Heat transfer, reactors walls

The mass- and ener -balance equations must be solved numerically in the general situation where heat is transferred to or from walls. There are three terms on the right side of the energy equation, heat flow with reactants and products, reaction heat, and heat transfer through walls. Flowever, the adiabatic reactor is a special case where we need to solve only one equation for a single reaction. 

De Wasch and Froment (1971) and Hoiberg et. al. (1971) published the first two-dimensional packed bed reactor models that distinguished between conditions in the fluid and on the solid. The basic emphasis of the work by De Wasch and Froment (1971) was the comparison of simple homogeneous and heterogeneous models and the relationships between lumped heat transfer parameters (wall heat transfer coefficient and thermal conductivity) and the effective parameters in the gas and solid phases. Hoiberg et al. (1971)... 

Gas plus catalyst soUd Usually BFB. For fast reactions, gas film diffusion may control and catalyst pore diffusion mass transfer may control if catalyst diameter >1.5 mm. Heat transfer heat transfer coefficient wall to fluidized bed is 20-40 X gas-wall at the same superficial velocity, h = 0.15-0.3 kW/m K. Nu = 0.5-2. Heat transfer from the bed to the walls U = 0.45 to 1.1 kW/m °C. from bed to immersed tubes U = 0.2 to 0.4 kW/m °C from solids to gas in the bed U = 0.017 to 0.055 kW/m °C. Fluidized bed usually expands 10-25 %. Backmix type reactor which increases the volume of the reactor and usually gives a loss in selectivity. Usually characterized as backmix operation or more realistically as a series of CSTR if the height/diameter > 2 Usually 1 CSTR for each H/D= 1. If the reactor operates in the bubble region, then much of the gas short circuits the catalyst so the overall apparent rate constant is lower by a factor of 10. 

In the above equation p and Cp are the density and heat capacity of the polymerizing mixture, (—AHr) is the heat of polymerization, Rp the rate of polymerization (see Equation 3.10), U the overall heat transfer coefficient, Aw the available area for heat transfer (usuallyreactor walls), T the reactor temperature, Tw the coolant (jacket) temperature finally, Qe represents the heat removed by sources other than reactor jacket (condenser and/or other (external) devices). 

Ultrafast pyrolysis in the vortex reactor is capable of pyrolyzing biomass at high heat-transfer rates on the reactor wall by ablation and has been... 

Traditionally, sodium dichromate dihydrate is mixed with 66° Bh (specific gravity = 1.84) sulfuric acid in a heavy-walled cast-iron or steel reactor. The mixture is heated externally, and the reactor is provided with a sweep agitator. Water is driven off and the hydrous bisulfate melts at about 160°C. As the temperature is slowly increased, the molten bisulfate provides an excellent heat-transfer medium for melting the chromic acid at 197°C without appreciable decomposition. As soon as the chromic acid melts, the agitator is stopped and the mixture separates into a heavy layer of molten chromic acid and a light layer of molten bisulfate. The chromic acid is tapped and flaked on water cooled roUs to produce the customary commercial form. The bisulfate contains dissolved CrO and soluble and insoluble chromic sulfates. Environmental considerations dictate purification and return of the bisulfate to the treating operation. 

A reaction A 2B runs in a tube provided with a cooling jacket that keeps the wall at 630 R. Inlet is pure A at 650 R and 50 atm. Other data are stated in the following. Find the profiles of temperature and conversion along the reactor, both with heat transfer and adiabatically. 

Vertical in-tube condensers are often designed for reflux or knock-back application in reactors or distillation columns. In this case, vapor flow is upward, countercurrent to the hquid flow on the tube wall the vapor shear ac4s to tliicken and retard the drainage of the condensate film, reducing the coefficient. Neither the fluid dynamics nor the heat transfer is well understood in this case, but Sohman, Schuster, and Berenson [J. Heat Transfer, 90, 267-276... 

Operating conditions The reactor is 10 cm ID, input of ethylbenzene is 0.069 kg mol/h, input of steam is 0.69 kgmol/h, total of 2,500 kg/h. Pressure is 1.2 bar, inlet temperature is 600 C. Heat is supplied at some constant temperature in a jacket. Performance is to be found with several values of heat transfer coeff cient at the wall, including the adiabatic case. 

Two complementai y reviews of this subject are by Shah et al. AIChE Journal, 28, 353-379 [1982]) and Deckwer (in de Lasa, ed.. Chemical Reactor Design andTechnology, Martinus Nijhoff, 1985, pp. 411-461). Useful comments are made by Doraiswamy and Sharma (Heterogeneous Reactions, Wiley, 1984). Charpentier (in Gianetto and Silveston, eds.. Multiphase Chemical Reactors, Hemisphere, 1986, pp. 104—151) emphasizes parameters of trickle bed and stirred tank reactors. Recommendations based on the literature are made for several design parameters namely, bubble diameter and velocity of rise, gas holdup, interfacial area, mass-transfer coefficients k a and /cl but not /cg, axial liquid-phase dispersion coefficient, and heat-transfer coefficient to the wall. The effect of vessel diameter on these parameters is insignificant when D > 0.15 m (0.49 ft), except for the dispersion coefficient. Application of these correlations is to (1) chlorination of toluene in the presence of FeCl,3 catalyst, (2) absorption of SO9 in aqueous potassium carbonate with arsenite catalyst, and (3) reaction of butene with sulfuric acid to butanol. 

Good heat transfer on the outside of the reactor tube is essential but not sufficient because the heat transfer is limited at low flow rates at the inside film coefficient in the reacting stream. The same holds between catalyst particles and the streaming fluid, as in the case between the fluid and inside tube wall. This is why these reactors frequently exhibit ignition-extinction phenomena and non-reproducibility of results. Laboratory research workers untrained in the field of reactor thermal stability usually observe that the rate is not a continuous function of the temperature, as the Arrhenius relationship predicts, but that a definite minimum temperature is required to start the reaction. This is not a property of the reaction but a characteristic of the given system consisting of a reaction and a particular reactor. 

A useful application is for tank and vessel heating, with the heater protruding into the vessel. Bayonet heat exchangers are used in place of reactor jackets when the vessel is large and the heat transfer of a large mass of fluid through the wall would be difficult or slow, because the bayonet can have considerably more surface area than the vessel wall for transfer. Table 10-43 compares bayonet, U-tube, and fixed-tubesheet exchangers. ... 

Ideally one would like a continuous reactor system to operate indefinitely at the desired steady-state. Unfortunately, a number of factors can cause shorter runs. Formation of wall polymer and latex flocculation is one such problem. This phenomenon can reduce reactor performance (for example, loss of heat transfer), lower product quality, and shorten run time. 

Reactor design can have a significant influence on reactor performance in a number of ways. Some aspects of reactor design such as heat transfer, structural design, etc., are reasonably well-understood. Other phenomena such as mixing details, latex flocculation, and the formation wall polymer are not completely understood. 

Heat transfer problems become more severe as reaction rates are increased and water-to-monomer ratios are reduced. In addition, as reactor sizes are increased for improved process economics, the amount of wall heat transfer surface area per unit volume will drop and result in a lower reactor space-time yield. 

As is common in most polymer reactor design problems, heat transfer is one of the major process concerns. For example, if the heat transfer is primarily through the wall of a jacketed reactor, the overall heat transfer coefficient is a function of both the agitator configuration and the degree of swelling of the particles. 

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