Tubular and stirred tank reactors

They are largely unproved and may still fail in fouling situations such as crosslinking or living polymerizations. 

Laminar-flow tubular reactors are occasionally used for bulk, continuous polymerizations. A monomer or monomer mixture is introduced at one end of the tube and, if all goes well, a high molecular weight polymer emerges at the other end. Practical problems arise from three types of instability  

Velocity profile elongation. Low fluid velocities near the tube wall give rise to high extents of polymerization, high viscosities, and to yet lower velocities. The velocity profile elongates, possibly to the point of hydrodynamic instability. 

Thermal runaway. Temperature control in a tubular polymerizer depends on convective diffusion of heat. This becomes difficult in a large-diameter tube, and temperatures may rise to a point where a thermal runaway becomes inevitable. 

Tube-to-tube interactions. The problems of velocity profile elongation and thermal runaway can be eliminated by using a multi-tubular reactor with many small-diameter tubes in parallel. Unfortunately, this may give rise to a new form of instability. Imagine a 1000-tube reactor with 999 of the tubes plugged with solid polymer  

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Homogeneous reactions are those in which the reactants, products, and any catalysts used form one continuous phase (gaseous or liquid). Homogeneous gas phase reactors are almost always operated continuously, whereas liquid phase reactors may be batch or continuous. Tubular (pipeline) reactors arc normally used for homogeneous gas phase reactions (e.g., in the thermal cracking of petroleum of dichloroethane lo vinyl chloride). Both tubular and stirred tank reactors are used for homogeneous liquid phase reactions. 

COMPARISON OF BATCH, TUBULAR AND STIRRED-TANK REACTORS FOR A SINGLE REACTION. REACTOR OUTPUT... 

It may be mentioned that batch, semibatch, tubular, and stirred-tank reactors serve as mere idealizations of actual reactors. Consider, for example. 

We shall now proceed to compare the three basic types of reactor—batch, tubular and stirred tank—in terms of their performance in carrying out a single first order irreversible reaction ... 

The book is divided into five sections. Each section corresponds to a particular reactor classification. The classifications covered here are trickle-bed reactors, slurry system and stirred tank reactors, novel reactors, packed-bed and tubular reactors, and catalyst deactivation. 

Photochemical tubular flow and stirred tank reactor O3-UV HjOj-UV Reaction simulation, reactor comparison Shimoda et al. (1997)... 

The shape of the reciprocal reaction rate curve in Fig. 8.3 suggests that a combination of tubular and stirred tank type reactors might have some advantages over either one of them used by itself. If we consider the feed extent to be zero and inlet temperature Py, then for the stirred tank... 

The maximum effect of the RTD on conversion is evident from the comparison of tubular-jflow and stirred-tank reactors. Figure 4-14 shows such a comparison for first-order kinetics, and it is apparent that the differences are sizable at high conversion levels. For example, when k d — 4.0 the conversion in the stirred-tank reactor is 80%, while in the tubular-flow unit... 

Cascades of tubular or stirred tank reactors are fairly simple to design as it is usually possible to treat the vessels independently. There do exist complex reactors which were developed empirically and are difficult to analyze from first principles. The transport equations still apply, but the hydrodynamics are too complex to allow modeling from first principles. Residence time measurements are a stimulus-response technique for appraising the hydrodynamics. They provide substantial insight in the performance of complex reactors, particularly when the performance is less than expected. See Nauman [2] or Nauman and Buffham [7] for a description of this technique. 

Reaction conditions depend on the composition of the bauxite ore, and particularly on whether it contains primarily gibbsite, Al(OH)2, or boehmite [1318-23-6] AlOOH. The dissolution process is conducted in large, stirred vessels or alternatively in a tubular reactor. The process originated as a batch process, but has been converted to a continuous one, using a series of stirred tank reactors or a tubular reactor. 

Specific reactor characteristics depend on the particular use of the reactor as a laboratory, pilot plant, or industrial unit. AH reactors have in common selected characteristics of four basic reactor types the weH-stirred batch reactor, the semibatch reactor, the continuous-flow stirred-tank reactor, and the tubular reactor (Fig. 1). A reactor may be represented by or modeled after one or a combination of these. SuitabHity of a model depends on the extent to which the impacts of the reactions, and thermal and transport processes, are predicted for conditions outside of the database used in developing the model (1-4). 

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. 

Continuous-flow stirred-tank reactors ia series are simpler and easier to design for isothermal operation than are tubular reactors. Reactions with narrow operating temperature ranges or those requiring close control of reactant concentrations for optimum selectivity benefit from series arrangements. 

In previous studies, the main tool for process improvement was the tubular reactor. This small version of an industrial reactor tube had to be operated at less severe conditions than the industrial-size reactor. Even then, isothermal conditions could never be achieved and kinetic interpretation was ambiguous. Obviously, better tools and techniques were needed for every part of the project. In particular, a better experimental reactor had to be developed that could produce more precise results at well defined conditions. By that time many home-built recycle reactors (RRs), spinning basket reactors and other laboratory continuous stirred tank reactors (CSTRs) were in use and the subject of publications. Most of these served the original author and his reaction well but few could generate the mass velocities used in actual production units. 

Hannon, J., Mixing and Chemical Reaction in Tubular Reactors and Stirred Tanks, PhD. Thesis, Cranfield Institute of Technology, U.K., 1992. 

There are a variety of ways of accomplishing a particular unit operation. Alternative types of process equipment have different inherently safer characteristics such as inventory, operating conditions, operating techniques, mechanical complexity, and forgiveness (i.e., the process/unit operation is inclined to move itself toward a safe region, rather than unsafe). For example, to complete a reaction step, the designer could select a continuous stirred tank reactor (CSTR), a small tubular reactor, or a distillation tower to process the reaction. 

The advantages of continuous tubular reactors are well known. They include the elimination of batch to batch variations, a large heat transfer area and minimal handling of chemical products. Despite these advantages there are no reported commercial instances of emulsion polymerizations done in a tubular reactor instead the continuous emulsion process has been realized in series-connected stirred tank reactors (1, . ... 

This chapter develops the techniques needed to analyze multiple and complex reactions in stirred tank reactors. Physical properties may be variable. Also treated is the common industrial practice of using reactor combinations, such as a stirred tank in series with a tubular reactor, to accomplish the overall reaction. 

The fractional tubularity model has been used to fit residence time data in flui-dized-bed reactors. It is also appropriate for modeling real stirred tank reactors that have small amounts of dead time, as would perhaps be caused by the inlet and outlet piping. It is not well suited to modeling systems that are nearly in piston flow since such systems rarely have sharp first appearance times. 

This section is concerned with batch, semi-batch, continuous stirred tanks and continuous stirred-tank-reactor cascades, as represented in Fig. 3.1 Tubular chemical reactor systems are discussed in Chapter 4. 

Because of the dilution that results from the mixing of entering fluid elements with the reactor contents, the average reaction rate in a stirred tank reactor will usually be less than it would be in a tubular reactor of equal volume and temperature supplied with an identical feed stream. Consequently, in order to achieve the same production capacity and conversion level, a continuous flow stirred tank reactor or even a battery of several stirred tank reactors must be much larger than a tubular reactor. In many cases, however, the greater volume requirement is a relatively unimportant economic factor, particularly when one operates at ambient pres-... 

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