Flash-zone pressure

Cohimn pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude-oil feed temper-... 

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. 

Column pressure at the reflux drum is established so as to condense totally the overhead vapor or some fraction thereof. Flash-zone pressure is approximately 69 kPa (10 psia) higher. Crude oil feed temperature at flash-zone pressure must be sufficient to vaporize the total distillates plus the overflash, which is necessary to provide reflux between the lowest sidestream-product drawoff tray and the flash zone. Calculations are made by using the crude oil EFV curve corrected for pressure. For the example being considered, percent vaporized at the flash zone must be 53.1 percent of the feed. 

Frequently vacuum gas oils are condensed in packed sections. If several zones are used, there will be opportunity for better heat efficiency. Structured packing, sheet-metal type, has largely displaced trays for contact in vacuum pumparound zones because packing requires less pressure drop. Thus, flash zone pressures can be lower and less vacuum pitch can be produced. Flash zone pressures as low as 25Torr (millimeters of mercury absolute) have become routine. For heat efficiency, a corrosion-resistant de-entrainment mesh pad is often installed above each pumparound packed zone. 

A decrease in flash-zone pressure increases velocity and promotes entrainment. Throttle the steam to the ejector system (to temporarily raise flash-zone pressure) and see if the HVGO color clears up. 

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. 

High flash-zone pressure. Perform a AP survey. 

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. 

As a first step, steam was supplied to the heater passes on the unit and the flash-zone pressure dropped from 25 in. Hg to 27 in. Hg. Gas oil recovery increased by 1,800 B/SD. 

Improve ejector performance. Providing a source of colder cooling water for. the interstage condensers is one method of achieving this objective. Lower top pressure will translate into lower flash-zone pressure. 

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... 

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. 

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. 

Any restriction to vapor flow above the flash zone must increase the flash-zone pressure. An increase in AP across the wash trays below the... 

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. 

Reduce the number of wash trays in the vacuum tower. In one installation, two bubble-cap trays replaced four tunnel-cap trays. The flash-zone pressure was therefore reduced. The net effect was beneficial in that the lower pressure drop was more useful than the loss in fractionation represented by the two trays removed. 

Replace pumparound trays with packing. Structured packing is a good bet for this application. The payoff is reduced AP and flash-zone pressure. For example, a West coast vaccum tower was revamjjed from bubble cap trays to packing as follows ... 

Reduced flash-zone pressure Lighter feed More velocity steam Increased charge rate Furnace tube peaking temperature... 

Indicates metals in FCCU feed Demister section upset Demister pad partially coked Too low a flash-zone pressure Wash oil flow too low... 

Inadequate velocity steam High flash-zone pressure Low flash-zone temperature Leaking TGO drawoff pan... 

Flash zone pressure should be set at the minimum possible level. This will maximize the crude vaporization or, if less than maximum vaporization is desired, will minimize the operating temperature level. 

At this point, the flash zone pressure is set as the sum of the reflux drum pressure and the combined pressure drops across the condensers and trays above the flash zone. A pressure drop of S psi between the flash zone and the furnace outlet is recommended. 

Convert the 14.7 psia EFV curves to the pressures which have been set for the furnace outlet and the entrance to the fla zone. This latter value is, for all practical purposes, equal to the flash zone pressure. The vapor pressure charts and nomographs in Section 4 of Maxwell are useful for these conversions. 

An increase in flash-zone pressure will increase draw tray temperatures. 

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