Co-current contacting

For co-current contact with immiscible solvents where the equilibrium curve is a straight line as in the present case ... 

Co-current contact with partially miscible solvents... 

Co-current contacting is applicable only for cases requiring less than 1.0 theoretical contact however, for those cases where it is appropriate, it can offer significant advantages. According to Baker and Rogers (1989) these advantages include... 

Packed Towers (also known as wet-scrubbers) A packed tower is a vessel that is filled with some suitable packing material. The liquid is introduced at the top of the tower and flows down through the packing to the bottom, and in so doing, exposes a large surface area to contact with the gas. The gas may be introduced at either end of the tower since the gas and liquid flows may be either co-current or counter-current. The counter-current operation is most commonly used. 

The term three-phase fluidization requires some explanation, as it can be used to describe a variety of rather different operations. The three phases are gas, liquid and particulate solids, although other variations such as two immiscible liquids and particulate solids may exist in special applications. As in the case of a fixed-bed operation, both co-current and counter- current gas-liquid flow are permissible and, for each of these, both bubble flow, in which the liquid is the continuous phase and the gas dispersed, and trickle flow, in which the gas forms a continuous phase and the liquid is more or less dispersed, takes place. A well established device for countercurrent trickle flow, in which low-density solid spheres are fluidized by an upward current of gas and irrigated by a downward flow of liquid, is variously known as the turbulent bed, mobile bed and fluidized packing contactor, or the turbulent contact absorber when it is specifically used for gas absorption and/or dust removal. Still another variation is a three-phase spouted bed contactor. 

In designing a unit, each side of the exchanger is independently tailored to the duty required, and the exchanger effectiveness (discussed in Section 9.9.4) can range from 2 5 per cent to values in excess of 98 per cent without fundamental design or construction problems arising. Countercurrent, co-current and cross flow contacting can be employed individually or in combination. 

Mass transfer processes involving two fluid streams are frequently carried out in a column countercurrent flow is usually employed although co-current flow may be advantageous in some circumstances. There are two principal ways in which the two streams may be brought into contact in a continuous process so as to permit mass transfer to take place between them, and these are termed stagewise processes and continuous differential contact processes. 

Coriolis flowmeter 267 Costich, E. W. 280,283,288.311 Coulson, J. M, 517, 565 Counter- and co-current flow, mass transfer 621 Countercurr ent contacting of phases 600 --flow 387... 

DDIF has been applied to understand two-phase flow (air and water) in a Berea sandstone sample and the relationship to the pore geometry [65], Several different states of saturation were studied full saturation and partial saturation by three methods, i.e., centrifugation, co-current imbibition and counter-current imbibition. Imbibition is a process in which a porous sample absorbs the wetting fluid through capillary force. In the case of co-current imbibition, the bottom of the rock sample was kept in contact with water, so the water is imbibed into the rock and the water and air flowed in the same direction. For counter-current imbibition, the whole sample was immersed and the water was drawn into the center of the rock as, the air was forced out in this case, the water and air flowed in opposite directions. 

The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column. The liquid flows down the column over the packing surface and the gas or vapour, counter-currently, up the column. In some gas-absorption columns co-current flow is used. The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed, and this is an important consideration in packed-column design. 

Hydrotreating units in the past were built with one reactor, to meet the 500 ppm S specification however, a second reactor is needed to cope with the actual requirements. Several attempts have been made also to change the operating conditions to improve performance of the units and to achieve the stipulated level of desulfurization. From a co-current fluid-dynamics, the first modification went into countercurrent feeding. In a countercurrent reactor, where hydrogen is fed at one end and the feed in the other, the most difficult to-desulfurize compounds, will react under the higher hydrogen concentration. The countercurrent operation introduces some other problems, such as hot spots and vapor-liquid contact. 

Figure 13.12. Calculation of number of stages for co-current multiple-contact process, using immiscible...

Tanja Obradovic, Ph.D., Absorption Systems, Oaklands Corporate Center, 440 Creamery Way, Suite 300, Exton, PA 19341, USA, Telephone +16102801406, Fax +1 6102803779, E-mail obradovic absorption.com (current contact information - Merck Co., Inc., 351 North Sumneytown Pike, UG4D-80, North Wales, PA 19454-2505, Phone +1267 305 1576, Fax +1 215-305-6428, Email tanja obradovic merck.com)... 

Trickle-bed reactors usually consist of a fixed bed of catalyst particles, contacted by a gas liquid two-phase flow, with co-current downflow as the most common mode of operation. Such reactors are particularly important in the petroleum industry, where they are used primarily for hydrocracking, hydrodesulfurization, and hydrodenitrogenation other commercial applications are found in the petrochemical industry, involving mainly hydrogenation and oxidation of organic compounds. Two important quantities used to characterize a trickle-bed reactor are... 

A heterogeneous tubular reactor that incorporates three phases where gas and liquid reactants are contacted with the solid catalyst particles, is classified as a trickle-bed reactor. The liquid is usually allowed to flow down over the bed of catalyst, while the gas flows either up or down through the void spaces between the wetted pellets. Co-current downflow of the gas is generally preferred because it allows for better distribution of liquid over the catalyst bed and higher liquid flow rates are possible without flooding. 

In ideal trickle flow reactors, all particles in the catalyst bed take part in the overall reactor performance, since each is surrounded (wetted) by the liquid phase that flows around it. Situations in which the liquid flows preferentially through a certain part of the bed, while the gas phase flows predominantly through another part, should be avoided [23]. In this case, part of the bed is not contacted by the liquid reactant at all and docs not contribute to the overall conversion. To avoid this maldistribution, Gierman [20] proposed the following criterion for the wetting number Wtx for co-current downflow operation ... 

Figure 1. Slurry reactors classified by the contacting pattern and mechanical devices (a) slurry (bubble) column (b) countercurrent column (c) co-current upflow (d) co-current downflow (e) stirred vessel (C) draft tube reactor (g) tray column (h) rotating disc or multi-agitated column reactor (i) three-phase spray column — liquid flow —> gas flow.

If the contact time of the liquid/solids suspension with the gas phase is relatively short and also for reactors of high H/dc values, it may be useful to distinguish a co-current and countercurrent contacting pattern and, especially in relation to the gas holdup, up-flow and down-flow. 

Figure 6.8 Two specific chemical engineering processes where the Fr number is applied. A device with trays for contacting two phases B two-phase pipe flow in co-current configuration. 1 bubble flow 2 aggregated flow 3 plug flow ...

As mentioned before, the unit operation of bed adsorption may be carried out in a moving-bed mode, either co-currently or countercurrently. When the breakthrough experiment is carried out, the superficial velocity should also be recorded. The reason is that adsorption is a function of the time of contact between the liquid phase containing the solute to be adsorbed and solid-phase carbon bed. Thus, for the breakthrough data to be applicable to an actual prototype adsorption column, the relative velocities that transpired during the test must be maintained in the actual column. When the relative velocities between the flowing water and the carbon bed are maintained, it is immaterial whether or not the bed is moving. 

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