Mode of feed introduction

In mis framework of mree broad categories of separation processes, former separation development/classi-flcation comes about due to me nature of me interaction between me basic separation phenomena in each category and the directions of bulk flow vis-k-vis me direction of force(s) responsible for me basic separation mechanism. Considerable additional separation development is achieved by reflux, recycle, creation of an additional property gradient in an external force field, mode of feed introduction, etc. These aspects have been addressed in me following sections reflux (Sections 2.3.2, 8.1.1, 8.1.4, 10.1.4.2, 10.2.2.1) recycle (Sections 2.2.2, 2.4.1, 7.2.1.1,... 

In many separation processes, the feed is introduced continuously at a fixed rate into one end, or in the middle, or other location, of the separator (Figure 6.1.9(a)) the direction, the flow rate and the location of feed introduction are invariant with time. Devices where two phases, regions or fractions flow perpendicular to the direction of the force are of prime interest (Chapter 8) in this mode of feed introduction. Although there is only one feed stream coming in, there will be a total of two/three streams, which will be entering the separator in such a case. The presence of a second mobile stream/phase/fraction within the separator allows species introduced via the feed to be removed continuously as separation is being implemented locally everywhere in the separator. 

A second mode of feed introduction involves a constant flow rate of feed into the separator at a fixed separator location for a fixed period of time, after which feed flow is stopped for a certain amount of time feed flow is restarted after this pause. The start-stop is carried out cyclically for as many cycles as is feasible/desirable (Figure 6.1.10(a)). Fixed-bed processes in a single-entry separator described in Sections 7.1.1-7.1.3,... 

A variant of this second mode of feed introduction involves a pulse of feed mixture introduced into the separator at a particular location periodically, as in the chromatographic processes discussed in Section 7.1.5 (Figure 6.1.10(b)). During the rest of the period, another non-feed stream is most likely to be introduced into the separator to continue separation and regenerate the separator. The last mode of feed introduction is simply an amount of fluid/ solid introduced into the separator in a batch fashion. The product withdrawal may be batchwise or semi-continuous (stopping when there is no feed). 

A third mode of feed introduction employs a constant flow rate of feed into the separator at one end for a fixed period of time then the same feed flow pattern is implemented at the other end of the separator such that the feed flow direction in the separator is reversed (Figure 6.1.10 (c)). Such a feed flow pattern is observed in continuous... 

Gentilcore (2002) has illustrated how the loss of the replacement solvent in the distillate can he considerably reduced hy continuously adding the replacement solvent under the condition of a constant-level control of the liquid in the pot (instead of a conventional hatch distillation). This case Illustrates how the mode of feed Introduction influences the separation achieved, even though the basic separation mechanism and the force vs. flow pattern remain unchanged. For a general and hroad introduction to hatch distillation in all forms, consult Diwekar (1996) and Seader and Henley (1998, chap. 13). Section 8.1.3 will focus on other flow vs. force configurations in hatch distillation. 

Reactor design is a key element in each process listed in Table XX. The method of feed introduction, the arrangement of the catalyst bed, and the mode of operation have an impact on the ability to process residua. For this reason, classification by reactor type provides a convenient and appropriate distinction for discussing hydroprocessing technology. The most common reactor designs include fixed beds, ebullated or expanded beds and slurry beds, and moving-bed reactors. These classifications are discussed in more detail next. 

The same time-invariant mode of feed stream introduction and feed-phase flow perpendicular to the force direction is also utilized in separations where any one stream enters the separator, provided at least two product streams are continuously withdrawn from the separator. Crossflow membrane separators and external force based separators studied in Sections 7.2 and 7.3 are particular examples. Of the at least two product streams, one is the depleted feed stream. The other product stream continuously removes selected species from the feed stream (Figure 6.1.9(b)). 

The ftxed-bed processes studied so far are generally useful for separating one solute or one ionic species from the solvent in some cases, more than one solute or one ionic species are also separated from the solvent Separation of each individual species present in a multicomponent gas mixture or liquid solution can be achieved in a fixed bed of adsorbent particles if the mode of operation, e.g. the method of feed introduction, is changed from that used so far. A number of methods are commonly used to this end. They are elution chromatography, displacement chromatography and firontal chromatography (see Figure 7.1.5(c)). 

Table 3 Influence of the mode of introduction of reagents into the feed on some properties of triple copolyesters il> prepared from the terephthaloyl dichloride (A), NMDEA (B) and phenolfluorene (C) (the data from [60])...

A distillation column contains perforated plates or trays, or is stacked with inert packing material made of stainless steel, ceramic, or polymer. Distillation may be carried out either in a batch or in a continuous mode of operation. For the latter introduction of the feed into, and removal of the product from, the system is carried out continuously. A low-boiling solvent can be removed from... 

Decisions as to whether a chemostat or a batch reactor is preferable for use in a particular application depend on a number of factors. However, an important consideration is whether or not the enhancement in productivity associated with the use of a continuous fermentation process is sufficient to offset the difficulties associated with this mode of operation the need for continuously feeding sterile supplies of the growth medium and (if appropriate) either air or oxygen, the potential for other modes of contamination of the fermentation broth, the potential for mutation of the microorganism, and so on. As an introduction to these considerations, let us consider the relative productivities of... 

Separation is most often implemented in open systems/devices with bulk flow(s) in and out The treatment of separation achieved in such separators is carried out in Chapters 6, 7 and 8. Chapter 6 begins with the sources and the nature of bulk flow in separation systems in a multiscale context as well as the feed introduction mode vis-a-vis time (Section 6.1). The various equations of change for species concentration in a mixture, the equation of motion of a particle in a fluid and the general... 

Chapter 7 will consider separations achieved under the bulk flow-force combination of (b). Separation systems utilizing the configurations of (c) are treated in Chapter 8. (There will be occasional examples of two combinations of bulk flow and force directions.) Chapters 6, 7 and 8 will generally employ one separator vessel. Reactive separations will be treated immediately alongside non-reactive separations as often as possible. Different feed introduction modes will be considered as required in all three configurations, (a), (b) and (c). Multistage separation schemes, widely used in the processes of gas absorption, distillation, solvent extraction, etc., are studied in Chapter 8 when only one vessel is used. When multiple devices are used to form a separation cascade, an introductory treatment is provided in Chapter 9. 

The feed mixture may be introduced into a separation device via a number of forces identified above. What is, however, much more important is the dependence of the feed introduction on the time and spatial coordinates in the separator. We will progressively learn in Sections 6.3, 6.4 and Chapters 7 and 8 that this feed introduction mode is intimately connected with the number of phases/regions... 

A major feature of the three techniques of continuous chromatography in the crossflow configuration is that the feed stream is introduced only over a small section of the flow cross section of the carrier fluid. Had the feed stream been introduced throughout the flow cross section in the carrier fluid flow direction, multicomponent separation would not have heen possible feed introduction mode is important. We see in Section 8.3.2 that introduction of the feed fluid across the whole flow cross section for this fluid in a crossflow system is a common feature of separation in a plate in a distillation column which can produce only a hinary separation. 

In the separation processes mentioned so far the introduction of the feed mixture is realised in a batch wise manner on one single column only. Especially for separations in a preparative scale, modes for continuous operation have to be considered to increase productivity, product concentration and to save fresh eluent. 

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