Continuous cocurrent operations

The continuous hydrogenation of butynediol in a cocurrently operated bubble column reactor is chosen as an example for a second order reaction where a sparingly soluble gas phase component (H2) reacts on the catalyst surface with a liquid phase component in excess (butyne-... 

We will first provide a very brief illustration of the governing equations for mass transport and the operating line for a two-phase continuous cocurrent separation system in a conventional chemical engineering context. This will be followed by a brief treatment of the multi-component separation capability of such a system. Cocurrent chromatographic separation in a two-phase system, where both phases are mobile and in cocurrent flow, will be introduced next. The systems of interest are micellar electrokinetic chromatography (MEKC) chromatography with two mobile phases, a gas phase and a liquid phase capillary electrochromatography, with mobile nanoparticles in the mobile liquid phase. Continuous separation of particles from a gas phase to a cocurrent liquid phase in a scrubber will then be illustrated. Finally, cocurrent membrane separators will be introduced. 

A trickle bed is a continuous three-phase reactor. Three phases are normally needed when one reactant is too volatile to force into the liquid phase or too nonvolatile to vaporize. Operation of a trickle bed is limited to cocurrent downflow to allow the vapor to force the liquid down the column. This contacting pattern gives good interaction between the gaseous and liquid reactants on the catalyst surface. 

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). 

For some reactions listed in Table 1-4A, the fixed-bed reactor is operated under cocurrent-upflow conditions. Unlike the trickle-flow condition, this type of operation is normally characterized by bubble-flow (at low liquid and gas rates) and pulsating-flow (at high gas flow rates) conditions. Normally, the bubble-flow conditions are used. In the SYNTHOIL coal-liquefaction process, both pulsating-and spray-flow conditions are used, so that the solid reactant (coal) does not plug the reactor. In bubble flow, the gas is the dispersed phase and the liquid Ls a continuous phase. In pulsating flow, pulses of gas and liquid pass through the reactor. In the spray-flow regime, the gas is a continuous phase and the liquid is a dispersed phase. 

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. 

For a bubble column operated under net liquid flow conditions, both the gas and the liquid are introduced and removed continuously. The liquid flow may be cocurrent or countercurrent. The boundary condition at the bottom depends on the way in which the liquid is introduced or taken away. Consider the bottom design as shown in Fig. 32. 

Gaseous isobutane purification process from C4 mixture by adsorption has been developed. For the purification, two adsorbents are applied. Zeolite A is used for the removal of paraffin components, and CMS or silver impregnated clay for the removal of C4 olefins. The continuous 3-bed process operated at ambient temperature was comprised of six steps, i.e., pressurization-1 by the cocurrent effluent gas from the other bed, pressurization-2 by isobutane product, adsorption, cocurrent depressurization, countercurrent blowdown, and low pressure purge by isobutane product. 

Co and Ci are the continuous phase solute concentrations in teed and inside the mixer, respectively, and are the feed rates of the continuous and emulsion phases, respectively, Cir is the internal reagent concentration (based on volume of emulsion), R is the emulsion globule radius, Kr is the emulsion phase holdup volume in the mixer, a is the partition coefficient for solute between external phase and emulsion, is the effective solute diffusivity in the emulsion, and % is the dimensionless reaction front position. Hatton and Wardius [51] also extended their analysis to develop simple graphical and numerical procedures for the prediction of multistage extraction performance of mixer-settler trains operating either cocurrendy or countercurrently without any external recycle over individual stages. For a typical stage i in a cocurrent mixer-settler, they defined the parameter 6 as... 

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... 

Static mixers are also used for continuous gas-liquid operations (see Section 9.9). The orientation of the mixer is important. A vertical orientation with both gas and liquids passing cocurrently downward is desirable. Considerable vendor information is available on gas-liquid dispersion in static mixers. 

Methyl Ester Hydrogenolysis. The flow sheet for the continuous methyl ester, catalyst slurry process is shown in Figure 1. The dry methyl ester, hydrogen, and catalyst slurry are fed cocurrently to a series of four vertical reactors operated at 250—300°C and 20,700 kPa (3000 psi). The reactors are unagitated, empty tubes, designed to provide adequate residence time, minimum backmixing, and a reasonable column height. Fresh catalyst powder is... 

Mode of operation (Semi)batch or continuous Continuous Continuous (+ quasi batch), cocurrent downward ... 

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