Liquid residence time

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Lapidus (LI) described liquid residence-time distribution studies for air-water and air-hydrocarbon in cocurrent, downward flow through a column of 2-in. diameter and 3-ft height. Spherical glass beads of 3.5. mm diameter and cobalt molybdate catalyst cylinders of -in. diameter were used as packing materials. 

Glaser and Lichtenstein (G3) measured the liquid residence-time distribution for cocurrent downward flow of gas and liquid in columns of -in., 2-in., and 1-ft diameter packed with porous or nonporous -pg-in. or -in. cylindrical packings. The fluid media were an aqueous calcium chloride solution and air in one series of experiments and kerosene and hydrogen in another. Pulses of radioactive tracer (carbon-12, phosphorous-32, or rubi-dium-86) were injected outside the column, and the effluent concentration measured by Geiger counter. Axial dispersion was characterized by variability (defined as the standard deviation of residence time divided by the average residence time), and corrections for end effects were included in the analysis. The experiments indicate no effect of bed diameter upon variability. For a packed bed of porous particles, variability was found to consist of three components (1) Variability due to bulk flow through the bed... 

Schoenemann (S4) reported qualitatively that the liquid residence-time distribution for cocurrent upward bubble flow was narrower than that observed in trickle-flow operation. 

Liquid residence-time distributions in mechanically stirred gas-liquid-solid operations have apparently not been studied as such. It seems a safe assumption that these systems under normal operating conditions may be considered as perfectly mixed vessels. Van de Vusse (V3) have discussed some aspects of liquid flow in stirred slurry reactors. 

The liquid residence-time distribution is close to plug flow in trickle-flow operation and corresponds to perfect mixing in the stirred-slurry operation, whereas the other types of bubble-flow operation are characterized by residence-time distributions between these extremes. 

Figure 1 Liquid Residence time distribution comparison - Experimental vs Numerical (a) counter-current operation Liquid flowrate l.SLmm (b) counter-current operation Liquid flowrate 3Lmin (c) co-current operation Liquid flowrate 1.5Lmm (d) co-current operation Liquid flowrate 3Lmm ...

Yields for the reactor system should be calculated on the basis of equal liquid residence times in the two reactors, with a negligible amount of unreacted chlorine in the vapour product streams. It may be assumed that the liquid product stream contains 1.5 wt per cent of hydrogen chloride ... 

Reactions involving gaseous and liquid reactants are carried out in various types of equipment. Packed columns, spray columns and bubble columns, as well as agitated tanks are all used (Fig. 2). Trickle-bed reactors are widely used in the petroleum industry for hydrodesulphurisation and related processes. In this type of reactor, liquid and gas both flow down through a bed of catalyst particles. The liquid flows around the particles as a thin film, thereby keeping the liquid residence time short and reducing undesirable side reactions. 

Glaser, M. B., and Lichtenstein, I. Interrelation of packing and mixed phase flow parameters with liquid residence time distribution. AJ.Ch.E. Journal 9, 30 (1963). (II,E)... 

Relationship between liquid residence time and the downward velocity of the trickling liquid reduces the range of possible gas and liquid flow-rates. 

In current design practice for downcomers, three parameters are commonly considered liquid residence time, liquid velocity, and downcomer backup. 

The residence-time concept is commonly misunderstood. The residence time is defined here as the downcomer volume divided by the liquid flow-rate. Typically, it is said that a liquid-residence time is required to allow adequate disengagement of vapour. Generally, two mechanisms are at work in a downcomer to provide the separation of vapour from liquid. The more obvious one is the relative velocity of the phases. If the downward velocity of the liquid exceeds the bubble-rise velocity, it does not matter how much residence time is provided, separation will not occur. This is true unless there is coalescence (the second mechanism), which there always is, to some extent. Coalescence is time dependent and therefore a residence-time criterion has some relevance. 

It is important to study the bubble rise velocity and its radial profile in a gas-liquid system as these are closely related to the hydrodynamics, and mass and heat transfer [25]. Bubble rise velocity and its radial profile have also significant influences on gas and liquid residence time distributions. A suitable bubble rise velocity and radial profile can improve production efficiency. Bubble rise velocities in a... 

Fig. 6. Qdyn/Qmax for BSA adsorption to fluidized Streamline DEAE at different linear flow rates. Original capacity data from Hjorth et al. [51], liquid residence time calculated from bed expansion data provided in Ref. [51]...

Linear flow rate (cm/min) Bed expansion (-) Liquid residence time (min) Dynamic capacity (mg/ml adsorbent)... 

Obviously liquid residence time is not an appropriate parameter to describe pore diffusion effects in fluidized bed adsorption. This may be elucidated by assessing particle side transport by a dimensionless analysis. Hall et al. [73] described pore diffusion during adsorption by a dimensionless transport number Np according to Eq. (17), De denoting the effective pore diffusion coefficient in case of hindered transport in the adsorbent pores and Ue the... 

Here L (1 — e) is equivalent to the number of binding sites available in the adsorbent bed. If during fluidization L is increased at higher U, (1 — e) is reduced, which is consistent with the fact, that the amount of matrix in the bed, which is available for protein binding, is independent from the fluidization conditions. Thus increased bed expansion does not affect pore diffusion as expressed by Np in spite of longer liquid residence time. The main influence on Np is found from the effective diffusion coefficient De and from the particle diameter dp. 

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