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of temperature and hydrocarbon partial pressure existing in the flash zone. The principles of this analysis are discussed as follows.

Flash Zone Pressure

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

The datum pressure in atmospheric tower design is the lowest permissible pressure in the towers' reflux accumulator. Since, in many cases, the tower overhead product stream cannot be totally condensed at a reasonably low pressure, there will often be a vapor distillate which must be removed from the tower system and sent either to the plant fuel gas system or to further processing. This is normally done by a compressor located as near as possible to the tower. In most refineries, this distillate vapor is fed to a gas plant for recovery of propane and heavier. In refineries running; heavy crudes, the volume of recoverable light ends may be too small to justify recovery facilities. In this case, the distillate vapor is sent to the plant fuel gas system. The author thas observed one operation where the distillate vapor was flared. This required a considerably higher reflux drum pressure than if compressors were used to feed other systems; Admittedly, this was a rare operation. For a grassroots design a reflux drum pressure as low as 0.2 psig is practical, provided that suction lines to the compressor can be kept;short. If the installation involves tying a new tower into an existing compressor which may be located a considerable distance away, a higher pressure may be required. The nofimal range of reflux drum pressures used is 0.2 to 1.0 psig!

Thet design pressure drop across the condensers) and associated piping should be set at 5 to 7 psi. When the system is clean, the normal pressure drop will be on the

EXTERNAL HEAT QUANTITIES LEAVING FLASH. ZONE Q'iFz= (Qf+Qsw ), BTU/hr. Q'Ofz = Qw, BTU/hr.

EXTERNAL HEAT QUANTITIES LEAVING FLASH. ZONE Q'iFz= (Qf+Qsw ), BTU/hr. Q'Ofz = Qw, BTU/hr.

Overflash Distillation

TD = TOTAL DISTILLATE PRODUCTS EXCLUSIVE OF OVERFLASH Lo= OVERFLASH Vso —STRIPOUT

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