Countercurrent mode

Sedimentation is also used for other purposes. For example, relative motion of particles and Hquid iacreases the mass-transfer coefficient. This motion is particularly useful ia solvent extraction ia immiscible Hquid—Hquid systems (see Extraction, liquid-liquid). An important commercial use of sedimentation is ia continuous countercurrent washing, where a series of continuous thickeners is used ia a countercurrent mode ia conjunction with reslurrying to remove mother liquor or to wash soluble substances from the soHds. Most appHcations of sedimentation are, however, ia straight sohd—Hquid separation. 

Firing. A hot-air oven having forced circulation in a countercurrent mode is used to dry the fermented tea leaves and inactivates the key enzymes required for fermentation. The firing process generally occurs over an 18—20-min period, which is optimum for normal process efficiencies. 

Types of air strippers include packed towers, tray towers, and spray towers. Packed towers are packed or filled with small forms made of polyethylene [9002-88-4] stainless steel, poly(vinyl chloride) (PVC) [9002-86-2] or ceramic that provide large surface area to volume ratios which increase transfer rates into the air stream. Packed towers operate in countercurrent mode, that is, the aqueous stream enters at the top of the tower while air is blown in from the bottom. An example of this type of unit is shown in Figure 1. Channeling or short circuiting of the aqueous stream is minimized by... 

One of the oldest filters applied throughout the chemical processing industry is the rotary vacuum drum filter, which is illustrated in Figure 8. This machine belongs to the group of bottom feed configurations. Rotary drum filters are typically operated in the countercurrent mode of operation. The principle advantage of these machines is the continuity of their operation. 

Gas separation performances (H2/n-butane, n-hexane/2-2 dimethylbutane) have been measured using a sweep gas (countercurrent mode) in order to increase the permeation driving force (no differential pressure was used) permeate and retentate compositions (see Figure 2) were analysed using on line gas chromatography. 

Figure 9. H2 ( ) / n-butane ( ) separaticm with the ccxnposite zeolite-alumina membrane (fluxes in the permeate as a function of the tenq>erature). A mixture of hydrogen, n-btitane and nitrogen (12 14 74) was fed in the tube (Fig. 2) with a flow rate of 4.8 1/h. Sweep gas (N2), countercurrent mode, flow rate 4.3 1/h.

Fixed-bed reactors are used for testing commercial catalysts of larger particle sizes and to collect data for scale-up (validation of mathematical models, studying the influence of transport processes on overall reactor performance, etc.). Catalyst particles with a size ranging from 1 to 10 mm are tested using reactors of 20 to 100 mm ID. The reactor diameter can be decreased if the catalyst is diluted by fine inert particles the ratio of the reactor diameter to the size of catalyst particles then can be decreased to 3 1 (instead of the 10 to 20 recommended for fixed-bed catalytic reactors). This leads to a lower consumption of reactants. Very important for proper operation of fixed-bed reactors, both in cocurrent and countercurrent mode, is a uniform distribution of both phases over the entire cross-section of the reactor. If this is not the case, reactor performance will be significantly falsified by flow maldistribution. 

Bubble columns and various modifications such as airlift reactors, impinging-jet-reactors, downflow bubble columns are frequently used in lab-scale ozonation experiments. Moderate /qa-values in the range of 0.005-0.01 s l can be achieved in simple bubble columns (Martin et al. 1994 Table 2-4 ). Due to the ease of operation they are mostly operated in a cocurrent mode. Countercurrent mode of operation, up-flow gas and down-flow liquid, has seldom been reported for lab-scale studies, but can easily be achieved by means of applying an internal recycle-flow of the liquid, pumping it from the bottom to the top of the reactor. The advantage is an increased level of the dissolved ozone concentration cL in the reactor (effluent), which is especially important in the case of low contaminant concentrations (c(M)) and/or low reaction rate constants, i. e. typical drinking water applications... 

Because of the long, narrow configuration, the equipment appears to function in countercurrent mode. Other data of experiments with gas permeators as continuous columns appear in Figures 19.6(b) and (c) the original paper has data on other binary and some complex mixtures. 

A cooling tower operates in the countercurrent mode as illustrated by Figure 5.13. Entering air has a 5% wet-bulb temperature of 65°F. Hot process water enters the tower at 118°F and cold water leaves at a 15° approach to the wet-bulb (i.e., at 80°F). The cross-sectional area of the tower is 676 ft2. Determine the number of transfer units (Ntu ) required to meet the process requirements. Air is supplied to the tower by a blower having a capacity of 250,000 cfm and the water loading is 1500 lb/(hr)(ft2). 

Experiments to measure pressure drop and flooding limits were performed in a set-up accommodating monoliths with diameters of 43 mm (Fig. 8.16), while the length of the monoliths varied up to total length of 1 meter. The liquid was distributed by a nozzle the gas was introduced in countercurrent mode via mass flow controllers in the system. At the outlet of the monolith, a special device was mounted (Fig. 8.17), which improved draining of the monolith. The pressure drop along the column was measured using differential pressure transmitters. All experiments were performed at room temperature and atmospheric pressure. 

Fig. 8.25. Schematic flowsheet for pilot-scale reactive stripping experiments in countercurrent mode.

Figure 3.3 Tubular reactor showing cocurrent and countercurrent modes of operation. (The inner rectangle with diagonals represents a bed of catalyst.)...

