Multiphase operations process

Ultrasound can thus be used to enhance kinetics, flow, and mass and heat transfer. The overall results are that organic synthetic reactions show increased rate (sometimes even from hours to minutes, up to 25 times faster), and/or increased yield (tens of percentages, sometimes even starting from 0% yield in nonsonicated conditions). In multiphase systems, gas-liquid and solid-liquid mass transfer has been observed to increase by 5- and 20-fold, respectively [35]. Membrane fluxes have been enhanced by up to a factor of 8 [56]. Despite these results, use of acoustics, and ultrasound in particular, in chemical industry is mainly limited to the fields of cleaning and decontamination [55]. One of the main barriers to industrial application of sonochemical processes is control and scale-up of ultrasound concepts into operable processes. Therefore, a better understanding is required of the relation between a cavitation coUapse and chemical reactivity, as weU as a better understanding and reproducibility of the influence of various design and operational parameters on the cavitation process. Also, rehable mathematical models and scale-up procedures need to be developed [35, 54, 55]. 

For multiphase flow processes, turbulent effects will be much larger. Even operability will be controlled by the generated turbulence in some cases. For dispersed fluid-fluid flows (as in gas-liquid or liquid-liquid reactors), the local sizes of dispersed phase particles and local transport rates will be controlled by the turbulence energy dissipation rates and turbulence kinetic energy. The modeling of turbulent multiphase flows is discussed in the next chapter. 

As discussed later regarding the role of multiphase operations for homogeneous catalysis, research has been focused mainly on the issue of recyclability of homogeneous catalysts. Relevant progress has been made in this aspect, but the other problem, which could be indicated as intensification of homogeneous catalysis processes, is the issue that should be solved, at least for medium-large scale industrial productions. 

In fine and specialty chemicals production the process cost is a less relevant aspect on the other hand, the time taken to realize the industrial production is typically the critical factor. This aspect, together with the limited resources dedicated for R D, determine the preference in companies for multipurpose catalysts with respect to optimized, but more specific, catalysts. This applies also to the process itself where simpler, not optimized, batch reactors are preferred to better, but less flexible, more complex operations. This is one of the key aspects to consider in evaluating the use of multiphase operations in the synthesis of this class of chemicals. 

We conclude here by indicating that heterogeneous catalysts are still preferable in developing sustainable chemical processes, but that the notable recent progress in multiphase operations to allow improved catalyst separation, recovery and recycle of homogeneous catalysts [63-67] indicates that the use of homogeneous catalysis in industrial processes will certainly increase in the near future. 

There are various possible approaches for multiphase operation of homogeneous catalysis, to improve their usability and recycle processes with organic/organic, organic/aqueous, or fluorous solvent pairs (solvent combinations), non-aqueous ionic solvents, supercritical fluids, and systems with soluble polymers. Figure 2.2 reports a general scheme of the possibilities for homogeneous catalysis. 

Table 2.1 gives a comparative overview of the pro and cons of the various options in processes with multiphasic operations. On going from left to right in the table the industrial relevance of the processes decreases. The RCH/RP oxo process and the Shell Higher Olefin Process (SHOP) discussed later belong to the first class (aqueous biphase) and second class (organic biphase), respectively. 

Table 2.1 Pro and cons of the various options in processes with multiphasic operation. Source adapted from Cornils et al. [63].

It might be added that the multiphase operation offers more than the decisive separation between desired products and catalyst, although there are differences between the various multiphase hquids [9]. It cannot be emphasized enough that the use of polar multiphase Hquids also separate the byproduct heavy ends from the catalyst in the system, thus avoiding a build-up in the catalyst recycle. In other processes (and probably also if very apolar fluorous liquids are used) an additional purge is needed to remove the high boilers from the catalyst, which then requires a further (and costly) separation or purification [10],... 

Table 1 Comparative approach to all processes with multiphasic operation Sol vents-l igands-catalysts.

