Reactors, batch back mixed

In addition to production of simple monofunctional products in hydrocarbon oxidation there are many complex, multifimctional products that are produced by less weU-understood mechanisms. There are also important influences of reactor and reaction types (plug-flow or batch, back-mixed, vapor-phase, Hquid-phase, catalysts, etc). 

Eig. 3. Plot of maximum yield as a % of maximum (zero conversion) efficiency to a primary intermediate x axis is ratio of oxidation rate constants ( 2 / i) for primary intermediate vs feed ( ) plug-flow or batch reactor (B) back-mixed reactor (A) plug-flow advantage, %. 

Chemical reactors basically come in the form of tanks, such as for batch reactors or back-mix flow reactors, large cylinders, such as for fluidized-bed or plug-flow reactors, or multiple tubes inside a cylindrical container, such as for plug-flow reactors when special needs exist for temperature control. High pressure and extremes of temperature as well as corrosive action of the materials involved can introduce complications in the design which must be handled by the design engineer. 

Fig. 33 Effect of monomer concentration Mpi fed to the first tubular seeding reactor with back mixing on the number of polymer particles produced (Sq (NaLS)=6.25 g/dm -water, Iq (KPS)=1.25 g/dm -water, Mpi (St)=variable 50 °C. Experimental data empty circles, particle number observed at t=40 min in a batch reactor filled circles, steady-state particle number observed in the first tubular seeding reactor operated with mean residence time r=40 min)...

Efficiency of Intermediate Formation. The variation of the efficiency of a primary intermediate with conversion of the feed hydrocarbon can be calculated (22). Ratios of the propagation rate constants ( 2 / i) reactor type (batch or plug-flow vs back-mixed) are important parameters. 

Eigure 2 shows that even materials which are rather resistant to oxidation ( 2/ 1 0.1) are consumed to a noticeable degree at high conversions. Also the use of plug-flow or batch reactors can offer a measurable improvement in efficiencies in comparison with back-mixed reactors. Intermediates that cooxidize about as readily as the feed hydrocarbon (eg, ketones with similar stmcture) can be produced in perhaps reasonable efficiencies but, except at very low conversions, are subject to considerable loss through oxidation. They may be suitable coproducts if they are also precursors to more oxidation-resistant desirable materials. Intermediates which oxidize relatively rapidly (/ 2 / i — 3-50 eg, alcohols and aldehydes) are difficult to produce in appreciable amounts, even in batch or plug-flow reactors. Indeed, for = 50, to isolate 90% or more of the intermediate made, the conversion must... 

Back-mixing (axial dispersion is not considered in dynamic similarity), for both the liquid and the gas, increases with increasing column diameter and gas superficial velocity. Consequently, if PFR exist in a small reactor it will be lost, and therefore the reactor performance will be reduced. One remedy is to extend the batch cycle-time. 

For the perfectly mixed continuous reactor, the CSTR, the ratio VT/ Fy only represents the mean residence time, /p,av however, it is still possible to compare the performance of the CSTR with the performance of the BR by letting the mean residence time fp av = t. Interestingly, when the reaction rate shows a positive dependence on reactants concentration, the BR is more effective than the CSTR. This is because the batch reactor experiences all the system compositions between initial and final values, whereas the CSTR operates at the final composition, where the reaction rate is smaller (under the above hypotheses). Finally, one can compare the two continuous reactors under steady-state conditions. The CSTR allows a more stable operation because of back-mixing, which however reduces the chemical performance, whereas the PFR is suitable for large heat transfer but suffers from larger friction losses. 

V = superficial linear velocity of gas (based on cross-sectional area of empty tower), ft/s Vb = volume of batch reactor, ft3 VB = volume of back-mix reactor, ft3... 

To guide the reactor selection process, Walas [7] has classified reactions according to the operating mode (batch or continuous), reactor type (tank, tank battery, tubular), flow type (back mixed, multistage back mixed), and the phases in contact. This reactor classification in Table 7.2 indicates if a particular reactor arrangement is commonly used, rarely used, or not feasible. 

Back-mix reactor design, 721, 728 Baffles in heat exchangers, 595, 610-612 pressure drop over, 599-602, 605-606 Bailee s liahihty, 263 Balance sheet, 140-142 Barometric-leg pumps, 523 Batch operation versus continuous, 35-36 Batch-reactor design, 721-727 Battery-limit additions, definition of, 167 Berl saddles, 688-690 cost of 710... 

Ratio factors for capital investmenf 180-184 Raw materials cost of 197,210 cost accounting for, 146-148 Reactor design, 714-732 back-mix, 728 batch, 726-727... 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

If either the batch or the tubular type of reactor is chosen, the reactor size and product distribution can be calculated by using the batch or longitudinal-flow equations. For a stirred-tank continuous reactor, the backmixing equations can be used. If a packed or baffled tower is used, then the calculations must be made for both the longitudinal and back-mixing cases. Proper extrapolation must then be made from empirical data or previous experience. 

The plug-flow tubular reactor (PFTR) rqnesents an extreme case. In it tho e is no back-mixing, and this model reactor is oftm consid ed as the spatial resolution of the temporal chemical process, inasmuch as the steady-state equations for the PFTR correspond to the time-depoident equations fm the wdl stirred batch reactor. Contonporary studies of PFTR date frmn 1956 ffilous and... 

While the PFTR presents a particularly simple model of a tubular reactor, reservations must be expressed at its ability to represent any real physical system satisfactorily. The solutions of the PFTR imply the existence of temperature and concentration gradients along the reactor but the model denies the possibility of any back-mixing . It is for this reason that the studies on PFTR are of more relevance to transient batch reactors than to the behaviour of real steady tubular reactors. 

In the third stage, polymerization and water removal continue in a pressurized vessel, with conditions that change with time (for batch nylon 6,6 production) or with position (for continuous nylon 6,6 production in tubular reactors [14] or in a series of back-mixed reactors) as shown in Figure 7.8. High pressure is required to maintain a sufficiently high... 

As with conventional CLC, two continuously operated and interconnected fluidized beds are a well-suited reactor configuration for CLPO due to the excellent gas-solid contacting pattern, and several studies have been conducted with this set-up [92,117,119,120]. Additionally, the back-mixing inherent in this reactor configuration results in a uniform oxygen availability as opposed to the temporal variation in the previously discussed batch-fluidized beds (which constitute integral reactors in which the lattice oxygen decreases with time). 

A reactor where mixing is important is the tank flow or continuously stirred tank reactor (CSTR) it is also referred to as a back-mix reactor. This type of reactor, like the batch reactor, essentially consists of a tank or kettle equipped with an agitator. 

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