Distillation-reaction packed column reactor

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Packed columns are used conventionally to obtain a low pressure drop or low liquid holdup when there is practically no heat to remove or supply or when the gas or the liquid is corrosive. They are not used when solids are present in the feed or are formed in the reaction. Although packed columns or reactors can be operated cocurrently, their operation is usually countercurrent. In particular, countercurrent use is preferred when a higher concentration driving force is needed, that is, for distillation or for most physical absorption. However, when irreversible reaction occurs between dissolved gases and the absorbent, the mean concentration driving force is the same for both modes of operation. In this case the capacity of cocurrent columns is not limited by flooding, and at any given flow rates... 

The transfer of mass within a fluid mixture or across a phase boundary is a process that plays a major role in various engineering and physiological applications. Typical operations where mass transfer is the dominant step are falling film evaporation and reaction, total and partial condensation, distillation and absorption in packed columns, liquid-liquid extraction, multiphase reactors, membrane separation, etc. The various mass transfer processes are classified according to equilibrium separation processes and rate-governed separation processes. Fig. 1 lists some of the prominent mass transfer operations showing the physical or chemical principle upon which the processes are based. 

Geelen and Wijffels (19) investigated the reaction of vinyl acetate with stearic acid in a distillation column to form vinyl stearate and acetic acid. A modified form of the McCabe-Thiele diagram was employed to obtain the number of theoretical plates for a given conversion. This was tested experimentally using a bubble-cap Oldershaw column for the distillation reactor followed by a packed column for separation of top products (vinyl acetate acetic acid). Theoretical considerations agreed reasonably well with experimental findings. 

Pilavakis (20, 29) investigated the esterification of methanol by acetic acid in a packed column. He assumed the reaction to be pseudo-first-order with respect to either methanol or acid over certain specified concentration ranges and incorporated the effect of heat of reaction not only in the enthalpy balances but also in the flux equations. The column was calculated by numerical solution of a set of differential equations. The top product was an azeotropic mixture of methanol and ester which could, however, be broken by introduction of acetic acid high up in the column rather than further down as a mixed feed with methanol. Consequently, in practice such a column will consist of a rectifying section, an extractive distillation section with acetic acid as the extractive solvent and a distillation reactor section. Good agreement was obtained between theory and experiment which, however, suffered from the fact that the hold-up of liquid in the column was small in comparison to the reboiler hold-up so that most of the reaction occurred in the latter location. 

Hegner and Molzahn (30) suggested the extension of their method for calculation of countercurrent separation processes accompanied by chemical reaction to differential-contact (packed column) distillation reactors. However, the method does not incorporate the constraint that liquid should stay at its boiling point and vapour at the condensation point and hence requires some modifications. 

The industrial process (Figure 1) consists of a reactor (acting as the reboiler), a packed column, a total condenser and two distillate vessels. The polymer is manufactured through reversible linear polycondensation or step-growth polymerisation. The overall reaction can be characterised by the following scheme ... 

According to components bubble point (table 2), the distillation involves the separation of the methanol and the resultant reactants (for the most part water) from the reaction mixture. Propylene glycol and by-products are then recovered from the boiler. The overhead batch distillation column consists of a packed column of 50 cm in length and 10 cm in diameter. A condenser equipped with a complex controlled reflux device completes this process. A heat transfer fluid supply reactor jacket with a temperature varying from 10 to 170°C according to the operating steps. 

If one of the products can be continuously removed by carrying out the reaction simultaneously with distillation, then the reaction will be driven further, thus increasing the conversion. The reaction can be carried out either in a simple batch reactor or in a continuous distillation column. The continuous column can be either a plate column (with variations in design) or a packed column. Several studies have been reported on the modeling of such units [see, e.g., the comprehensive reviews by Malone and Doherty (2000) and Chopade and Sharma (1997)]. 

A more recent innovation in MTBE synthesis is the use of catalytic distillation in which the reactor and MTBE fractionator are combined in one vessel [130,131]. The reactive distillation unit is basically a tray distillation column with catalyst held in a proprietary packing placed on the trays. In this way the heat of reaction is recovered and used for the distillation and recovery of the MTBE. Among the major benefits of this design are efficient conversion of the isobutylene [130] and lower operating and capital costs [131], Many processes based on this technolo have been established recently [126,132]. 

Another multiphase reactor that achieves reaction with separation is catalytic distillation. In this reactor a catalyst is placed on the trays of a distillation column or packed into a distillation column, as shown in Figure 12-18. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

In the reaction path from chloro-difluoro-ethane (CDFE) to VDF, a small amount of chlorine works as a catalyst. The mixture is preheated and then fed into a metal nickel tubular reactor, which is housed in a high temperature fired furnace at 550°C-600°C. The reactor effluent is then cooled and enters HCl adsorption tower for the removal of HCl. The gaseous effluent from the HCl adsorption tower is scrubbed with dilute caustic soda and is dried in a packed molecular sieve column. It then enters a distillation column... 

Catalytic distillation (CD) is an unit operation combining reaction and separation in a single reactor/distillation column. CD belongs to the general class of two-phase flow fixed-bed catalytic reactor. An upward flow of vapor and downward flow of liquid conqjrise the two flowing phases in the CD reactor. Solid catalyst packed in a distillation column not only accelerates a chemical reaction but also supplies a packing surface for vapor-liquid mass transfer to separate the reactants... 

The mass balances (Eqs. (10.3) and (10.4)) assume plug-flow behavior for both the vapor and the liquid phase. However, real flow behavior is much more complex and constitutes a fundamental issue in multiphase reactor design. It has a strong influence on the column performance, for example via backmixing of both phases, which is responsible for significant effects on the reaction rates and product selectivity. Possible development of stagnant zones results in secondary undesired reactions. To ensure an optimum model development for catalytic distillation processes, we performed experimental studies on the nonideal flow behavior in the catalytic packing MULTIPAK [77]. 

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