The reflux is generated by a pumparound system taking the liquid from the top
section of the tower and cooling it by heat exchange with cold crude oil. This cooled
stream is then returned to the top tray of the tower. This is a conventional
pumparound system. About three trays are used for this pumparound section in
the tower. The tower top temperature remains as the dew point of the overhead
product distillate at its partial pressure with steam. The temperature of the liquid
leaving the tower as the pumparound will be between 5
F and 7
F higher than the
tower top temperature. The internal pumparound section may be assumed to be
equivalent to one half an actual tray for mass transfer.
Discussion
All three of these configurations have their advantages and disadvantages. These
are discussed briefly in the following paragraphs:
Configuration 1. As noted earlier, this is the most common configuration. It is
relatively easy to control with really only two set points that one has to be
concerned with. They are the drum pressure and the tower overhead temperature. There is good heat transfer between the condensing vapors and the crude
oil. The overall heat transfer coefficient is high at around 70–80 Btu/h/sqft/
F,
and advantage is taken in the LMTD of the latent heat of the vapors being
transferred to the low temperature of the incoming crude. The biggest disadvantage with this system is that the crude to the first condenser is of course at a
much higher pressure than the vapors condensing on the shell side. The rupture
of a tube in the exchanger bundle or a leak in the tube plate will cause crude to
contaminate the overhead product and “dirty” the tower overhead trays. This
can be quite a disaster if the downstream units (particularly the Cat Reformer)
are fed directly from the condensate drum without some intermediate surge
drum or tankage.
Configuration 2. This has the advantage of providing a very high LMTD in
condensing with heat exchange against crude. This is due to the higher temperature now experienced in the vapors leaving the tower. As in the case of
configuration 1, the overall heat transfer coefficient in this case is also high.
There still remains though the problem of contamination, but in this case
downstream units have a measure of protection in that only the vapors from
the reflux drum are condensed to make the product. The top trays of the tower
however will be contaminated as before. The two-drum system’s biggest advantage however is more aligned to the installed cost of the unit. Moderately sized
equipment are much cheaper than large items which may have to be field
fabricated. Cost savings may in any case be achieved in the transportation
costs of the drums and heat exchangers.
Configuration 3. The only major advantage in this configuration is that the liquid
pumparound can be circulated at a higher pressure than the incoming crude.
Thus, the problem associated with tube fracture or leakage contaminating the
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
139
section of the tower and cooling it by heat exchange with cold crude oil. This cooled
stream is then returned to the top tray of the tower. This is a conventional
pumparound system. About three trays are used for this pumparound section in
the tower. The tower top temperature remains as the dew point of the overhead
product distillate at its partial pressure with steam. The temperature of the liquid
leaving the tower as the pumparound will be between 5
F and 7
F higher than the
tower top temperature. The internal pumparound section may be assumed to be
equivalent to one half an actual tray for mass transfer.
Discussion
All three of these configurations have their advantages and disadvantages. These
are discussed briefly in the following paragraphs:
Configuration 1. As noted earlier, this is the most common configuration. It is
relatively easy to control with really only two set points that one has to be
concerned with. They are the drum pressure and the tower overhead temperature. There is good heat transfer between the condensing vapors and the crude
oil. The overall heat transfer coefficient is high at around 70–80 Btu/h/sqft/
F,
and advantage is taken in the LMTD of the latent heat of the vapors being
transferred to the low temperature of the incoming crude. The biggest disadvantage with this system is that the crude to the first condenser is of course at a
much higher pressure than the vapors condensing on the shell side. The rupture
of a tube in the exchanger bundle or a leak in the tube plate will cause crude to
contaminate the overhead product and “dirty” the tower overhead trays. This
can be quite a disaster if the downstream units (particularly the Cat Reformer)
are fed directly from the condensate drum without some intermediate surge
drum or tankage.
Configuration 2. This has the advantage of providing a very high LMTD in
condensing with heat exchange against crude. This is due to the higher temperature now experienced in the vapors leaving the tower. As in the case of
configuration 1, the overall heat transfer coefficient in this case is also high.
There still remains though the problem of contamination, but in this case
downstream units have a measure of protection in that only the vapors from
the reflux drum are condensed to make the product. The top trays of the tower
however will be contaminated as before. The two-drum system’s biggest advantage however is more aligned to the installed cost of the unit. Moderately sized
equipment are much cheaper than large items which may have to be field
fabricated. Cost savings may in any case be achieved in the transportation
costs of the drums and heat exchangers.
Configuration 3. The only major advantage in this configuration is that the liquid
pumparound can be circulated at a higher pressure than the incoming crude.
Thus, the problem associated with tube fracture or leakage contaminating the
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
139