Reactive absorption

The main purposes of absorption processes are the removal of one or more components from the gas phase, the production of particular substances in the liquid phase, and gas mixture separation [1]. Industrial absorption operations are usually realized by combining absorption and desorption units. 

An example given in Fig. 9.1 illustrates this combination of two processes. In an absorber, one or several gas components are absorbed by a lean solvent, either physically or chemically. A rich solvent, after preheating in heat exchangers Hi and H3, is transported to the top of a desorption unit which usually operates under a pressure lower than in absorber. Part of the gas absorbed by the rich solvent is desorbed due to flashing and heating. The other part has to be desorbed in the stripper with the 

Integrated Chemical Processes. Edited by K. Sundmacher, A. Kienle and A. Seidel-Morgenstern Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30831-8 

Usually, a small amount of fresh solvent should be added to the column in order to equalize the solvent loss due to evaporation in the desorber or underwent irreversible chemical reaction occurring in the whole system [1]. 

Reactive absorption represents a process in which a selective solution of gaseous species by a liquid solvent phase is combined with chemical reactions. As compared to purely physical absorption, reactive absorption does not necessarily require elevated pressure and high solubility of absorbed components because of the chemical reaction, the equilibrium state can be shifted favorably, resulting in enhanced solution capacity [2]. Most of the reactive absorption processes involve reactions in the liquid phase only, but in some of them both liquid and gas reactions occur [3, 4]. 

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For physical absorption, values of the mass-transfer coefficients may not vary greatly, so a mean value could be adequate and coiild be taken outside the integral sign, but for reactive absorption the variation usually is too great. 

Derive an equilibrium expression for the reactive absorption of HiS in diethanolamine DEA . The molarity of DEA is 2 kmol/m. The following reaction takes place ... 

Develop equilibrium equations for the reactive absorption of CO2 into ... 

Example 11.7 Carbon dioxide is sometimes removed from natural gas by reactive absorption in a tray column. The absorbent, typically an amine, is fed to the top of the column and gas is fed at the bottom. Liquid and gas flow patterns are similar to those in a distillation column with gas rising, liquid falling, and gas-liquid contacting occurring on the trays. Develop a model for a multitray CO2 scrubber assuming that individual trays behave as two-phase, stirred tank reactors. 

Equipment for reactive absorption is of the same types as for physical absorption, towers of various kinds and stirred tanks. Packed or tray towers are the most common, but spray or bubble towers are used for their mechanical simplicity and when there is a likelihood of clogging. Thus S02 is scrubbed from air with a spray of lime slurry in a tower. Fluorine waste gases form solids on contact with water, so they are scrubbed by bubbling the gas through water in an empty tower. 

A reactive absorption is done in a countercurrent packed tower. The material balance is made in terms of solute-free quantities. Inlet conditions are X2 = 0, Yx - 0.5 outlet Xx =0.8, Y2 = 0.1. Bottom is 1, top is 2. 

Mehendale HM. 1977a. Chemical reactivity-absorption, retention, metabolism, and elimination of hexachlorocyclopentadiene. Environ Health Perspect 21 275-278. 

Reactive absorption is probably the most widely applied type of a reactive separation process. It is used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. It is also often employed in gas purification processes, e.g., to remove carbon dioxide or hydrogen sulfide. Other interesting areas of application include olefin/paraffin separations, where reactive absorption with reversible chemical complexation appears to be a promising alternative to the cryogenic distillation (62). 

Reactive distillation Membrane-based reactive separations Reactive adsorption Reactive absorption Reactive extraction Reactive crystallization... 

Reactive absorption is very old as a processing technique and has been used for production purposes in a number of classical bulk-chemical technologies, such as nitric or sulfuric acid. The Raschig process for the production of hydroxylamine, an important intermediate in classical caprolactam technologies (Stamicarbon, Inventa), is also an example of a multistep reactive absorption process. Here, water, ammonia, and carbon dioxide react together in an absorption column to give a solution of ammonium carbonate, which subsequently forms an alkaline... 

Carbon dioxide removal by reactive absorption in amine solutions is also applied on the commercial scale, for instance, in the treatment of flue gas (see later in this chapter). Another possible application field of the technique is gas desulfurization, in which H2S is removed and converted to sulfur by means of reactive absorption. Aqueous solutions of ferric chelates (160-162) as well as tetramethylene sulfone, pyridine, quinoline, and polyglycol ether solutions of S02 (163,164) have been proposed as solvents. Reactive absorption can also be used for NOx reduction and removal from flue or exhaust gases (165,166). The separation of light olefins and paraffins by means of a reversible chemical com-plexation of olefins with Ag(I) or Cu(I) compounds in aqueous and nonaqueous solutions is another very interesting example of reactive absorption, one that could possibly replace the conventional cryogenic distillation technology (167). 

