Periodic countercurrent process

Belter and co-workers developed a periodic countercurrent process for treating a fermentation broth to recover novobiocin. They found that they were able to scaleup the laboratory results to production operations if the two systems have similar mixing patterns and the same distribution of residence times in the respective columns. The mixing patterns are the same when the space velocity (F/%) and the volume ration (f /c) are the same. This is shown in Fig. 22 for the effluent concentration of novobiocin from laboratory and production columns. 

FIGURE 13 Schematic diagram showing the sequence of column interchange in a periodic countercurrent separation process. 

Periodic countercurrent systems are widely used in ion exchange and water purification systems for the removal of trace concentrations of organic components with beds of activated carbon.At low sorbate concentrations the adsorption isotherms for many organic pollutants are linear or only slightly favorable so that the mass transfer zone is wide, and in a simple two-bed cyclic batch system the LUB would therefore be uneconomically large. For such systems, in order to obtain an economic process, some form of counter-current operation is necessary in order to reduce the adsorbent inventory. 

Chen, J.W. Cunningham, F.L.. and Buege, J.A.. Computer simulation of plant-scale multicolumn adsorption processes under periodic countercurrent operation, Ind. Eng. Chem. Process Des. Dev.. 11(3), 430-434 (1972). 

Firing. A hot-air oven having forced circulation in a countercurrent mode is used to dry the fermented tea leaves and inactivates the key enzymes required for fermentation. The firing process generally occurs over an 18—20-min period, which is optimum for normal process efficiencies. 

Considerable difficulty was experienced throughout the entire period of plant-scale operation of the DBBP countercurrent extraction process in adjusting the CAW solution to the desired pH of 0.75. Several factors contributed to these difficulties. Lack of any buffering capacity in the CAW solution made it easy to overshoot or undershoot the desired pH. The two-step neutralization procedure and equipment aided considerably in achieving proper feed acidity. But, even with this approach, inadequate mixing coupled with unsophisticated and insensitive monitoring and control instrumentation made it impossible to routinely achieve reliable adjustment of feed acidity to its optimum range. 

Actually this dilution phenomenon is partially avoidable in three-zone SMB separation processes when a simulated countercurrent is employed instead of a true countercurrent when port motion occurs, a fully rinsed column (thus full of eluent) is added to the end of zone 3. As a matter of fact, during a first part of the switching period, pure eluent will exit from the raffinate outlet. Of course, this can be discarded or recycled to the eluent tank. During the... 

The dark-colored clarified liquor is presure-filtered and concentrated to 60 percent solids in multiple-effect evaporators. The concentrated liquor is decolorized with granular carbon in columns 12 ft in diameter and 30 ft high in a countercurrent manner that is, liquor flows upward in the columns, while a portion of the carbon is removed from the bottom periodically. Carbon is used at a rate of 2.5 percent of dry solid processed, and approximately 5 percent of carbon is lost during revivification.91 In some cases, granular carbon has been replaced by synthetic polymeric adsorbents to decolorize the syrup. Low ash syrups usually are deionized with ion-exchange resins. The processed liquor is evaporated to a final solids content of 75-85 percent in a single-effect evaporator. 

He) was introduced into the column, and the column pressure reached the adsorption pressure Pa. This process took about 5 seconds. Valve V-2 was then opened, and adsorption in the column from the inlet gas took place (adsorption step). During this period, the flow rate and concentration of CO, Gu and C/ were measured. (2) Valves V-1 and V-2 were closed, V-4 was opened, and the column was evacuated (desorption step). At the end of the desorption step, the pressure was below 13 Pa after 600 seconds. (3) As V-3 was also opened under evacuation, helium was supplied as a countercurrent puige to remove CO thoroughly (countercurrent purge step). (4) With V-3 closed and V-4 still open, the column was re-pressurized to the adsorption pressure with helium. The measurement conditions are summarized in Table 1, and the samples that were screened are listed in Table 2. The adsorption temperature was one parameter examined in this study, and a sequence controller was programmed for each set of conditions, so that steps (1) ( 4) were repeated over more than 4 hours. The total amount of desorbed gas was determined from the gas collected at the exit of the rotary flowm er. 

The processing of soybeans has been described in more detail elsewhere than can be done here (132-134). Oil is recovered today by either mechanical means or through the use of organic solvents. In the preindustrial revolution period, soybeans were merely pressed with lever or animal-driven screw-operated batch presses. Around the turn of the Twentieth Century, when soybeans became a viable commercial crop in the United States, steam-powered hydraulic batch presses were used. Today, electric-powered continuous screw-presses, often referred to as expellers (but this is a trademarked name for screw presses manufactured by one supplier), or continuous countercurrent solvent extractors are used. 

In this approach, an equivalent countercurrent movement of solid is assumed instead of the SMB process. This equivalent true moving bed (TMB) neglects the d3mamics associated with periodic switching and produces mean concentration profiles over a switching period. 

---