To judge the different processes of multiphase catalyzes is rather unfair because of the wide range in the status of the variants, which include commercially operating processes such as Ruhrchemie/Rhone-Poulenc and SHOP, some processes that are in the first stages of commercial demonstration of their reliabilty, and what are more or less process proposals in the other cases. This is specially relevant when economic figures have to be compared. But it is possible to recognize and distinguish basic trends. 

As far as other chemical engineering aspects are concerned, the scaling-up of the newer proposals for a multiphasic operation ought to be scrutinized. Once more, the commercial processes are out of the discussion. All new processes, ionic liquids, SCCO2, and fiuorous systems, should be able to be scaled up without problems. Unless serious developments occur, this seems to be valid also for polymer-bound systems - with the prerequisite of membrane devices for the separation of the products from the composite of polymer and catalyst. Moreover, imdustrial interest in soluble polymer-bound catalysts has been closely linked to the development of membranes with sufficient long-term stability in organic solvents. The discoveries attained with polymer-bound catalysts may fertilize new... 

Section 7.3 covers separation operations/processes/ techniques in which an external force field is applied perpendicular to the direction of bulk flow of a singlephase solution or a dispersed/particulate multiphase mixture (Figures 7.0.1(i)-(p)). The external force fields considered are electrical, centrifugal, gravitational and magnetic. Specific processes treated include, among others, electrophoresis (Figure 7.0.1(i)), dielectrophoresis. 

Note that filter aid selection must be based on planned laboratory tests. Guidelines for selection may only be applied in the broadest sense, since there is almost an infinite number of combinations of filter media, filter aids, and suspensions that will produce varying degrees of separation. The hydrodynamics of any filtration process are highly complex filtration is essentially a multiphase system in which interaction takes place between solids from the suspension, filter aid, and filter medium, and a liquid phase. Experiments are mandatory in most operations not only in proper filter aid selection but in defining the method of application. Some general guidelines can be applied to such studies the filter aid must have the minimum hydraulic resistance and provide the desired rate of separation an insufficient amount of filter aid leads to a reduction in filtrate quality — excess amounts result in losses is filtration rate and it is necessary to account for the method of application and characteristics of filter aids. 

Balanced bellows type valves are normally used where the relief valves are piped to a closed flare system and the back-pressure exceeds 10% of the set pressure, where conventional valves can t be used because back-pressure is too high. They are also used in flow lines, multiphase lines, or for ptu affinic or asphaltic crude, where pilot-operated valves can t be used due to possible plugging of the pilot line. An advantage of this type of relief valve is, for corrosive or dirty service, the bellows protects the spring from process fluid. A disadvantage is that the bellows can fatigue, which will allow process fluid to escape through the bonnet. For HjS service, the bonnet vent must be piped to a safe area. 

Flowever, information concerning the characteristics of these systems under the conditions of a continuous process is still very limited. From a practical point of view, the concept of ionic liquid multiphasic catalysis can be applicable only if the resultant catalytic lifetimes and the elution losses of catalytic components into the organic or extractant layer containing products are within commercially acceptable ranges. To illustrate these points, two examples of applications mn on continuous pilot operation are described (i) biphasic dimerization of olefins catalyzed by nickel complexes in chloroaluminates, and (ii) biphasic alkylation of aromatic hydrocarbons with olefins and light olefin alkylation with isobutane, catalyzed by acidic chloroaluminates. 

Multiphase reactors can be batch, fed-batch, or continuous. Most of the design equations derived in this chapter are general and apply to any of the operating modes. Unsteady operation of nominally continuous processes is treated in Chapter 14. 

This review paper is restricted to stirred vessels operated in the turbulent-flow regime and exploited for various physical operations and chemical processes. The developments in the field of computational simulations of stirred vessels, however, are not separated from similar developments in the fields of, e.g., turbulent combustion, flames, jets and sprays, tubular reactors, and multiphase reactors and separators. Fortunately, there is a strong degree of synergy and mutual cross-fertilization between these various fields. This review paper focuses on aspects specific to stirred vessels (such as the revolving impeller, the resulting strong spatial variations in turbulence properties, and the macroinstabilities) and on the processes carried out in them. 

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