Energy integration through retrofitting of heat exchanger network at Equinor Kalundborg Oil Refinery

Heat integration studies are commonly performed in the wider chemical industry to identify current energy utilization and detect potential improvements with respect to energy efficiency. In this regard, there are several established methodologies, such as: Pinch analysis, Mathematical Programming (MP) and Hybrid methods. In pinch analysis, the objective is to remove cross pinch heat transfer and configure appropriate utilities, based on a minimum approach temperature ∆ T_min. The Minimum Energy Required (MER) for the network can then be calculated. However, a drawback is that the user must specify the changes of the HEN to achieve MER, which may not be the best economical solution. In MP the latter problem can be expressed as an optimization problem. However, due to the complexity of HEN in the industry, pinch analysis is typically the preferred method (Sreepathi and Rangaiah 2014). A similarity for all 3 retrofit solutions, are the challenges regarding data collection and the associated uncertainty. To accommodate for this, we present a methodology that involves an iterative application of a process simulator with plant data (to match the heat flows) and the uncertainty of the pinch point(s).

The pinch analysis was constrained to 1 reforming section and 3 hydrofining sections. Average temperature, pressure, volume flow rate and assay of the heavy feeds and residues were taken over a month. One month was selected, when the refinery had been cleaned and flowrates were in the normal ranges of operation. After balancing mass and energy based on the SRK EOS, enthalpies were segmented and exported into UniSim ExchangerNet. Based on a minimum approach temperature of 20^°C, the cold pinch temperature was found to be 127.5^°C and the hot pinch temperature was 147.5^°C, with a total of 9MW cross pinch. A feasible retrofit solution could not be achieved for the heat exchanger with the highest cross pinch of 2.88 MW. Nonetheless, a retrofit solution was possible for the heat exchanger with the second-highest cross pinch at 1.16MW. However, the payback time exceeded the specified requirement, which made the retrofit economically infeasible. Nevertheless, the uncertainty analysis showed that 2 possible pinch points existed. The uncertainty of the pinch point would change the retrofit considerably and therefore also the economical potential of the retrofit.