Refineries are constantly pushing to improve their processes in order to improve profitability. Because refining is such an energy-intensive process and many refineries are using outdated technology, there is room for improvement.
One way refiners have been able to improve profitability is by using welded plate heat exchangers in place of shell-and-tube heat exchangers. Traditionally, shell-and-tube exchangers have been the primary heat exchangers used in nearly every refinery process for transferring heat. While effective some applications, when compared to alternative designs, they are large, relatively inefficient and do not inherently produce high wall shear — the force a fluid puts on the exchanger surface to deter particle buildup — to combat fouling.
In the past 25 years, welded plate exchangers have found wider use within refinery applications. When the scope of features a welded plate heat exchanger can provide is realized by an end user, he or she may ask, “Why have these not been implemented everywhere?” One simple answer is conservatism. By nature, refiners are understandably conservative when it comes to introducing new technology into a process. In my experience, however, the frequency with which welded plate exchangers are being used in refinery applications has increased considerably in the past few years.
What Is a Welded Plate Heat Exchanger?
The principle of the plate heat exchanger is not new. Nearly 100 years ago, the original plate exchanger was designed with gaskets between each plate. That design is still used in many services today.
Plate heat exchanger technology has evolved from the gasketed plate heat exchanger to many different types of semi-welded and fully welded options. A primary type of plate heat exchanger used in refinery applications is the welded block exchanger (figure 1). The value achieved with this type of exchanger is that the corrugated plates give three to five times higher U-values than traditional shell-and-tube exchangers. They also have five to 10 times higher wall shear stress, assuming similar pressure drop is used. These advantages allow the welded block heat exchanger to provide a compact solution that resists fouling buildup on the plates due to the greatly increased wall shear. The fluid will enter the exchanger and, depending upon the design, will have a certain number of passes inside the unit prior to leaving. This allows for tight temperature approaches in a small package. Figure 2 shows a size comparison of a welded plate heat exchanger and a shell and tube with the same duty.
The most energy-intensive process heating application in a refinery is the crude preheat train. This is where raw crude is heated to separate components in the atmospheric distillation unit. At times accounting for 30 percent of the energy used in the entire refinery, the crude preheat train is a costly but mandatory process heating application. With that said, how can a refinery increase the efficiency of this process while reducing cost? One solution is to use welded plate heat exchangers, which can reduce energy consumption by up to 30 percent and use less than a quarter of the space.
Considerations for Welded Heat Exchangers in Refineries
To employ welded plate heat exchangers in the crude preheat train, the first step is to add a filter in the crude line at the welded heat exchanger inlet. The filter is especially important if the fluid is coming from a shell-and-tube exchanger or equipment that might send large or small particles in large volume into the exchanger. A typical filter is a 0.125” mesh screen placed at the crude line inlet. Second, the baffles on the crude side must be reinforced so they do not bend under the potentially large pressure differentials that can occur during the fouled state. In applications retrofitted with a welded plate block exchanger equipped with a filter and reinforced baffles, I have seen increased uptimes and increased performance compared to the shell-and-tube exchangers that were replaced.
Hydrotreating is another process that is suited to the use of welded plate heat exchangers. The government recently passed a standard — Tier 3 Desulfurization — that limits the amount of sulfur allowed in gasoline. Instead of the 20 ppm of sulfur allowed in gasoline before the revised regulation, refineries must ensure that gasoline contains only 10 ppm of sulfur.
The process of using heat from the effluent to preheat the combined feed for the naphtha hydrotreater (prior to it going into the reactor) can be improved in several ways. Increasing the flow rate will increase the amount of heat needed for the process steps that follow the combined feed effluent exchangers. This heat-recovery position has typically been performed using four to eight shell-and-tube exchangers connected in series. The same function can be achieved with one or two welded plate heat exchangers.
In conclusion, the idea of replacing shell-and-tube exchangers with welded plate heat exchangers to increase efficiency and wall shear stress capacity while reducing space requirements is applicable to almost all processes within a refinery. Refineries that need to optimize their process heating should take a look at welded heat exchangers. Welded plate heat exchangers provide several process and cost benefits that apply to atmospheric distillation units, hydrotreating, hydrocracking, fluid catalytic cracking, alkylation, isomerization, reforming, vacuum distillation units, sulfur-recovery units, wastewater treatment plants and gas plants.