Pipe still

In addition to the distillation of crude oil coming into the refinery, stills of various designs are used in other types of service throughout the refinery. Cracked products are separated in distillation equipment which is very similar to an atmospheric crude pipe still. The principal difference is that these products are hot from the cracking operation, so that a fired heater is not required. 

Petroleum products may be treated with various solvents for the removal by selective solubility of undesirable constituents or for the recovery of by-products. The solvent and solute must be separated to yield the desired product and to recover the solvent for reuse. The solvents normally boil at a lower temperature than the products from which they are to be removed and so are generally distilled off as overhead products. The pipe stills used for this service may be single-stage or multi-stage units, depending on the service involved. Some solvents can be removed by the use of steam heated stills. In other cases, the high temperature required necessitates the use of fired heaters and vacuum towers. 

Critical temperatures throughout the tower are controlled by automatic instruments and products are withdrawn under various combinations of flow and level control. A pipe still is capable of mnning for days on end with only minor adjustment by the operators except, of course, when a change in crude type or in product distribution is required. 

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... 

Cracking imposes an additional penalty in a vacuum unit in that it forms gas which cannot be condensed at the low pressures employed. This gas must be vented by compressing it to atmospheric pressure. This is accomplished by means of steam jet ejectors. Ideally, it would be possible to operate a vacuum pipe still without ejectors, with the overhead vapors composed only of steam. In practice, however, leakage of air into the system and the minor cracking which occurs make it necessary to provide a means of removing non-condensibles from the system. In addition to the distillation of atmospheric residuum, the lube vacuum pipe still is also used for rerunning of off specification lube distillates. 

A single-stage pipe still used for distilling whole crude is shown in Figure 1. The unit shown is designed to separate six products gas, overhead distillate, three sidestream distillates, and undistilled residue, or bottoms. Modem pipe stills usually have several sidestreams. 

Liquid products are withdrawn from plates 9, 16, and 22 as sidestreams. These are contacted with steam in external stripping columns having 4 to 6 plates in order to remove light dissolved hydrocarbons. Since the liquid withdrawn from any plate in a pipe still is in equilibrium with all the lighter vapor passing that plate, it always will contain material which is lower in boiling point than the desired product. 

The lube vacuum pipe still, shown in Figure 2, was developed in the period between the two World Wars. As the name implies, this type of unit was designed to meet the growing demand for heavy lubricating oil distillates. With this unit, it is possible to produce distillates which boil as high as 1050°-1100°F at atmospheric pressure. 

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. 

The fuel vacuum pipe still is also used to recover cracked gas oil from the tar formed in residuum cracking (visbreaking) processes. In this service, it it frequently referred to as a vacuum, flash unit. Pipe stills designed for the production of asphalt are usually the fuel type of unit. 

Normally, all of the heat is removed from the fractionator by three or more circulating reflux streams. The proportion of gas and naphtha in the cracked products is much higher than in crude oil, so it is seldom possible to reduce the diameter of the tower top as in atmospheric pipe still design. Due to the low operating pressure, it is necessary to provide expensive compression capacity to permit recovery of these light hydrocarbons in subsequent equipment. 

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