Cascade arrangement

FIG. 22-64 Cascade arrangement for membrane processing to maintain cross-flow velocity as permeate is removed. 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

Particularly in large-scale operation, it is often desirable to carry out a process continuously and a tank reactor may be operated in this way with constant streams feeding reactants in and taking products out as shown in Fig. 1(b) such an arrangement is known as a continuous stirred tank, or back-mix, reactor and several tanks are often used in a series or cascade arrangement. 

Compared to the single-autoclave process, the cascade arrangement gives more flexibility to adjust the properties of LDPE resins. 

The Buss mixers are of two types the PRD mixer serves for pre-mixing, homogenization and cold crushing of materials and the DKG machines are provided with a shaft and a granulating screw in an air-granulation cascade arrangement (Table 4) 27). 

The new concept for processing of bimodal polyolefins was first presented in 2004 at the Kunststoffmesse in Dtisseldorf. The system, known as the ZSK-NT, combines two corotating, twin-screw kneaders in a cascade arrangement. Figure 15.1 shows the setup of the two ZSKs operating as ZSK-NT. ... 

Fig. 30.8. The downstream processing cascade arranged by final product requirement.

Measurement of the steam or refrigerant flow can provide a good indication of heat duty. If there are multiple users, which cause disturbances to the utility, then a temperature to flow cascade control arrangement should be considered. In such a cascade arrangement, the temperature controller output provides the setpoint for the flow controller. The flow controller minimizes the effect of utility stream disturbances and hnearizes the temperature control loop. 

It may be noted that the cascade arrangement of Figure 19.7, if suitably disentangled, will correspond to a multistage operation as encountered in absorption, stripping, and distillation practices. 

Further contributions to this subject have been made by Jackson and Pigford [67], Rosen [68] and Brodsky [38]. The books of London and Cohen [69] deal with the whole theory of isotope separation. This includes the optimization of cascade arrangements which is of great importance in isotope separation as well as in the separating plants of the chemical industry [71]. 

Unless the cascade arrangement in item 2 above is used, the differential pressure control method will lead to overrefluxing and overreboiling at low rates and will therefore be energy-wasteful. 

A schematic of the co-current flash cascade arrangement is shown in Fig. 6.7. There are two sections. In the rectifying cascade, vapor from each flash reactor... 

Fig. 6.7 A CO current flash cascades arrangement. The top half is the rectifying cascade and the bottom half is the stripping cascade...

In most evaporator applications the control of product quality is constantly affected by variations in feed rate and composition to the evaporator. In order to counter these load variations, the manipulated variable must attain a new operating level. In pure feedback or cascade arrangements, this new level is achieved by trial-and-error as performed by the feedback controller. 

It is noteworthy that a true continuously stirred tank reactor is perfectly mixed a recycle loop cannot be used to further improve agitation or the value of In practice, particularly useful methods of improving the overall fractional conversion are (1) batch recirculation of the electrolyte through the reactor from a storage tank (section 2.5.4) or (2) the use of a number of reactors in hydraulic series, i.e. a cascade arrangement (see section 2.5.5). 

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