Diameter tubular reactors

In the diacid mediod, die PA salt is made first. A solution of this PA salt in water can be used for the polymerization. In the temperature range where the reaction rates are high, the diamines are volatile, and thus, it is preferable to carry out the prepolymerization under pressure. The prepolymerization can be carried out either at 220-250°C for 1 h or at 280-320°C in a matter of seconds. In the latter case, die reaction is carried out in a small-diameter tubular reactor.64 Although a prepolymerization under pressure is preferred, Nielinger28 has described a polymerization at atmospheric pressure at 210°C, whereby the loss in diamine is compensated for. 

This points out several major characteristics of premixed flames (1) They cannot be controlled stably within a constant-diameter tubular reactor configuration and (2) the chain... 

Processes for Paraffin Nitrations. Propane is thought to be the only paraffin that is commercially nitrated by vapor-phase processes. Temperature control is a primary factor in designing the reactor, and several approaches have been investigated. A spray mtrator in which liquid nunc acid is spiayed into hoi propane is used industiially. Relatively small-diameter tubular reactors, fluidized-bed reactors, and molten salt reactors have all been successfully used in laboratory units. 

P14-5j A gas-phase reaction is being carried out in a 5-cm-diameter tubular reactor that is 2 m in length. The velocity inside the pipe is 2 cm/s. As a very first approximation the gas properties can be taken as those of air (kinematic viscosity = 0.01 cmVs) and the diffusivities of the reacting species are approximately 0.005 cmVs. 

Reactant A is converted irreversibly and exothermically to products in a 2-in.-inner-diameter tubular reactor via first-order chemical kinetics. The reactive mixture in the inner pipe is cooled using a concentric double-pipe heat exchanger. The nonreactive cooling fluid in the annular region flows countercurrently with respect to the reactive fluid. The radius ratio of the double-pipe configuration is If = Rinside/ outside = 0.5, the inlet temperature of the reactive fluid is 340 K,... 

Calculate the temperature profiles for the following reaction carried out in a 2-cm-diameter tubular reactor with feed and jacket temperatures of about 350 K. 

The following procedure is suggested for predicting the product composition that will be obtained in large diameter tubular reactors (such as are used in commercial units) ... 

Two factors must be considered in extrapolating the results to commercial reactors. First, as also discussed earlier (1), the small diameter tubular reactors used in this investigation accentu-... 

If the reaction is to be carried out in a 2.0 ft ID (inside diameter) tubular reactor with an inlet volume flow rate of 2 ft /s, what reactor volume is required to attain an 80% conversion Also calculate the length of the reactor. 

Tubular reactors, as previously stated, are also advantageous for high-pressure reactions where smaller-diameter cylindrical vessels can be used to allow thinner vessel walls. Tubular reactors should be avoided when carrying out multiphase reactions, since it is often difficult to achieve good mixing between phases. 

Flow in tubular reactors can be laminar, as with viscous fluids in small-diameter tubes, and greatly deviate from ideal plug-flow behavior, or turbulent, as with gases, and consequently closer to the ideal (Fig. 2). Turbulent flow generally is preferred to laminar flow, because mixing and heat transfer... 

In tubular reactors of only a few cm in diameter, the temperature is substantially uniform over the cross section so only an axial gradient occurs in the heat balance. 

The alternate possibility of building a laboratory tubular reactor that is shorter and smaller in diameter is also permissible, but only for slow and only mildly exothermic reactions where smaller catalyst particles also can be used. This would not give a scaleable result for the crotonaldehyde example at the high reaction and heat release rates, where flow and pore-ditfusion influence can also be expected. 

Figure 2.2.4 (Berty 1983) shows a tubular reactor that has a thermosiphon temperature control system. The reaction is conducted in the vertical stainless steel tube that can have various diameters, 1/2 in. being the preferred size. If used for fixed bed catalytic studies, it can be charged with a single string of catalytic particles just a bit smaller than the tube, e.g., 5/16 particles in a l/2 O.D. tube. With a smaller catalyst, a tube with an inside diameter of up to three to four particle diameters can be used. With such catalyst charges and a reasonably high Reynolds number— above 500, based on particle diameter—this reactor...

