds to the reaction rate and the heat balance is a simplified version showing only the heat production by the reaction and the heat removal by the cooling system, both terms resulting in heat accumulation. This system presents the property of parametric sensitivity, meaning that a small change in one of the parameters may lead to dramatic changes in the solution of the system of equations, that is, in the behavior of the reactor. This is an old [10-12], but always real, problem [13, 14]. This behavior may be observed for batch reactors and for tubular reactors (plug flow reactors) and also for bed reactors [15,16]. Calorimetric methods make it possible to... 

For a plug-flow tubular reactor, the flow velocity v, through the reactor can be related to the distance travelled along the reactor or tube Z, and to the time of passage t, where... 

In Sect. 3.2, the development of the design equation for the tubular reactor with plug flow was based on the assumption that velocity and concentration gradients do not exist in the direction perpendiculeir to fluid flow. In industrial tubular reactors, turbulent flow is usually desirable since it is accompanied by effective heat and mass transfer and when turbulent flow takes place, the deviation from true plug flow is not great. However, especially in dealing with liquids of high viscosity, it may not be possible to achieve turbulent flow with a reasonable pressure drop and laminar flow must then be tolerated. 

When processing in a set-up with a short, curved flow element (0.3 m long bendt Teflon tube of 0.3 mm inner diameter) between the funnel and straight tubular reactor plugging occurred after only 30 s (see Table 1.10) [53], Hence the insertion of curved flow passages is detrimental, even for only short paths. 

In the next two chapters of this book we turn to the chemical reactor that is probably the most challenging the tubular or plug flow reactor. The inherent distributed nature of the unit (variables change with axial and radial position) gives rise to complex behavior, which is often counterintuitive and difficult to explain. The increase in the number of independent variables makes the development and solution of mathematical models more complex compared to the perfectly mixed CSTR and batch reactor. 

In view of the high length-to-diameter ratio of most tubular reactors the flow through them can in most cases be described adequately as a so-called plug flow. It is assumed that ... 

Equations (59) and (60) are in a form often used for tubular, idealized, plug-flow reactors when the reaction rate is based on the volume of the reactor. When the reaction is heterogeneous, such as one occurring on the surface of a catalyst, it is common practice to base the reaction rate on the mass of the catalyst rather than on the volume of the reactor and substitute ric for r,. The resulting design equation equivalent to Eq. (60) is... 

IDEAL TUBULAR-IiLOW (PLUG-FLOW) REACTORS... 

In Sec. 6-5 a non-steady-state mass balance for a tubular-flow reactor (plug flow except for axial dispersion) was used to evaluate an effective diffusivity. Now we consider the problem of calculating the conversion when a reaction occurs in a dispersion-model reactor operated at steady-state conditions. Again a mass balance is written, this time for steady state and including reaction and axial-dispersion terms. It is considered now that the axial diffusivity is known. 

Reactors may be divided into three simple, idealized model categories batch reactor, tubular or plug flow reactor, and the continuous stirred tank reactor (CSTR). 

Extent of reaction specified Two-phase, chemical equilibrium Multiphase, chemical equilibrium Continuous-stirred tank reactor Plug-flow tubular reactor Pump or hydraulic turbine Compressor or turbine Pressure drop in a pipe Stream multiplier Stream duplicator... 

The performance of a PSR will be bench marked against two reference cases (i) a multi-tubular cooled plug flow reactor (PFR) containing only a catalyst and cooled co- or counter currently, and (ii) a combination of a PSA vessel containing a sorbent to separate A, and a multi-tubular cooled PFR containing a catalyst and receiving the A-enriched feed from the PSA. 

Figure 4.1. A differential volume element (dVr) in a tubular (or plug flow) reactor with F and f being the flow rate and fractional conversion, respectively, of the limiting reactant A.

There are two major types of continuous reactors plug-flow tubular reactor (PFTR) and CSTR. The polymerization kinetics with PFTR is similar to that of batch reactor. In PFTR, the concentrations of reactants and products vary with location but not with time. The rate equations derived for batch processes can be easily transformed into their corresponding equations for PFTR using... 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

Continuous-Flow Stirred-Tank Reactor. In a continuous-flow stirred-tank reactor (CSTR), reactants and products are continuously added and withdrawn. In practice, mechanical or hydrauHc agitation is required to achieve uniform composition and temperature, a choice strongly influenced by process considerations, ie, multiple specialty product requirements and mechanical seal pressure limitations. The CSTR is the idealized opposite of the weU-stirred batch and tubular plug-flow reactors. Analysis of selected combinations of these reactor types can be useful in quantitatively evaluating more complex gas-, Hquid-, and soHd-flow behaviors. 

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases, and consequently closer to the ideal (Fig. 2). Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer... 

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