Temperature reactive distillation

Figure 2 illustrates the three-step MIBK process employed by Hibernia Scholven (83). This process is designed to permit the intermediate recovery of refined diacetone alcohol and mesityl oxide. In the first step acetone and dilute sodium hydroxide are fed continuously to a reactor at low temperature and with a reactor residence time of approximately one hour. The product is then stabilized with phosphoric acid and stripped of unreacted acetone to yield a cmde diacetone alcohol stream. More phosphoric acid is then added, and the diacetone alcohol dehydrated to mesityl oxide in a distillation column. Mesityl oxide is recovered overhead in this column and fed to a further distillation column where residual acetone is removed and recycled to yield a tails stream containing 98—99% mesityl oxide. The mesityl oxide is then hydrogenated to MIBK in a reactive distillation conducted at atmospheric pressure and 110°C. Simultaneous hydrogenation and rectification are achieved in a column fitted with a palladium catalyst bed, and yields of mesityl oxide to MIBK exceeding 96% are obtained. 

This low viscosity resin permits cure at low (70°C) temperatures and rapidly develops excellent elevated temperature properties. Used to increase heat resistance and cure speed of bisphenol A epoxy resins, it has utihty in such diverse appHcations as adhesives, tooling compounds, and laminating systems. A moleculady distilled version is used as a binder for soHd propellants (see Explosives and propellants) and for military flares (see Pyrotechnics). Its chief uses depend on properties of low viscosity and low temperature reactivity, particularly with carboxy-terminated mbbers. 

EfiBdent hydrogen supply iiom decalin was only accomplished by the si terheated liquid-film-type catalysis under reactive distillation conditions at modaate heating tempaatures of 210-240°C. Caibcm-supported nano-size platinum-based catalysts in the si ietheated liquid-film states accelerated product desorption fixjm file catalyst surface due to its temperature gradient under boiling conditions, so that both hi reaction rates and conversions were obtained simultaneously. 

EfiSdent hydrogai supply firm decalin at modoate temperatures of below 250°C was acomplished by utilizing the superheated liqirid-film- pe catalysis under reactive distillation conditions in the present study. The composite catalysts in Ak liquid-film states improved dehydrogenation activities for decalin. 

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. 

Two options are being developed at the moment. The first is to produce 1,2-propanediol (propylene glycol) from glycerol. 1,2-Propanediol has a number of industrial uses, including as a less toxic alternative to ethylene glycol in anti-freeze. Conventionally, 1,2-propanediol is made from a petrochemical feedstock, propylene oxide. The new process uses a combination of a copper-chromite catalyst and reactive distillation. The catalyst operates at a lower temperature and pressure than alternative systems 220°C compared to 260°C and 10 bar compared to 150 bar. The process also produces fewer by-products, and should be cheaper than petrochemical routes at current prices for natural glycerol. The first commercial plant is under construction and the process is being actively licensed to other companies. 

Figure 33.5 shows the composition, temperature and reaction rate profiles in the reactive distillation column. The ester product with traces of methanol is the bottom product, whereas a mixture of water and fatty acid is the top product. This mixture is then separated in the additional distillation column and the acid is refluxed back to the RDC. The fatty ester is further purified in a small evaporator and methanol is recycled back to the RDC (Figures 33.3 and 33.4). 

Selective tertiary-huimoX (tBA) dehydration to isobutylene has been demonstrated using a pressurized reactive distillation unit under mild conditions, wherein the reactive distillation section includes a bed of formed solid acid catalyst. Quantitative tBA conversion levels (>99%) have been achieved at significantly lower temperatures (50-120°C) than are normally necessary using vapor-phase, fixed-bed, reactors (ca. 300°C) or CSTR configurations. Substantially anhydrous isobutylene is thereby separated from the aqueous co-product, as a light distillation fraction. Even when employing crude tBA feedstocks, the isobutylene product is recovered in ca. 94% purity and 95 mole% selectivity. 

From a thermodynamics basis, the transesterification reaction favors the formation of methylphenyl carbonate (Equation 7.4), whilst its further disproportionation in a second-stage continuous reactive distillation column affords DPC with selectivity >99%. Although both reactions occur at a relatively high temperature ( 473 K), optimization of the reaction conditions and engineering design would allow a productivity that fitted with the economics [17, 27]. 

Table 1 gives an overview of the possible applications of reactive distillation reported in the literature. Very few of them have been realized so far on the commercial scale. One of the common factors that hinders a broader application of reactive distillation is a small feasible operation window. The overlap region in the pressure-temperature domain, in which chemical reaction and separation and apparatus design are feasible, is usually quite narrow (see Figure 2 in Chapter 9). A possible remedy for this limitation is sought in the development of new types of catalysts that would allow one to significantly broaden the feasible operation window for chemical reaction. 

The analysis presented in this chapter is an example of how the principles of thermodynamics can be applied to establish efficiencies in separation units. We have shown how exergy analysis or, equivalently, lost work or availability analysis can be used to pinpoint inefficiencies in a distillation column, which in this case were the temperature-driving forces in the condenser and the reboiler. The data necessary for this analysis can easily be obtained from commonly used flow sheeters, and minimal extra effort is required to compute thermodynamic (exergetic) efficiencies of various process steps. The use of hybrid distillation has the potential to reduce column inefficiencies and reduce the number of trays. We note that for smaller propane-propene separation facilities (less than 5000bbl/day [10]), novel technologies such as adsorption and reactive distillation can be used. 

