fractionation tray. Consider the loading on the bottom side stream and pumparound
drawoff tray. This commences with a heat balance from below this tray and
includes the streams shown in the envelope in Fig. 23.
The heat into and out of the envelope as shown is provided by the following
streams:
Heat in:
The crude vapor and liquid feed at the flash zone conditions
The steam to the residue stripper
The overflow liquid from the drawoff tray
Heat out:
The residue product
The vapor of the total distillate products from the tray below the drawoff
The overflow vapor from the tray below
The steam from the flash zone at the conditions of the tray below
The temperature and quantities of these streams are known (from the material
balance), except the quantity of overflow. Their temperatures are also known from
the tower temperature profile or from previous calculations. To complete the
calculation, let the lbs of overflow be x, and give all the streams their enthalpy
value. Then solve the equation
Heat in ¼ Heat out
to find the value of x.
Repeat the heat balance calculation for the envelope ending below the next
pumparound drawoff tray. In this case the vapor leaving the envelope will contain
the overflow vapor from the pumparound tray and all the distillate products except
the heavier product drawn off from the pumparound tray below. It will also contain
all the steam from the flash zone and from the side stream stripper of the heavier
product. The remaining heat out of the envelope will be the stripped heavier product
and the bottom pumparound duty in Btu/h. The heat into the envelope will be as
before except this time the unknown overflow quantity will be that flowing from the
lighter pumparound drawoff tray. It will also include the steam from the heavier
product side stream stripper.
The Tower Diameter
The tower diameter may now be calculated from the vapor flow to the tray and the
total liquid load on the tray. The quantity of the vapor to the tray has already been
established. The liquid load on the tray will be:
The product drawoff liquid
The pumparound liquid
The calculated over flow liquid
These quantities must be in terms of weight and volume at the tray conditions of
temperature and pressure.
152
D.S.J. Jones
drawoff tray. This commences with a heat balance from below this tray and
includes the streams shown in the envelope in Fig. 23.
The heat into and out of the envelope as shown is provided by the following
streams:
Heat in:
The crude vapor and liquid feed at the flash zone conditions
The steam to the residue stripper
The overflow liquid from the drawoff tray
Heat out:
The residue product
The vapor of the total distillate products from the tray below the drawoff
The overflow vapor from the tray below
The steam from the flash zone at the conditions of the tray below
The temperature and quantities of these streams are known (from the material
balance), except the quantity of overflow. Their temperatures are also known from
the tower temperature profile or from previous calculations. To complete the
calculation, let the lbs of overflow be x, and give all the streams their enthalpy
value. Then solve the equation
Heat in ¼ Heat out
to find the value of x.
Repeat the heat balance calculation for the envelope ending below the next
pumparound drawoff tray. In this case the vapor leaving the envelope will contain
the overflow vapor from the pumparound tray and all the distillate products except
the heavier product drawn off from the pumparound tray below. It will also contain
all the steam from the flash zone and from the side stream stripper of the heavier
product. The remaining heat out of the envelope will be the stripped heavier product
and the bottom pumparound duty in Btu/h. The heat into the envelope will be as
before except this time the unknown overflow quantity will be that flowing from the
lighter pumparound drawoff tray. It will also include the steam from the heavier
product side stream stripper.
The Tower Diameter
The tower diameter may now be calculated from the vapor flow to the tray and the
total liquid load on the tray. The quantity of the vapor to the tray has already been
established. The liquid load on the tray will be:
The product drawoff liquid
The pumparound liquid
The calculated over flow liquid
These quantities must be in terms of weight and volume at the tray conditions of
temperature and pressure.
152
D.S.J. Jones