Tower pressure selection

Assuming the tower pressure as set and an average of top and bottom temperatures can be selected (these may become variables for study), read equilibrium Kj values from charts for each component in gas. 

On the basis of this better performance of the Pall ring, a smaller diameter tower must be selected and the tower reevaluated based on the new mass flow rates with this packing. The economics require that the higher packing cost, smaller tower diameter, new total packing volume, and tower pressure drop be considered. 

Tower diameter selection is usually made with the maximum expected gas and liquid flow rates and depends on the size and type of tower packing The portion of Fig. 3 with solid lines illustrates the pressure-drop relationship for 1.5-m. [3.81 -cm] Pull rings supplied by the manufacturer for typical superficial gas and liquid flow rates used in the chemical processing industry.1 This relationship is usually available in graphical form from any manufacturer of packed-tower packing elements. While these... 

Naturally, we do not want to overpressure the tower, and pop open the safety relief valve. Alternatively, if the tower pressure gets too low, we could not condense the reflux. Then, the liquid level in the reflux drum would fall and the reflux pump would lose suction and cavitate. But assuming that we have plenty of condensing capacity, and are operating well below the relief valve set pressure, why do we attempt to fix the tower pressure Further, how do we know what pressure target to select ... 

The first two factors help make fractionation better, the last factor makes fractionation worse. How can an operator select the optimum tower pressure, to maximize the benefits of enhanced relative volatility, and reduced tray deck dumping, without unduly promoting jet flooding due to entrainment ... 

Figure 3.5 illustrates this relationship. Point A is the incipient flood point. In this case, the incipient flood point is defined as that operating pressure that maximizes the temperature difference across the tower at a particular reflux rate. How, then, do we select the optimum tower pressure, to obtain the best efficiency point for the trays Answer—look at the temperature profile across the column. 

Similar kinds of constraints involve the reflux ratio in distillation, which must exceed the minimum value for the required separation. If the distillation tower pressure is adjusted, the minimum reflux ratio will change and the actual ratio must be maintained above the minimum value. Even when optimization is not performed, the decision variable values must be selected to avoid violating the inequality constraints. In some cases, the violations can be detected when examining the simulation results. In other cases, the imit subroutines are unable to solve the equations as, for example, when the reflux ratio is adjusted to a value below the minimum value for a specified split of the key components. 

Regardless of the method selected, the principal concept of tower pressure control is the same. We control the pressure in the reflux drum by manipulating the temperature in the reflux drum. The tower pressure then floats on the reflux drum pressure. To lower the tower pressure, we must first cool the reflux drum. This reduces the vapor pressure of the liquid in the reflux drum. 

Equation 39 can often be simplified by adopting the concept of a mass transfer unit. As explained in the film theory discussion eadier, the purpose of selecting equation 27 as a rate equation is that is independent of concentration. This is also tme for the Gj /k aP term in equation 39. In many practical instances, this expression is fairly independent of both pressure and Gj as increases through the tower, increases also, nearly compensating for the variations in Gj. Thus this term is often effectively constant and can be removed from the integral ... 

Ethylene Stripping. The acetylene absorber bottom product is routed to the ethylene stripper, which operates at low pressure. In the bottom part of this tower the loaded solvent is stripped by heat input according to the purity specifications of the acetylene product. A lean DMF fraction is routed to the top of the upper part for selective absorption of acetylene. This feature reduces the acetylene content in the recycle gas to its minimum (typically 1%). The overhead gas fraction is recycled to the cracked gas compression of the olefin plant for the recovery of the ethylene. 

Spray Drying and Agglomeration. Most instant coffee products are spray-dried. Stainless steel towers with a concurrent flow of hot ak and atomized extract droplets are utilized for this purpose. Atomization, through pressure nozzles, is controUed based on selection of the nozzles, properties of the extract, pressures used, bulk density, and capacity requkements. Low inlet ak temperatures (200—280°C) are preferred for best flavor quaHty. The spray towers must be provided with adequate dust coUection systems such as cyclones or bag filters. The dried particles are coUected from the conical bottom of the spray drier through a rotary valve and conveyed to bulk storage bins or packaging lines. Processors may screen the dry product to... 

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

Reaction in a Centrifugal Pump In the reaction between acetic acid and gaseous ketene to make acetic anhydride, the pressure must be kept low (0.2 atm) to prevent polymerization of ketene. A packed tower with low pressure drop could be used but the required volume is very large because of the low pressure. Spes (Chem. Ing. Tech., 38, 963-966 [1966]) selected a centrifugal pump reactor where... 

However, low velocities require towers with large cross-sectional areas to handle a given gas flow, and allow the wet gas to channel through the desiccant bed and not be properly dehydrated. In selecting the design velocity therefore, a compromise must be made between the tower diameter and the maximum use of the desiccant. Figure 8-22 shows a maximum design velocity. Smaller velocities may be required due to pressure drop considerations. 

Eckert [125] provides some basic guidelines to good packing selection for various system performance requirements. Kunesh [126] illustrates the often-determined pressure drop advantage of random packed towers over the usual valve tray. See Figure 9-19 [126],... 

Kunesh [126] presents tm overview of the basis for selecting rsuidom packing for a column application. In first deciding between a trayed tower or a packed one, a comparative performance design and its mechanical interpretation should be completed, considering pressure drop, capacity limitations, performance efficiencies (HETP), material/heat balances for each alternate. For one example relating to differences in liquid distribution performance, see Reference 126. 

These ratios are useful in dealing with small towers, and serve as guides for the borderline cases of others. There are no guides to the smallest sized packing to place in a tower. However, /4-in. is about the smallest ceramic used with %-in. and 1-in. being the most popular. Operating and pressure drop factors will usually control this selection. 

Low to medium pressure column operation, select design pressure drop of 0.40 to 0.60 in. water/ft of packing height, although some towers will operate... 

The economics of forced and induced draft cooling tower operation require a study of fan and water pump horsepower and usually dictate a fan static pressure requirement not to exceed 0.75-1.0 in. of water. For atmospheric and natural draft towers the economics of pumping water are still very important. This means that the ground area must be so selected as to keep the height dovm while not dropping the unit rates so low that performance becomes poor. This then, is a balance of ground area versus total deck height. Pritchard [16] presents an... 

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