It is generally desirable to minimize the diameter of a tubular reactor, because the leak rate in case of a tube failure is proportional to its cross-sectional area. For exothermic reactions, heat transfer will also be more efficient with a smaller tubular reactor. However, these advantages must be balanced against the higher pressure drop due to flow through smaller reactor tubes. 

The precondensation can be earned out continuously with the use of a tubular reactor at a temperature of 290-310°C.56 The tubular reactor is a 4-m-long coiled pipe with a diameter of 4 mm which is heated at 300°C. At the end of the pipe is a valve which is regulated so that the pressure is 1.5 bar. The residence time in the pipe is only seconds. The prepolymer obtained can be postcondensed in the solid state to a high molecular weight. 

Figure 1. Typical reactor temperature profile for continuous addition polymerization a plug-flow tubular reactor. Kinetic parameters for the initiator 1 = 10 ppm Ea = 32.921 kcal/mol In = 26.492 In sec f = 0.5. Reactor parameter [(4hT r)/ (DpCp)] = 5148.2. [(Cp) = heat capacity of the reaction mixture (p) = density of the reaction mixture (h) = overall heat-transfer coefficient (Tf) = reactor jacket temperature (r) = reactor residence time (D) = reactor diameter].

It is common practice to use geometric similarity in the scaleup of stirred tanks (but not tubular reactors). This means that the production-scale reactor will have the same shape as the pilot-scale reactor. All linear dimensions such as reactor diameter, impeller diameter, and liquid height will change by the same factor, Surface areas will scale as Now, what happens to tmix upon scaleup ... 

The dAc/dz term is usually zero since tubular reactors with constant diameter are by far the most important application of Equation (3.7). For the exceptional case, we suppose that Afz) is known, say from the design drawings of the reactor. It must be a smooth (meaning differentiable) and slowly varying function of z or else the assumption of piston flow will run into hydrodynamic as well as mathematical difficulties. Abrupt changes in A. will create secondary flows that invalidate the assumptions of piston flow. 

Depart from Geometric Similarity. Adding length to a tubular reactor while keeping the diameter constant allows both volume and external area to scale as S if the liquid is incompressible. Scaling in this manner gives poor results for gas-phase reactions. The quantitative aspects of such scaleups are discussed... 

This section has based scaleups on pressure drops and temperature driving forces. Any consideration of mixing, and particularly the closeness of approach to piston flow, has been ignored. Scaleup factors for the extent of mixing in a tubular reactor are discussed in Chapters 8 and 9. If the flow is turbulent and if the Reynolds number increases upon scaleup (as is normal), and if the length-to-diameter ratio does not decrease upon scaleup, then the reactor will approach piston flow more closely upon scaleup. Substantiation for this statement can be found by applying the axial dispersion model discussed in Section 9.3. All the scaleups discussed in Examples 5.10-5.13 should be reasonable from a mixing viewpoint since the scaled-up reactors will approach piston flow more closely. 

FIGURE 13.8 Temperature profiles using a simplified model of a tubular reactor with pure styrene feed = 135 C and = 20°C. The parameter is the tube diameter in meters. 

Tubular reactors are used for some polycondensations. Para-blocked phenols can be reacted with formalin to form linear oligomers. When the same reactor is used with ordinary phenol, plugging will occur if the tube diameter is above a critical size, even though the reaction stoichiometry is outside the region that causes gelation in a batch reactor. Polymer chains at the wall continue to receive formaldehyde by diffusion from the center of the tube and can crosslink. Local stoichiometry is not preserved when the reactants have different diffusion coefficients. See Section 2.8. 

The importance of the linear arrangement of mixer/funnel/tubular reactor is shown when processing in a set-up with a curved flow element (0.3 m long bent Teflon tube of 0.3 mm inner diameter) in between the funnel and tubular reactor [78]. If a straight tube of equal dimensions as given above is used, plugging occius after 30 s. Hence even short curved flow passages are detrimental for micro-chan-nel-based amidation studies. 

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