Counter-current contacting

Fields of Application One of the major advantages of the gravity-bed technique is that it lends itself well to true intimate counter-current contacting of solids and gases. This provides for efficient heat transfer and mass transfer. Gravity-bed contacting also permits the use of the sohd as a heat-transfer medium, as in pebble heaters. 

Counter-current rinsing and rinse-water reuse are useful tips for reducing usage. Counter-current contact systems are more efficient in promoting heat and mass exchanges, which are important to gas absorption, extraction, and many types of chemical reactions. 

The direct counter-current contact of a hot gas with a cool immiscihle liquid is effectively used in certain hydrocarbon cracking processes for the quenching of hot gases/vapors. Sometimes, the liquid used is oil and followed hy water quench, as is typical in ethylene plants cracking naphtha or other hydrocarbon as feed stock. 

In the extraction process, the LPG from the prewash tower enters the bottom of an extractor column. The extractor is a liquid/liquid contactor in which the LPG is counter-currently contacted by a caustic solution. Another option is the use of a fiber film contacting device. The mercaptans dissolve in the caustic (Equation 1-14). The treated LPG leaves the top of the extractor and goes on to a settler, where entrained caustic is separated. 

The most recent UK Royal Ordnance Factory (ROF) continuous process for the manuf of TNT is described by Thomas (Ref 90b). It uses a novel method of achieving continuous counter-current contacting between heavy and light phases in the trinitration section and some of the washing stages . The plant is also designed to operate based on the use of 96—100% sulfuric acid in the trinitration stage instead of the oleum used at Radford TNT Purification. 

Parastiilation. This relatively new approach to distillation, as proposed by A.F..O. Jenkins in 1983. is a method for multistage, counter-current contact between vapor and liquid and is reputed to provide 33 3 more ideal stages than traditional factional distillation at the same tray spacing. The... 

Figure 6 shows a simplified C02 capture process with two major process units absorber and stripper Zhang et al., 2009. A lean amine solvent (low C02 loading) is fed into the top of the absorber and is in counter-current contact with the gas containing C02. The C02 is chemically absorbed by the amine solvent and the treated gas exits the top of the absorber. The rich (high C02 loading) amine leaves the bottom of the absorber and is preheated by a cross heat exchanger before... 

Problem. A hydrocarbon gas stream is to be purified by continuous counter-current contact with a liquid organic solvent in an absorption column. The inlet gas contains 1.5% by volume of toxic DMSO of which 95% is to be removed. The gas flows at a rate of G = 0.1 kmol s 1 on a DMSO-free basis. Calculate... 

This overall flow pattern in a distillation column provides counter-current contacting of vapor and liquid streams on all the trays through the column. Vapor and liquid phases on a given tray approach thermal, pressure, and composition equilibria to an extent dependent upon the efficiency of the contacting tray. 

The distillation process utilizes the difference in vapor pressures of isotopic species. Because these differences are small, the process has to be repeated many times in a fractionating column, resulting in a cascade of several stages. Liquid is boiled at the bottom and vapor is condensed at the top, so that a continuous counter-current contact is established. 

The solvent, alone or in a mixture, with water added if necessary, is placed in counter-current contact with the feed, and carries off the aromatics. The introduction of a reflux helps to remove nearly all the non-aromatics. The solvent is regenerated by distillation or reextraction. Distillation is normally carried out in two steps. The first exploits the extractive distillation property, thus increasing the purity of the aromatics, by using the paraffins leaving at the top (light paraffins and entrained benzene) as a backwash reflux. In the second, the solvent is separated by simple distillation and liberates the pure aromatics. The yield in the operating conditions selected is always very high (Table 4.4). 

About the same time a novel liquid-liquid or solid-liquid counter-current contacting system was being developed by W. H. Morris (30, 31) at ROF Pembrey who proposed its use for the manufacture of TNT as one of a number of applications (including the manufacture of antibodies and other solvent-extraction applications and ion exchange).cThe equipment... 

Current American practice entails continuous treatment of molten TNT (> 80 ) with an aq soir containing about 0.1 part anhyd Na sulfite per part TNT. This corresponds to about four times the theoretical requirement, according to the equation given above. The liqs are counter-currently contacted in two washers, each of which is followed by a gravity separator (Ref 9). See Fig 1 in the section on Preparation for a general view, and Fig 5 for more detail... 

The separation is achieved, therefore, without the continuous consumption of chemicals and the operating costs should be reduced. Several counter-current contact stages will be necessary to achieve both an efficient extraction of the phenols from the nitro product and also their recovery from the aqueous extract pheise into the organic solvent. The principles of the flow sheet are illustrated in Fig.12. 

Den Hollander et al. [ 14,16] investigated the enzymatic hydrolysis of penicillin G to phenylacetic acid and 6-aminopenicillanic acid in biphasic aqueous-organic systems without pH-control. In a preliminary study, the two phases were counter-currently contacted in a discrete manner, so that equilibriiun was reached in each stage. Sets of three and five shake flasks served to mimic equilibrium stages in the counter-current set-up. It was shown, that counter-current contact leads to significant improvement of the equilibrium conversion relative to the batch or co-current situation. When penicillin G was fed in an intermediate stage, either exit contained mainly one of the two products. This simplifies product recovery. 

This section discusses a solvent recovery process developed by Kureha Engineering Co. Ltd., Japan. In this continuous process, spherical particles of activated carbon (AC) circulate in the adsorption and desorption columns by fluidization. In the adsorption column, the particles form fluidized beds on multi-trays to adsorb the solvent in counter-current contact with the feed gas. The cleaned gas is released from the top of the adsorption-column to the atmosphere. 

Continuous fractionation is carried out in countercurrent flow. For the continuous fractionation of a solution containing two dissolved substances, a split column is used (Fig. 7-30). In the upper column section, the crystallization column, the less soluble component crystallizes during the counter-current contact of the mother liquor and the crystal. In the lower section, the concentrating or enrichment column, the more soluble component is favorably extracted from... 

Extraction - Extraction is performed by a counter-current contact of dissolution liquor vwth the selected solvent. Phoqihoric acid transfers selectively from the aqueous dissolution liquor to the organic solvent phase the resultant extract and calcium-chloride brine (raffinate) contain substantiaDy all the impurities, such as fluorine and iron. 

Weiss, D. E. and Swinton, E. A. Methods and Apparatus for Obtaining Continuous Counter-current Contact between Solid Particles and a Liquid. U.S. Patent 2,765,913 (1956). 

The multiple stages of counter-current contact with mass transfer in a distillation column cause a temperature profile to... 

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