Cocurrent chemical reactions

Pipe Lines The principal interest here will be for flow in which one hquid is dispersed in another as they flow cocurrently through a pipe (stratified flow produces too little interfacial area for use in hquid extraction or chemical reaction between liquids). Drop size of dispersed phase, if initially very fine at high concentrations, increases as the distance downstream increases, owing to coalescence [see Holland, loc. cit. Ward and Knudsen, Am. In.st. Chem. Eng. J., 13, 356 (1967)] or if initially large, decreases by breakup in regions of high shear [Sleicher, ibid., 8, 471 (1962) Chem. Eng. ScL, 20, 57 (1965)]. The maximum drop size is given by (Sleicher, loc. cit.)... 

Wei, F., Chen, W., Jin, Y., and Yu, Z. Axial solids mixing in cocurrent downflow circulating fluidized bed, in Proceeding of 5th National Meeting on Chemical Reaction Engineering (in Chinese), Tianjing (1993). 

For adsorption plus chemical reaction, consider cocurrent falling film multitube system. See also long tube falling film evaporator. Section 16.11.4.1. 

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. 

Flooding is virtually impossible to induce in cocurrent flow, but extremely easy to produce in a countercurrent absorber. Cocurrent absorbers also require a lower pressure drop across the packing for equivalent throughputs, and required rates for the liquid absorbent are usually lower in a cocurrent absorber. Furthermore, if an absorbing liquid is chosen such that a chemical reaction occurs between the liquid and gas phases, conditions may be adjusted so that a constant concentration... 

Figure 4-5 Sensitivity of reactant conversion to changes in flow rate ratio for nonisother-mal plug-flow tubular reactors with exothermic chemical reaction and cocurrent cooling in a concentric double-pipe configuration with radius ratio k = 0.5. The inlet tempoatures are 340 K for the reactive fluid and 335 K for the cooling fluid.

Cocurrent Cooling Fluid That Undergoes a Strongly Endothermic Chemical Reaction... 

Consider a liquid-phase plug-flow tubular reactor with irreversible nth-order endothermic chemical reaction. The reactive mixture is heated with a fluid that flows cocurrently in the annular region of a double-pipe configuration. The mass and heat transfer Peclet numbers are large for both fluids. All physical properties of both fluids are independent of temperature and conversion, and the inlet conditions at z = 0 are specified. What equations are required to investigate the phenomenon of parametric sensitivity in this system ... 

Miscellaneous Applications. A variety of special evaporation problems have been solved in thin-film evaporators evaporation and chemical reaction two-phase flow (immiscible fluids, slurries, suspensions) cocurrent evaporation high overhead splits multiple-effect evaporation end others. 

In Section 5.4.4, we studied a variety of chemical reaction facilitated separation where the reaction was taking place in a thin liquid layer acting as the liquid membrane Figure 5.4.4 illustrated a variety of liquid membrane permeation mechnisms. Here we will identify first the structural configuration of the liquid membranes as they are used in separators with countercurrent flow pattern (as well as for the cocurrent flow pattern). There are three general classes of liquid membrane structures emulsion liquid membrane (ELM) supported liquid membrane (SLM) or immobilized liquid membrane (ILM) hollow fiber contained liquid membrane (HFCLM). Each will be described very briefly. 

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. 

In addition, as shown in Section 1.3, the fluxes of the concentration of chemical components are determined generally by the differences in the thermodynamic rushes of corresponding reaction groups rather than by true thermodynamic forces (chemical affinities of the reactions). Consider the simplest pathway of cocurrent transformations, which includes two parallel channels of independent reversible transformation of initial reactant R into products Pi and P2 ... 

Monolithic Loop Reactor A novel MLR was developed af Air Products and Chemicals (Figure 17) (144). The reactor contains a monolithic catalyst operating under cocurrent downflow condifions. Because the residence time in the monolith is short and the heat of reaction has to be removed, the liquid is continually circulated via an external heat exchanger until the desired conversion is reached. The concept was patented for the hydrogenation of dinifrofoluene fo give toluenediamine (37). 

In general, the procedure for designing a bubble column reactor (BCR) (1 ) should start with an exact definition of the requirements, i.e. the required production level, the yields and selectivities. These quantities and the special type of reaction under consideration permits a first choice of the so-called adjustable operational conditions which include phase velocities, temperature, pressure, direction of the flows, i.e. cocurrent or countercurrent operation, etc. In addition, process data are needed. They comprise physical properties of the reaction mixture and its components (densities, viscosities, heat and mass diffusivities, surface tension), phase equilibrium data (above all solubilities) as well as the chemical parameters. The latter are particularly important, as they include all the kinetic and thermodynamic (heat of reaction) information. It is understood that these first level quantities (see Fig. 3) are interrelated in various ways. 

Oxidation of organic and inorganic species in aqueous solutions can find applications in fine chemical processes and wastewater treatment. Here, the oxidant, often either air or pure oxygen, must undergo all the mass transfer steps mentioned above in order for the reaction to proceed. During the last decade, increased environmental constraints have resulted in the application of novel processes to the treatment of waste streams. An example of such a process is wet air oxidation. Here, the simplest reactor design is the cocurrent bubble column. However, the presence of suspended organic and inert solids makes the use of monolith reactors favorable. 

Despite the experience with batch reactors it may be worthwhile to operate continuous reactors also for fine chemicals. Continuously operated reactors only demand for one start-up and one shut-down during the production series for one product. This increases the operating time efficiency and prevents the deactivation of dry catalysts this implies that the reactor volume can be much smaller than for batch reactors. As to the reactor type for three phase systems an agitated slurry tank reactor [5,6] is not advisable, because of the good mixing characteristics. Specially for consecutive reaction systems the yields to desired products and selectivities will be considerably lower than in plug flow type reactor. The cocurrent down flow trickle flow reactor... 

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. 

---