Tower Flash Zone

In the vacuum tower, heavy liquids from the atmospheric tower, referred to as reduced crude, are fed to a vacuum furnace and heated to about 750—800 °F. To suppress coking in the furnace tubes, steam is added to increase the velocity of the hydrocarbon in the tubes. The reduced crude enters the vacuum tower flash zone, where the pressure is maintained at 20—30 mmHg absolute (2600-4000 Pa). When foaming occurs, fouling of the demister pads (sets of grids designed to minimize entrainment of heavier liquids into the upper sections of the tower) above the flash zone can occur, side stream gas oil products will be discolored, and gas oil end-point specification cannot be met. Again, silicone additives are used. The antifoam is normally injected into the feed to the... 

Trim gas oil (TGO) is a black oil stream withdrawn immediately above the vacuum-tower flash zone. It consists of 20%-50% resid and 50%-80% gas oil. This relatively light material may cause the bottoms pump to cavitate when it overflows its trapout pan at a nonuniform rate. Reduce the TGO pan level to see if this helps the NPSH problem on the bottoms pump. 

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. 

Do not forget to take advantage of lower crude rates by reducing the fractionator operating pressure. Very substantial energy savings will result if the tower flash zone pressure can be cut by 5 or 10 psi. Chapter 9 discusses a number of ideas along these lines. 

Try the following problem to sharpen your skills in working with material and energy balances. Crude oil is heated to 525° K and then charged at a rate of 0.06 m /hr to the flash zone of a pilot-scale distillation tower. The flash zone is maintained at an absolute temperature of 115 kPa. Calculate the percent vaporized and the amounts of the overhead and bottoms streams. Assume that the vapor and liquid are in equilibrium. 

In two stage units, it is often economical to distill more gas oil in the vacuum stage and less in the atmospheric stage than the maximum attainable. Gas formed in the atmospheric tower bottoms piping at high temperatures tends to overload the vacuum system and thereby to reduce the capacity of the vacuum tower. The volume of crude vaporized at the flash zone is approximately equal to the total volume of distillate products. Of course, the vapor at this point contains some undesirable heavy material and the liquid still contains some valuable distillate products. The concentration of heavy ends in the vapor is reduced by contact with liquid on the trays as the vapor passes up the tower. This liquid reflux is induced by removing heat farther up in the tower. 

Before desalters came into common use, crude pipe stills were frequently equipped with flash drums to minimize salt deposition on hot surfaces. In the flash drum system, the crude is heated to about 300°F. under enough pressure to suppress vaporization. The pressure is released as the crude enters the flash drum and all of the water (along with a small amount of crude) is flashed off, leaving the salt as a suspension in the oil. The flashed vapor is recombined with the crude near the furnace outlet or in the flash zone of the fractionating tower. 

To obtain a low flash zone pressure, the number of plates in the upper section of the vacuum pipe still is reduced to the minimum necessary to provide adequate heat transfer for condensing the distillate with the pumparound streams. A section of plates is included just above the flash zone. Here the vapors rising from the flash zone are contacted with reflux from the product drawoff plate. This part of the tower, called the wash section, serves to remove droplets of pitch entrained in the flash zone and also provides a moderate amount of fractionation. The flash zone operates at an absolute pressure of 60-90 mm Hg. 

Feed rate to tower, lb mols/hr or, mols of feed, (batch distillation) entering flash zone/time all components except non-condensable gases Factor for contribution of other feed flow to minimum reflux Mols of liquid feed Mols of vapor feed... 

One method of maximizing the LCO end point is to control the main fractionator bottoms temperature independent of the bottoms pumparound. Bottoms quench ( pool quench ) involves taking a slipstream from the slurry pumparound directly back to the bottom of the tower, thereby bypassing the wash section (see Figure 9-9). This controls the bottoms temperature independent of the pumparound system. Slurry is kept below coking temperature, usually about 690°F, while increasing the main column flash zone temperature. This will maximize the LCO endpoint and still protect the tower. 

