Pressure drop considerations, heat exchangers

Polley, G. T., Panjeh Shahi, M. H., and Jegede, F. O., Pressure Drop Considerations in the Retrofit of Heat Exchanger Networks, Trans. IChemE, part A, 68 211, 1990. 

Ahmad S, PoUey GT, PetelaEA (1989) Retrofit of heat exchanger networks subject to pressure drop considerations. Paper No. 34a, AlCHE Meeting, Houston, April. 

Polley GT, Panjeh Shahi MH, Jegede FO (1990) Pressure drop considerations in the retrofit of heat exchanger networks. Transactions of IChemE, Part A, 68, 211. 

The feed is composed of two streams. The first stream is a hydrocarbon stream that contains 30 mol % isobutene and 70 mol % 1-butene. The second stream, consisting of pure methanol, is in 5 per cent molar excess of the reaction stoichiometry. The hydrocarbon feed rate is 1000 kg h . Both streams are at 30°C and 1500 kPa. The reactor inlet temperature should be controlled at 70°C. The reactor outlet temperature will be higher than the inlet, since the reaction is exothermic and a considerable amount of heat is released. This has the effect of limiting the conversion of isobutene in the reactor. The reactor product should be cooled to around 40°C so that a second reaction stage can increase the isobutene conversion to around 99 per cent. The reactor pressure drop is 140 kPa, and the pressure drops through the exchangers are 70 kPa. The exchanger volumes can be estimated at 0.1 m each. 

Friction Coefficient. In the design of a heat exchanger, the pumping requirement is an important consideration. For a fully developed laminar flow, the pressure drop inside a tube is inversely proportional to the fourth power of the inside tube diameter. For a turbulent flow, the pressure drop is inversely proportional to D where n Hes between 4.8 and 5. In general, the internal tube diameter, plays the most important role in the deterrnination of the pumping requirement. It can be calculated using the Darcy friction coefficient,, defined as... 

Flow Maldistribution. One of the principal reasons for heat exchangers failing to achieve the expected thermal performance is that the fluid flow does not foUow the idealized anticipated paths from elementary considerations. This is referred as a flow maldistribution problem. As much as 50% of the fluid can behave differently from what is expected based on a simplistic model (18), resulting in a significant reduction in heat-transfer performance, especially at high or a significant increase in pressure drop. Flow maldistribution is the main culprit for reduced performance of many heat exchangers. 

Higher overall heat transfer coefficients are obtained with the plate heat exchanger compared with a tubular for a similar loss of pressure because the shell side of a tubular exchanger is basically a poor design from a thermal point of view. Considerable pressure drop is used without much benefit in heat transfer efficiency. This is due to the turbulence in the separated region at the rear of the tube. Additionally, large areas of tubes even in a well-designed tubular unit are partially bypassed by liquid and low heat transfer areas are thus created. 

Considerable interest has been generated in turbulence promoters for both RO and UF. Equations 4 and 5 show considerable improvements in the mass-transfer coefficient when operating UF in turbulent flow. Of course the penalty in pressure drop incurred in a turbulent flow system is much higher than in laminar flow. Another way to increase the mass-transfer is by introducing turbulence promoters in laminar flow. This procedure is practiced extensively in enhanced heat-exchanger design and is now exploited in membrane hardware design. 

Taking into account typical numbers for a and D, this underlines that the channel width should be considerably smaller than 1 mm (1000 pm) in order to achieve short residence times. Actually, heat exchangers of such small dimensions are not completely new, because liquid cooled microchannel heat sinks for electronic applications allowing heat fluxes of 790 watts/cm2 were already known in 1981 [46]. About 9 years later a 1 cm3 cross flow heat exchanger with a high aspect ratio and channel widths between 80 and 100 pm was fabricated by KFK [10, 47]. The overall heat transport for this system was reported to be 20 kW. This concept of multiple, parallel channels of short length to obtain small pressure drops has also been realized by other workers, e.g. by PNNL and IMM. IMM has reported a counter-current flow heat exchanger with heat transfer coefficients of up to 2.4 kW/m2 K [45] (see Fig. 3). 

Considerable engineering judgment and effort are needed to ensure that the heat recovery is efficient, yet has low pressure drop. Since a large portion of the heat contained in the reactor effluent has to be transferred back to the cold recycle gas, this exchanger arrangement received our special attention. 

Selection of the laboratory reactor requires considerable attention. There is no such thing as a universal laboratory reactor. Nor should the laboratory reactor necessarily be a reduced replica of the envisioned industrial reactor. Figure 1 illustrates this point for ammonia synthesis. The industrial reactor (5) makes effective use of the heat of reaction, considering the non-isothermal behavior of the reaction. The reactor internals allow heat to exchange between reactants and products. The radial flow of reactants and products through the various catalyst beds minimizes the pressure drop. In the laboratory, intrinsic catalyst characterization is done with an isothermally operated plug flow microreactor (6). 

Chapter 7, Reactor Design, discusses continuous and batch stirred-tank reactors and die packed-bed catalytic reactor, which are frequently used. Heat exchangers for stirred-tank reactors described are the simple jacket, simple jacket with a spiral baffle, simple jacket with agitation nozzles, partial pipe-coil jacket, dimple jacket, and the internal pipe coil. The amount of heat removed or added determines what jacket is selected. Other topics discussed are jacket pressure drop and mechanical considerations. Chapter 7 also describes methods for removing or adding heat in packed-bed catalytic reactors. Also considered are flow distribution methods to approach plug flow in packed beds. 

Sometimes the design is governed by considerations that have little to do with heat transfer, such as the space available for the equipment or the pressure drop that can be tolerated in the fluid streams. Tubular exchangers are, in general, designed in accordance with various standards and codes, such as the Standards of the Tubular Exchanger Manufacturers Association (TEMA) and the ASME-API Unfired Pressure Vessel Code. ... 

In addition, when blowers and pumps are used for the fluid flow, they are generally head-limited, and the pressure drop itself can be a major consideration. Also, for condensing and evaporating fluids, the pressure drop affects the heat transfer rate. Hence, the Ap determination in the exchanger is important. As shown in Eq. 17.177, the pressure drop is proportional to D, 3 and hence it is strongly influenced by the passage hydraulic diameter. 

General Considerations. The importance of fouling phenomena stems from the fact that the fouling deposits increase thermal resistance to heat flow. According to the basic theory, the heat transfer rate in the exchanger depends on the sum of thermal resistances between the two fluids, Eq. 17.5. Fouling on one or both fluid sides adds the thermal resistance R, to the overall thermal resistance and, in turn, reduces the heat transfer rate (Eq. 17.4). Simultaneously, hydraulic resistance increases because of a decrease in the free flow area. Consequently, the pressure drops and the pumping powers increase (Eq. 17.63). 

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