Of all process operations in any refinery or petrochemical plant, heating and cooling streams are the most common. In fact, a typical refinery has anywhere from 200 exchangers on the low end to as many as 350 exchangers for a large, complex refinery. Plants in other process industries may not have quite as many exchangers, but the number is still quite significant.
Many of these heat exchangers are in relatively clean services with only a small probability of failure. The balance is used in in “dirtier” services: Process streams where corrosive materials could be present. It is these exchangers that are much more prone to foul, leak or experience corrosion problems. Often, these problems develop slowly, but they also can be sudden.
When failures or excessive fouling occur, the usual result is either a reduction in unit throughput or a complete unit shutdown. For the most part, refiners have learned to live with these problems, limping through to the next turnaround. Yet, this is not the only option available. Temporary process cooling equipment can be placed into service. This allows the process plant to clean, repair or replace the fouled heat exchanger without having to reduce production.
Three common situations in which temporary heat exchangers can be helpful are:
- To serve as an emergency replacement for fouled, leaking or corroded equipment.
- To provide supplemental capacity to overcome cooling limitations.
- To alleviate bottlenecks and allow process optimization.
Manufacturers that cannot afford to be without reliable temperature control should explore whether temporary heat exchangers can help.
Before any product can be sent to a storage tank, it must be cooled to near ambient temperature. This is especially important for petroleum products because as the temperature rises, the amount of material that vaporizes increases. This can pose an explosion danger.
Typically, the last exchanger in any product-cooling heat-exchanger train will cool the petroleum product indirectly using a heat exchanger and water from a cooling tower as the cooling media. There may be times when a refinery runs into a problem with one of these exchangers, resulting in a loss of cooling.
For instance, one plant had to take heat exchangers used to cool the product out of service for emergency cleaning. This loss of cooling was so significant that the product could no longer meet the minimum safe storage temperature. The unit feed-rate would need to be reduced to be consistent with the equipment’s capacity to cool the product to the required temperatures — potentially resulting in huge losses.
By losing an upstream exchanger, this placed additional duty on the remaining downstream cooling water exchangers. However, it was unrealistic to expect these exchangers to be able to adequately compensate for the loss of the upstream exchangers.
A temporary solution was required to compensate for the lost cooling capacity from the upstream exchanger. This solution needed to be in place until the upstream exchanger could be cleaned and brought back into service. A replacement heat exchanger was obtained for the unit that was taken out of service, allowing the plant to maintain normal production rates until the upstream exchanger could be brought back online.
When refinery temperatures are too warm, its products produce vapors that can ignite. For this reason, it is essential that every product in an oil refinery be adequately cooled from process conditions to near ambient temperatures in order to be safely stored. Often, the final cooling process is accomplished through a series of heat exchangers, with the last exchanger usually being against cooling water.
For a refiner located in the warm southern part of the United States, the diesel product from the crude unit was not being adequately cooled and presented a possible safety hazard. The assumption was that the cooling water exchanger was fouled. However, the refinery did not want to shut down the entire plant to fix the exchanger. At the same time, the plant operator did not want to reduce crude rates enough to allow the available cooling in that exchanger to reduce diesel temperatures enough to send it to storage. Instead, the plant ordered a 400-ton, closed-loop chiller system from a third-party temperature control solution provider in an effort to solve the problem.
Upon installation, the technicians observed, after initial system startup, that the diesel temperature would not reduce enough to safely send the crude to storage. They assessed that the cooling water exchanger was more fouled than initially estimated. The chiller’s return temperature was actually much higher that it should have been and was causing mechanical issues with the chiller. The technicians knew this was likely due to a severe restriction on the cooling-water side of the diesel/cooling-water exchanger.
The refinery wanted to order another 200-ton chiller. However, a specialized team of experienced chemical, mechanical and electrical engineers, provided by the third-party temperature control provider, were instead called in to evaluate the process cooling issues and offer expert recommendations. The team reviewed process data and recommended the refinery change the closed-loop configuration to a once-through configuration using only the 400-ton chiller.
Instead of the chiller cooling-water discharge being routed directly back to the chiller, it was routed to the cooling tower as it had been before the temporary chiller was installed. The diesel-to-storage temperature dropped below the safety limit, and the refinery did not need the additional chiller, which provided a cost savings.
Many process plants are trying to maximize production capacity. Given that they are usually operating far beyond what they were designed for, heat exchangers can be a limit to increasing throughput, or maintaining product split. However, a temporary exchanger often can be used to alleviate a bottleneck.
A leading petrochemical producer was faced with a leaking recycle gas heat exchanger. The process gas side of the exchanger was operating near the reaction pressure, which was higher than the cooling water side pressure. Unfortunately, the customer suspected that process side material was entering the cooling water in the recycle gas heat exchanger, which posed a potential safety hazard.
The producer ordered a replacement heat exchanger, but they were facing a long lead time of approximately 12 weeks until the exchanger arrived. The plant estimated that if it had to shut down due to this problem, the losses would be roughly $700,000 profit per day. However, continuing to operate the leaking exchanger posed a significant risk.
To alleviate the issue, a system was designed whereby a heat exchanger was cooled by a chiller system. It was designed such that the heat exchanger was operating at a higher pressure than the process gas stream. The solution was installed in only six days and consisted of multiple 1,000-ton cooling towers, 2,000-ft2 shell-and-tube heat exchangers, high pressure pumps and high pressure piping that was custom made for the project in order to meet the petrochemical producer’s welding specs.
The three-month installation enabled the plant to continue operations, resulting in $64 million in profit that would have otherwise been lost due to the leaking heat exchanger. The final return on investment for the project was 40:1.
In conclusion, the applications described in this article are but a few examples of how temporary heat exchangers can help mitigate the operational process hazards and costs associated with petrochemical and refining equipment emergency maintenance, temperature limitations and unforeseen bottlenecks. The ultimate benefit of these engineered systems can be realized by partnering with a provider with technical, engineering and project management expertise.