Towers and columns

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. 

In this subsection, the terms gas and vapor are used interchangeably. Vapor is more precise for distillation, where the gas phase is at equilibrium. Also, the terms tower and column are usedinterchangeably. 

At near atmospheric pressures, oxidation and absorption rates are slow, and earlier atmospheric and low-pressure plants used between five and ten very large towers. Maximum strengths attainable were in the range of 42% to 52% HNO3. To reduce capital investment and to obtain increased operating efficiencies, absorption under pressure was developed, using various types of equipment, e.g., cascade coolers, packed columns, spray towers, and columns incorporatir bubble plates, sieves, and such special devices as the Kuhlman tray. 

Distillation Fractional Distillation (see also Towers and Columns)... 

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. 

Towers and columns are heat- and mass-transfer devices in which reactions may occur. Reactors fi equently are large enoui to require the structural design techniques used with towers. The term reactor tower mi t be used to describe a tower that does reactor functions. 

Separate ketdes and backwash towers are frequendy used to convert ion-exchange resins from one ionic form to another prior to packaging, and to cleanse the resin of chemicals used in the functionalization reactions. Excess water is removed from the resin prior to packaging by a vacuum drain. Both straight line filters and towers or columns are used for this purpose. 

The effective interfacial area depends on a number of factors, as discussed in a review by Charpentier [C/j m. Eng.J., 11, 161 (1976)]. Among these factors are (1) the shape and size of packing, (2) the packing material (for example, plastic generally gives smaller interfacial areas than either metal or ceramic), (3) the liquid mass velocity, and (4), for smaU-diameter towers, the column diameter. 

Determine the values of the plate activation velocities (or load points), Fh, for the minimum as well as maximum liquid loads at top and bottom of the tower and any intermediate points exhibiting significant change in flow rates. For partial column area... 

Relative Performance Characteristics of Tower Packing and Column Trays ... 

The gas oil feed for the conventional cat cracker comes primarily from the atmospheric column, the vacuum tower, and the delayed coker. In addition, a number of refiners blend some atmospheric or vacuum resid into the feedstocks to be processed in the FCC unit. 

Considering the countercurrent flow of water and air in a tower of height z (Figure 13.15), the mass rate of flow of air per unit cross-section G is constant throughout the whole height of the tower and, because only a small proportion of the total supply of water is normally evaporated (1 -5 per cent), the liquid rate per unit area ll can be taken as constant. The temperature, enthalpy, and humidity will be denoted by the symbols 6, H, and Tf respectively, suffixes G, L, 1, 2, and / being used to denote conditions in the gas and liquid, at the bottom and top of the column, and of the air in contact with the water. 

Equation 13.50 gives the relation between liquid temperature, air enthalpy, and conditions at the interface, for any position in the tower, and is represented by a family of straight lines of slope —(ht/hop). The line for the bottom of the column passes through the point A( u,Hgi) and cuts the enthalpy-temperature curve for saturated air at the point C, representing conditions at the interface. The difference in ordinates of points A and C is the difference in the enthalpy of the air at the interface and that of the bulk air at the bottom of the column. 

In a countercurrent packed column, n-butanol flows down at a rate of 0.25 kg/m2 s and is cooled from 330 to 295 K. Air at 290 K, initially free of n-butanol vapour, is passed up the column at the rate of 0.7 m /m2 s. Calculate the required height of tower and the condition of the exit air. 

Spray towers, and plate and packed columns are used, as well as a variety of proprietary designs. Spray towers have a low pressure drop but are not suitable for removing very fine particles, below 10 /j.m. The collecting efficiency can be improved by the use of plates or packing but at the expense of a higher pressure drop. 

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. 

Values of the ratio V(IVR given in Table 24.1 emphasize that most of the volume in a tower reactor (apart from a bubble column, data for which would be similar to a sparger-equipped tank) is occupied by the gas phase, and conversely for a tank reactor. This means that a, a in a tower and a, - a t in a tank. For mass transfer-controlled situations, a, is the more important quantity, and is much greater in a tower. For reaction-controlled situations, in which neither ai nor a is important, a sparger-equipped tank reactor, the cheapest arrangement, is sufficient. 

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