Vt = Total vapor leaving flash zone/unit time at specific temperatm-e and pressure or total overhead vapor from tower, mols/hr or mols of component in vapor phase or volumetric flowrate for incoming fresh air Quantity of vapor, mols... 

Foaming in the atmospheric column can sometimes occur but is not normally encountered. In the atmospheric column, the crude, at about 700 °F, enters a flash zone at the bottom of the tower, where the vapors and liquid are separated. The vapors travel up the tower, where the various fractions condense. When foaming does occur, difficulties may be encountered in meeting the specified cut points of the fractions. Again the 60,000-cSt silicones are used at dosages of 2—10 ppm. 

Tp2 = Temperature of the fractionator flash zone Tpf = Preflash tower feed temperature... 

Low flash-zone temperature. Have the instrument mechanic check the furnace outlet thermocouple. The optimum tower top temperature for a vacuum tower equipped with a precondenser is usually not the minimum temjjerature. As the tower top temperature is raised, heavy naphtha boiling-range materials are flashed overhead into the precondenser. Acting as an absorption oil, they absorb a portion of the light hydrocarbons that would otherwise overload the jets. However, getting the vacuum tower top too hot can overload the precondensers. By field trials, find the tower top temperature (usually 230°F to 280°F), that minimizes flash-zone pressure. 

Effective steam stripping is indicated by a 30°F-50°F temperature drop between the flash zone and the tower bottoms. A lower AT means that the steam is not effectively contacting the resid. This can be due to upset or corroded trays or tray flooding. Often, poor level control in the bottom of the tower will permit resid to back up and flood the stripping trays. 

It seems as if the vacuum-lower transfer line is a weak point in many crude units. It is possible, because of an incorrectly sized transfer line, to approach sonic velocity in these lines. Such superhigh velocities have led to rapid erosion and failure of the transfer line. If a unit s transfer line is experiencing an accelerated rate of failures, the operating engineer should consider several questions. Has the flash-zone pressure been substantially reduced Has the furnace charge rate (including velocity steam) been increased Is the vacuum-tower feed lighter than it used to be ... 

Vacuum tower wash oil, 165 Vacuum lowers, 281—300 bottoms-pump suction pressure loss, 281— 285 high flash-zone pressure, 285-288 ejector problems, 288-292 black gas oil, 292-293 trim gas oil production, 293 pumparound draw temperatures, 293 light resid, 294 steam-to-healer passes, 295-297 gas-oil recovery improvement, 297-298 transfer-line failures, 298-299 troubleshooting problems, 299-300... 

A vacuum tower s flash zone typically operates at 1-2 psia and 720°F to 780°F. The tower is designed to tolerate a small degree of thermal cracking. A sketch of a typical vacuum tower is shown in Figure 13-1. Some of the more common troubles associated with operating a vacuum tower are ... 

The reduction of resid in a vacuum tower is a function of the flash-zone temperature and pressure. A rise in this pressure increases production of resid at the expense of the more valuable gas-oil product. 

The key tool in troubleshooting flash-zone pressure problems is a vacuum-tower pressure survey. The time to initiate this survey is just after start-up when the trays, demister, and ejector system are clean and in good condition. Pressures are best measured with a portable mercury-filled vacuum manometer. Using a vacuum pressure gauge will reduce the accuracy of observed pressure drops. Relying on permanently installed gauges for pressure drop data will not give reliable results. 

A high tower bottoms level will cause liquid to flood the bottom stripping section. This backs up resid into the flash zone. When the frothy liquid level rises to the flash zone inlet nozzle, entrainment of bottoms will carry up into the wash oil section, which will also flood. 

The process conditions shown in Figure 13-7 were the design-basis operating parameters. Note that the 715°F flash-zone temperature and the 25-in. Hg flash-zone pressure (128 mm Hg) are indicative of an operation that results in excessive gas oil left in the vacuum tower bottoms. This downgrades virgin gas oil from FCCU feedstock to delayed coker feed at a penalty of 5/bbI. A properly designed and operated vacuum column that employs steam stripping of the heater coils operates at 27-in. Hg flash-zone pressure and 760°F flash-zone temperature. 

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