allow for the steam inlet distributor and a surge holdup of the residue product. This
surge holdup is based on the company’s operating policy, but as a guide:
• If the product is routed to storage, 5 min holdup.
• If the product is to be fed into a fired heater of a downstream unit, allow 15 min
holdup.
The Stripper Column
This column is sized using the same procedure as used for the main column sizing.
As mentioned earlier the strippers for the various side streams are usually stacked
one above the other to form a single uniform diameter tower. The only diameter that
may change is that for the holdup section in the bottom product stripper. This may
be increased to adjust the height of the column to ensure the free flow of unstripped
feed to each stripper from its respective drawoff tray in the main tower.
The number of trays for steam stripping is usually four. The tray spacing is also
usually 24 in. For the height of the tower, begin with setting the height of the bottom
stripper’s bottom tangent line above grade. This should be at least 15 f. to allow a
reasonable available NPSH for the product pump. From there allow a 5–15 min
surge capacity at the calculated tower diameter and some 12 in. for the steam inlet
distributor below its bottom stripping tray. Note the same comments apply to surge
capacities in this tower as given for the residue stripper product.
The Crude Feed Preheat Exchanger System Design
All crude distillation units preheat the incoming crude oil feed by heat exchange with
hot product and reflux streams. The preheated crude is then partially vaporized to
satisfy the flash zone conditions by a fired heater. The degree of preheat is a question
of an economic balance between the cost of the hardware and the savings in utilities
due to the recovery of heat from the hot product and reflux streams. The exercise to
arrive at this economic balance is a critical analysis of the respective enthalpy and the
temperature levels of the various streams to be considered for heat exchange.
It requires also good cost data for the heat equipment included in the system. This
equipment must include the cost of the heat exchangers (usually shell and tube), cost
of the final product coolers, and cost of the final crude feed fired heater.
More than one scheme can be developed and their capital costs calculated from
up to date equipment vendor data, if possible. Next, the utility requirements and
cost for each system are developed using the company’s unit utility costs. For an
examination and comparison of these systems, these cost data must be on the same
basis for each of the schemes. The following procedure is one of several that can be
used for this economic analysis:
Step 1. Construct the enthalpy curve for the crude feed. This stream will remain in
the liquid phase through the preheat train. The enthalpy curve therefore needs
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
155
surge holdup is based on the company’s operating policy, but as a guide:
• If the product is routed to storage, 5 min holdup.
• If the product is to be fed into a fired heater of a downstream unit, allow 15 min
holdup.
The Stripper Column
This column is sized using the same procedure as used for the main column sizing.
As mentioned earlier the strippers for the various side streams are usually stacked
one above the other to form a single uniform diameter tower. The only diameter that
may change is that for the holdup section in the bottom product stripper. This may
be increased to adjust the height of the column to ensure the free flow of unstripped
feed to each stripper from its respective drawoff tray in the main tower.
The number of trays for steam stripping is usually four. The tray spacing is also
usually 24 in. For the height of the tower, begin with setting the height of the bottom
stripper’s bottom tangent line above grade. This should be at least 15 f. to allow a
reasonable available NPSH for the product pump. From there allow a 5–15 min
surge capacity at the calculated tower diameter and some 12 in. for the steam inlet
distributor below its bottom stripping tray. Note the same comments apply to surge
capacities in this tower as given for the residue stripper product.
The Crude Feed Preheat Exchanger System Design
All crude distillation units preheat the incoming crude oil feed by heat exchange with
hot product and reflux streams. The preheated crude is then partially vaporized to
satisfy the flash zone conditions by a fired heater. The degree of preheat is a question
of an economic balance between the cost of the hardware and the savings in utilities
due to the recovery of heat from the hot product and reflux streams. The exercise to
arrive at this economic balance is a critical analysis of the respective enthalpy and the
temperature levels of the various streams to be considered for heat exchange.
It requires also good cost data for the heat equipment included in the system. This
equipment must include the cost of the heat exchangers (usually shell and tube), cost
of the final product coolers, and cost of the final crude feed fired heater.
More than one scheme can be developed and their capital costs calculated from
up to date equipment vendor data, if possible. Next, the utility requirements and
cost for each system are developed using the company’s unit utility costs. For an
examination and comparison of these systems, these cost data must be on the same
basis for each of the schemes. The following procedure is one of several that can be
used for this economic analysis:
Step 1. Construct the enthalpy curve for the crude feed. This stream will remain in
the liquid phase through the preheat train. The enthalpy curve therefore needs
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
155