Cocurrent operations

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. 

Gas flow in these rotary dryers may be cocurrent or countercurrent. Cocurrent operation is preferred for heat-sensitive materials because gas and product leave at the same temperature. Countercurrent operation allows a product temperature higher than the exit gas temperature and dryer efficiency may be as high as 70%. Some dryers have enlarged cylinder sections at the material exit end to increase material holdup, reduce gas velocity, and minimize dusting. Indirectly heated tubes are installed in some dryers for additional heating capacity. To prevent dust and vapor escape at the cylinder seals, most rotary dryers operate at a negative internal pressure of 50—100 Pa (0.5—1.0 cm of water). 

Column reactors are the second most popular reactors in the fine chemistry sector. They are mainly dedicated reactors adjusted for a particular process although in many cases column reactors can easily be adapted for another process. Cocurrently operated bubble (possibly packed) columns with upflow of both phases and trickle-bed reactors with downflow are widely used. The diameter of column reactors varies from tens of centimetres to metres, while their height ranges from two metres up to twenty metres. Larger column reactors also have been designed and operated in bulk chemicals plants. The typical catalyst particle size ranges from 1.5 mm (in trickle-bed reactors) to 10 mm (in countercurrent columns) depending on the particular application. The temperature and pressure are limited only by the material of construction and corrosivity of the reaction mixture. 

Removal of reaction products can shift the equilibrium, forcing the reaction to go to completion. This can be effected by evaporation of products from the reaction mixture (reactive distillations), extraction (including supercritical extraction) of products from the reaction mixture (reactive extractions), or membrane processes. Counter- and cocurrent operation also falls within this category. If the reaction is equilibrium-limited or inhibited by reaction products countercurrent operation outperforms cocurrent operation. 

Related Calculations. This example assumes standard cocurrent operation of the column with downflow feed and downflow regeneration. Countercurrent operation is a special case that is best handled by a manufacturer of ion-exchange equipment. 

Figure 4.11. Heat exchangers with countercurrent and cocurrent operations.

A summary of the available experimental data for gas liquid mass transfer under trickle-flow conditions is given in Tabic 6-8. A significant portion of this table is derived from Table 4 of Charpentier.12 The liquid-phase mass-transfer coefficient is affected by both gas and liquid flow rates. At high gas and liquid rates, the values of fcLaL may exceed 1 s l, a value normally not achieved in any other type of gas-liquid contactor. When the liquid is trickling over the packing, kLaL values for the cocurrent operation are of the same order of magnitude as those obtained in countercurrent operation under similar working conditions. 

The slope of the line would be an indication of the reactor stability to variations in the inlet coolant temperature. Clearly, cocurrent operation provides better sensitivity and this conclusion is a general one. The reason for this is that by operating in a cocurrent manner the greatest ability to remove heat [largest AT coolant)] can occur in the region of highest... 

Countercurrent operation can have advantages over cocurrent operation in situations where the desired conversion is suppressed by (by-)products generated in the process that act as a catalyst poison (product inhibition) or when the conversion is limited by thermodynamic... 

An example of an equilibrium-limited reaction is the hydrogenation of aromatics in petroleum fractions. This is illustrated by Fig. 3, which shows that under the prevailing conditions, saturation of aromatics is kinetically limited at temperatures below about 370°C, but that above this temperature, thermodynamic limitation occurs. In cocurrent operation, the hydrogen partial pressure will be lowest at the reactor outlet, due to the... 

If octanol is the stationary phase and water the mobile phase, Pq/w is the oetanol-water partition eoeffieient without any assumption. Correlations of the or log Pq/ values obtained with the same liquid system by the shake flask method and by CCC produce straight lines with a slope unity and a negligible intercept. The validity and solidity of the method was assessed by Gluck and Martin for P / coefficients [2]. The P / range that can be obtained directly by CCC is 0.05-200 [1]. It is limited on the high side by the experiment duration. A P / value of 200 corresponds to a Vr retention volume of 6 L with a Vs value of only 30 mL [Eq. (1)]. This is 1200 min or 20 h with a 5-mL/min flow rate. The lower-side limitation is due to experimental precision. The difference between the retention volume Vr and the dead volume Vm is equal to PVs [Eq. (1)]. With a 30-mL Vs volume, the P / value of 0.05 corresponds to a - Vm value of only 1.5 mL. Such a low value may be difficult to evaluate with an acceptable accuracy. To increase the measurable Pq/w range, the fact that the CCC stationary phase is a liquid can be used. This led to the dual-mode use of CCC and the cocurrent operation. 

