Temperature mass transfer

Pathway temperatures must be strictly controlled (especially in single-phase systems) to create a balance between low-temperature oxide dissolution and high-temperature mass transfer limitations. 

Just as heat transfer refers to the energy transfer within a system or between a system and its surroundings occurring because of a difference in temperature, mass transfer refers to the transfer of mass (i.e., matter) which occurs within a system or between the system and its surroundings due to a difference in the concentration of a particular component between two points which are not in equilibrium. 

Other factors that may affect protein extraction are volume ratio of organic to aqueous phases (Vo g/V q), cosurfactant, temperature, mass transfer efficiency, protein charge, electrostatic potential of the RMs, and presence of other ions such as Ca+, Mg+, Ba+, etc. 

Changes in the particle structure have a strong effect on combustion behavior, influencing the particle temperature, mass transfer and pore diffusion rates, and consequently the rate-control regime of the process ( 5 ) Tlie changes in size and density of particles that have a homogeneous pore structure and small pore sizes (relative to particle size) are related to fractional burn-off, u, by... 

In the design of antimicrobial films or packages various factors may be of importance. These include the chemical nature of films and lasting antimicrobial activity, storage temperature, mass transfer coefficients, and physical properties of packaging materials (Quintavalla and Vicini, 2002). 

This case study is concerned with a three-phase gas-liquid-solid (catalytic) reaction. A systematic stepwise procedure has been described for determining the rate-controlling step, which depends on the catalyst type, particle size, operating pressure and temperature, mass transfer coefficient, and concentrations of reactants and products. As indicated, the rate-controlling step may change with location in a continuous reactor and with time in a batch reactor. 

Fig. 5. Theoretical heat-transfer e-folding time and warm-temperature mass-transfer time, Model II gas thermometer. Cold-volume temperature change = 0.5°K.

For mass transfer diffusion, is so difficult to find for most insulators that 2 could not be computed. An indication is given of whether each system appears to be in the high-temperature or low-temperature mass transfer regime (described further in the Discussion). Coverages are not reported because selfdiffusion is often involved, or because coverages are often so poorly defined. For several systems, the original anthors reported a product of a D° times the thickness 5 of a hypothetical diffusional layer. This layer is typically conceived to have a thickness between 0.3 and 1 nm [76Furl] to convert the reported values into standard nnits of D° for Table 21, 5 = 1.0 nm was nsed by the present authors. 

Figure 5.7 shows the specific mass flow of CO2 required in an extraction cascade to achieve the indicated degree of decaffeination for an extraction period of 12 hours. Data are shown for two pressures at the same temperature. Mass transfer tests of this type enable optimum mass flow rates to be determined for a given efficiency of extraction. It should be noted that the energy consumption of the bean extraction process is directly proportional to the amount of CO2 recirculated. 

The compatibility of INOR-8 and sodium is adequate in the temperature range presently contemplated for molten-salt reactor heat-exchanger operation. At higher temperatures, mass transfer could become a problem, and therefore the fabrication of duplex tubing has been investigated. Satisfactory duplex tubing has been made that consists of Inconel clad with type-316 stainless steel, and components for a duplex heat exchanger have been fabricated, as shown in Fig. 13-20. 

In general, one may say that at low temperatures and for small particles the system tends to region I, while at high temperatures mass transfer and pore diffusion become important, causing the system to tend to regime II or III. 

To speed up the process of attainment of the temperature steady value one can use special operations calculation without a kiln rotation, using large time intervals and calculation in two-dimensional R-tp geometry without regard for heat and mass transfer along an axis The program for realization of discussed simulation algorithms enables to calculate temperature in cells, a total number of which can not exceed 130 thousands A circular kiln structure can contain up to three layers. 

The effect can be important in mass-transfer problems (see Ref. 57 and citations therein). The Marangoni instability is often associated with a temperature gradient characterized by the Marangoni number Ma ... 

Temperature and pressure are not considered as primary operating variables temperature is set sufficiendy high to achieve rapid mass-transfer rates, and pressure is sufficiendy high to avoid vaporization. In Hquid-phase operation, as contrasted to vapor-phase operation, the required bed temperature bears no relation to the boiling range of the feed, an advantage when heat-sensitive stocks are being treated. 

