Opportunities and Perspectives for a Sustainable Process Design

PI is a hot topic in chemical and process engineering, and is now reaching a maturity that is seeing PI concepts applied to a wide range of processes and technologies. Already, over 30 industrial examples are available, but this number is rapidly increasing. Originally developed for the bulk chemical industry, PI developments have more recently been focused on the higher added-value chemicals and pharmaceutical active ingredient sectors. Examples reported by the Process Intensification Network (www.pinetwork.org) and the BHR Group (www.bhrgroup.co.uk) indicate:

• a 99% reduction in impurity levels, resulting in significantly more valuable product;

• a 70% plus reduction in energy usage and hence a substantial reduction in operating cost;

• a 93% yield first time out - better than fully optimized batch process;

• a 99.8% reduction in reactor volume for a potentially hazardous process, leading to inherently safe operation.

Although these values cannot always be obtained, they give an indication of the potential that may be reached by using the PI concepts. The general approach of PI is to reconsider from scratch the process or unit operation design, and determine the key issues in a process based on recent advances in chemical engineering knowledge. Distillation for example is all about gas-liquid mass transfer, and the keys to mass transfer for a given system are the following:

• well mixed liquid and gas phases,

• high interfacial surface area,

• counter-current operation.

Well mixing ofgas and liquid phases is a clear issue. Smaller, finer packing allows us to increase the surface area between the gas and liquid contact, for example, the mass transfer could be increased in a column with very fine packing with counter-current gas flow. However, a liquid film running through a bed of fine material is problematic when the liquid film thickness is around the same as the clearance between the bits ofpacking. Liquid flow essentially stops and the column floods. The key is the thickness of the liquid film and the factors that control that. The equations describing the problem indicate that most factors relate to the physical properties of

Controlled feed jets

Liquid film flow over disc

Optional gas entry

Falling film on walls

Products ^ I

Controlled feed jets

Liquid film flow over disc

Optional gas entry

Falling film on walls

Products ^ I

Reactor Jet Caco3

Temperature controlled walls

Rotating shaft

Heat exchange fluid Figure 3.1 Simplified scheme of a spinning disc reactor (SDR).

Temperature controlled walls

Rotating shaft

Heat exchange fluid Figure 3.1 Simplified scheme of a spinning disc reactor (SDR).

the fluid, but one is independent, namely, gravity. The higher the applied gravity the thinner the film and the smaller the packing can be. One way to enhance throughputs and the interphase transfer rate is to replace the gravitational field with centrifugal fields that are higher by a few orders of magnitude. Process intensification research has therefore naturally focused on the use of rotating packed beds for the miniaturization of reactors and separators.

One example of "high-g" equipment is the spinning disc reactor (SDR), developed by Ramshaw's group at Newcastle University (Newcastle, UK) and commercialized, for example, by Protensive (www.protensive.co.uk). Figure 3.1 reports a simplified scheme of the equipment. An SDR can operate as a multifunction device. The disc can be horizontally or vertically mounted on an axle. Liquid fed near or at the center flows across the surface of a spinning disc under the influence of centrifugal force. This force stretches and contorts the film. The thin liquid film allows high rates of mass transfer and it favors absorption, stripping, mixing and reaction processes. Residence times on the disc are low, typically in the range of 3 s down to tenths of a second. Both film thickness and residence time depend on fluid physical properties, rotational speed and radial location of the fluid. On exiting the periphery of the disc, the liquid is thrown onto an enclosing wall whereupon it drains away. Heating or cooling can be applied to the disc surface and the enclosing wall to control fluid temperature.

Centrifugal forces are used not only in SDRs. High gravity (HIGEE) technology [5] is another example (Figure 3.2). The main difference with respect to SDR is the

Higee Reactor

Liquid outlet

Figure 3.2 (a) Scheme of a HIGEE unit; (b) high-gravity rotating packed bed reactor for CaCO3 nanoparticles production with a capacity of 10000tons a~ . Source: courtesy of Research Center of the Ministry of Education for High Gravity Engineering & Technology, Beijing.

Liquid outlet

Figure 3.2 (a) Scheme of a HIGEE unit; (b) high-gravity rotating packed bed reactor for CaCO3 nanoparticles production with a capacity of 10000tons a~ . Source: courtesy of Research Center of the Ministry of Education for High Gravity Engineering & Technology, Beijing.

presence of a packing bed. The liquid flows as a thin film over the packing due to the high centrifugal acceleration (100-1000g) and therefore raises the upper limit of flooding and permits the use of packing of high surface area, in the range of 1000-5000 m2 m~3, which is 3-10-times that used in conventional packed columns. The rotating-bed equipment, originally dedicated to separation processes (such as absorption, extraction and distillation), can also be utilized for reacting systems. It potentially can be applied not only to gas/liquid systems but also to other phase combinations, including three-phase gas/liquid/solid systems. The HIGRAVITEC Center (Beijing, China) has successfully applied rotating (500-2000 rpm) packed beds on a commercial scale for deaeration of flooding water in oil fields. The equipment, about 1-m diameter, replaced conventional vacuum towers over 30 m high [5].

Heat exchangers are another example where PI concepts largely enhance performance. The heat transfer area is the critical factor, but conventional heat exchangers are based on pipes that have a minimum surface area, because the design reflects more mechanical engineering considerations than process ones. Core to the heat transfer performance of an SDR or analogous equipment is the characteristic of the film as it moves across the disc. Waves tend to form in the film that significantly enhance heat transfer (as well as mass transfer), for example, with high film coefficients. However, the film coefficient is only part of the problem. The film coefficient for the heating/ cooling fluid and the thermal resistance ofthe disc itselfare also critical components. By incorporating special channels for the service fluids in the rotating disc it is possible to further enhance the heat transfer thanks to the fin effect ofthe channels. When used with suitable heat transfer fluids, it can significantly enhance the effective transfer coefficient. Therefore, an SDR has an overall heat transfer coefficient of approximately 10kWm~2 K_1 even for organic liquids. This is typically 5-10-times that achieved by most heat transfer devices and enables small discs with low process fluid inventory to handle significant thermal duties.

Often, slow reactions (in a batch-type reactor) are not due to intrinsic slow chemical kinetics but, instead, heat transfer, mass transfer or mixing limitations of the reactor are the factors that control the rate. An exothermic reaction might need an hour to be carried out in a batch reactor, not because of kinetic constraints but because it takes an hour to remove the heat of reaction. A reaction that involves a gas to liquid interface might well be controlled by the mass transfer between phases. A liquid-liquid reaction might be controlled by mixing rather than chemical kinetics. By using a SDR (or similar equipment like the HIGEE) to carry out a chemical reaction it is possible to greatly reduce these limitations.

Not only the reaction rate increases, but often there are improvements in the selectivity of the reaction as the short residence time, coupled with rapid quenching, will reduce by-product formation caused by further reaction of the desired product. Other benefits in the selectivity are likely to come from the lack of back-mixing (plug flow) and the intense mixing of the SDR, which will minimize concentration gradients of the same reactants.

Further examples and concepts are discussed in the following sections, but already this introduction evidences the possibilities and opportunities for a sustainable industrial chemistry that derive from the application of process intensification equipment and methodology.

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