Counter-current process

Removal of Free Fatf Fields. Alkali treatment of the oil is accompHshed by the use of caustic soda solutions to neutralize the excess free fatty acids. Because castor oil readily forms emulsions with water and/or alkaline solutions, special techniques have been developed to neutralize the acids. A continuous counter-current process was developed using a stationary contact reactor (15). Treatment in the presence of a solvent is also utilized (16). 

Continuous ion-exchange separation technology, which adopts counter-current processing in a rotating turntable, is currently making an impact and 70% of the world s lysine manufacture units use this technology. 

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. 

Counter-current processes involving, for example, the use of spinning disc or pulse column techniques do not require the same degree of premixing and achieve separation in a shorter period of time. However, as a continuous process it becomes essential to correctly size the plant required in order to maintain an adequate stock. Unlike the batch process, counter-current techniques are not always easily able to accommodate a sudden escalation in product order requirements. 

Using equation (151), Wo 5 values of 0.99906 for An and 0.0182 for Ln are obtained, giving a DFl of 55. This modification of the counter-current process thus offers a substantial increase in DFLn at the expense of a small reduction in An recovery. 

Different problems are modeled by two-point boundary value differential equations in which the values of the state variables are predetermined at both endpoints of the independent variable. These endpoints may involve a starting and ending time for a time-dependent process or for a space-dependent process, the boundary conditions may apply at the entrance and at the exit of a tubular reactor, or at the beginning and end of a counter-current process, or they may involve parameters of a distributed process with recycle, etc. Boundary value problems (BVPs) are treated in Chapter 5. 

Needless to say, the assumption of plug flow is not always appropriate. In plug flow we assume that the convective flow, i. e., the flow at velocity qjAt = v that is caused by a compressor or pump, is dominating any other transport mode. In practice this is not always so and dispersion of mass and heat, driven by concentration and temperature gradients are sometimes significant enough to need to be included in the model. We will discuss such a model in detail, not only because of its importance, but also because the techniques used to handle the ensuing boundary value differential equations are similar to those used for other diffusion-reaction problems such as catalyst pellets, as well as for counter-current processes. 

Concentration of urine is made possible by generation of osmotic gradients in the renal medulla. This involves a counter-current process which pumps ions without water from the loops of Henle into the medullary interstitial fluid. When the filtrate passes down the collecting ducts lying parallel to the loops of Henle, water passes out of the ducts along an osmotic gradient, provided ADH is present. A hypertonic urine is therefore produced. 

In columns with a fixed bed, ion-exchanger capacity is not completely used since the process is performed up to breakthrough, that is until the appearance of the substance injected into the column in the outlet of the column. So a considerable part of ion-exchange capacity which is within the zone of the sorption front is not used. The counter-current process is carried out so that completely exhausted ion exchanger is in equilibrium with the injected solution when it leaves the column. In this case the capacity is fully used. 

This type of error is always liable to occur when our data is existing plant records winch we observe. It is less likely to occur if the data is obtained by actual experiment. Thus in an experiment if we raise the temperature Ti to a chosen value, and the running of the plant as measured by the dependent variable y (which may be yield, quality, etc.) improves, then for practical purposes it may be sufficient to say run the plant with T, at this value. However, this still may not be real evidence of the causative effect of T i upon y, for the plant may be such that in raising Ti to the new value we have at the same time affected the real causative variable Tj. Thus, for example, in a counter-current process, raising the temperature at one point will probably affect the temperature at other points. 

However, Dijkstra states that current industrial bleaching processes are scaled up laboratory experiments. Accordingly, spent bleaching earth has been in adsorption equilibrium with bleached oil and thus has a much lower adsorbate loading than if this spent earth had been in equilibrium with nonbleached oil. He then raises the question of how much bleaching earth could possibly be saved if spent earth were to be in equilibrium with nonbleached earth as in a truly counter-current process with respect to the present, cocurrent processes. 

However, the scenario is more complex. The process can be looked upon as a semicontinuous counter current process. Excavation of furnaces after cooldown, have given a good understanding of the process. In Fig. 1.1, the inner zone of an electric arc furnace is shown just before stoking of the furnace. 

Here Vsoiid is the apparent solid flow rate, HA and ffB describe the slopes of the adsorption isotherm, which are calculated in the nonlinear case by linearization of the adsorption isotherm for the feed concentration Cfeed, . The transformation reflects the fact that, in a counter-current process, it is not the net flow rates that are important but rather their values relative to the apparent solid movement. For this reason, Morbidelli et al. introduced the m factors in their graphical design (known as the triangle theory) (Biressi et al., 2000 and Mazzotti et al., 1997c). 

Despite the success of the counter-current process in comparison with single batch or crosscurrent processes, a DFl value of 10 is modest in view of the hundredfold difference between and. Better decontamination can be achieved if a washing or scrub section is incorporated in the process. This involves contacting the loaded organic product stream with an aqueous phase having a low or zero Ln concentration. The raffinate from this section is then combined with the aqueous feed to the extraction section, as illustrated in Figure 31. The mass fraction, of... 

The various process modes will be applied primarily on the basis of the properties of the feed mixture. Solids will be processed mainly in single stage or multiple stage operational mode, fluid feed mixtures containing compounds of similar solubility in the solvent, will be treated in a multiple stage counter current process. Separation of isomers will be the field of chromatography or selectively catalyzed reactions. Chemical reactions, e.g., esterification of free fatty acids, may make the separation process easier. 

Percolation leaching may also be regarded as a partially counter-current process. In its simple form the process involves the passing of the leaching acid down a column, or through a bed, of ore which has been ground to a suitable particle size. A suitable grid or filter medium is used as a support... 

The hydrofluorination reaction is not very efficient, as measured by the usage of hydrogen fluoride, when carried out in a single furnace. This is because a large excess of the gas is required for the conversion of the last few per cent of oxide to fluoride. A method of improving the efficiency is to hydrofluorinate in a number of reactors in series, the first being removed from the line at intervals, and replaced by a new one at the end of the line. This then becomes a counter-current process. The excess hydrogen fluoride is absorbed in potassium hydroxide solution in a suitable scrubber. 

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