Tower pressure lowering

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. 

Fire prevention may include providing degassing boots/vent stacks to the top of risers where flammable combustible vapors can be entrained in the water system, i.e. water pressure lower than process pressure. Combustible gas detection at the top ofthe vent stacks can be used to detect flammable material presence in cooling tower. However, the hostile environment and difficult to access location makes the detection challenging to maintain. 

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 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... 

Separation of the oxide and the organic byproducts is accomplished by distillation in two towers. Feed from the saponifier contains oxide, aldehyde, dichloride, and water. In the first tower, oxide and aldehyde go overhead together with only small amounts of the other substances the dichloride and water go to the bottom and also contain small amounts of contaminants. Two phases will form in the lower section of this tower this is taken off as a partial side stream and separated into a dichloride phase which is sent to storage and a water phase which is sent to the saponifier as recycle near the top of that vessel. The bottoms are a waste product. Tower pressure is 20 psig. Live steam provides heat at the bottom of this column. 

A combination of lower pressure drop per stage and high efficiency for vacuum systems results in low tower pressure drops. 

Pressure Lower pressure typically saves energy. This is because the lower the tower pressure the less heat required for liquid to vaporize and thus less energy required. This results in better fractionation as it is easier for vapor to penetrate into liquid on the tray deck. 

The condenser pressure controls the tower pressure and thus the feed tray pressure. There is a pressure valve in the overhead, which can be used to control tower pressure. The lower limit of the tower pressure is defined by the column overhead condensing duty, net gas compressor capacity, and column flood condition. During extended turndown periods, reducing pressure up against an equipment limit can avoid dumping. Many of the new APC systems have pressure control implemented. 

It is generally known that reducing the operating pressure of separation columns reduces energy consumption. This is because the lower the tower pressure, the less heat required for liquid to vaporize (thus less energy required) and the easier for vapor to penetrate into liquid on the tray deck (thus better separation). Yet many columns are operated well above their potential minimum pressure. One may ask If benefit of reducing pressure is well known, why is it not widely implemented There appears to be three primary reasons for this. 

Because this problem occurred in December, it was possible to operate the isostripper well below the design pressure. To increase the volumetric vapor flow (but not the mass flow) through the tray decks, we suggested that the tower pressure be lowered from 120 psig to 65 psig. The objective was to increase the dry tray pressure drop by about 1 in. of liquid per tray. 

Reduce tower pressure. The lower tower pressure will create higher vapor velocities and thus minimize tray leakage. This idea is discussed... 

If an ejector is not overloaded at a normal gas rate, reducing the gas load will not result in greatly improved vacuum. The ejector is simply oversized at the lower charge rate and wastes steam without obtaining any appreciable benefit in lower vacuum tower pressure (see Chapter 13). To save this wasted steam, new ejector internals are needed. The internals... 

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... 

When the relief valve nozzle plugged, the PRC (tower pressure control) began to show a lower operating pressure. 

Top Temperature. The temperature at the top of the tower must be just high enough to allow complete vaporization of the overhead product. A lower temperature will condense a part of the desired overhead product and incorporate it in the first side-draw product, and a higher temperature will cause the inclusion of high-boiling materials which are not desired in the overhead product. If the top of the tower is at atmospheric pressure and no steam is used, the 100 per cent point of the equilibrium vaporization curve of the overhead product is the top temperature. Such a rimple case is seldom encountered, and hence the top temperature at 760 mm must be corrected for the tower pressure and for the partial-pressure effect of steam or gas. 

By controlling the tower pressure and the bottom temperature, the vapor pressure of the crude oil leaving the bottom of the tower can be controlled. At a set tower pressure, the crude product s vapor pressure can be lowered by increasing the bottom temperature or... 

A.djustingTrocess to Optimi AT. At first glance, there appear to be only three or four utiUty levels (temperatures), and these can be 50°C apart. Different ways to increase the options include using multieffect distillation, which spreads the AT across two or three towers using waste heat for reboil and recovering energy from the condenser. To make these options possible, the pressure in a column may have to be raised or lowered. 

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