Countercurrent operation of shaft and rotary kiln furnaces leads to extensive heat transfer between feedstock and outgoing products. The sensible heat of the products remains within the system which enhances its thermal yield. Cocurrent operation shows none of these advantages yet it is a simple and efficient method to avoid tar problems. Indeed the tar is extensively cracked while moving through the hearth zone. Also the problem of wastewater is somewhat simplified. Crosscurrent operation is only seldom used. 

Countercurrent operation is the most widely used absorption equipment arrangement. As the gas flow increases at constant liquid flow, liquid holdup must increase. The maximum gas flow is limited by the pressure drop and the liquid holdup that will build up to flooding. Contact time is controlled by the bed depth and the gas velocity. In countercurrent flow mass transfer driving force is maximum at the gas entrance and liquid exit. Cocurrent operation can be carried out at high gas velocities because there is no flooding limit. In fact, liquid holdup decreases as velocity increases. However, the mass transfer driving force is smaller than in countercurrent operation. 

For separation processes, countercurrent operation is usually preferred over cocurrent operation because, in the former, the driving force is maintained at a larger value along a boundary. 

While tray columns and packed columns are invariably the devices used for stripping, other devices are occasionally used for absorption or scrubbing. These are often proprietary, and the vendor does the design based on the specifications provided by the user and/or process designer. While the foregoing material is based on countercurrent contacting of gas and liquid, there may be some instances when cocurrent operation has advantages ... 

COCURRENT FLOW OPERATION. When the chemical reaction is essentially irreversible and the equilibrium partial pressure of the solute is zero, the number of transfer units for a given separation is the same for countercurrent operation or for cocurrent flow of liquid and gas. Figure 22.24 shows typical operating lines for both cases. In this diagram x is the total solute absorbed and reacted and not the amount of solute present in the original form. For cocurrent operation with the feeds at the top, the gas leaving at the bottom is exposed to rich liquid, which has absorbed a lot of solute, but if jf = 0, the driving force is just y, and Noy is calculated from Eq. (22.50), as for countercurrent flow. 

The advantage of cocurrent operation is that there is no flooding limitation, and a gas flow rate much greater than normal can be used. This reduces the required column diameter, and the corresponding increase in liquid and gas mass velocity gives high mass-transfer coefRcients. Liquid rates as high as 50,000 to... 

In view of the heat sensitivity of the solids, cocurrent operation will be used. The outlet gas temperature is found from Eq. (24.8) for adiabatic drying. Assume the number of transfer units is 1.5, The inlet wet-bulb temperature from Fig. 23.2, is 102°F. Since T j is 260°F, Eq. (24.8) gives... 

This is the equation of a straight line on the XY diagram through the points (Xp Tj) and (X2, Y2) with slope - LJVS, the operating line for cocurrent operation. Figures... 

Therefore, Ts (min) = 6.09 x0.488 = 2.957 kg/s and Ls = 2.0 x 2.957 = 5.920 kg/s. Notice that the mass velocity of the silica gel required for cocurrent operation is about 11 times that required for countercurrent operation. This example dramatically illustrates the fact that the driving force for mass transfer is used much more efficiently in countercurrent than in cocurrent operation. 

An often used gas-liquid reactor is the bubble column. The gas is usually fed from the bottom through a sparger and the liquid flows either cocurrently or counter-currently. Counter-current operation is more efficient than co-current, but for certain types of parallel reactions, cocurrent operation can give better selectivity. Bubble columns are often operated in semi-batch mode the gas bubbles through the liquid. This mode of operation is attractive in the production of fine chemicals which are produced in small quantities - especially in the case of slow reactions. The flow patterns can vary a lot in a bubble column. Generally, as a rule of thumb, the liquid phase is more back-mixed than the gas phase. The plug flow model is suitable for the gas phase whereas the liquid phase can be modelled with the backmixed, dispersion, or plug flow model. 

This special case in Equation 3.28 is valid for any system, regardless of the phase equilibrium properties of the system. The significance of = — 1 is that this represents the point where the vapor flowrate in the CS tends to zero, and can be seen as the switching point from a counter-current mode of operation to a cocurrent operation mode. The only pinch point at these conditions is thus at Xa and profiles will resemble the mixing only process shown in Figure 3.12b. More importantly, though, this shows that any pinch point loci will always move through Xa, irrespective of phase equilibrium. 

Vickers- Zimmer multitubular reactor with downward cocurrent operation evaporative cooling with inert solvents... 

For cocurrent operation (case 3a in Figure 3.1) both the case design and simulation are simple. The four balance equations (3.18 through 3.21) supplemented by a suitable drying rate and heat flux equations are solved starting at inlet end of the dryer, where all boundary conditions (i.e., all parameters of incoming streams) are defined. This situation is shown in Figure 3.4. 

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