reactive azeotropes and last but not least reduction of number of units (investment cost) and ..... and the azeotropic data of the system are shown in Table 1.
ABSTRACT The integration of an in-situation separation function within the reactor holds the promise of increased conversion, higher selectivity and reduced capital investment. The introduction of an in-situation separation function within the reaction zone leads to complex interactions between vapour liquid equilibrium, vapour
liquid mass transfer, intra-catalyst dilution (for
heterogeneously catalysed processes) and chemical kinetics. Such interactions have been shown to lead to the phenomenon of multiple steady-states and complex dynamics, which have been verified in experimental laboratory and pilot plant units. In this paper we have discussed the Dynamics and Kinetics of four major separation processes namely Distillation; Extraction; Absorption and Adsorption. Multi-functional reactor is discussed which is often used to embrace reactive separations technology, which promises reduction in capital costs, increased conversion and reduced by-product formation. The design and operation issues for reactive separation systems are considerably more complex than those involved for either conventional reactors or conventional separation columns. In heterogeneous reactive separation the problem is exacerbated by the fact that these transfer processes occur at length scales varying from 1 nm (pore diameter in micro gels) to say a few meters (column dimensions).The time scales vary from 1 ms (dilution within gels) to say a few hours (column dynamics). The phenomena at different scales interact with each other. Such interactions, along with the strong nonlinearities introduced by the coupling between dilution and chemical kinetics in counter-current contacting, have been shown to lead to the phenomenon of multiple steady-states and complex dynamics, which have been verified in experimental laboratory and pilot plant units. The main underlying principle behind the application of reactive distillation for selectivity enhancement is to facilitate the separation of selected components and favorably manipulate the composition profiles in the reactive zone to get desired reaction. Reactive distillation today is a well established operation that combines reaction and distillation in a single stage which offers enhancement in the overall performance of the process. While there has been substantial experimental & theoretical work reported on equilibrium related reactions, its practical applications and its usage in selectivity engineering has not been well established. The basic principle behind the application of this process for selectivity engineering is that one can 1
favorably manipulate the composition profiles with the aid of distillation attributes to expedite the desired reaction. A need for such progress has been emphasized here. Survey has been done on the
recent developments in reactive separation technology,
emphasising the available alternatives and pointing out obstacles in the way of successful implementation of this technology.
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Chapter 1: Introduction To Reactive Separation Technology The traditional flow sheet of a chemical process consists of a reactor followed by a separation unit to remove the unconverted reactants from the desired product and recycle these to the reactor. This is done to maximize the conversion of reactants and improve selectivity to the desired product, thereby reducing the costs associated with the separation step. Strategies for arriving at the “ideal” reactor configuration have been discussed in the literature. In recent years there has been considerable academic and industrial interest in the area of reactive separations wherein the separation function is integrated within the reactor; a variety of separation principles and concepts can be incorporated into the reactor. The term multifunctional reactor is often used to embrace reactive separations technology, which promises reduction in capital costs, increased conversion and reduced by-product formation . When chemical reactions and physical separations have some overlapping operating conditions the combination of these tasks in a single process unit can offer significant benefits. These benefits could involve: avoidance of reaction equilibrium restrictions, higher conversion, selectivity and yield, removal of side reactions and recycling streams, circumvention of nonreactive azeotropes and last but not least reduction of number of units (investment cost) and energy demands (heat integration). Nowadays, the focus of the chemical and process industry has shifted towards the development and application of integrated processes. This trend is motivated by benefits such as a reduction in equipment and plant size and improvement of process efficiency and safety, and hence a better process economy. Reactive distillation and Reactive Absorption is an important example of a reactive separation process. Especially for equilibrium reactions like esterifications, ester hydrolysis and etherification, the combination of reaction and separation within one zone is a well-known alternative to conventional processes with sequential reaction and separation steps . Chemical manufacturing companies produce materials based on chemical reactions between selected feed stocks. In many cases the completion of the chemical reactions is limited by the equilibrium between feed and product. The process must then include the separation of this equilibrium mixture and recycling of the reactants. Usually reaction and separation stages are carried out in discrete equipment units, and thus equipment and energy costs are added up from these major steps. In recent decades, a combination of separation and reaction inside a single unit has become more and more popular. This combination has been recognised by the chemical 3
process industries for having favourable economics of carrying out reaction simultaneously with separation for certain classes of reacting systems, and many new processes (called reactive separations) have been invented based on this technology. The most important examples of reactive separation processes (RSP) are reactive distillation (RD) and reactive absorption (RA. Chemical process industries have shown increasing interest in the development of reactive separation processes (RSP) combining reaction and separation mechanisms into a single, integrated unit. Such processes bring several important advantages among which are increase of reaction yield and selectivity, overcoming thermodynamic restrictions, e.g. azeotropes, and considerable reduction in energy, water and solvent consumption. Important examples of reactive separations are reactive distillation (RD) and reactive absorption (RA). Due to strong interactions of chemical reaction and heat and mass transfer, the process behavior of RSP tends to be quite complex. When considering in-situ separation of product, it is important to stress that often removing only one of the products of the reaction is sufficient to drive the equilibrium to the right or prevent unwanted side reactions. There is usually a choice with respect to the product to separate from the reaction zone.
