Tower pressure increase

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. 

The bypassed vapor heats up the liquid there, thereby causing the pressure to rise. WTien the bypass is closed, the pressure falls. Sufficient heat transfer surface is provided to subcool the condensate, (f) Vapor bypass between the condenser and the accumulator, with the condenser near ground level for the ease of maintenance When the pressure in the tower falls, the bypass valve opens, and the subcooled liquid in the drum heats up and is forced by its vapor pressure back into the condenser. Because of the smaller surface now exposed to the vapor, the rate of condensation is decreased and consequently the tower pressure increases to the preset value. With normal subcooling, obtained with some excess surface, a difference of 10-15 ft in levels of drum and condenser is sufficient for good control, (g) Cascade control The same system as case (a), but with addition of a TC (or composition controller) that resets the reflux flow rate, (h) Reflux rate on a differential temperature controller. Ensures constant internal reflux rate even when the performance of the condenser fluctuates, (i) Reflux is provided by a separate partial condenser on TC. It may be mounted on top of the column as shown or inside the column or installed with its own accumulator and reflux pump in the usual way. The overhead product is handled by an alter condenser which can be operated with refrigerant if required to handle low boiling components. 

The DIP process control was accomplished with a Distributed Control Stystem (DCS) system equipped with an APC algorithm. The controller was set to target 10% nCs in the overhead and 10% /C5 in the product, and would increase reboiler steam and reflux rate until reaching the maximum limits for these flows. The tower pressure was also controlled within a specified range by the APC, and this could indirectly limit the reboiler duty as well, if the tower pressure increased beyond its maximum limit. Inferential estimates for product tCs and Cs qualities were calculated based on tower temperatures and pressures, and a bias for these values was continually updated based on daily laboratory (lab) data. 

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. 

A stripper operating at a high pressure drop will require a smaller volume than a similar stripper at a lower pressure drop. This reduces the capital cost for the tower, but increases the blower cost. Towers designed and built to operate at a low pressure drop have the flexibility to increase the gas flow rate and hence the air/water ratio, should the future influent concentration increase or the effluent limitation decrease. Towers designed for high pressure drops do not have this flexibility in operation and would need to decrease the liquid loading to increase the air/water ratio. 

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. 

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. 

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 ... 

Increasing the tower bottoms liquid level. However, should this level reach the reboiler return nozzle, thermosyphon flow will be restricted, or even stop. Then, the reboiler heat duty will be reduced, and the tower pressure will drop. 

In case 2, the shell-side reboiler temperature rises from 240 to 280°F (one reason for such a rise in temperature could be an increase in tower pressure). Now AT = (320°F - 280°F) = 40°F. Looking at the equation above, it looks as if Q will drop in half to 5000 lb/h (which is about the same as 5,000,000 Btu/h). Thus, the flow of steam to the reboiler has been cut in half, even though the control valve position has not moved. 

The pressure inside the deaerator started to drop, as there was not enough steam flow to keep the water in the drum at its boiling point. The reduction in the deaerator pressure increased the volume of steam flow through the bottom tray of the stripping tower. Why ... 

If an increase in the tower-top reflux rate causes the top of the tower to flood, how should the operator respond She should then increase the pumparound flow to reduce the pounds of vapor flow to tray 5, in Fig. 12.4. But suppose this causes the pumparound trays 6, 7, and 8 to flood, because of the extra liquid flow She should increase the cold liquid flow through the pumparound heat exchanger. If this cannot be done, either, then the tower pressure can be increased. This will increase the density of the flowing vapors and shrink the volume of the vapors which the trays must handle. 

Figure 13.4 Elevation increase of reflux drum increases tower pressure.

As the butane liquid level in the condenser increased, the area of the exchanger exposed to the condensing vapors would decrease. Let s assume that the tower s reboiler duty was constant. The vapor flow rate to the condenser would then be constant. To condense the same flow rate of vapor, with a shrinking exchanger surface area, the pressure of condensation must increase. The tower pressure would also go up, as the condenser pressure rose. 

The oldest, most direct method of pressure control is throttling on the cooling-water supply. This scheme is shown in Fig. 13.5. Closing the water valve to the tube side of the condenser increases the condenser outlet temperature. This makes the reflux drum hotter. The hotter liquid in the reflux drum creates a higher vapor pressure. The higher pressure in the reflux drum increases the pressure in the tower. The tower pressure is the pressure in the reflux drum, plus the pressure drop through the condenser. 

As the hot-vapor bypass valve opens, the condensate level in the shell side of the condenser increases to produce cooler, subcooled liquid. This reduces the surface area of the condenser exposed to the saturated vapor. To condense this vapor, with a smaller heat-transfer area, the pressure of condensation must increase. This, in turn, raises the tower pressure. This then is how opening the hot bypass pressure-control valve increases the tower pressure. 

The lower velocity in the throat does not affect the jet s performance, as long as the velocity remains above the speed of sound. If the velocity in the throat falls below the speed of sound, we say that the jet has been forced out of critical flow. The sonic pressure boost is lost. As soon as the sonic boost is lost, the pressure in the vacuum tower suddenly increases. This partly suppresses vapor flow from the vacuum tower. The reduced vapor flow slightly unloads condenser 1 and jet 2 shown in Fig. 16.2. This briefly draws down the discharge pressure from jet 1. The pressure in the diffuser throat declines. The diffuser throat velocity increases back to, or above, sonic velocity. Critical flow is restored, and so is the sonic boost. The compression ratio of the jet is restored, and the vacuum tower pressure is pulled down. This sucks more vapor out of the vacuum tower, and increases the loads on condenser 1 and... 

Operating the column at the minimum pressure minimizes the energy cost of separation. Towering this pressure increases the relative volatility of distillation components and thereby increases the capacity of the reboiler by reducing operating temperature, which also results in reduced fouling. Reducing pressure also affects other parameters, such as tray efficiencies and latent heats of vaporization. 

The first fraction consisting of benzene, chlorobenzene and phenyltrichlorosilane is separated in two stages. First, when the temperature of the tower top is 60-80 °C and residual pressure is 210 GPa, the distillation gives low-boiling components, benzene and chlorobenzene then, with the pressure increased to 80 GPa and the temperature on top increased to 195 °C the distillation produces high-boiling components, phenyltrichlorosilane and to some extent diphenyl. The end of the separation of low-boiling substances is determined by the reduction in chlorine content [to 26-25% (mass)] and the fraction density [to 1.200 g/cm3]. 

A fouling-induced pressure increase in the effluent tower leads to decreased profits due to a lower HVGO recovery rate. The prevention of such fouling is therefore of considerable interest. 

The most conservative definition of vapor flooding capacity is the load for which tower pressure drop exhibits a sharp increase, signifying liquid buildup at some tray. It is, however, possible to operate the tower at somewhat higher load, perhaps 10% more. As the load is increased, reboiler pressure rises, allowing a semistable operation, albeit with reduced separating... 

The dependence of a on temperature and pressure, as computed from Eq. (13.144), is shown in Fig. 13.33. In the cold tower an increase in pressure decreases a because it increases the concentration of Hi S in the liquid more than it decreases the concentration of Hj 0 in the vapor. In the hot tower, an increase in pressure increases a because it decreases the concentration of HiO in the vapor more than it inaeases the concentration of HjS in the liquid. 

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