Page 280 - Sustainability in the Process Industry Integration and Optimization

I n d u s t r i a l A p p l i c a t i o n s a n d Ca s e S t u d i e s 257

Gases

Water
Light Distillate Steam

Steam
Crude Oil
Steam
Atmospheric residue
Diesel Oil
Lower pumparound
Heavy Distillate
FIGURE 11.3 Flowsheet of the crude-oil distillation unit.

pump-around and from some of the products before the furnace. To
bring the final crude oil temperature to the column’s required target,
the remaining heat duty is provided by the combustion of
noncondensed light gases and of additional natural gas.
The process features the following characteristics: (1) temperature-
dependent heat capacities; (2) continuous partial phase change of the
crude oil; (3) temperature variations, which are small owing to
specifications of the distillation process; and (4) large variations of
the hot stream flow rates due to changing stockfeed composition.
The three utility sources available for HEN operation are light
gases (as a furnace fuel for higher temperature levels), steam at 1 atm
(1.01325 bar), and cooling water at 18–35°C. The data used to estimate
the utility cost are $68.74/kWy for furnace heating, $103.10/kWy for
steam, and $30.00/kWy for cooling water. The following area cost
law was used:
Capital cost [$ = 25,000 + 680 Area 0.81 [m ] (11.1)
2

The plant processes a range of alternative feedstocks. Three
crude-oil types were selected to represent this range. Feed 1 is a light
crude oil; it contains a significant amount of the lightest fractions,
which require a large amount of cooling in the condensers. However,
the relatively small amount of the heavy fraction means that less
reboiler heating is required. Feed 2 is medium crude oil and feed 3 is
heavy crude oil. These two feedstocks are characterized by a relatively
greater amount of the heavy fractions (diesel and atmospheric
residue), which means that more heat is available for recovery in the
higher temperature range. It was assumed that, over a year’s worth of
operation, the plant uses equal amounts of the three feedstocks. Each
alternative feedstock is associated with a unique operating point.