The most important examples of reactive separation processes (RSPs) are reactive distillation (RD), reactive absorption (RA), and reactive extraction (RE). In RD, reaction and distillation take place within the same zone of a distillation column. Reactants are converted to products, with simultaneous separation of the products and recycling of unused reactants. The RD process can be efficient in both size and cost of capital equipment and in energy used to achieve a complete conversion of reactants. Since reactor costs are often less than 10% of the capital investment, the combination of a relatively cheap reactor with a distillation column offers great potential for overall savings. Among suitable RD processes are etherifications, nitrations, esterifications, transesterifications, condensations, and alcylations (2). 

Reactive absorption, distillation, and extraction have much in common. First of all, they involve at least one liquid phase, and therefore the properties of the liquid state become significant. Second, they occur in moving systems, thus the process hydrodynamics plays an important part. Third, these processes are based on the contact of at least two phases, and therefore the interfacial transport phenomena have to be considered. Further common features are multicomponent interactions of mixture components, a tricky interplay of mass transport and chemical reactions, and complex process chemistry and thermodynamics. 

Reactive absorption can be realized in a variety of equipment types, e.g., in him absorbers, plate columns, packed units, or bubble columns. This process is characterized by independent how of both phases, which is different from distillation and permits both cocurrent (downflow and uphow) and countercurrent regimes. 

Reactive absorption is essentially an old process, known since the foundation of modem industry. This is a very important process, too, being the basic operation in many technological chains. More recently, the role of RA as a key environmental protection process has grown up significantly. 

This chapter concerns the most important reactive separation processes reactive absorption, reactive distillation, and reactive extraction. These operations combining the separation and reaction steps inside a single column are advantageous as compared to traditional unit operations. The three considered processes are similar and at the same time very different. Therefore, their common modeling basis is discussed and their peculiarities are illustrated with a number of industrially relevant case studies. The theoretical description is supported by the results of laboratory-, pilot-, and industrial-scale experimental investigations. Both steady-state and dynamic issues are treated in addition, the design of column internals is addressed. 

Reactive absorption, reactive distillation, and reactive extraction occur in multicomponent multiphase fluid systems, and thus a single modeling framework for these processes is desirable. In this respect, different possible ways to build such a framework are discussed, and it is advocated that the rate-based approach provides the most rigorous and appropriate way. By this approach, direct consideration... 

Noeres C, Kenig EY, Gorak A. Modelling of reactive separation processes reactive absorption and reactive distillation. Chem Eng Process 2003 42 157-178. 

Kenig EY, Wiesner U, Gorak A. Modeling of reactive absorption using the Maxwell-Stefan equations. Ind Eng Chem Res 1997 36 4425-4434. 

Schneider R, Kenig EY, Gorak A. Dynamic modelling of reactive absorption with the Maxwell-Stefan approach. Trans IchemE 1999 77(part A) 633-638. 

Kenig EY, Schneider R, Gorak A. Rigorous dynamic modelling of complex reactive absorption processes. Chem Eng Sci 1999 54 5195-5203. 

Usually, the effect of chemical reactions in reactive absorption processes is advantageous only in the region of low gas-phase concentrations due to limitations by the reaction stoichiometry or equilibrium [5]. Further difficulties of reactive absorption applications may be caused by the reaction heat through exothermic reactions and by relatively difficult solvent regeneration [6, 7]. Most of the reactive absorption processes are steady-state operations, either homogeneously catalyzed or auto-catalyzed. Recently, an application of a reactive absorption process based on using secondary amine groups on solid supports as immobilized activators has been reported [8]. 

Reactive absorption processes are predominantly used for the production of basic chemicals, e.g., sulfuric or nitric acids, and for the removal of harmful substances such as H2S from gas streams. Absorbers or scrubbers in which reactive absorption is performed are often considered as gas-liquid reactors [9], If more attention is paid to the mass transport, these apparata are treated rather as absorption units. Some important industrial applications of reactive absorption are given in Tab. 9.1. 

Despite the clear importance of reactive absorption, its behavior is still not properly understood. This can be attributed to a very complex combination of process thermodynamics and kinetics, with intricate reaction schemes including ionic species, reaction rates varying over a wide range, and complex mass transfer-reaction coupling. As compared to distillation, reactive absorption is a fully rate-controlled process and it occurs definitely far from the equilibrium state. Therefore, both practitioners and theoreticians are highly interested to establish a proper understanding and description of this process. 

This chapter provides a comprehensive overview of reactive absorption and discusses in detail the following issues ... 

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