Improved Heat Transfer

Classical bubbles do not exist in the vigorously bubbling, or turbulent fluidization regimes. Rather, bubbles coalesce constantly, and the bed can be treated as a pseudohomogenous reactor. Small bubble size improves heat transfer and conversion, as shown in Figure 5b. Increasing fines levels beyond 30—40% tends to lower heat transfer and conversion as the powder moves into Group C. 

Reductive alkylations and aminations requite pressure-rated reaction vessels and hiUy contained and blanketed support equipment. Nitrile hydrogenations are similar in thein requirements. Arylamine hydrogenations have historically required very high pressure vessel materials of constmction. A nominal breakpoint of 8 MPa (- 1200 psi) requites yet heavier wall constmction and correspondingly more expensive hydrogen pressurization. Heat transfer must be adequate, for the heat of reaction in arylamine ring reduction is - 50 kJ/mol (12 kcal/mol) (59). Solvents employed to maintain catalyst activity and improve heat-transfer efficiency reduce effective hydrogen partial pressures and requite fractionation from product and recycle to prove cost-effective. 

Batch reaclors are tanks, usually provided with agitation and some mode of heat transfer to maintain temperature within a desirable range. They are primarily employed for relatively slow reactions of several hours duration, since the downtime for filling and emptying large equipment may be an hour or so. Agitation maintains uniformity and improves heat transfer. Modes of heat transfer are illustrated in Figs. 23-1 and 23-2. 

Spiral baffles, which are sometimes installed for hquid services to improve heat transfer and prevent channeling, can be designed to serve as reinforcements. A spiral-wound channel welded to the vessel wall is an alternative to the spiral baffle which is more predictable in performance, since cross-baffle leakage is eliminated, and is reportedly lower in cost [Feichtinger, Chem. Eng., 67, 197 (Sept. 5, I960)]. 

A novel variation is a cyhndrical model equipped with a tube bundle to resemble a sheU-and-tube heat exchanger with a bloated shell [Chem. Proce.s.s., 20 (Nov. 15, 1968)]. Conical ends provide for redistribution of burden between passes. The improved heat-transfer performance is shown by Fig. 11-61. 

Refrigeration units modified for free cooling do not include the hq-uid-refrigerant pump and cooler spray header nozzles. Without the cooler refrigerant agitation for improved heat transfer, this arrangement allows up to about 20 percent of rated capacity. Expected capacities for both tnermocycle and free cooling are indicated in Fig. 12-21. 

Stirred Vessels Gases may be dispersed in hquids by spargers or nozzles and redispersed by packing or trays. More intensive dispersion and redispersion is obtained by mechanical agitation. At the same time, the agitation will improve heat transfer and will keep catalyst particles in suspension if necessaiy. Power inputs of 0.6 to 2.0 kW/m (3.05 to 10.15 np/1,000 gal) are suitable. 

Figure 1.7 Simple detail of shell-and-tube heat exchanger. The water box may be designed for as many as eight passes, and a variety of configurations of shell-side baffles may be used to improve heat transfer, (a) Several water box arrangements for tube-side cooling, (b) Assembly of simple two-pass exchanger with U-tubes. [Fig. 38.2, The Nalco Water Handbook, 1st ed. (1979), reprinted with permission from McGraw-Hill, Inc.)...

In order to improve cost effectiveness, the following parameters must be considered. First, the limitations on improved porosity adsorbents will be explored and then a number of advanced cycles will be reviewed. All would benefit from improved heat transfer which leads to more compact and hence less costly machines. 

The carbons are broadly comparable in terms of their maximum concentration and implied energy efficiency but the two monolithic forms offer the advantage of smaller pressure vessel sizes and improved heat transfer. 

The applieation of aetivated earbons in adsorption heat pumps and refrigerators is diseussed in Chapter 10. Sueh arrangements offer the potential for inereased efficiency because they utilize a primary fuel source for heat, rather than use electrieity, which must first be generated and transmitted to a device to provide mechanical energy. The basic adsorption cycle is analyzed and reviewed, and the ehoiee of refrigerant-adsorbent pairs discussed. Potential improvements in eost effeetiveness are detailed, including the use of improved adsorbent carbons, advanced cycles, and improved heat transfer in the granular adsorbent earbon beds. 

