Jul-2020

Improving hydrotreater performance with welded plate heat exchangers

Pressure has never been higher on refiners to improve the efficiency of their energy-intensive processes.

WIVIKA LAIKE and CHRIS WAJCIECHOWSKI
Alfa Laval

Article Summary

Recent volatile crude oil prices have led to the growth of several alternative methods for crude oil extraction around the globe. These extraction methods can often produce sour crude oil with a high sulphur content. At the same time, the demand for high sulphur products is decreasing as the understanding of the environmental effects of burning high sulphur fuels grows. This necessitates that refiners are now required to reduce the sulphur content of their products before they can be sold. This is most commonly done by hydroprocessing, such as hydrotreating and hydrocracking.

Focusing on hydrotreating as a process for removing unwanted impurities such as sulphur, nitrogen, and metals, there are several alternative configurations. However, at the heart of hydrotreating there is always the reactor section, featuring a high pressure reaction vessel as well as reactor internal technology and catalyst, enabling the feed to react with hydrogen. This unit operation is common for hydrodesulphurisation, but also in other fuel upgrading technologies such as isomerisation and catalytic saturation. These processes are energy intensive and as such require a high degree of heat integration to lower the energy operating expenditures (opex). Earlier, this heat integration was performed with shell-and-tube (S&T) heat exchangers. However, more recently refineries have been maximising energy efficiency by using welded plate heat exchangers by Alfa Laval in the main heat recovery positions. This article explains the advantages of using welded plate heat exchangers in key heat integration positions in hydrotreaters among several refiners around the world.

Process layout
Figure 1 shows a general process layout of a hydrotreater. Hydrogen is added to the feed stream which is then vaporised and superheated in a heat exchanger. The reactor effluent is used as a heating medium; this in turn needs to be cooled and condensed before being separated into various products. The more you heat up the feed, the less energy has to be used in the furnace. And the more you cool down the effluent, the less energy has to be used in the subsequent (air) cooler. Therefore process designers usually focus on minimising the hot end approach temperature (HAT) and internal pinch (minimum delta T) of the combined feed/effluent (CFE) exchanger. Typical internal pinch temperature for this position when S&T is used is between 20°C and 40°C (36-72°F). However, with plate technology the temperature difference can easily be reduced to less than 6-10°C (11-18°F, see Figure 2). 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.

Traditionally, a maximum of eight S&Ts in series have been used in such a service, as an optimum with respect to investment cost versus achieving more heat recovery. As the efficiency of these heat exchangers affects the surrounding process equipment, their performance should preferably be fixed at an early stage.
 
Benefits of Compabloc/Compabloc+
Welded plate heat exchangers, such as the Compabloc, extend the practical performance limits of heat recovery by using engineered corrugated heat transfer surfaces to generate three to five times the heat transfer coefficient compared to traditional technologies. At the same time, fouling rate is minimised. The flow geometry also achieves very close to counter-current flow, perfect for small temperature differences and high heat recovery. Further on, as already explained, a single Compabloc can replace several S&T heat exchangers, significantly reducing the amount of space required for installation by up to 90% compared to traditional S&T heat exchangers.

The heat recovery capability of Compabloc means that less fossil fuel is consumed, and that emissions and carbon footprint are reduced. A compact design makes installation easier and more cost effective. By freeing up space, Compabloc resolves bottleneck issues, enabling new ways to increase production and heat recovery.

Compabloc operates with superior shear stress, minimises fouling, and allows operation with cleaning intervals substantially longer than traditional S&T heat exchangers. It is equipped with four removable panels to allow full access to the heat transfer area for cleaning or inspection. The fully cleanable design with cleaning lanes at each side of the plates means that the exchanger can be returned to 100% performance when cleaning the plate pack by hydrojetting, ensuring a long and highly efficient operational life cycle. This is especially important in duties like naphtha hydrotreater CFE where gumming and salting can occur.

The classic range of Compabloc can go up to 38 or 42 bar depending on the model. The Compabloc+ range however allows operation up to 60 bar because of new features, and brings the benefits of Compabloc technology to a wider range of positions.

Compabloc+ is equipped with +Seal, a sealing concept that allows the panels to be reliably sealed at high pressure. In Compabloc+ design, the graphite gasket is fully contained in a groove and the compression is controlled with a metal- to-metal contact which prevents overtightening and related damage.

Several cases where Compabloc welded plate heat exchangers have been successfully utilised in hydrotreater systems will be presented in this article.

Effect of acid dew point on boiler efficiency
After the combustion process, the flue gas follows the radiant zone, convection zone, economiser, and flue path and proceeds by cooling. The fuel used contains carbon, hydrogen, and sulphur. After combustion, sulphur is oxidised to SO2 and SO3.4 SOx compounds react with moisture going from the combustion chamber to the stack. The H2SO4 formed presents a danger of corrosion. H2SO4 may stick to the tubes where it is formed and cause leakage in the tubes. Factors that trigger condensation are reduced flue gas temperature, increased oxygen percentage in the flue gas, or the presence of V2O5 catalyst when fuel oil is burned.