Flow distributions

It is important to obtain good flow distribution for all reformer streams. The piping should be designed sueh that the variation in gas flow to the reformer tubes and to the burners does not exceed plus or minus 2.5 percent. Otherwise, the tubewall temperatures may not be sufficiently uniform. 

Special consideration must be given to the PSA offgas flow since it is typically available to the burners at only about 3 psig. Even greater consideration must be given for preheated combustion air (if used), since the differential air pressure across each burner is t5 ically no more than 2 inches H2O. 

To help ensure good distribution, the piping should be as synunetrical as possible, and detailed pressure drop computations should be made and analyzed. 

Generally impingement plates should not be perforated. There may be times when a flow distribution plate also serves as an impingement plate. There is seldom sufficient space to install a plate to serve both as an impingement plate and as a flow distribution plate unless all tubes have been omitted from the baffle windows. 

AXIAL NOZZLE IN CONICAL HEAT WITH internal INFFUSER COi S 

Flow distribution is also important on the shellside, especially in cases where parallel flow baffles or Rod Baffles are required. Vapor belts, or distribution belts, serve to distribute shellside fluids in many cases. Vapor belts may be used to permit fluid to enter the shell over the entire circumference or they may be designed such that the fluid enters only a specified area of the shell. Vapor belts reduce entrance velocities and entrance pressure drops while effecting good distribution. 

Shellside distribution may also be effected by installing perforated plates downstream of the inlet nozzle. This approach is often used for condensers when the shellside fluid makes only one pass across the tube bundle. The perforated plate must be adequately positioned above the tube bundle to prevent excessive jetting and impingement. 

Many details must be considered when installing heat transfer equipment. Many of these may seem of minor importance but it is precisely these small details that often lead to poor performance and increased maintenance. 

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Finite element modelling of flow distribution in an extrusion die... 

Figure 5.20 (a) The predicted non-uniform exit flow distribution in a co-extrusion die. 

Nassehi, V. and Pittman,. 1. F. T., 1989. Finite element modelling of flow distribution in an extrusion die. In Bush, A. W., Lewis, B.A. and Warren, M.D. (eds), Flow Modelling in Industrial Processes, Chapter 8, Ellis Horwood, Chichester. 

In addition to the reduction in performance, flow maldistribution may result in increased corrosion, erosion, wear, fouling, fatigue, and material failure, particularly for Hquid flows. This problem is even more pronounced for multiphase or phase change flows as compared to single-phase flows. Flow distribution problems exist for almost all types of exchangers and can have a significant impact on energy, environment, material, and cost in most industries. 

It has become quite popular to optimize the manifold design using computational fluid dynamic codes, ie, FID AP, Phoenix, Fluent, etc, which solve the full Navier-Stokes equations for Newtonian fluids. The effect of the area ratio, on the flow distribution has been studied numerically and the flow distribution was reported to improve with decreasing yiR. 

A numerical study of the effect of area ratio on the flow distribution in parallel flow manifolds used in a Hquid cooling module for electronic packaging demonstrate the useflilness of such a computational fluid dynamic code. The manifolds have rectangular headers and channels divided with thin baffles, as shown in Figure 12. Because the flow is laminar in small heat exchangers designed for electronic packaging or biochemical process, the inlet Reynolds numbers of 5, 50, and 250 were used for three different area ratio cases, ie, AR = 4, 8, and 16. 

The quahtative flow distribution in a manifold can be estimated by examining a streamline plot. Figure 13 shows the streamline plot for the manifold having AR = 4. Note that the same amount of fluid flows between two consecutive streamlines. The area ratio is an important parameter affecting the flow distribution in a manifold, as shown in Figure 14a, which shows the percent flow rate in each channel for three cases. As the area ratio increases, the percent flow rate increases in channels no. 1 and no. 8, whereas the percent flow rate decreases in the middle channels. 

The flow distribution in a manifold is highly dependent on the Reynolds number. Figure 14b shows the flow distribution curves for different Reynolds number cases in a manifold. When the Reynolds number is increased, the flow rates in the channels near the entrance, ie, channel no. 1—4, decrease. Those near the end of the dividing header, ie, channel no. 6—8, increase. This is because high inlet velocity tends to drive fluid toward the end of the dividing header, ie, inertia effect. 

Figure 15 shows the effect of the width ratio DJthe ratio of the combining header width to the dividing header width, on the flow distribution in manifolds for Reynolds number of 50. By increasing DJthe flow distribution in the manifold was significantly improved. The ratio of the maximum channel flow rate to the minimum channel flow rate is 1.2 for the case of D /= 4.0, whereas the ratio is 49.4 for the case oiDjD,=0.5. 

The improvement of the flow distribution by increasing the value of DJD results from the decrease of momentum gain in the combining header. 

A. K. Singhal, L. W. Keeton, A. K. Majundar, and T. Mukerjee,M Improved Mathematical Formulation for the Computations of Flow Distributions in Manifolds for Compact Heat Exchangers, paper presented at The ASME Winter Annual Meeting, Anaheim, Calif., 1986, p. 105. 

