Pipestill Furnace

The furnace partially vaporizes the feed to the tower. A typical furnace has multiple parallel passes and the outlets are combined as feed to the distillation tower. Steam may be injected into the vacuum furnace coil to increase vaporization of feed at a lower temperature and to reduce the residence time. The vacuum cut points are set by the extent of the vaporization in the flash zone where temperatures may range from 390-420°C. Furnace firing is controlled to achieve the desired vacuum cut point. 

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Pipestill furnaces vary greatly in size, shape, and interior arrangement and can accommodate 25,000 bbl or more of crude petroleum per day. The walls and... 

Figure 7-10 A pipestill furnace (heater) and a bubble (distillation) tower.

Pipestill A furnace containing a series of pipes through which oil is pumped and heated. During heating, the oil is vaporized prior to introduction into the distillation unit or thermal cracking unit. 

The first petroleum fractionator simulated is a simple distillation column that removes some of the light material in the crude. Figure 11.16 gives the Aspen Plus flowsheet of this unit. There are two crude feed streams that are combined and heated in a furnace in which the feed is partially vaporized before entering the bottom of the column. There is no reboiler. Live steam is introduced in the bottom of the column to strip out some of the light components in the bottoms stream, which is fed to a pipestill to be considered in Section 11.4. 

The feed is partially vaporized 2278 lb mol/h of vapor with a feed of 3644 lb mol/h. It is introduced into the flash zone on Stage 22. There are three stages below the flash zone that are used to strip out any light material that is in the liquid leaving the flash zone. Open steam is fed to the bottom of the column at a rate of 12,000 Ib/h. The bottoms stream from the pipestill ( reduced crude ) goes to a downstream vacuum pipestill in which more gas oil is recovered. The low pressure in the vacuum furnace produces more vapor for the same furnace temperature. 

The evaluation of the percentage of radiant absorption that will occur in a particular pipestill, and the distribution of this radiation throughout the furnace cavity, has always been difficult. However, in recent years,... 

Radiant-absorption Rate. Radiation between solid surfaces is dependent upon the fourth power of the temperature difference and upon a constant, the value of which is dependent on the kind of material and the condition of the surface. Radiation from a flame, as in pipestill or boiler furnaces, is governed by the same laws except that the size of the flame... 

The rate of radiant absorption varies at different parts of the radiant section. In ordinary stills the exact distribution of radiation is not important but for (1) heating sensitive stocks such as treated lubricating oils, (2) heating to very high temperatures as in cracking stills, and (3) heating two different stocks in separate coils in the same radiant section, the rate of heat absorption in the different parts of the furnace box becomes very important. Data on several stills have been reported that indicate radian tion rates that varied by 400 per cent in different parts of the radiant section. Combustion space is not of direct importance in pipestills. 

Pipestill Losses. The heat losses in pipestills, when based on the net heating value, consist mainly of two items (1) the sensible heat content of the stack gases above 60°F and (2) the losses through the furnace walls, header boxes, roof, floor, etc. Pipestills are usually fired with 25 per cent or more excess air, and hence carbon monoxide is seldom reported in the flue gas, and no loss occurs by incomplete combustion of the fuel. No loss due to the heat contained in the cinder or combustible in the cinder is necessary unless coal is fired. 

Larger yields are obtained in this manner, and for very large plants the use of selective cracking is always justified. In smaller plants the erection of several pipestills is costly, or the operation of several coils in one furnace is difficult, and hence the position of selective cracking is not clear. Separate evaporators may be used for each stock, but common practice is to use a single evaporator into which all stocks are led (Fig. 19-21). 

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