Humidification. For wiater operation, or for special process requirements, humidification maybe required (see Simultaneous HEAT and mass transfer). Humidification can be effected by an air washer which employs direct water sprays (see Evaporation). Regulation is maintained by cycling the water sprays or by temperature control of the air or water. Where a large humidification capacity is required, an ejector which direcdy mixes air and water in a no22le may be employed. Steam may be used to power the no22le. Live low pressure steam can also be released directly into the air stream. Capillary-type humidifiers employ wetted porous media to provide extended air and water contact. Pan-type humidifiers are employed where the required capacity is small. A water filled pan is located on one side of the air duct. The water is heated electrically or by steam. The use of steam, however, necessitates additional boiler feed water treatment and may add odors to the air stream. Direct use of steam for humidification also requires careful attention to indoor air quahty. 

Below about 0.5 K, the interactions between He and He in the superfluid Hquid phase becomes very small, and in many ways the He component behaves as a mechanical vacuum to the diffusional motion of He atoms. If He is added to the normal phase or removed from the superfluid phase, equiHbrium is restored by the transfer of He from a concentrated phase to a dilute phase. The effective He density is thereby decreased producing a heat-absorbing expansion analogous to the evaporation of He. The He density in the superfluid phase, and hence its mass-transfer rate, is much greater than that in He vapor at these low temperatures. Thus, the pseudoevaporative cooling effect can be sustained at practical rates down to very low temperatures in heHum-dilution refrigerators (72). 

Fluid mixing is a unit operation carried out to homogenize fluids in terms of concentration of components, physical properties, and temperature, and create dispersions of mutually insoluble phases. It is frequently encountered in the process industry using various physical operations and mass-transfer/reaction systems (Table 1). These industries include petroleum (qv), chemical, food, pharmaceutical, paper (qv), and mining. The fundamental mechanism of this most common industrial operation involves physical movement of material between various parts of the whole mass (see Supplement). This is achieved by transmitting mechanical energy to force the fluid motion. 

Manufacture and Processing. Mononitrotoluenes are produced by the nitration of toluene in a manner similar to that described for nitrobenzene. The presence of the methyl group on the aromatic ring faciUtates the nitration of toluene, as compared to that of benzene, and increases the ease of oxidation which results in undesirable by-products. Thus the nitration of toluene generally is carried out at lower temperatures than the nitration of benzene to minimize oxidative side reactions. Because toluene nitrates at a faster rate than benzene, the milder conditions also reduce the formation of dinitrotoluenes. Toluene is less soluble than benzene in the acid phase, thus vigorous agitation of the reaction mixture is necessary to maximize the interfacial area of the two phases and the mass transfer of the reactants. The rate of a typical industrial nitration can be modeled in terms of a fast reaction taking place in a zone in the aqueous phase adjacent to the interface where the reaction is diffusion controlled. 

The physical mass-transfer rate of o2one into water is affected by the gaseous o2one concentration, temperature, pressure, gas dispersion, turbulence, mixing, and composition of the solution, ie, pH, ionic strength, and the presence of reactive substances. Mass transfer of gaseous o2one into... 

Reaction rates typically are strongly affected by temperature (76,77), usually according to the Arrhenius exponential relationship. However, side reactions, catalytic or equiHbrium effects, mass-transfer limitations in heterogeneous (multiphase) reactions, and formation of intermediates may produce unusual behavior (76,77). Proposed or existing reactions should be examined carefully for possible intermediate or side reactions, and the kinetics of these side reactions also should be observed and understood. 

Using this simplified model, CP simulations can be performed easily as a function of solution and such operating variables as pressure, temperature, and flow rate, usiag software packages such as Mathcad. Solution of the CP equation (eq. 8) along with the solution—diffusion transport equations (eqs. 5 and 6) allow the prediction of CP, rejection, and permeate flux as a function of the Reynolds number, Ke. To faciUtate these calculations, the foUowiag data and correlations can be used (/) for mass-transfer correlation, the Sherwood number, Sb, is defined as Sh = 0.04 S c , where Sc is the Schmidt... 

Although equation 35 is a simple expression, it tends to be confusing. In this equation the enthalpy difference appears as driving force in a mass-transfer expression. Enthalpy is not a potential, but rather an extensive thermodynamic function. In equation 35, it is used as enthalpy pet mole and is a kind of shorthand for a combination of temperature and mass concentration terms. 

Thus, the enthalpy and temperature of the vapor—Hquid interface are related to the Hquid temperature and gas enthalpy at any point in the column through a ratio of heat- and mass-transfer coefficients. 

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