Lowering the Tower Pressure

In general, distillation columns should be operated at a low pressure. For example. Fig. 6.3 shows an isobutane-normal butane stripper. This fractionator is performing poorly. A computer simulation of the column has been built. The column has 50 actual trays. But in order to force the computer model to match existing operating parameters (reflux rate, product compositions), 10 theoretical separation stages (i.e., 10 trays, each 100 percent efficient) must be used in the model. This means that the trays are developing an actual tray efficiency of only 20 percent. 

A field measurement indicated a pressure drop of 2.0 psi. Assuming a specific gravity of 0.50, then the pressure drop per tray in inches of liquid is  

As the weir height of the trays is 3 inch, it is a safe assumption that the low tray efficiency is due to tray deck dumping, rather than flooding. As shown in Fig. 6.3, this column has no reflux. This is a typical design for strippers when feed is introduced on the top tray, there is no need for reflux. 

In order to improve tray efficiency, it will be necessary to increase the vapor velocity through the trays, so as to increase the pressure drop to at least 4 or 5 inch of liquid per tray. If the reboiler duty were simply increased, the concentration of the heavy component—normal butane—in the light overhead product—isobutane— would escalate exponentially. Another method, however, that does not involve increasing either the reboiler duty or the mass flow of vapor through the trays can be used to increase vapor velocity. 

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 

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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. 

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. 

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. 

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. 

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. 

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. 

The production of cracked gas in the heater is largely a function of the peak temperature developed inside the heater coils. When the peak temperature is suppressed, the load of cracked gas to the vacuum tower overhead steam ejectors is reduced. The ejectors can, therefore, pull a deeper vacuum, lowering the tower flash-zone pressure and increasing gas oil recovery. 

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... 

In using this scheme, check to see that the partial pressure of water in the overhead vapor is sufficiently low to preclude water condensation in the upper few trays of the tower. If water condensation does occur, it will be necessary to redesign the system to require some pumpback reflux from the condenser to the top tray. This will lower the partial pressure of water in the gross overhead vapor. Only enough pumpback reflux should be used to avoid the possibility of water condensation, the remainder of the heat removal being accomplished by the pumparound system. 

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... 

The turboexpander lowers the temperature of the product to -100°F, causing it to liquify. Now at 350 psig pressure, the liquid from this process enters the demethanizer tower where it mingles with the previously introduced stream of liquid. The turboexpanders provide a 92% recovery rate while the former system, a backup Joule-Thomson valve, was able to provide only a 60% recovery rate. The volume of gas entering the turboexpanders can vary up to 10% yet, the different flowrates do not significantly affect the efficiency of these units, which are rated at 2,400 hp at 16,000 rpm. 

The CO2 rich solvent is drained from the bottom of the tower, and led first to a hydraulic turbo-expander and then to four flash drums connected in series, where CO2 is de-absorbed as the pressure is lowered. Lean solvent is pumped back to the top of the absorber tower... 

The inlet gas to a spray tower is at 1600°F. It is piped tlirougli a 3.0 ft inside diameter duct at 25 ft/s to the spray lower. The scrubber cools the gas to 500°F. In order to maintain the velocity of 25 ft/s, what size duct would be required at the outlet of the unit Neglect Uie pressure across the spray lower and any moisture considerations. 

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