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
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
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