Reactors, continuous backmix

Gas phase olefin polymerizations are becoming important as manufacturing processes for high density polyethylene (HOPE) and polypropylene (PP). An understanding of the kinetics of these gas-powder polymerization reactions using a highly active TiCi s catalyst is vital to the careful operation of these processes. Well-proven models for both the hexane slurry process and the bulk process have been published. This article describes an extension of these models to gas phase polymerization in semibatch and continuous backmix reactors. 

The kinetic models for the gas phase polymerization of propylene in semibatch and continuous backmix reactors are based on the respective proven models for hexane slurry polymerization ( ). They are also very similar to the models for bulk polymerization. The primary difference between them lies in the substitution of the appropriate gas phase correlations and parameters for those pertaining to the liquid phase. 

Continuous Model "C0NGAS". This model predicts performance of an ideal continuous wellstirred polyreactor. The model system consists of a continuous backmix reactor in which the total powder volume is held constant. There are four inlet streams 1) Makeup of pure propylene, 2) Catalyst feed, 3) Hydrogen feed, and 4) Recycle. The single effluent powder stream is directed through a perfect separator that removes all solids and polymer and then the gases are recycled to the reactor. The makeup propylene is assumed to disperse perfectly in the well-mixed powder. 

Figure 6. Simulation of a continuous backmix reactor (propylene gas phase polymerization—kg° = 0,0249 cm/sec, X = 9.68 hr, 400 psia reactor gas composition—99% CsH6,1% inerts)...

Yields from a Continuous Backmix Reactor, Simulated with C0NGAS... 

The effects of diffusion and catalyst decay cause yields from a continuous backmix reactor to be 25 to 30% lower than from a semibatch reactor at the same residence time. This yield penalty can be reduced by staging backmix reactors in series. 

Mixing of product and feed (backmixing) in laboratory continuous flow reactors can only be avoided at very high length-to-diameter (aspect) ratios. This was observed by Bodenstein and Wohlgast (1908). Besides noticing this, the authors also derived the mathematical expression for reaction rate for the case of complete mixing. 

The yield that can be attained by a semibatch process is generally higher because the semibatch run starts from scratch, with maximum values of both variables Cg (o) = Cg and k] (o) = k . However, the yield from a continuous run in which t equals the batch time is governed by the product of Cg (t) and kj (t), so > and k (t) = k °. Because neither of these conditions is likely to be fulfilled completely, a continuous polymerization in a backmix reactor will probably always fail to attain the Y attainable by a semibatch reactor at the same t. However, several backmix reactors in series will approach the behavior of a plug flow continuous reactor, which is equivalent to a semibatch reactor. 

Continuous solution Free radical (backmixed reactor) Styrene monomer Recycled solvent W or W/O initiator Good Temperature Control Good for copolymers Good clarity and color Uniform product Limited in final conversion Limited in product range Pumping difficulties High capital Low-cost process for high-volume GP... 

The analytical problems associated with differential reactors can be overcome by the use of the recirculation reactor. A simplified form, called a Schwab reactor, is described by Weisz and Prater . Boreskov.and other Russian workers have described a number of other modifications " . The recirculation reactor is equivalent kinetically to the well-stirred continuous reactor or backmix reactor , which is widely used for homogeneous liquid phase reactions. Fig. 28 illustrates the principle of this system. The reactor consists of a loop containing a volume of catalyst V and a circulating pump which can recycle gas at a much higher rate, G, than the constant feed and, withdrawal rates F. 

A type of reactor used commonly in industrial processing is the stirred tank operated continuously (Figure 1-7). It is referred to as the contimious-srirred tank reactor (CSTR) or vat, or backmix reactor, and is used primarily for liquid... 

Continuous homogeneous reactors Longitudinal tubular reactor (no backmixing)... 

