ranges, such as naphtha and SAF, thus making it possible to maximise the yield of the most high- value product. Because the pressure in the column is decided by the overhead vapour condenser, optimal condenser design and technology are key parameters in minimising the column pressure. When conventional shell-and-tube or air heat exchangers are used as overhead vapour condensers, a higher temperature approach to the supply temperature of the cooling media is required. Hence, a higher pressure in the column is needed to achieve a certain liquid yield at the condenser outlet. When a WPHE is used as an overhead condenser, it is possible to operate with only a 3°C temperature approach to the cooling media. As a result, the same liquid yield can be achieved at the condenser outlet at a much lower operating pressure. Moreover, thanks to the multiple short, parallel channels in the WPHE design, the condenser pressure drop can be reduced compared to conventional heat exchangers. Together, these factors minimise the necessary column operating pressure. Depending on the supply temperature of the cooling media, the operating pressure in the column can sometimes be reduced by 2 bar or more when using a WPHE. This reduces the fractionator reboiler duty and increases the difference between the naphtha and SAF boiling temperatures (see Figure 4 ).
Fractionator optimisation: condenser and run-down coolers Several process streams, including overhead vapour and run-down streams, are usually cooled utilising either cooling water or air, as recovering low-grade energy from these streams is difficult and expensive with conventional shell-and- tube heat exchangers. Such coolers require a large amount of either cooling water or electrical power, which puts high demands on the utility system of the HVO complex. In this way, the coolers increase both the project investment and the plant operating cost. With WPHEs, it is possible to maximise the recovery of low-grade energy, using it to generate both low-pressure steam and hot water. The steam can then be used to evaporate the wastewater (as described in the effluents from the PTU section) or to produce electricity by means of Organic Rankine Cycle (ORC) systems. The hot water can be used as boiler feed water or for tank or plant heating, and it can even be supplied to district heating networks. Recovering otherwise wasted heat in this way turns process cooling from a cost generator into a profit generator (see Figure 5 ). Fractionator optimisation: final product and vapour trim coolers and condensers The final optimisation step within the scope of this article is to minimise the fractionator’s cooling water requirement in all final product and vapour trim coolers and condensers.
Cooling water
P min
Renewable diesel
O gas
T & dP min
Green naphtha
m opt .
dT max
Product fractionator
M
m opt . SAF
M
Stripper bottoms
Q min
P min
Figure 4 Improved product fractionator design using a WPHE to minimise column pressure
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