Improve heat transfer coefficient by forcing flow past coil surfaces. 

Figure 10-93B. Process vessel with internal coil and agitation to improve heat transfer. (Used by permission Engineering Manual Dowtherm Heat Transfer Fluids, 1971. The Dow Chemical Co.)...

See Figures I0-93A and I0-93B as limited examples of reaction and other process vessels that require heat transfer for proper processing. Markovitz reports improved heat transfer for the inside of jacketed vessels when the surface has been electropolished, which gives a fine, bright surface. 

Markovitz, R. E., Improve Heat Transfer with Electropol-ished Glad Reactors, Hydro. Proc., V. 52. No. 8, p. 117 (1973). 

If water is required below 5°C, the approach to freezing point brings considerable danger of ice formation and possible damage to the evaporator. Some closed systems are in use and have either oversize heat exchange surfaces or high-efficiency-type surfaces. In both of these, the object is to improve heat transfer so that the surface in contact with the water will never be cold enough to cause ice layers to accumulate. 

For economy of cost, and to reduce the viscosity (and so improve heat transfer), solutions weaker than eutectic are normally used, provided there is no risk of freezing at the evaporator. 

With ceiling inlet and extract systems, the opportunity is presented to remove heat from light troughs. This can reject a proportion of the cooling load, possibly as high as 20%, in the exhaust air. The recirculated air is also warmer, improving heat transfer at the cooling coil. (See also Example 26.3.)... 

Tubes may be arranged in either a horizontal and vertical alignment pattern (which provides for easier cleaning and inspection) or in a staggered layout (which provides for improved heat transfer) because of the more circuitous flow of water around the tubes). 

Boiler surfaces tend to be cleaner with little or no suspended solids or sludge present, resulting in improved heat transfer. 

Additionally, the surfactant properties of filmers reduce the potential for stagnant, heat-transfer-resisting films, which typically develop in a filmwise condensation process, by promoting the formation of condensate drops (dropwise condensation process) that reach critical mass and fall away to leave a bare metal surface (see Figure 11.2). This function, together with the well-known scouring effect on unwanted deposits keeps internal surfaces clean and thus improves heat-transfer efficiencies (often by 5-10%). 

Wall temperatures drop after reaching the maximum in the case of the two highest heat flux levels in Fig. 8, and this is due to increasing convective heat transfer through the steam film, which now completely blankets the surface. The improved heat transfer is caused by the higher flow velocities in the tube as more entrained liquid is evaporated. Finally, at about 100% quality, based on the assumption of thermal equilibrium, only steam is present, and wall temperatures rise once more due to decreasing heat-transfer coefficients as the steam becomes superheated. 

In most cases where convective heat transfer is taking place from a surface to a fluid, the circulating currents die out in the immediate vicinity of the surface and a film of fluid, free of turbulence, covers the surface. In this film, heat transfer is by thermal conduction and, as the thermal conductivity of most fluids is low, the main resistance to transfer lies there, Thus an increase in the velocity of the fluid over the surface gives rise to improved heat transfer mainly because the thickness of the film is reduced. As a guide, the film coefficient increases as (fluid velocity)", where 0.6 < n < 0.8, depending upon the geometry. 

The full utilization of improved heat transfer in a given reactor can only be made when the molecular weight-conversion relationships are carefully studied with various initiator types at different heat transfer levels. Then a particular initiator system must be selected for a maximum conversion improvement for a specified product. 

Optimized molecular weight-conversion relationship is related to the system heat transfer coefficient. The degree of conversion improvement from improved heat transfer depends on the average molecular weights of polymer being produced for a given initiator system. 

In turbulent flow, part of this extra energy buys something. It increases turbulence and improves heat transfer and mixing. 

The application of microchannel technology is a natural fit for the production of synthetic fuels via the FT process. The primary limitations of conventional FT technology include the removal of process heat that can produce hot spots and severely shorten catalyst life, and effective management of two-phase flow as synthesis gas transforms into hquid hydrocarbons. Both these issues can be addressed with microchaimel technology, which greatly improves heat transfer and precisely controls flow through thousands of parallel chaimels. 

Improving heat-transfer rates to match the exothermicity of a reaction. 

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