Liquid mass velocity effect

Summarizing then, it has been shown that for at least three reactions, that under strong mass transfer control with a limiting gas phase conponent, liquid mass velocity effects on the global reaction rates showing a minimum can be obtained due to the countering effects of liquid velocity on gas/catalyst and liquid/cata-lyst contacting. 

To quantify this model further a more thorough knowledge of the physics of the flow in trickle beds is needed. However, at this time it can be used as a criteria for when to expect a liquid mass velocity effect and as a vehicle for checking the consistency of experimental data. 

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

The effect of hills is interesting, in that no credit can be taken for the downhill side of the pipeline. The sum of all the uphill elevations appears as a pressure loss in actual operating practice. Baker includes an elevation correction factor which attempts to allow for the fact that the fluid-mixture density in the inclined uphill portion of the line is not accurately known. The gas mass-velocity seems to be the major variable affecting this correction factor, although liquid mass-velocity, phase properties. 

A review of previous reaction studies used to support the development of trickle-bed reactor models is presented. This review suggests that these previous models have neglected the effect of incomplete liquid-solid contacting even though it was experimentally observed in a number of cases. For the few cases where it was included, it was used as an adjustable parameter to match the measured conversion versus liquid mass velocity data to the model. 

Although the value of critical liquid mass velocity for kapp/kv 1 is above 0.3 g cm-2 s it is not a universal constant. It is a function of reactor geometry and catalyst-bed packing and, in some instances, no effect of mass velocity on the contacting effectiveness can be observed. 

At high liquid mass velocities, good contacting effectiveness can be obtained even for reactor/catalyst diameter ratios as low as 1.4/1. 

Ibs/ft hr (0.10 kg/m s) in minimum as liquid mass velocity utilized in a pilot plant test and a correction method involving mainly effects of feedstock properties on desulfurization activity are proposed to get better agreement in evaluating aging performance and cycle length of a catalyst system with multiple catalysts between pilot and commercial units. 

Catalyst lives were compared with reactor metal distribution, obtained from micro, pilot and commercial units. It was found that a micro reactor with low liquid mass velocity caused metal pass-through to down stream catalysts, resulting in a shorter life of catalyst system than that in the commercial operation. Therefore in this paper, effects of liquid mass velocity and feedstock properties on catalyst aging performance of the catalyst system are discussed. 

An absorber packed with 1-in. Intalox saddles operates at 50 C and 10 atm with a liquid mass velocity five times the gas mass velocity. Assuming the gas and liquid are similar to air and water, what gas mass velocity will give a pressure drop of 0.5 in. HjO/ft packing Use the generalized correlations to get the effect of changed physical properties, and apply a correction to the data of Fig. 22.4. 

Fig. 18 - Ratio of apparent and effective diffusivlty as a function of the superficial liquid mass velocity...

Solid-Liquid Mass Transfer There is potentially a major effect of both shear rate and circulation time in these processes. The sohds can either be fragile or rugged. We are looking at the slip velocity of the particle and also whether we can break up agglomerates of particles which may enhance the mass transfer. When the particles become small enough, they tend to follow the flow pattern, so the slip velocity necessary to affect the mass transfer becomes less and less available. 

Ggi, = mass velocity of liquid, Ib/hr (fF). For outside horizontal tubes, use projected area (diameter X length) of the tube, not the outside surface area. This assumes that only half of the tube is effective for bubble release. This does not apply to actual heat transfer area. 

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

The above experimenters have used the technique described to obtain flow rate measurements of the liquid wall-film at various mass velocities, tube dimensions, etc., and some typical results from Staniforth and Stevens (S7) are shown in Fig. 7. Also shown are the values of burn-out heat flux obtained at the four different mass velocities indicated. It can be seen that the liquid-film flow rate decreases steadily with increasing heat flux until at burn-out the flow rate becomes zero or very close to zero. We thus have confirmation of a burn-out mechanism in the annular flow regime which postulates a liquid film on the heated wall diminishing under the combined effects of evaporation, entrainment, and deposition until at burn-out, the film has become so thin that it breaks up into rivulets which cause dry spots and consequent overheating. 

The above conclusion must certainly be taken with a measure of reserve as regards the mass velocity, for at very low velocities it appears reasonable to expect that the relative motion between vapor and liquid in a boiling channel will be affected sufficiently to influence the burn-out flux. Barnett s conclusion also applies to simple channels, whereas Fig. 35 discussed in Section VIII,C shows that a rod-bundle system placed in a horizontal position is likely to incur a reduction in the burn-out flux at mass velocities less than 0.5 x 106 lb/hr-ft2, presumably on account of flow stratification. Furthermore, gravitational effects induced in a boiling channel by such means as swirlers placed inside a round tube can certainly increase the burn-out flux as shown by Bundy et al. (B23), Howard (H10), and Moeck et al. (Ml5). 

