Countercurrently

FLASH determines the equilibrium vapor and liquid compositions resultinq from either an isothermal or adiabatic equilibrium flash vaporization for a mixture of N components (N 20). The subroutine allows for presence of separate vapor and liquid feed streams for adaption to countercurrent staged processes. 

Gas-liquid mixtures are sometimes reacted in packed beds. The gas and the liquid usually flow cocurrently. Such trickle-bed reactors have the advantage that residence times of the liquid are shorter than in countercurrent operation. This can be useful in avoiding unwanted side reactions. 

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. 

Tunnel dryers are shown in Fig. 3.15a. Wet material on trays or a conveyor belt is passed through a tunnel, and drying takes place by hot air. The airflow can be countercurrent, cocurrent, or a mixture of both. This method is usually used when the product is not free flowing. 

The shell-and-tube heat exchanger is probably the most common type of exchanger used in the chemical and process industries. The simplest type of such device is the 1-1 design (1 shell pass, 1 tube pass), as illustrated in Fig. 7.7a. Of all shell-and-tube types, this comes closest to pure countercurrent flow and is designed using the basic coimtercurrent equation ... 

FIgura 7.7 1-1 shells approach pure countercurrent flow, whereas 1-2 shells exhibit partial countercurrent and partial cocurrent flow. 

If exchangers are countercurrent devices, then the number of units equals the number of shells, providing indithdual shells do not exceed some practical upper size limit. If, however, equipment is used that is not completely countercurrent, as with the 1-2 shell and tube heat exchanger, then... 

Example 7.4 For the process in Pig. 6.2, the stream and utility data are given in Taible 7.1. Pure countercurrent (1-1) shell and tube heat exchangers are to be used. 

Benfield process Removal of carbon dioxide from fuel gases, such as those obtained by gasifying coal in the Lurgi process, by countercurrent scrubbing of the gases by hot potassium carbonate solution. 

The theory behind countercurrent extractions is outlined in Appendix 6. 

Let s assume that the solute to be separated is present in an aqueous phase of 1 M HCl and that the organic phase is benzene. Because benzene has the smaller density, it is the upper phase, and 1 M HCl is the lower phase. To begin the countercurrent extraction the aqueous sample containing the solute is placed in tube 0 along with a portion of benzene. As shown in figure A6.1a, initially all the solute is present in phase Lq. After extracting (figure A6.1b), a fraction p of the solute is present in phase Uq, and a fraction q is in phase Lq. This completes step 0 of the countercurrent extraction. Thus far there is no difference between a simple liquid-liquid extraction and a countercurrent extraction. 

In a countercurrent liquid-liquid extraction the lower phase in each tube remains in place, and the upper phase moves from tube 0 to higher numbered tubes. This difference in the movement of the phases is indicated by referring to the lower phase as a stationary phase and the upper phase as a mobile phase. With each transfer some of the solute in tube r is moved to tube r -I- 1, and a portion of the solute in tube r - 1 is moved to tube r. As a result, a solute introduced at tube 0 moves with the mobile phase. The solute, however, does not move at the same rate as the mobile phase since, at each step, a portion of the solute is extracted into the stationary phase. A solute that is preferentially extracted into the stationary phase spends proportionally less time in the mobile phase and moves at a slower rate. As the number of steps increases, solutes with different values of q separate into completely different sets of extraction tubes. 

Figure A6.1 and Table A6.1 show how a solute s distribution changes during the first four steps of a countercurrent extraction. Now we consider how these results can be generalized to give the distribution of a solute in any tube, at any step during the extraction. You may recognize the pattern of entries in Table A6.1 as following the binomial distribution...

Fraction of Solute Remaining in Tube r After Extraction Step n for a Countercurrent Extraction... 

Progress of a countercurrent extraction for Example A6.1 after 30 steps. 

Two solutes, A and B, with distribution ratios of 9 and 4, respectively, are to be separated by a countercurrent extraction in which the volumes of the upper and lower phases are equal. After 100 steps, determine the 99% confidence interval for the location of each solute. 

Since the two confidence intervals overlap, a complete separation of the two solutes cannot be achieved in a 100-step countercurrent extraction. The complete distribution of the solutes is shown in Figure A6.4. 

For the countercurrent extraction in Example A6.2, calculate the recovery and separation factor for solute A if the contents of tubes 85-99 are pooled together. 

From Example A6.2 we know that after 100 steps of the countercurrent extraction, solute A is normally distributed about tube 90 with a standard deviation of 3. To determine the fraction of solute in tubes 85-99, we use the single-sided normal distribution in Appendix lA to determine the fraction of solute in tubes 0-84 and in tube 100. The fraction of solute A in tube 100 is determined by calculating the deviation z (see Chapter 4)... 

The Phillips process is a two-stage crystallisation process that uses a pulsed column in the second stage to purify the crystals (79,80). In the pulsed column, countercurrent contact of the high purity PX Hquid with cold crystals results in displacement of impurities. In the first stage, a rotary filter is used. In both stages, scraped surface chillers are used. This process was commercialized in 1957, but no plants in operation as of 1996 use this technology. 

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