Finally, several alternate names have been used for what here is called the perfectly mixed flow reactor. One of the earliest was continuous stirred tank-reactor, or CSTR, which some have modified to continuous flow stirred tank reactor, or CFSTR. Other names are backmix reactor, mixed flow reactor, and ideal stirred tank reactor. All of these terms appear in the literature, and must be recognized. 

Continuous flow stirred-tank reactors are normally just what the name implies tanks into which reactants flow and from which a product stream is removed on a continuous basis. CFSTRs, CSTRs, C-star reactors, and backmix reactors are only a few of the names applied to the idealized stirred-tank flow reactor model. We will use the letters CSTR in this book. The virtues of a stirred-tank reactor lie in its simplicity of construction and the relative ease with which it may be controlled. These reactors are used primarily for carrying out liquid phase reactions in the organic chemicals industry, particularly for systems that are characterized by relatively slow reaction rates. If it is imperative that a gas phase reaction be carried out under efficient mixing conditions similar to those found in a stirred-tank reactor, one may employ a tubular reactor containing a recycle loop. At sufficiently high recycle rates, such systems approximate the behavior of stirred tanks. In this section we are concerned with the development of design equations that are appropriate for use with the idealized stirred-tank reactor model. 

One of the most Important parameters to characterize continuous flow reactors is the degree of backmixing. In the ideal mixed reactor the concentrations and the temperature within the reactor volume are uniform. In consequence, the whole volume occupied by the reaction mixture can be taken as the system volume for the mass balance (see Figure 2.1). 

Fluidized beds give relatively higher performance, but within a narrow operating window. Another type of reactors, the slurry reactor, effectively utilizes the catalyst because of their small particle size in the micrometer range. However, catalyst separation is difficult and a filtration step is required to separate fine particles from the product. Moreover, when applied in the continuous mode, backmixing lowers the conversion and usually the selectivity [2]. Conventional continuous tubular reactors are used as falling film or wall reactor with catalyst coated on the wall however, supply/removal of heat and often broad residence time distribution because of large reactor diameters are two main drawbacks commonly encountered with such reactors. 

Following on the work of Teramoto, Hoffman, Sharma and Luss (28) have performed an analysis of the adiabatic gas-liquid reactor operating in continuous backmixed flow of the liquid phase for this consecutive (1,1) - (1,1) reaction. They used data relative to the system chlorine/n-decane with a selectivity ratio of k /kp = 1. 1 The boundary conditions were formulated in terms of overall material balances on the gas and liquid phases, so that for component A, the boundary condition at the film-bulk junction is given by... 

Continuous-Flow Stirred Tank Reactor (CSTR) Flow reactor designed to achieve a perfect mix of all reactants in its tank sometimes called a backmix reactor. 

It was shown that the residence time distribution in the reactor may have a considerable effect, since this influences the concentration profiles of the reactants in time, TTiere are significant differences between batch (or plug flow) reactors and continuous mixed reactors. When the undesired reaction is of a higher order, the CSTR has the highest selectivity. For reactors with other residence time distributions, the qualitative effect of backmixing was discussed in section 7,2.1 A, Quantitative effects can be computed by numerical methods. 

The continuous stirred-tank reactor is also known as a continuous backmix, bachnaed, or mixed flow reactor. In addition to the catalytic reactors mentioned in the preceding paragraph, the reactors that are used for certain continuous polymerizations, e.g., the polymerization of styrene monomer to polystyrene, closely approximate CSTRs. 

Solution, l ree possibilities are sketched in Fig. 12.4. With a semibatch reactor, the more reactive monomer is replenished as the reaction proceeds to maintain/i (and therefore FJ constant. A method for calculating the appropriate rate of addition has been described. In a continuous stirred tank (backmix) reactor, both fi and Fx are constant with time. In a continuous plug-flow reactor, the variation in Fx can be kept small by limiting the conversion per pass in the reactor. Note that the last two techniques require facilities for separating unreacted monomer from the polymer, and in most cases, recycling it. 

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