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. 

At high temperatures and pressures, the iodine partial pressure in the gas phase above low iodine content liquid HI/I2/H20 phases (representative of the top of the reactive distillation column) is higher than predicted by Neumann s model. Since this partial pressure limits HI decomposition, the reactive distillation column will be less effective than expected. 

In some liquid-vapour equilibrium experiments with low iodine contents (Larousse, 2009), an unexpected decrease of the pressure with time, especially at elevated temperatures, is observed (Figure 3). A possible, though not proven, explanation would be HI decomposition with condensation of the resulting I2 in the liquid phase, just like what is anticipated to take place in the reactive distillation column. However, the observed kinetics are about 105 faster than what is expected from gas phase composition, which could be an indication of liquid phase decomposition. 

The third reaction takes place in a reactive distillation tower where the iodhiric acid is concentrated and decomposed simultaneously, to produce hydrogen at a temperature range from 200 to 310°C and pressure up to 22 bar (Brown, 2003). This section requires analysis of iodhidric acid leak and hydrogen explosions, though these sections are not covered in this paper as they will be the subject of future developments. 

In the first case, product purities are controlled indirectly by controlling front positions. In distillation columns the front positions are easily controlled with cheap, reliable and fast online temperature measurements on sensitive trays inside the column [27]. A similar procedure was recently proposed for moving-bed chromatographic processes with UV rather than temperature measurement [37]. However, the performance of such an approach is usually limited. Exact product specifications cannot be guaranteed because of this indirect approach. Furthermore, in combined reaction separation processes the relationship between the measured variable and the variable to be controlled is often non-unique, which may lead to severe operational problems as shown for reactive distillation processes [23], It was concluded that these problems could be overcome if in addition some direct or indirect measure of conversion is taken into account. 

At first sight, adsorption and reaction are well-matched functionalities for integrated chemical processes. Their compatibility extends over a wide temperature range, and their respective kinetics are usually rapid enough so as not to constrain either process, whereas the permeation rate in membrane reactors commonly lags behind that of the catalytic reaction [9]. The phase slippage observed in extractive processes [10], for example, is absent and the choice of the adsorbent offers a powerful degree of freedom in the selective manipulation of concentration profiles that lies at the heart of all multifunctional reactor operation [11]. Furthermore, in contrast to reactive distillation, the effective independence of concentration and temperature profiles... 

The best-known and (commercially) most successful example of combining reaction and separation is the reactive distillation. Reactive distillation has been investigated widely [3, 4 see also Chapters 3, 4, and 5] and is applied to many processes [5], However, the integration of more than one functionality in an apparatus leads to a loss in degrees of freedom. For a successful integration, the feasible window of operation concerning process conditions such as pressure and temperature must coincide for the reaction, the separation and the apparatus (Fig. 8.1). 

For a number of processes, reactive distillation is not possible, as some of the reactants are destroyed or degraded in side reactions by heating them up to boiling temperature. Examples of such processes are the Knoevenagel-condensation of aldehydes or ketones with components of high CH-acidity, the production of enam-ines or carbonic acid amides, or the esterification of fatty acids with fatty alcohols to fatty esters [7]. 

Reactive stripping has its own importance, in addition to reactive distillation. Several situations exist in which reactive stripping becomes more interesting for example, in the production of high-boiling esters and ethers, especially, when reactants or products are temperature-sensitive, and also in gas-liquid-solid processes, where product inhibition may play a role. 

However, conventional batch distillation with chemical reaction (reaction and separation taking place in the same vessel and hence referred to as Batch REActive Distillation- BREAD) is particularly suitable when one of the reaction products has a lower boiling point than other products and reactants. The higher volatility of this product results in a decrease in its concentration in the liquid phase, therefore increasing the liquid temperature and hence reaction rate, in the case of irreversible reaction. With reversible reactions, elimination of products by distillation favours the forward reaction. In both cases higher conversion of the reactants is expected than by reaction alone. Therefore, in both cases, higher amount of distillate (proportional to the increase in conversion of the reactant) with desired purity is expected than by distillation alone (as in traditional approach) (Mujtaba and Macchietto, 1997). 

Lehtonen et al. (1998) considered polyesterification of maleic acid with propylene glycol in an experimental batch reactive distillation system. There were two side reactions in addition to the main esterification reaction. The equipment consists of a 4000 ml batch reactor with a one theoretical plate distillation column and a condenser. The reactions took place in the liquid phase of the reactor. By removing the water by distillation, the reaction equilibrium was shifted to the production of more esters. The reaction temperatures were 150-190° C and the catalyst concentrations were varied between 0.01 and 0.1 mol%. The kinetic and mass transfer parameters were estimated via the experiments. These were then used to develop a full-scale dynamic process model for the system. 

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. 

The first two items are particularly powerful. The result is that a reactive distillation setup offers the possibility of achieving simultaneously high conversion for both reactants, with stoichiometric consumption of reactants at optimal selectivity. The third item indicates that the reactive distillation is of great interest for equilibrium constrained reactions. Taking advantage of exothermal reactions depends on the temperature level that can be allowed by the phase equilibrium. 

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. 

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