Flash-zone temperature

In single-stage units which do not produce kerosene or other critical stocks, flash zone temperatures may be as high as 750 - 775 F. The principal limitation is the point at which cracking of distillates to less valuable gas or the rate of coke formation in the furnace tubes becomes excessive. 

The heated oil is flashed into the VPS flash zone where vapor and liquid separate. Split between distillate and bottoms depends on flash zone temperature and pressure. Separation of vapor and liquid in the flash zone is incomplete, since some lower boiliug sidestream components are present in the liquid while bottoms components are entrained in the vapor. The liquid from the flash zone is steam stripped in the bottom section of the VPS to remove the lower boiling components. 

Since cracking stocks generally do not have to meet the color specifications that lube distillates do, higher flash zone temperatures (up to 8(X)°F) can be tolerated. Fuel units are normally designed to distill material boiling up to 1100°F (at atmospheric pressure) from the feed, and some units have distilled beyond 12(X)°F at low feed rates. 

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. 

Example The dew point is defined an all-vapor system except for one very small increment of liquid. Now take the feed again, and consider the fact that this is a flash-off crude still sidestream vapor at 20 psig (34.7 psia). The dew point is to be determined to set the proper sidestream stripper overhead temperature for this desired product. This problem is worked similarly to the bubble point. Simply hold the pressure (34.7 psia) constant, and vary the temperature. Note that when you find the temperature at which just a small amount of liquid is formed, the sign SYSTEM IS ALL VAPOR goes off and the flash component summary appears. Note also that the previous flash summary will remain on the screen (if you had a previous run) until you input a flash zone temperature and click on Run Prog. 

From the American Petroleum Institute s (API) Technical Data Book—Petroleum Refining, specific heats, specific gravities, latent heats of vaporization, and percent vaporization can be obtained, for a given oil, as a function of flash-zone temperature (percent vaporization and flash-zone temperature are functionally related because the flash vaporization takes place adiabatically). This suggests a trial-and-error procedure Assume a flash-zone temperature and the associated percent vaporization then make an energy balance to check the assumptions. Finally, complete the material balance. 

Related Calculations. The problem can be worked in similar fashion using values from enthalpy tables. In this case, the datum temperature is below the flash-zone temperature therefore, sensible heat in the two exiting streams must be taken into account. 

Distillation column controls combined with pressure minimization and flash zone temperature control to maintain yields of the most valuable products while minimizing energy use in reboiling or feed heater... 

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. 

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. 

Increasing the flash-zone temperature will reduce the gas oil left in the resid. Unfortunately, thermal cracking rates double for every increase of 20 F-25°F. As shown in Figure 13-4, the production of noncondensable... 

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. 

The increase in heater temperature raised the flash-zone temperature to 723"F from the previous 715°F. Pressure rose a bit to 26.9 in. Hg., but the gas oil production rose another 800 B/SD. 

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

The calculations outlined in this subsection are based on having predetermined the maximum temperature of the oil leaving the furnace. Occasionally, one may define the thermal basis as the flash zone temperature. If this is the basis, the procedure described in the chapter covering the vacuum tower-specifically Figure 3.8—will apply for flash zone calculations. 

Assume that the leinperafulT"dTthe.net bottoms liquid, W, is 30 degrees F lower than the flash zone temperature. Calculate the heat quantity leaving the system in this stream. 

Using Maxwell s nomogram, the 14.7 psia./74.5 vol. percent temperature of 648 F converts to 690 F at 22.4 psia. This is the flash zone temperature. 

In the design of any vacuum tower, the first question to be settled is the selection of the optimum operating pressure of the system. In order to simplify this discussion, let us consider certain facts, assuming that a maximum allowable flash zone temperature has been set. 

The general criteria for establishing the design material balance were outlined in the opening section. The exact material balance for design purposes is determined by calculation of either how much oil can be lifted from the charge at the optimum flash zone temperature-pressure or the relative distillate-residuum separation required to produce a desired asphalt quality. The relative yields of the various distillate fractions will almost invariably be determined by the owner s specifications. For this reason, no discussion of the properties of vacuum distillates is offered here. 

The maximum allowable flash zone temperature has been established. As pointed out in Chapter 2, this has been the subject of much discussion throughout the industry. Normally, maximum flash zone temperatures range from 775 to 800 degrees F. 

Knowing the flash zone temperature and the feed vaporization requirements, the hydrocarbon partial pressure, in the flash zone is found by inspection of the vacuum region EFV curves which were developed earlier in this chapter. The difference between the partial pressure and the total pressure in the flash zone must be made up by the air leakage and by steam. The required steam to the flash zone is calculated as... 

The flash zone temperature is determined from the phase diagram as the temperature of the total products leaving the feed section at the total hydrocarbon partial pressure existing there. 

Absorption Factor 117 Flash Zone Temperature, Atmospheric Tower 20 ... 

Bottom stage temperature Model prediction must be lower than flash zone temperature due to isenthalpic cooling. The temperature of the bottoms stream leaving the column model should be lower than feed temperature of the crude (5-7 °C). 

He gives a guideline to estimate these gas streams and corresponding compositions to adjust the atmospheric residue. We summarize the rules and automate the calculations by using MS Excel. Figure 3.5 represents the MS Excel interface we have developed to estimate these gas streams. The spreadsheet requires the atmospheric residue flow rate and flash zone temperature (Cell B2 and B3) to calculate the flow rate of gas streams. 

Figure 3.13 shows the process flowsheet ofthe simplified VDU model built by Aspen HYSYS. Since the first absorber (flash and striping sechon) is an absorber without any side draw and side pumparound and the condihons of atmospheric residue and steam are fixed, there is no addihonal specificahon required to run the first column. It is unlikely to change any process variables such as flash zone temperature and VR yield. However, we can add a heat flow to the feed stage to ensure that the flash-zone temperature matches the plant measurement (Figure 3.14). 

Figure 3.14 Additional heat flow to tune flash zone temperature (simplified model).

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