Tower or Column Reactors

The types of reactors used for fluid-fluid reactions may be divided into two main types (1) tower or column reactors, and (2) tank reactors. We consider some general features of these in this section and in Section 24.3. In Sections 24.4 and 24.5, we treat some process design aspects more quantitatively. 

As its name implies, a tower reactor typically has a height-to-diameter (h/D) ratio considerably greater than 1. Types of tower or column reactors (the words tower and column may be used interchangeably) go by descriptive names, each of which indicates a particular feature, such as the means of creating gas-liquid contact or the way in which one phase is introduced or distributed. The flow pattern for one phase or for both phases may be close to ideal (PF or BMF), or may be highly nonideal. 

---

Figure 24.1 Types of tower or column reactors for gas-liquid reactions (a) packed tower, (b) plate tower, (c) spray tower, (d) falling-film tower, (e) bubble column...

Many chemical and biological processes are multistage. Multistage processes include absorption towers, distillation columns, and batteries of continuous stirred tank reactors (CSTRs). These processes may be either cocurrent or countercurrent. The steady state of a multistage process is usually described by a set of linear equations that can be treated via matrices. On the other hand, the unsteady-state dynamic behavior of a multistage process is usually described by a set of ordinary differential equations that gives rise to a matrix differential equation. 

These processes are carried out in a variety of equipment ranging from a bubbling absorber to a packed tower or plate column. The design of the adsorber itself requires models characterizing the operation of the process equipment and this is discussed in Chapter 14. The present chapter is concerned only with the rate of reaction between a component of a gas and a component of a liquid—it considers only a point in the reactor where the partial pressure of the reactant A in the gas phase is and the concentration of A in the liquid is C, that of B, Cg. Setting up rate equations for such a heterogeneous reaction will again require consideration of mass and eventually heat transfer rates in addition to the true chemical kinetics. Therefore we first discuss models for transport from a gas to a liquid phase. 

The above experiments, if done under conditions equivalent to full scale ones with a well-mixed stirred tank reactor at steady state, give the basic rate of overall reaction plus information on what influences it. These can be used for scale-up calculations, either keeping to a stirred tank, or where appropriate, scaling up a different type of reactor, e.g. a bubble column for Regime I, a cascade of stirred tanks if plug flow is required in Regime II, or a packed tower or gas-liquid annular flow tubular reactor for Regime III or for gas-fllm controlled mass transfer. 

Blenke-cascade reactor Baffled tower separating the reactor into different sections being mixed by upward flowing gas. Similar to plate columns. Liquid or liquid-solid mixture can be operated co- or countercurrently... 

Hence in laboratory bubble column reactors, eqs. (92), (93) and/or (94) can be used, where X-, S- and 0i should be replaced by their mean values in the tower. 

An interesting situation arises in processes where the reaction product P evaporates and is taken out of the reactor with the gas phase (the supply phase). Let us assume that there are no chemical reactions in the gas phase, e.g., l ause the liquid phase reaction is catalysed. We consider the case of rapid reactions, so that all the desired product P is formed in the diffusion layer in the liquid phase, close to the interface. When P can undergo undesired reactions in the liquid phase it is essential to remove P as effectively as we can, e.g., by creating a large surface area and very high gas-phase mass transfer coefficients. At the same time it is essential that the volume of the liquid phase is minimized, since decomposition of P will occur just there. The obvious choice would then be a configuration where the liquid is the dispersed phase, such as in a spray tower or a spray cyclone, provided the heat removal rate is sufficient. Another suitable arrangement could be a gas/liquid packed bed or a wetted wall column. The latter reactor type is very suitable for heat removal (section 4.6.3.1)... 

Continuous reactors are at work all the time. This means newly introduced reactants mix to some extent with products. This extent is termed backmixing. A tower has many plates or baffles in it and experiences less backmixing as, for instance, a tank with no plates. Continuous reactors can then be found within towers and columns. Towers may be packed or plate (bubble cap or sieve tray) type. Optimum reactor design attempts to curtail the amount of dead space or areas where no reaction is taking place. It is also possible to have reactants take a shorter path than is necessary for optimum reaction. This is called shortcircuiting. 