Figure 4.17. Temperature profiles for heat exchangers operated in cocurrent and countercurrent modes.

Montecatini A process for making nitric acid by oxidizing ammonia. It differs from related processes in the equipment used to absorb the gases connected horizontal steel chambers operated in a countercurrent mode. Widely operated in Europe in the early 20th century. 

The three-phase continuous countercurrent fluidized-bed reactor and the spouted-bed reactor have been used on the laboratory scale. Pruden and Weber88 have shown that the countercurrent mode of operation for hydrogenation of a-methyl styrene performs better than the cocurrent fixed-bed operation under similar reaction conditions. 

The countercurrent mode of fixed-bed operation is normally used for physical absorption or gas-liquid reaction processes, rather than gas liquid-solid reaction processes. An extensive literature review on this type of column, as applied to the former processes, has already been reported11-45 and will not be repeated here. Two flow regimes in countercurrent operation, namely trickle-flow and bubble-flow, may be useful for gas-liquid-solid reactions. The majority of the discussion in this chapter will, therefore, be restricted to these flow regimes. 

The three-phase fluidized-bed reactor with countercurrent mode of operation was used by Pruden and Weber 10 to study the hydrogenation of a-methyl styrene to cumene in the presence of palladium black catalysts. They used low gas velocities so that the gas was dispersed as bubbles in the slurry. They showed that the countercurrent mode of operation was better than the slurry operation (with no liquid flow), due to improved catalyst usage and improved gas holdup characteristics. 

At the same time, in the case of separating mixtures of dissolved substances, use of a countercurrent mode opens up the possibility of developing processes which are inaccessible to an ion-exchanger fixed bed. 

Brine processing is carried out at a bromine concentration of 1 g/L and higher by steam stripping after preliminary acidification and by oxidative chlorination in a countercurrent mode. In this case, the raw bromide is obtained in one stage and then further reftned. 

Sloot et al. [1990] presented a simplified isothermal CNMR/ORG model which assumes that the two chambers divided by the membrane are well mixed. In practical applications, the model needs to be incorporated into a more complex model which, for example, considers the effect of flow configuration (cocurrent or countercurrent mode). In their model, mass transfer in the direction perpendicular to a flat membrane (i.e., y-direction) is considered for a general instantaneous, reversible reaction... 

An interesting monolithic configuration has recently been disclosed that can be suitable for three-phase processes carried out in countercurrent mode [10]. This can be particularly important for processes in which both thermodynamic and kinetic factors favor countercurrent operation, such as catalytic hydrodesulfurization. The flooding of a reactor is a considerable limitation for the countercurrent process run in conventional fixed-bed reactors. Flooding will not occur to that extent in the new monolith. A configuration of channels of the new monolith is such that subchannels open to the eentcrline are formed at the walls. The liquid flows downward, being confined in these subchannels and kept there by surface tension forces. The gas flows upward in the center of the channel. The results of studies on the new monolith concept are presented in Chapter 11 of this book. 

Kobe Steel Co. [42] has patented a monolithic process for oxidation of Fe to Fe in acidic aqueous solutions using monolith made of carbon. The monoliths were prepared by mixing active carbon with a binder, extrusion, and thermal treatment. Slices 150 mm in diameter of cell density 20-60 cells/cm were tested. Monolithic slices 30 mm thick were stacked and separated one from another with turbulizers. The reactor was operated in the countercurrent mode with the gas flowing upward. The liquid was recirculated. The liquid flow rate was varied from 250 to 333 cm sec and the gas flow rate ranged from 83 to 250 cm sec . Pressure was up to 0.31 MPa. The oxidation efficiency was 34-80% at circulation time of 1800 sec, and rose to 89-93% at 3600 sec. 

In most applications of trickle-flow reactors, the conversions generate heat that causes a temperature rise of the reactants, since the industrial reactors are generally operated adiabatically. In the cocurrent mode of operation, both the gas and the liquid rise in temperature as they accumulate heat, so there is a significant temperature profile in the axial direction, with the highest temperature at the exit end. When the total adiabatic temperature rise exceeds the allowable temperature span for the reaction, the total catalyst volume is generally split up between several adiabatic beds, with interbed cooling of the reactants. In the countercurrent mode of operation, heat is transported by gas and liquid in both directions, rather than in one direction only, and this may increase the possibility of obtaining a more desirable temperature profile over the reactor. 

Figure I also shows the hydrogen sulfide pressure profile in the case of countercurrent operation. It can be seen that the main part of the bed where high catalyst activity is needed now operates under H2S-lean conditions. Only a relatively small part of the bed operates under H2S-rich conditions, and in this part suppression of catalytic activity is less serious, since here the conversion of relatively reactive compounds takes place. Therefore, the countercurrent mode of operation will be clearly superior, and this is the more true the deeper the desulfurization target.

For the above reasons, countercurrent operation in a packed catalyst bed is seldom applied in practice. One example is the Arosat process [7], where a relatively small catalyst bed is operated in the countercurrent mode at the tail end of a conventional trickle-bed reactor to allow deeper hydrogenation of aromatics in kerosine. Since the absolute amounts of aromatics to be converted is small, little hydrogen is required, and consequently gas velocities can be low, while catalyst utilization is not so important, since the conversion is largely equilibrium limited. A more recent example is the Synsat process [8], which... 

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