Reactor continuous, design

Friis and Hamielec (48) offered some comments on the continuous reactor design problem suggesting that the dispersed particles have the same residence time distribution as the dispersing fluid and the system can be modeled as a segregated CSTR reactor. 

In batch mode, conversions of up to 92.5% were achieved under the above mentioned conditions (Table 6.5). More complete conversions for a continuous reactor design would require intermittent removal of excess methanol and water, possible by flashing the wet methanol. To simulate a continuous process with removal of water a batch experiment was subjected to three repeat cycles of flashing water and methanol, resulting in an acid value 0.5 mg KOH/g for the crude ester product. 

For an excellent discussion of continuous reactor designs for mass styrene polymerization see Meister BJ, Malanga MT (Moore ER, Editor) (1989) Ency Polym Sci Eng 16 48-52... 

It is perhaps not generally realised that a switch from batch to continuous reactor design has intrinsic beneficial PI and safety implications when exothermic reactions and their associated runaway risks are involved. For batch operation, the time during which the reaction exotherm is generated is only a fraction of the batch cycle time. In order to control the reactions, it is imperative that provision is made to cope with the maximum likely heat evolution load so as to inhibit runaway. On the other hand, the heat exchanger provision for a continuous process operating at the same production rate needs to be considerably less than that for the batch equivalent, because the heat load is uniformly time distributed rather than being concentrated in a fraction of the batch residence time. Hence, continuous versions of batch processes have both safety and intensification benefits. 

Hydrothermal Synthesis Systems. Of the unit operations depicted in Figure 1, the pressurized sections from reactor inlet to pressure letdown ate key to hydrothermal process design. In consideration of scale-up of a hydrothermal process for high performance materials, several criteria must be considered. First, the mode of operation, which can be either continuous, semicontinuous, or batch, must be determined. Factors to consider ate the operating conditions, the manufacturing demand, the composition of the product mix (single or multiple products), the amount of waste that can be tolerated, and the materials of constmction requirements. Criteria for the selection of hydrothermal reactor design maybe summarized as... 

Fig. 4. Continuous tubular reactor design. Courtesy of BatteUe.

Nuclear power has achieved an excellent safety record. Exceptions are the accidents at Three Mile Island in 1979 and at Chernobyl in 1986. In the United States, safety can be attributed in part to the strict regulation provided by the Nuclear Regulatory Commission, which reviews proposed reactor designs, processes appHcations forUcenses to constmct and operate plants, and provides surveillance of all safety-related activities of a utiUty. The utiUties seek continued improvement in capabiUty, use procedures extensively, and analy2e any plant incidents for their root causes. Similar programs intended to ensure reactor safety are in place in other countries. 

Among continuous reactors, the dominant system used to produce parasubstituted alkylphenols is a fixed-bed reactor holding a soHd acid catalyst. Figure 3 shows an example of this type of reactor. The phenol and alkene are premixed and heated or cooled to the desired feed temperature. This mix is fed to the reactor where it contacts the porous soHd, acid-impregnated catalyst. A key design consideration for this type of reactor is the removal of the heat of reaction. 

Nuclear Reactors. Nuclear power faciUties account for about 20% of the power generated in the United States. Although no new plants are plaimed in the United States, many other countries, particularly those that would otherwise rely heavily on imported fuel, continue to increase their nuclear plant generation capacity. Many industry observers predict that nuclear power may become more attractive in future years as the price of fossil fuels continues to rise and environmental regulations become more stringent. In addition, advanced passive-safety reactor designs may help allay concerns over potential safety issues. 

Table I provides an overview of general reactor designs used with PS and HIPS processes on the basis of reactor function. The polymer concentrations characterizing the mass polymerizations are approximate there could be some overlapping of agitator types with solids level beyond that shown in the tcd>le. Polymer concentration limits on HIPS will be lower because of increased viscosity. There are also additional applications. Tubular reactors, for example, in effect, often exist as the transfer lines between reactors and in external circulating loops associated with continuous reactors.

Because of the difficulties of presses with HIPS cited earlier, it is usual to transfer the syrup to a suspension reactor containing water and a suspending agent for the completion of polymerization. Design problems for suspension reactors will be discussed in the next section. Design problems for HIPS prepoly batch-mass reactors are analogous to HIPS continuous reactors as discussed in Section 2.3. 

Perhaps the most challenging reactor design problem is the design of the continuous precipitation polymerization reactor. Although several patents (45,46,47) are concerned with this problem, the topic has been generally neglected in the reactor-... 

Reactor Design. The continuous polymerization reactions in this investigation were performed in a 50 ml pyrex glass reactor. The mixing mechanism utilized two mixing impellers and a Chemco magnet-drive mechanism. 

This chapter treats the effects of temperature on the three types of ideal reactors batch, piston flow, and continuous-flow stirred tank. Three major questions in reactor design are addressed. What is the optimal temperature for a reaction How can this temperature be achieved or at least approximated in practice How can results from the laboratory or pilot plant be scaled up ... 

There is a continuing emphasis on numerical solutions. Numerical solutions are needed for most practical problems in chemical reactor design, but sophisticated numerical techniques are rarely necessary given the speed of modern computers. The goal is to make the techniques understandable and easily accessible and to allow continued focus on the chemistry and physics of the problem. Computational elegance and efficiency are gladly sacrificed for simplicity. 

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