Fig 1: (a) Conventional 5xed-bed reactor train, with inter-stage sulphur removal by condensation, for Claus process. (b) Fixed bed reactor, with in-situ sorption of water by zeolite adsorbent. Adapted from Agar (1999).[1]
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Fig 2:Various in-situ separation function integrated into the reactor[1]
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Chapter 2: Introduction To Reactive Absorption Process Many present-day commercial gas absorption processes involve systems in which chemical reactions take place in the liquid phase. These reactions generally enhance the rate of absorption and increase the capacity of the liquid solution to dissolve the solute, when compared with physical absorption systems. A necessary prerequisite to understanding the subject of absorption with chemical reaction is the development of a thorough understanding of the principles involved in physical absorption. Reactive Absorption (RA) represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. In RA reactions occur simultaneously with the component transport and absorptive separation. These processes are predominantly used for the production of basic chemicals, e.g. sulphuric or nitric acids and the removal of components from gas and liquid streams. This can be either the cleanup of process gas streams or the removal of toxic or harmful substances in flue gases. Absorbers or scrubbers where RA is performed are often considered as gas liquid reactors. If more attention is paid to the mass transport, these apparatuses are rather treated as absorption units . The typical flow sheet of a RA process for gas cleaning includes an absorption column to perform the removal of the compounds from the feed gas. The outlet gas leaves the column nearly free of unwanted compounds (clean gas).Reactive Absorption is an important industrial operation for production of some basic chemicals and for the removal of harmful substances from gas streams. In recent decades, this process has become especially important for the purification of gases to high purities. Reactive absorption is able to provide high throughput at moderate partial pressures and without requiring large amounts of solvent. Most RA processes are steady-state operations involving reactions in the liquid phase, although some applications involve both liquid-phase and gas-phase reactions. In reactive absorption, a fluid-fluid reaction takes place between a gas phase and a liquid phase. At the same time, mass transfer from the gas to the liquid phase is also occurring. To be able to completely understand reactive absorption, one must first have some understanding of the kinetics of fluid-fluid reactions. RA is a complex rate-controlled process that occurs far from thermodynamic equilibrium. Therefore, the equilibrium concept is often insufficient to describe it, and instead accurate and reliable models involving the process kinetics are required. The 6
effectiveness of online model based applications, such as process control and optimization, depends strongly on the quality of the available model predictions. When considering in-situ separation of product, it is important to stress that often removing only one of the products of the reaction is sufficient to drive the equilibrium to the right or prevent unwanted side reactions. There is usually a choice with respect to the product to separate from the reaction zone Compared to reactive distillation, the absence of a reboiler and a condenser makes reactive absorption a simpler process. However, the drawback is the small number of degrees of freedom that makes it difficult to set the reactants feed ratio correctly and consequently to avoid impurities in the products. Reactive absorption offers indeed significant advantages such as minimal capital investment and operating costs, as well as no catalyst-related waste streams and no soap formation. However, the controllability of the process is just as important as the capital and operating savings. Therefore, it is important to note that reactive absorption has less degrees of freedom and therefore more difficult to control than reactive distillation. Reactive Absorption is usually dominated by the mass transport kinetics. Besides, in reactive processes, chemical reactions must be taken into account. Mass transfer in RA is explained using the two-film theory. The boundary between the gas phase and the liquid phase is presumed to consist of a gas film adjacent to a liquid film. Flow in both of these films is assumed to be laminar or stagnant. The main-body gas phase and liquid phase are assumed to be completely mixed in turbulent flow so that no concentration gradient exists in the main body of either phase. The solute concentration in the gas film at the interface is assumed to be in equilibrium with the solute concentration in the liquid film at the interface. There is a solute concentration gradient across both the gas film and the liquid film.
2.1 Characteristics Of Reactive Absorption. There is no sharp dividing line between pure physical absorption and absorption controlled by the rate of a chemical reaction. Most cases fall in an intermediate range in which the rate of absorption is limited both by the resistance to diffusion and by the finite velocity of the reaction. Consider an absorber with a reaction occurring in the liquid phase. Since the reaction is in the liquid phase, the gas-phase rate coefficient KG is not affected. If the reaction is extremely fast and irreversible, the rate of absorption may be governed completely by the resistance to diffusion
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in the gas phase. Therefore the absorption rate can be estimated by knowing the gas-phase rate coefficient KG. The liquid-phase rate coefficient K6 +L is strongly affected by fast chemical reactions and generally increased with increasing reaction rate.
The highest possible absorption rates will occur under conditions in which the liquid phase resistance is negligible and the equilibrium back pressure of the gas over the solvent is zero. This condition can be attained if KL is very large. Frequently, even though reaction consumes the solute as it is dissolving, thereby enhancing both the mass transfer coefficient and the driving force for absorption, the reaction rate is slow enough that the liquid-phase resistance must be taken into account.
This may be due to an insufficient supply of a second reagent or to an inherently slow chemical reaction. What this all boils down to is that the liquid-phase rate coefficient KL in the presence of a chemical reaction normally is larger than the value found when only physical absorption occurs. To account for the effects of chemical reaction, the liquid enhancement factor, E is introduced.
E
=
