Nov-2023

Enhancing profitability using high-efficiency heat exchangers (RI 2023)

Alfa Laval has been in the refinery industry for more than 40 years, with more than 3,000 high-efficiency heat exchangers installed worldwide. The company’s technology provides significant performance and economic benefits in refinery processes.

Salwati Ahmad
Alfa laval

Article Summary

Crude distillation unit
Crude distillation is the heart of the refinery, and optimal performance of this process is key to profitability. Alfa Laval has more than 1,000 Compabloc welded heat exchangers installed worldwide in atmospheric and vacuum distillation processes. There are 25 preheat trains in which every single heat exchanger is a Compabloc.

Energy consumption in atmospheric and vacuum distillation processes accounts for more than 30% of the total energy consumption of a refinery. Therefore, maximising energy efficiency to reduce energy consumption has a direct impact on the profitability of the refinery.

Using Compabloc heat exchangers in the crude preheat train could help reduce energy consumption in the fired heater in two ways. The first is by maximising heat recovery from the hot fractions leaving the distillation column. The second is by reducing the pressure drop in the overhead vapour system, thereby reducing the operating pressure in the flash zone of the column. This means energy consumption in the fired heater can be reduced by at least 25%. With low-fouling heat exchangers, the end-of-run energy efficiency of the plant will not be far from the start-of-run efficiency. Another opportunity to improve the energy efficiency of the distillation processes is to recover more waste heat. Using Compabloc heat exchangers for run-down cooling or vapour condensing enables maximum recovery of low-grade energy for the generation of superheated steam or boiler feed water preheating or district heating.

Crude Preheating Case Study
A major US refinery installed nine Compabloc heat exchangers in the crude preheat train as part of a revamp project. The main objective was to increase the heat recovery while using higher alloys that allowed the refiner to process more advantageous crude oils. The results from a pinch analysis showed that the Compabloc solution would allow the refiner to increase the inlet temperature to the heater by an additional 100°F, saving millions of BTU in energy as well as a significant emission reduction. Compabloc were put into service in several locations: atmospheric OVHDS, naphtha kerosene, and vacuum tower bottom vs crude positions. Actual operating data from the crude confirms the pinch analysis performance expectations. In addition, the performance and temperature of the Compabloc have remained stable throughout the operating period, proving that low fouling can be expected with the heat exchangers when designed correctly.

Naphtha hydrotreating unit
The need for hydrotreating is increasing as refiners adapt to changing sulphur regulations in engine fuels. To help meet these regulations for gasoline, a naphtha hydrodesulphurisation unit is used, generically called a naphtha hydrotreater (NHT). NHTs are critical refinery process units that remove sulphur from gasoline components by reacting with hydrogen in the presence of specialised catalysts. The process is energy intensive and requires a high degree of heat integration to lower the energy operating expenditures. Over the past decades, refineries have been boosting process efficiency by using Compabloc plate heat exchangers in key heat recovery positions.

Naphtha combined feed/effluent exchanger (CFE)
In this exchanger, process feed is combined with hydrogen. It is heated and boiled to a superheated vapour in a series of heat exchangers. The heat source is reactor product vapours, which need to be cooled, condensed, and then separated into various components. Maximising this heat exchanger’s performance by minimising the feed/effluent approach temperature is critical as it drives the energy efficiency of the entire process. Heat exchanger designers focused on the hot-end approach temperature (HAT) and the minimum internal delta temperature (pinch) as limitations to the thermal design. Pinch temperatures are a function of service and heat transfer technology.

For a shell-and-tube unit in the hydrotreater CFE service, typical the internal pinch is between 20 and 40˚C. However, with Compabloc technology, the pinch can easily be reduced to less than 6 to 10°C. This means that by using a traditional technology, the number of S&T in series and the heat transfer area needed to do the same duty will be significantly higher, as will the cost of the heat exchangers.

Naphtha CFE case study
A major refinery in Europe needs to debottleneck an existing naphtha hydrotreater and identified the CFE heat exchanger heat recovery as a limiting factor to its project goals. Additional S&Ts in series with the existing S&T CFE train were evaluated, but the performance of the additional shells fell short of its goals and the new train consumed too much pressure drop. Finally, as is often the case, insufficient space was available to install the additional S&Ts, so the project was not feasible with this technology. Concurrently, the refinery evaluated Compabloc heat exchangers to be placed on the hot end of the feed side, vaporising and superheating the feed with hot reactor effluent while still using the S&Ts on the cold end. The operating parameters were optimised by Alfa Laval specialists and the customer’s process specialists, who iterated the Compabloc and S&T performances at different temperature approaches. The optimum point, in this case, was to design the Compablocs for a 12°C pinch temperature and a 22°C HAT, limited by the performance of the existing S&T on the cold end. This performance met the project’s goals, and the solution was implemented in 2016.

The refinery started up the Compablocs in early 2017. The thermal and hydraulic performance was exactly as expected. However, after several months of operation, increases in pressure drop were detected on the hot end of the feed side in the Compabloc, located at the dry point of the exchanger. The root cause of fouling was determined to be an excess of corrosion inhibitor being dosed in the naphtha upstream, causing the filming agent to be deposited at the dry point of the feed. The Compabloc was cleaned of the fouling material, and a process adjustment was made, after which performance returned to the typical unit cycle length.

This short article originally appeared in the 2023Refining India Newspaper, which you can VIEW HERE