Reactive-distillation solution

The dehydrogenation of decalin to naphthalene has been investigated on Pt/C, Pt/A1(0H)0 and Pt/Al203 catalysts. The maximum conversion of decalin on 3.9% Pt/C, which did not repel decalin, was observed at 483 K under the conditions of 0.3 g of the catalyst and 1ml of decalin, which was corresponded to the liquid film state under reactive distillation conditions. However such a maximum was not observed on Pt/Al(OH)0 and Pt/Al203, which repelled decalin. Furthermore it was found that the reaction temperature, at which the maximum hydrogen evolution was observed on Pt/C, was shifted from the boiling point of decalin to that of naphthalene with increasing the amormt of naphthalene in the reaction solution. 

Removal of formaldehyde from aqueous 2-butyne-l,4-diol, or a similar solution, which is relevant in the subsequent manufacture of c -2-butene-l,4-diol, by batch reactive distillation with methanol or ethylene glycol in the presence of Indion 130 as catalyst has also been reported 98% conversion of formaldehyde was obtained by reactive distillation with 7 times the stoichiometric quantity of methanol, compared to 15% conversion obtained in a closed system (Kolah and Sharma, 1995). 

To a 500 g sample of acidic, dealuminized mordenite (CBV-20A from PQ Corporation, 1.5 mm diameter extrudates that had been calcined at 540 °C, overnight) was added a solution of 33 ml of 48% HF in 1633 ml of distilled water, and the mix cooled in ice, stirred on a rotary evaporator overnight, then filtered to recover the extruded solids. The extrudates were further washed with distilled water, dried in vacuo at 100 °C, and then calcined at 540 °C, overnight. Analyses of the treated mordenite showed 1.2% fluoride, 0.49 meq/g acidity. Samples were charged to the reactive distillation unit either as 20/40 mesh granules, or as ca. 1.5 mm extrudates. 

In numerous cases, membrane-separation processes operate at much lower temperature, especially when compared with thermal processes such as reactive distillation. As a consequence they might provide a solution for the limited thermal stability of either catalyst or products. Furthermore, by membrane-separation processes is possible also to separate nonvolatile components. 

The outline of this chapter is as follows First, some basic wave phenomena for separation, as well as integrated reaction separation processes, are illustrated. Afterwards, a simple mathematical model is introduced, which applies to a large class of separation as well as integrated reaction separation processes. In the limit of reaction equilibrium the model represents a system of quasilinear first-order partial differential equations. For the prediction of wave solutions of such systems an almost complete theory exists [33, 34, 38], which is summarized in a second step. Subsequently, application of this theory to different integrated reaction separation processes is illustrated. The emphasis is placed on reactive distillation and reactive chromatography, but applications to other reaction separation processes are also... 

In the scalar case (i.e., N = 1), wave solutions are easily constructed with the equilibrium diagram y(x) or Y(X). According to the above considerations, typical scalar problems are a binary nonreactive distillation process, a ternary reactive distillation process with a single chemical reaction, a reactive distillation process with Nc components and Nc - 2 chemical reactions, or a chromatographic reactor with Ns solutes... 

Note that the system (2.45) is a DAE system of nontrivial index, since z cannot be evaluated directly from the algebraic equations. A solution for the variables z must be obtained by differentiating the constraints k(x) = 0. For most chemical processes, such as reaction networks (Gerdtzen et al. 2004), reactive distillation processes (Vora 2000), and complex chemical plants (Kumar and Daoutidis 1999a), the z variables can be obtained after just one differentiation of the algebraic constraints ... 

The mathematical model comprises a set of partial differential equations of convective diffusion and heat conduction as well as the Navier-Stokes equations written for each phase separately. For the description of reactive separation processes (e.g. reactive absorption, reactive distillation), the reaction terms are introduced either as source terms in the convective diffusion and heat conduction equations or in the boundary condition at the channel wall, depending on whether the reaction is homogeneous or heterogeneous. The solution yields local concentration and temperature fields, which are used for calculation of the concentration and temperature profiles along the column. 

Therefore, adopting the solution of reactive distillation instead of separate reaction and separation units does not lead automatically to a more efficient process. Matching the conditions of separation and reaction in the same device requires careful design. The element with the highest impact is the chemical reaction. The key condition for an efficient and competitive process by reactive distillation is the availability of a superactive catalyst capable to compensate the loss in the driving force by phase equilibrium, but at the same time ensuring a good selectivity pattern. 

Any of the global Newton methods can be converted to a relaxation form in Ketchum s method by making both the temperatures and the liquid compositions time dependent and by having the time step increase as the solution is approached. The relaxation technique should be applied to difflcult-to-solve systems and the method of Naphtali and Sandholm (42) is best-suited for nonideal mixtures since both the liquid and vapor compositions are included in the independent variables. Drew and Franks (65) presented a Naphtali-Sandholm method for the dynamic simulation of a reactive distillation column but also stated that this method could be used for finding a steady-state solution. 

A sensitivity analysis on optimal solutions obtained for a reactive distillation column... 

