Recirculating tubular reactor

With a reaction-limited deposition process, the film should have uniform thickness as long as the partial pressures ofreactants do not vary with position. In a tubular reactor, the conversion of reactants must be kept small or the film thickness will depend on the location of the sohd in the reactor, with upstream regions having a greater deposition rate. It is therefore common to use a gas recirculation reactor (a recycle PFTR) so that the composition of the reactants is independent of position in the reactor to assure uniform film thickness. 

For the same type of catalyst we have observed in a recirculation laboratory reactor multiplicity, periodic and chaotic behavior. Unfortunately, so far we are not able to suggest such a reaction rate expression which would be capable of predicting all three regimes [8]. However, there is a number of complex kinetic expressions which can describe periodic activity. One can expect that such kinetic expressions combined with heat and mass balances of a tubular nonadiabatic reactor may give rise to oscillatory behavior. Detailed calculations of oscillatory behavior of singularly perturbed parabolic systems describing heat and mass transfer and exothermic reaction are apparently beyond, the capability of both standard current computers and mathematical software. 

Various laboratory reactors have been described in the literature [3, 11-13]. The most simple one is the packed bed tubular reactor where an amount of catalyst is held between plugs of quartz wool or wire mesh screens which the reactants pass through, preferably in plug flow . For low conversions this reactor is operated in the differential mode, for high conversions over the catalyst bed in the integral mode. By recirculation of the reactor exit flow one can approach a well mixed reactor system, the continuous flow stirred tank reactor (CSTR). This can be done either externally or internally [11, 12]. Without inlet and outlet feed, this reactor becomes a batch reactor, where the composition changes as a function of time (transient operation), in contrast with the steady state operation of the continuous flow reactors. 

The second issue for cooled tubular reactors is how to introduce the coolant. One option is to provide a large flowrate of nearly constant temperature, as in a recirculation loop for a jacketed CSTR. Another option is to use a moderate coolant flowrate in countercurrent operation as in a regular heat exchanger. A third choice is to introduce the coolant cocurrently with the reacting fluids (Borio et al., 1989). This option has some definite benefits for control as shown by Bucala et al. (1992). One of the reasons cocurrent flow is advantageous is that it does not introduce thermal feedback through the coolant. It is always good to avoid positive feedback since it creates nonmonotonic exit temperature responses and the possibility for open-loop unstable steady states. 

Kinetic measurements in flow systems are usually made with the catalyst packed in a tubular reactor. Three methods of operation are possible (/) as an integral reactor ( ) as a differential reactor, and Hi) as a recirculation or recycle reactor. 

The second-step oxidation is normally by means of nitric acid, but catalytic air oxidation results in good yields of adipic acid. In recent practice, the refined first-step product of cyclohexane oxidation freed of unconverted hydrocarbon and a 50-60 per cent nitric acid solution containing copper-vanadium catalyst are separately and continuously fed to a jacketed reaction vessel at a ratio such that weight ratio of 100 per cent nitric acid to organic feed is between 2.5 and 6. The reaction mixture is rapidly recirculated through a tubular reactor at 60-80°C, and fresh feed is admitted to give about 5 min time of contact. Yields are improved by reheating the continuously withdrawn effluent stream to 95-100°C for a... 

Equations will be derived for three representative cases the tubular reactor of annular cross-section the isothermal, well-mixed batch reactor and the isothermal batch reactor inside a recirculating system. The chosen exemplifications will deal with these types of reactors that are, without doubt, the most widely used. 

Several types of tubular reactors have been reported. Recirculation loop reactors have a pump which continuously circulates the reacting dispersion through a tube loop with raw materials being introduced at one location and product latex removed at another. Material passing through the pump is remixed with each pass. If the circulation rate is significantly higher than the feed and effluent rates, which is usually the case, the residence time distribution approaches that of a well-mixed CSTR, Hence this reactor cannot be used to produce latexes with narrow PSDs. 

Figure 9 shows the flowsheet. The packed-bed adiabatic tubular reactor has a gas-phase exothermic irreversible reaction A + B C. Two gaseous fresh feed streams are introduced into a recirculating gas loop. The fresh feed and recycle streams are heated in a feed-effluent heat exchanger (FEHE) to conserve energy and finally in a furnace to achieve the desired reactor inlet temperature. Since the reactor is adiabatic and the reaction is exothermic, a... 

Medium and low molecular weight polymers are produced in solution processes in light hydrocarbons in CSTR reactors or in recirculating tubular (loop) reactors. The molecular weight is controlled by monomer purity, initiator, water content, and temperature. 

Balakotaiah, V., Christoforatou, E.L. and West, D.H. (1999) Transverse concentration and temperature non-uniformities in adiabatic packed-bed reactors. Chemical Engineering Science, 54,1725-34. Berezowski, M., (2003), Fractal solutions of recirculation tubular chemical reactors. Chaos, Solitons... 

Description of Apparatus and Biocide Test Procedures. Sessile bacteria for use in biocide tests were colonized in Robbins devices as described by McCoy et al. The apparatus (Fig. 1) consists of a tubular reactor with 48 removable studs through which S L of field brine (containing bacteria) are recirculated. The removable test surfaces (studs), detailed in Fig. 2, were made of admiralty brass. 

Scheme 10 (a) Single CSTR, (b) tubular reactor, (c) CSTR in series, and (d) tubular reactor with recirculation. 

Figure 13.5 shows a flowsheet for the manufacture of phthalic anhydride by the oxidation of o-xylene. Air and o-xylene are heated and mixed in a Venturi, where the o-xylene vaporizes. The reaction mixture enters a tubular catalytic reactor. The heat of reaction is removed from the reactor by recirculation of molten salt. The temperature control in the reactor would be diflficult to maintain by methods other than molten salt. 

A tubular bioreactor design with operational may lead to a CSTR, having sufficient recycle ratio for plug flow that behave like chemostat. The recirculation plug flow reactor is better than a chemostat, with maximum productivity at C, 3 g-m 3. Combination of plug flow with CSTR which behave like chemostat was obtained from the illustration minimised curve with maximum rate at CSf = 3 g-m-3. 

Figure 11.9 Different arrangements and modes of operation for membrane bioreactors Continuous Stirred Tank Reactor (CSTR) with recirculation arrangement (a), dead-end cell (b), tubular with entrapped enzyme (c).

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