Tower pressure

The absolute tower pressure (in psia) increased by 17 percent, and hence the volume (as well as the velocity of vapor through the valve tray caps) declined by 17 percent. The reduced vapor velocity reduced the dry tray pressure drop, thus reducing both the spray height above the tray deck and the liquid backup in the downcomers. 

Another reason to raise tower pressure is to permit higher reflux rates. If the pressure controller in Fig. 3.1 is set too low, then during hot weather, when condenser capacity becomes marginal, the level in the reflux drum will be lost. If we then raise the pressure set point, the drum will refill—but why  

Raising the tower pressure also increases the reflux drum pressure, raising, in turn, the temperature at which the vapors condense. The rate of condensation is then calculated from the following  

A = heat-exchanger surface area, ft2 U = heat-transfer coefficient, Btu/[(h)(ft2)(°F)] 

Tc = condensation temperature of vapors, °F Ta = temperature of air or cooling water, °F 

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Onee expander ease pressure is above 150 psi or equal to tower pressure, aetivate XYZ. This will open both expander outlet (diseharge) and eompressor inlet valves. 

Problem Three chemical plant recovery train towers were limited to half of design rates by bottoms pump cavitation and high tower pressure drop. 

Figure 6-4 shows the cold feed distillation tower of Figure 6-3. The inlet stream enters the top of the tower. It is heated by the hot gases bubbling up through it as it falls from tray to tray through the downcomers, A flash occurs on each tray so that the liquid is in near-equilibrium with the gas above it at the tower pressure and the temperature of that particular tray.

Calculate the temperature required at the base of the tower to develop this liquid. This is the temperature at the bubble point for the tower pressure and for the assumed outlet composition. Since the composition and pressure are known, the temperature at its bubble point can be calculated. 

The temperature at the base of the de-butanizer determines the vapor pressure of the gasoline product. If its vapor pressure is too high, the temperature must be increased or the tower pressure decreased to drive more butanes-minus out of the bottoms liquids. 

Because the process system that will receive vents from this condensing system is operating at 5 psig, allow 5 psi pressure drop to ensure positive venting and set top of tower pressure at 10 psig. 

Allow 10 psi tower pressure drop, this makes bottom pressure = 20 psig = 1,800 mm Hg. 

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. 

B. A second and also successful method accounts to a certain extent for the aeration effect, based on test data from many references. This method is not quite as conservative when estimating total tower pressure. This follows the effective head concept of Hughmark et al. [31]. Effective head, hg, is the sum of the hydrostatic head plus the head to form the bubbles and to force them through the aerated mixture. Figure 8-130 is the correlation for hg plotted against submergence, hji [31]. See Dynamic Liquid Seal. ... 

Revised Packed Tower Pressure Drop Correlation Constants for Towers Operating Below Flooding Region... 

Figure 9-79E. CO2 absorption from atmosphere effect of tower pressure on ICqs at veirious liquor rates. Reproduced by pemrrission of the American Institute of Chemical Engineers, Spector, N. A., emd Dodge, B. F., Trans. AI.Ch.E., V. 42 (1946) p. 827 all rights reserved.

Calculate tower pressure drop from Figure 9-21 for packing, and Figures 9-37-41 for support and grids, g. Make specification sheet. 

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. 

The tower pressure losses are (1) tower packing or fill (70-80% of loss) (2) air inlet if induced draft (3) mist eliminators at top (4) air direction change losses and entrance to packing on forced draft units. These losses are a function of air velocity, number and spacing of packing decks, liquid rate and the relation between L and Ga. 

Leva, M., "Reconsider Packed-Tower Pressure-Drop Correlations, Chem. Eng. Progress, V. 88, No. 1 (1992), p. 65. 

Reformulated gasoline specifications require lower vapor pressure in the blended gasoline. It also requires maximum feed to the alkylation unit. This puts more pressure on the gas plant, particularly the debutanizer. Floating the tower pressure is often the best way to meet both constraints. 

Leva M. Reconsider packed tower pressure drop correlations. Chem Eng Prog 88 65-72, 1992. 

This equation is based on experimental data obtained from packed towers under preluading conditions where pressure drop is low and is a linear function of gas and liquid mass flow rates Although the I,cva equation is useful for an estimation of the packed-tower pressure drop. 

While we realize that distillation towers are designed with a control scheme to fix the tower pressure, why is this necessary ... 

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 liquid on the tray deck was at its bubble, or boiling, point. A sudden decrease in the tower pressure caused the liquid to boil violently. The resulting surge in vapor flow promoted jet entrainment, or flooding. 

The vapor flowing between trays was at its dew point. A sudden increase in tower pressure caused a rapid condensation of this vapor and a loss in vapor velocity through the tray deck holes. The resulting loss in vapor flow caused the tray decks to dump. 

Either way, erratic tower pressure results in alternating flooding and dumping, and therefore reduced tray efficiency. While gradual swings in pressure are quite acceptable, no tower can be expected to make a decent split with a rapidly fluctuating pressure. 

To lower the tower pressure, the hot-vapor bypass pressure recorder controller (PRC) valve is closed. This forces more vapor through the condenser, which, in turn, lowers the temperature in the reflux drum. As the liquid in the reflux drum is at its bubble point, reducing the reflux drum temperature will reduce the reflux drum pressure. As the stripper tower pressure floats on the reflux drum pressure, the pressure in the tower will also decline. 

The net effect of reducing the stripper pressure was to greatly reduce the amount of isobutane in the heavier normal butane bottoms product. Undoubtedly, most of the improvement in fractionation was due to enhanced tray efficiency, which resulted from suppressing tray deck leaking, or dumping. But there was a secondary benefit of reducing tower pressure increased relative volatility. 

As an operator reduces the tower pressure, three effects occur simultaneously ... 

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 ... 

To answer this fundamental question, we should realize that reducing the tower pressure will also reduce both the tower-top temperature... 

Figure 3.5 Point A represents the optimum tower pressure.

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. 

The prior discussion assumes that the feed rate, feed composition, and heat content (enthalpy) are fixed. My purpose in presenting this review of the phase rule is to encourage the routine manipulation of tower operating pressures, in the same sense, and with the same objectives, as adjusting reflux rates. Operators who arbitrarily runs a column, at a fixed tower pressure, discards one-third of the flexibility available to them, to operate the column in the most efficient fashion. And this is true, regardless of whether the objective is to save energy or improve the product split. 

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