The reactive distillation operation is obviously not limited to zeolite catalysts. It can also be carried out with homogeneous acids such as sulfuric acid or p-toluene-sulfonic acid. Since they lack shape selectivity, these catalysts first convert phenyl-ethanol to the corresponding ether and only then to styrene. Hence, the reaction proceeds in a solution of heavy products that have accumulated over time. Additives have been developed to control the oligomerization reactions and keep the liquid viscosity at a workable level [35]. The heavy liquid medium needs to be bled. Its contamination with strong acid makes its disposal costly, however. 

The production of high-purity methyl acetate was the first large-scale application of reactive distillation the process was developed by Agreda and coworkers at Eastman Chemical Company [27]. The reaction of methanol with acetic acid takes place in solution with an acid catalyst ... 

Integration of reaction and separation in a single unit is a powerful tool to increase efficiency and economic advantages of many chemical processes. Reactive distillation, extraction, and adsorption are well-known examples of this technological resource. Recently, a very promising solution is offered by membrane reactors... 

Jacobs R. and Krishna R. (1993). Multiple solutions in reactive distillation for methyl tert-butyl ether synthesis. Industrial and Engineering Chemistry Research 32 (8), 1706-1709. 2.1, 2.9, 2.9.1.1,... 

Flowsheets for processes are sometimes generated without following the hierarchy of properties described previously. As an example, Siirola [20] proposed a reactive-distiUation solution to make methyl acetate. Unit operations that combine the property differences present abrupt departures from common methodologies. With the advent of various pieces of equipment, such as differential side-stream feed reactors (i.e., semicontinuously fed batch reactors), continuous evaporator-reactors (e.g., wiped-film evaporators), and reactive distillation columns, one can consider these unit operations in the development of conceptual designs. As an example, Doherty and Malone [21] have presented systematic methods for reactive distillation design. 

Pervaporation, as a non-integrated process, is typically utilized for dehydration and for the recovery or removal of organics from aqueous solutions and sometimes also for the separation of organic mixtures (Neel, 1995). Also many hybrid processes have been developed where PV is coupled with other processes, such as different membrane processes (e.g., reverse osmosis, or organophilic pervaporation coupled with hydrophilic pervaporation), distillation, reactive distillation and, of course, reaction. With these aspects in mind, PV appears particularly suitable to keep the concentration of a by-product low, or to continuously recover a product while it is formed. Note that these are the main objectives typically pursued in membrane reactors. 

Thermodynamic effects. Many performance limitations are related to a thermodynamic equilibrium, which is the case for numerous reversible reactions. Existing solutions consist in coupling the reaction with a separation system (reactive distillation, reactive chromatography, etc.) and can even be coupled to geometric structuring [11]. 

Despite these many studies, reactive distillation is not the best solution to couple reaction and separation, mainly for three reasons (1) in most of the cases, the esterification medium (reagents plus products) is a nonideal system (not really fit for vapor-equilibrium-based technology such as reactive distillation) (2) pure water cannot be selectively removed from the top or the bottom of the column and (3) reactive distillation is a high-energy consumption technology (because separation is based on conventional distillation) (Lim, Park, Hung, Sahimi, Tsotsis, 2002 Drioli Giomo, 2010). 

Reactive distillation. To avoid conversion of product into unwanted components or to drive equilibrium reactions forward, reactive distillation is used to distill out one of the products immediately as it is formed. A conventional equilibrium process consists of a reactor and a distillation column. One of the products is removed in the column, and the bottom is recycled (see Fig. 6.22). In reactive distillation the column is located on top of the reactor, obviating the need for a reboiler (see Fig. 6.23). Reaction of an organic chlorine compound with an aqueous caustic solution can be carried out in a reactive distillation column, which will combine functions such as dissolution of... 

Mechanical stable polymer based catalytic packings for reactive distillation columns are not available up to date. An Intermediate solution Is to fill small beads of lonexchange resin into wire nets or glassfiber cloth bags. These bags or bales are introduced into the... 

Rigorous models of staged distillation processes are formulated by setting up material balance equations, equilibrium relations, summation equations, and enthalpy balance equations (MESH equations). In these models, the extent of nonlinearity may be very severe, particularly for azeotropic and reactive distillation systems. MESH system based mathematical models can thus yield multiple solutions (multiple steady states), a fact which has been observed by many researchers. 

Phase and chemical equilibrium calculations are essential for the design of processes involving chemical transformations. Even in the case of reactions that cannot reach chemical equilibrium, the solution of this problem gives information on the expected behaviour of the system and the potential thermodynamic limitations. There are several problems in which the simultaneous calculation of chemical and phase behaviour is mandatory. This is the case, for example, of reactive distillations where phase separation is used to shift chemical equilibrium. Also, the calculation of gas and solid solubility in liquids of high dielectric constants requires at times the resolution of chemical equilibrium between the different species that are formed in the liquid phase. Several algorithms have been proposed in the literature to solve the complex non-linear problem however, proper thermodynamic model selection has not received much attention. 

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