Radial density gradients in FCC and other large-diameter pneumatic transfer risers reflect gas—soHd maldistributions and reduce product yields. Cold-flow units are used to measure the transverse catalyst profiles as functions of gas velocity, catalyst flux, and inlet design. Impacts of measured flow distributions have been evaluated using a simple four lump kinetic model and assuming dispersed catalyst clusters where all the reactions are assumed to occur coupled with a continuous gas phase. A 3 wt % conversion advantage is determined for injection feed around the riser circumference as compared with an axial injection design (28). 

Distributors in industrial units typically have large numbers of injection points of quite diverse design characteristics, some of which are depicted in Eigure 16 for fluidized-bed appHcations. Flow variations through these parallel paths can lead to poor flow distributions within a reactor, thus reducing product yields and selectivity. In some circumstances, undesirable side products can foul portions of the distributor and further upset flow patterns. Where this is important, or where the possibiHties and consequences are insufficiently understood and independent means caimot be employed to assure adequate distribution, the pilot plant must be sized to accommodate such a distributor. Spacing should be comparable to those distributors that are anticipated to be... 

The flow along the membranes also improves the mass transport there, and the separators between the membranes are constmcted to provide good flow distribution and mixing on the membrane surfaces. Membrane sizes are often about 0.5 x 1 m, spaced about 1 mm apart. Many types of polymers are used to manufacture these ion-exchange-selective membranes, which are often reiaforced by strong fabrics made of other polymers or glass fibers. 

Reactor design for glucose isomerization ia the United States has been documented (75). The diameter of the reactor is normally between 0.6 and 1.5 m. Typical bed height is 2—5 m. The ratio between the bed height and diameter of a reactor should be at least 3 1 to ensure good flow distribution. Plants that produce more than 1000 t of HECS per day, based on dry matter, use at least 20 separate reactors. 

H. Weltans and co-workers. Optimisation of Catalytic Converter Gas Flow Distribution by CFD Prediction, SAE 930780, Society of Automotive Engineers, Warrendale, Pa., 1993. 

Impingement baffles or flow-distribution devices are recom-men oed for axial tube-side nozzles when entrance velocity is high. 

The membranes are supported and kept apart by feed spacers. A typical cell gap is 0.5-2 mm. The spacer also helps control solution distribution and enhances mass transfer to the membrane. Given that an industrial stack may have up to 500 cell pairs, assuring uniform flow distribution is a major design requirement. 

An industrial chemical reacdor is a complex device in which heat transfer, mass transfer, diffusion, and friction may occur along with chemical reaction, and it must be safe and controllable. In large vessels, questions of mixing of reactants, flow distribution, residence time distribution, and efficient utilization of the surface of porous catalysts also arise. A particular process can be dominated by one of these factors or by several of them for example, a reactor may on occasion be predominantly a heat exchanger or a mass-transfer device. A successful commercial unit is an economic balance of all these factors. 

Topics that acquire special importance on the industrial scale are the quality of mixing in tanks and the residence time distribution in vessels where plug flow may be the goal. The information about agitation in tanks described for gas/liquid and slurry reactions is largely apphcable here. The relation between heat transfer and agitation also is discussed elsewhere in this Handbook. Residence time distribution is covered at length under Reactor Efficiency. A special case is that of laminar and related flow distributions characteristic of non-Newtonian fluids, which often occiu s in polymerization reactors. 

FIG. 23-24 Reactors with moving catalysts, a) Transport fluidized type for the Sasol Fischer-Tropsch process, nonregenerating, (h) Esso type of stable fluidized bed reactor/regeuerator for cracldug petroleum oils, (c) UOP reformer with moving bed of platinum catalyst and continuous regeneration of a controlled quantity of catalyst, (d) Flow distribution in a fluidized bed the catalyst rains through the bubbles. 

Alternatively, reactant and product gases can be distributed to and removed from individual cells through internal pipes in a design analogous to that of filter presses, (iare must be exercised to assure an even flow distribution between the entiv and exit cells. The seals in internally manifolded stacks are generally not subject to electrical, thermal, and mechanical stresses, but are more numerous than in externally manifolded stacks. 

Flow distribution in a packed bed received attention after Schwartz and Smith (1953) published their paper on the subject. Their main conclusion was that the velocity profile for gases flowing through a packed bed is not flat, but has a maximum value approximately one pellet diameter from the pipe wall. This maximum velocity can be 100 % higher than the velocity at the center. To even out the velocity profile to less than 20 % deviation, more than 30 particles must fit across the pipe diameter. 

There are many reasons for division into trays. The best known reason is to limit the temperature change by having a heat exchanger between trays. The other is to give an opportunity to inject a reactant, the concentration of which was limited by safety or selectivity reasons. The final reason is to compensate for uneven flow distribution, the result of uneven catalyst packing across a bed, which happens during catalyst charging. The channels of low resistance to flow have a tendency to extend themselves. 

Katsanis, T., Use of Arbitrary Quasi-Orthogonals for Calculations Flow Distribution in the Meridional Plane of a Turbomachine, NASA TND-2546, 1964. 

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