The purpose of the main fractionator, or main column (Figure 1 -1 o i, is to desuperheat and recover liquid products from the reactor vapors. The hot product vapors from the reactor flow into the main fractionator near the base. Fractionation is accomplished by condensing and revaporizing hydrocarbon components as the vapor flows upward through trays in the tower. 

This situation describes an emulsion reactor in which reacting drops (such as oil drops in water or water drops in oil) flow through the CSTR with stirring to make the residence time of each drop obey the CSTR equation. A spray tower (liquid drops in vapor) or bubble column or sparger (vapor bubbles in a continuous liquid phase) are also segregated-flow situations, but these are not always mixed. We wiU consider these and other multiphase reactors in Chapter 12. 

Figure 12-9 Bubble column and spray tower reactors. Large drop or bubble areas increase reactant mass transfer,...

Figure 12-10 Sketches of reactant concentration Ca around a spherical bubble or drop that reacts after migrating from ftie gas phase into the liquid phase in bubble column and spray tower reactors.

If we simply turn the drawing of the bubble column upside down, we have a spray tower reactor. Now we have dense liquid drops or solid particles in a less dense gas so we spray the liquid from the top and force the gas to rise. The same equations hold, but now the mass transfer resistance is usually within the hquid drop. 

The bubble column and spray tower depend on nozzles to disperse the drop or bubble phase and thus provide the high area and small particle size necessary for a high rate. Drop and bubble coalescence are therefore problems except in dilute systems because coalescence reduces the surface area. An option is to use an impeller, which continuously redisperses the drop or bubble phase. For gases this is called a sparger reactor, which might look as shown in Figure 12-16. 

The spray tower is a heterogeneous gas-liquid reactor. The gas passing up the column obeys plug flow conditions, and the liquid sprayed into the column behaves either as plug flow or as batch for individual droplets falling down the tower. 

Purification. The effluent is sent to a series of distillations. The first (30 to 35 trays) separates a methanol/water mixture at the top, which is then sent to a dehydration tower (25 to 30 trays), while the bottom is sent to a column in winch the ester is fractionated under partial vacuum (40 to 45 trays). Methyl toluate and excess p-xylene leave at the top and are recycled to the oxidation reactor. The withdrawal, consisting of crude terephthalate, is redistilled under vacuum to remove heavy compounds (20 trays), and then sent to a vacuum crystallizer (40 to 50 kPa absolute) using methanol as solvent. This may be followed by a second crystallization, or a countercurrent washing with methanol to complete the purification. The dimethyl terephthalate is finally centrifuged, melted to remove residual methanol, and vacuum distilled (30 trays). The molar yield of the operation in relation to p-xylene is about 87 per cent... 

Bipolar Trickle Tower Reactor British Technology Group Series (<60) of bipolar elements in a column Discontinuous via leaching or removal of the carbon bed No / /... 

Fluid phase only Countercurrent flow Absorber Countercurrent flow Absorber Countercurrent flow Spray tower Co-current or countercurrent Bubble column Absorber or Reactor Venturi Static mixers Falling film, etc. 

Stream Information. Directed arcs that represent the streams, with flow direction from left to right wherever possible, are numbered for reference. By convention, when streamlines cross, the horizontal line is shown as a continuous arc, with the vertical line broken. Each stream is labeled on the PFD by a numbered diamond. Furthermore, the feed and product streams are identified by name. Thus, streams 1 and 2 in Rgure 3.19 are labeled as the ethylene and chlorine feed streams, while streams 11 and 14 are labeled as the hydrogen chloride and vinyl-chloride product streams. Mass flow rates, pressures, and tempera-mres may appear on the PFD directly, but more often are placed in the stream table instead, for clarity. The latter has a column for each stream and can appear at the bottom of the PFD or as a separate table. Here, because of formatting limitations in this text, the stream table for the vinyl-chloride process is presented separately in Table 3.6. At least the following entries are presented for each stream label, temperature, pressure, vapor fraction, total and component molar flow rates, and total mass flow rate. In addition, stream properties such as the enthalpy, density, heat capacity, viscosity, and entropy, may be displayed. Stream tables are often completed using a process simulator. In Table 3.6, the conversion in the direct chlorination reactor is assumed to be 100%, while that in the pyrolysis reactor is only 60%. Furthermore, both towers are assumed to carry out perfect separations, with the overhead and bottoms temperatures computed based on dew- and bubble-point temperatures, respectively. 

---