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Optimized Furnace Design

The previous sections quantify the general principle that the flash zone temperature should be maximized while the pressure should be minimized. What may not be obvious is how the unit design can affect the maximum temperature of the vacuum tower feed before it reaches the flash zone, and the resultant amount of cracking experienced. The top, red curve shows the temperature/pressure profile in a non-optimized vacuum furnace and transfer line design.

The vacuum tower feed enters the furnace at the left of the curve at low temperature and high pressure. As the oil flows through the furnace, the oil is heated and the two phase flow causes the pressure to drop. The non-optimized example shown has a furnace outlet temperature of 775° F to achieve a flash zone temperature of 725° F. The pressure drop in the restrictive transfer line piping causes a significant pressure drop between the furnace outlet and the vacuum tower flash zone. This pressure drop causes significant flashing in the transfer line and hence a large temperature loss. The latter portions of the radiant tubes also display high pressure drops causing high temperatures to be maintained throughout much of the heater increasing the fouling susceptibility.

The lower, blue curve presents an optimized furnace and transfer line design. To achieve the same flash zone temperature of 725° F, the furnace outlet temperature is only 750° F. Also, the latter radiant coils are significantly larger reducing the pressure drop in the high temperature section of the heater. The overall benefit of the cooler temperature profile in the optimized design is represented by the difference in area between the two curves. Also, the optimized design will have a lower residence time reducing cracking and coking tendencies.

The advantage of the lower pressure drop design is that the flash zone temperature can be increased without increasing the cracking/coking tendency of the vacuum tower feed.




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