The process parameters influencing droplet sizes may include liquid pressure, flow rate, velocity ratio of air to liquid (mass flow rate ratio of air to liquid), and atomizer geometry and configuration. It has been clearly established that increasing the velocity ratio of air to liquid is the most important practical method of improving atomization)211] In industrial applications, however, the use of mass flow rate ratio of air to liquid has been preferred. As indicated by Chigier)2111 it is difficult to accept that vast quantities of air, that do not come into any direct contact with the liquid surface, have any influence on atomization although mass flow rates of fluids include the effects of velocities. 

The possible existence of an interface resistance in mass transfer has been examined by Raimondi and Toor(12) who absorbed carbon dioxide into a laminar jet of water with a flat velocity profile, using contact times down to 1 ms. They found that the rate of absorption was not more than 4 per cent less than that predicted on the assumption of instantaneous saturation of the surface layers of liquid. Thus, the effects of interfacial resistance could not have been significant. When the jet was formed at the outlet of a long capillary tube so that a parabolic velocity profile was established, absorption rates were lower than predicted because of the reduced surface velocity. The presence of surface-active agents appeared to cause an interfacial resistance, although this effect is probably attributable to a modification of the hydrodynamic pattern. 

The influence of pressure on the mass transfer in a countercurrent packed column has been scarcely investigated to date. The only systematic experimental work has been made by the Research Group of the INSA Lyon (F) with Professor M. Otterbein el al. These authors [8, 9] studied the influence of the total pressure (up to 15 bar) on the gas-liquid interfacial area, a, and on the volumetric mass-transfer coefficient in the liquid phase, kia, in a countercurrent packed column. The method of gas-liquid absorption with chemical reaction was applied with different chemical systems. The results showed the increase of the interfacial area with increasing pressure, at constant gas-and liquid velocities. The same trend was observed for the variation of the volumetric liquid mass-transfer coefficient. The effect of pressure on kia was probably due to the influence of pressure on the interfacial area, a. In fact, by observing the ratio, kia/a, it can be seen that the liquid-side mass-transfer coefficient, kL, is independent of pressure. 

Previous workers have studied the influence of the ratio of the cross-section area of the downcomer to the riser [4,5], the reactor height [6,7], the gas-liquid separator configuration [8], and the distributor type and location [9]. All these affect the flow characteristics and mass transfer. Most previous works focus on global parameters, such as the liquid circulation velocity [10-13] and the average gas holdup in the riser [14-16]. Although much work has been carried out on EL-ALRs, the proper design and scale-up of an EL-ALR is still difficult because any variation in the physical properties of the gas or the liquid and the reactor structural feathers can have a considerable effect on the hydrodynamics... 

Internal recycle reactors are designed so that the relative velocity between the catalyst and the fluid phase is increased without increasing the overall feed and outlet flow rates. This facilitates the interphase heat and mass transfer rates. A typical internal flow recycle stirred reactor design proposed by Berty (1974, 1979) is shown in Fig. 18. This type of reactor is ideally suited for laboratory kinetic studies. The reactor, however, works better at higher pressure than at lower pressure. The other types of internal recycle reactors that can be effectively used for gas-liquid-solid reactions are those with a fixed bed of catalyst in a basket placed at the wall or at the center. Brown (1969) showed that imperfect mixing and heat and mass transfer effects are absent above a stirrer speed of about 2,000 rpm. Some important features of internal recycle reactors are listed in Table XII. The information on gas-liquid and liquid-solid mass transfer coefficients in these reactors is rather limited, and more work in this area is necessary. 

Gas-liquid mass transfer can have a strong effect on TBR overall performance therefore its accurate evaluation is essential for achieving successful design and scale-up. In spite of the vast information available on gas-liquid mass transfer characteristics of atmospheric TBRs [1,2] only a few researchers have studied how interfacial areas, a, and volumetric liquid-side mass transfer coefficients, kLa, evolve at elevated pressures. For example, it has been reported that both a and kLa increase as gas density is rised while the gas superficial velocity is kept constant [3-5], Similar observations regarding gas hold-up and two-phase pressure drop, as well as the delay in the onset of pulsing have also been reported [6],... 

As shown in Fig. 4, the effect of pressure is to increase the corresponding kLa. Similarly to a, kLa depends on pressure only for gas and liquid velocities above some critical velocities. The effect of pressure can be due either to an increase in kL or in a, or in both. A literature review on the pressure effect on kL in different gas-liquid contactors reveals that kL may be considered as independent of reactor pressure. Hence, kLa should vary with pressure only via the effect of the interfacial area. Following the assumption of the presence of small bubbles in the liquid films, gas-liquid mass transfer can be split into a mass transfer from the continuous gas to the liquid film, with a mass transfer coefficient equal to the one at atmospheric conditions and a mass transfer from the bubbles to the surrounding liquid, as if bubbles were suspended in a stagnant medium. Then, contribution brought about by bubbles is calculated as the product of the excess interfacial area ab and the mass transfer coefficient of a bubble in a stagnant medium (Sh = 2) ... 

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