Counter-current mechanisms

Kidney medulla From the metabolic point of view the kidney is virtually two organs, the cortex and the medulla. The cortex contains the glomeruli, through which the blood is filtered, the proximal tubules and part of the distal tubules, from which ions and molecules are reabsorbed. The cortex is well supplied with blood so that ATP is generated by the oxidation of fuels. The medulla is metabolically quite different. Here the ATP is required for the reabsorption of ions from the loop of Henle. Some ATP is generated by anaerobic glycolysis, since the supply of blood, and therefore of oxygen, to the medulla is much poorer than to the cortex. This reflects control of the uptake of water and Na+ ions into the blood by the counter current mechanism. This depends on a slow flow of the blood in the capillaries. 

In the distal part of the nephron, urine is concentrated and the likelihood of crystalline precipitation increases substantially, particularly if urinary pH favors decreased solubility. As the urinary concentrating process also involves the counter-current mechanism, solute concentrations in the medullary interstitium can reach values several times higher than tissues elsewhere in the body. Finally during the process of renal excretion, a particular drug may undergo bioactivation resulting in reactive metabolites [2]. 

Eor one shell and multipass on the tube side, it is obvious that the fluids are not in true counter-current flow (nor co-current). Most exchangers have the shell side flowing through the unit as in Eigure 10-29C (although some designs have no more than two shell-side passes as in Eig-ures 10-IJ and 10-22, and the tube side fluid may make two or more passes as in Eigure 10-IJ) however, more than two passes complicates the mechanical construction. 

F = Correction factor to LMTD for counter-current flow for various mechanical pass configurations, see Figures lO-SfA-J. 

Layered-type structure, 92-100 Lewis acidity, 15 Liquid - liquid extraction collective, 276-277 counter-current scheme, 272 equipment, 273 extractants, 279-281 mechanism, 274-276 multistage counter-current, 271 multistage cross-current, 270 one-stage process, 270 principals, 267-269 process performance, 282-283 process schemes, 277,278, selective, 277-278 with 2-octanol, 284-291... 

In wet scrubbing the dust is removed by counter-current washing with a liquid, usually water, and the solids are removed as a slurry. The principal mechanism involved is the impact (impingement) of the dust particles and the water droplets. Particle sizes down to 0.5 /i.m can be removed in suitably designed scrubbers. In addition to removing solids, wet scrubbers can be used to simultaneously cool the gas and neutralise any corrosive constituents. 

The main function of the Loop at Henie (LoH) in the context of diuretics is further reabsorption and conservation of water and eiectroiytes from the remaining 30% of the fiitrate emerging from the PCT. This is achieved by a combination of counter-current muitipiier and counter-current exchange mechanisms in the LoH and the vasa recta respectiveiy which first concentrate and then diiute the filtrate during its passage. 

Cotton or wood cellulose is introduced into an iron box 1 equipped with a metal mesh bottom occupying the outside position. By means of a chain and sprocket mechanism the box is moved in position 2 and later to the position 3. Meanwhile another box 1 is loaded and so on. Finally the box from position 3 is transferred to the neighbouring chamber, position 4, to be shifted gradually downwards to the positions 5 and 6, and thereafter to the position 1 and outside again. There the dried cotton is unloaded and replaced by a fresh batch. Drying cellulose in chamber (I) is co-current process, while in the chamber (II) a counter-current process is carried out. The movement of shelves carrying the cellulose is marked as a thick line, while a thin line indicates the direction of air. 

In 1979 Chem Systems initiated a program to develop a liquid-entrained catalyst reactor which would provide improved contacting of syngas with the catalyst in a three phase system (ref. 38). This reactor system uses much finer catalyst particles than the fluidized bed reactor, and the catalyst-liquid slurry circulates through the reactor. The syngas can be contacted with the catalyst-liquid slurry either counter currently or co-currently. It appears that this process is more efficient than the original fluidized bed process. However, a major problem with this type of three phase system will no doubt be the development of a suitable catalyst since it is unlikely that conventional co-precipitated Cu-ZnO-A Oj catalysts will have the desired characteristics, particularly mechanical strength. 

The jet flow is issued from the Jet/Counter-current facility at the Fluid Mechanics Research Laboratory at Florida State University (Fig. 1.2). To make the PIV measurement, the flame is seeded with... 

---