Reactor tube-CSTR

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. 

There are two important types of ideal, continuous-flow reactors the piston flow reactor or PFR, and the continuous-flow stirred tank reactor or CSTR. They behave very diflerently with respect to conversion and selectivity. The piston flow reactor behaves exactly like a batch reactor. It is usually visualized as a long tube as illustrated in Figure 1.3. Suppose a small clump of material enters the reactor at time t = 0 and flows from the inlet to the outlet. We suppose that there is no mixing between this particular clump and other clumps that entered at different times. The clump stays together and ages and reacts as it flows down the tube. After it has been in the piston flow reactor for t seconds, the clump will have the same composition as if it had been in a batch reactor for t seconds. The composition of a batch reactor varies with time. The composition of a small clump flowing through a piston flow reactor varies with time in the same way. It also varies with position down the tube. The relationship between time and position is... 

Figure 14. Conversion transient for MMA polymerization in the tube-CSTR reactor system...

Gonzalez placed a continuous tubular reactor in iront (upstream) of the CSTR. In this case the particle seed was formed in the tube from a recipe that did not contain seed- Gonzalez found the tube-CSTR system to be quite stable so long as the conversion in the tube was adequate to prevent significant particle formation in the CSTR. 

Reactor CSTR Long tube CSTR CSTR Loop Fluid Bed... 

Figure 8.2 Particle-size histograms for polystyrene particles from a tube-CSTR reactor system d%) and (dp) are average diameters from the tube (seed) and CSTR respectively. Xi and Xi are the monomer conversions from the tube and CSTR. [2]...

The material of construction of the tubular reactor can also influence fouling and plugging. Tubes made from polymeric materials, especially fluoropolymers, have been more successful than those made of glass and metals. Fouling and plugging can also be minimized by only carrying out the early part of the reactions in the tube, such as done with tube-CSTR systems [20-22]. 

Daniels and Lenney [32] give a detailed process description of a two-reactor CSTR system used to produce ethylene-vinyl acetate (EVA) copolymer latexes. A seed latex is fed to the first reactor. Korte and Silling [33] employed a tube-CSTR system for producing acrylonitrile-vinylacetate-acetate-styrene copolymers. The PFT generated the seed for the CSTR which was also fed with a stream which by-passed the tube. 

Gonzalez (6,7) used a tubular pre-reactor upstream of a CSTR to generate a particle seed to feed into the CSTR. The motivation for this work was to control the conversion oscillations often observed in CSTR systems. A tube-CSTR series has a number of potential advantages which will be reviewed in this paper. 

If severe heat-transfer requirements are imposed, heating or cooling zones can be incorporated within or external to the CSTR. For example, impellers or centrally mounted draft tubes circulate Hquid upward, then downward through vertical heat-exchanger tubes. In a similar fashion, reactor contents can be recycled through external heat exchangers. 

Cooking extmders have been studied for the Uquefaction of starch, but the high temperature inactivation of the enzymes in the extmder demands doses 5—10 times higher than under conditions in a jet cooker (69). Eor example, continuous nonpressure cooking of wheat for the production of ethanol is carried out at 85°C in two continuous stirred tank reactors (CSTR) connected in series plug-fiow tube reactors may be included if only one CSTR is used (70). 

The first of the relations in Equation (4.9) is valid for any flow system. The second applies specifically to a CSTR since p = pout- It is not true for a piston flow reactor. Recall Example 3.6 where determination of t in a gas-phase tubular reactor required integrating the local density down the length of the tube. 

CSTRs, shell-and-tube reactors, and single-tube reactors, particularly a single adiabatic tube. Realistically, these different reactors may all scale similarly e.g., as but the dollar premultipliers will be different, with CSTRs being more expensive than sheU-and-tube reactors, which are more expensive than adiabatic single tubes. However, in what follows, the same capital cost will be used for all reactor types in order to emphasize inherent kinetic differences. This will bias the results toward CSTRs and toward shell-and-tube reactors over most single-tube designs. 

Why are the CSTRs worth considering at all They are more expensive per unit volume and less efficient as chemical reactors (except for autocatalysis). In fact, CSTRs are useful for some multiphase reactions, but that is not the situation here. Their potential justification in this example is temperature control. BoiUng (autorefrigerated) reactors can be kept precisely at the desired temperature. The shell-and-tube reactors cost less but offer less effective temperature control. Adiabatic reactors have no control at all, except that can be set. 

The above computation is quite fast. Results for the three ideal reactor t5T)es are shown in Table 6.3. The CSTR is clearly out of the running, but the difference between the isothermal and adiabatic PFR is quite small. Any reasonable shell-and-tube design would work. A few large-diameter tubes in parallel would be fine, and the limiting case of one tube would be the best. The results show that a close approach to adiabatic operation would reduce cost. The cost reduction is probably real since the comparison is nearly apples-to-apples. ... 

A modem polystyrene process consists of a CSTR followed by several stirred tube reactors in series. A description of this typical process is given in... 

Two types of reactors were used One was a CSTR type consisting of an Y2O3 (8mol%)-stabilized Z1O2 (YSZ) tube (length 15 cm, diameter 2 cm) closed flat at one end with an appropriately machined water-cooled stainless steel reactor cap attached to the other end, thus allowing for continuous gas feed and... 

The next two steps after the development of a mathematical process model and before its implementation to "real life" applications, are to handle the numerical solution of the model s ode s and to estimate some unknown parameters. The computer program which handles the numerical solution of the present model has been written in a very general way. After inputing concentrations, flowrate data and reaction operating conditions, the user has the options to select from a variety of different modes of reactor operation (batch, semi-batch, single continuous, continuous train, CSTR-tube) or reactor startup conditions (seeded, unseeded, full or half-full of water or emulsion recipe and empty). Then, IMSL subroutine DCEAR handles the numerical integration of the ode s. Parameter estimation of the only two unknown parameters e and Dw has been described and is further discussed in (32). 

The FTS experiments were conducted in a 1 L CSTR equipped with a magnetically driven stirrer with turbine impeller, a gas inlet line, and a vapor outlet line with an stainless steel (SS) fritted filter (7.0 microns) placed external to the reactor. A tube fitted with an SS fritted filter (2.0 micron opening) extends below the liquid level of the reactor for withdrawing reactor wax to maintain a nearly constant liquid level in the reactor. Another SS dip tube (1/8 inch OD) extends to... 

In many reacting flows, the reactants are introduced into the reactor with an integral scale L that is significantly different from the turbulence integral scale Lu. For example, in a CSTR, Lu is determined primarily by the actions of the impeller. However, is fixed by the feed tube diameter and feed flow rate. Thus, near the feed point the scalar energy spectrum will not be in equilibrium with the velocity spectrum. A relaxation period of duration on the order of xu is required before equilibrium is attained. In a reacting flow, because the relaxation period is relatively long, most of the fast chemical reactions can occur before the equilibrium model, (4.93), is applicable. 

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