Chemical flow distribution

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. 

We prepared microchannel reactor employing stainless steel sheet 400tan thick patterned microchannel by a wet chemical etching. The microchannel shape and dimension were decided by computer simulation of flow distribution and pressure drop of the reactants in the microchaimel sheet. Two different types of patterned plates with mirror image were prepared [5]. The plate has 21 straight microchannels which are 550/an wide, 230/an deep and 34mi long as revealed in Fig. 1(b). 

Fig 18. Experimental trickle-bed system A, tube bundle for liquid flow distribution B, flow distribution packing of glass helices C, activated carbon trickle bed 1, mass flow controllers 2, gas or liquid rotameters, 3, reactor (indicating point of gas phase introduction) 4, overflow tank for the liquid phase feed 5, liquid phase hold-up tank 6, absorber pump 7, packed absorption column for saturation of the liquid phase 8, gas-liquid disengager in the liquid phase saturation circuit. (Figure from Haure et ai, 1989, with permission, 1989 American Institute of Chemical Engineers.)... 

It is much more difficult to describe the relationship of the bulk field gradients, easily recognised in the flow of water in clouds and of oxygen in the ozone layer described in Section 3.4, to that of the gradients controlling the chemical flow in cell liquids. The effects of electric fields due to charge distribution in various parts of the cell is an obvious possibility. 

In chemical process engineering these so-called unit-operations are components of every complex plant As catalyst screening contains many different processes, such as heat exchange, flow distribution, sampling, analysis and reaction, such a subdivision in unit operations is justified. An overview of the modules that are part of the screening set-up is given in Fig. 4.6. 

CT) scanner with 7-ray source at the Chemical Reaction Engineering Laboratory (CREL), Washington University [5] has been used successfully in the past to characterize flow distribution of liquid and gas over non-structured packing [6]. Hence, it is the tool of choice for studies on structured packing. 

A combined evaporator and methanol reformer was developed by Park et al. [124] to power a 5 W fuel cell. However, the device was still electrically heated by heating cartridges. Both the evaporator and the reformer channels, which were identical in size, were prepared on metal sheets 200 pm thick by wet chemical etching. The channel dimensions were length 33 mm, width 500 pm and depth 200 pm. Therefore, the channels were completely etched through the sheets and the channel depth could be varied by introducing several of these sheets into the reactor. The flow distribution between the 20 channels of the device was performed by triangular inlet and outlet fields. Both devices had outer dimensions of 70 mm x 40 mm x 30 mm. 

More than 100 micro structured devices are listed on the homepage of the pChemTec consortium [24]. The devices cover physical applications such as flow distribution, mixing, heat transfer, phase transfer, emulsification and suspension, as well as chemical applications such as chemical and biochemical processing. Some separation units such as membrane separation and capillary electrophoresis are also offered. Control devices such as valves, micro pumps for product analysis and mass flow controllers supplement the catalog. 

Qualitative inspection of the tracer response can go a long way toward identifying flow distribution problems. Additional references on tracers are Wen and Fan Models for Flow Systems in Chemical Reactors, Marcel Dekker, 1975) and Levenspiel (Chemical Reaction Engineering, 3d ed., Wiley, 1999). 

In fixed bed chemical reactor analysis it is common to assume uniform flow distribution within the bed. The reality however is different. Due to a change of the average porosity near the wall [ 1, 2,3],(Figure 1.) -e=1 at the wall - the flow velocity increases until close to the wall and is reduced again because of the non slip condition (Figure 2.) The artificial flow profile is described by the Brinkman equation... 

Lerou and Froment [10] found by calculations that a reactor may ignite under non constant flow conditions while it is still stable if constant flow is assumed. Kalthoff and Vortmeyer [11],(Figure 4) found an improved agreement between measured and calculated ranges of multiple solutions for non -uniform flow. From the previous work therefore can be concluded that non-uniform porosity and flow distributions effect the chemical reactor performance. The question however, whether real improvements are obtained has to be subject to a comparison of experimental results with calculations. 

Geometrical illustrations of the efficiency of thermodynamic description of the stationary flow distribution problems as applied to the analysis of closed active and open passive hydraulic circuits were already presented in Section 3.2. The geometrical interpretation of the general models for the nonstationary flow distribution in the hydraulic circuit ((23)—(28)) and chemical systems with the set redundant mechanism of reaction ((29)-(34)) is still to be carried out which will obviously require a number of nontrivial problems to be solved. 

Takeuchi et al. 7 reported a membrane reactor as a reaction system that provides higher productivity and lower separation cost in chemical reaction processes. In this paper, packed bed catalytic membrane reactor with palladium membrane for SMR reaction has been discussed. The numerical model consists of a full set of partial differential equations derived from conservation of mass, momentum, heat, and chemical species, respectively, with chemical kinetics and appropriate boundary conditions for the problem. The solution of this system was obtained by computational fluid dynamics (CFD). To perform CFD calculations, a commercial solver FLUENT has been used, and the selective permeation through the membrane has been modeled by user-defined functions. The CFD simulation results exhibited the flow distribution in the reactor by inserting a membrane protection tube, in addition to the temperature and concentration distribution in the axial and radial directions in the reactor, as reported in the membrane reactor numerical simulation. On the basis of the simulation results, effects of the flow distribution, concentration polarization, and mass transfer in the packed bed have been evaluated to design a membrane reactor system. 

Systemization. The Distribution Manager must relate the creative, kaleidoscopic activities of chemicals physical distribution with information concepts, systems, and hardware to achieve appropriate control. The Distribution Manager who masters this very complex simultaneous equation will achieve for his company a continuous flow network with the end costs of the delivered material progressively improving. The most complex and challenging system which the Distribution Manager will need to develop will be himself and his people organization. 

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