Jul-2017

Best practice desalting Part 1: crude flexibility window

A replacement programme of desalter treatment chemistry transformed the operation of crude units struggling to meet performance targets

BRAD MASON
Nalco Champion

Article Summary

Reliability remains king in the refining environment today, which can sometimes go against best laid plans when processing challenging crudes. This is the ongoing juggling act: to capitalise on the discounts presented by these crudes while balancing all the potential negative impacts; and how to safely navigate these waters.

The articles that will follow convey a vision for reaching a new level of performance across the crude unit. The following articles will detail a ‘crude unit of the future’ concept, whereby refiners will have the ability to make process changes to operations in near-real time with the same crude blend, while also maintaining safety limits and key performance indicators (KPI). This programme involves state of the art technologies from the desalter, extended to the wastewater treatment plant, and across the crude unit overhead.

Before that, however, this article will focus on best practice desalting as a mandatory requirement. Without a solid, best practice mechanical-operational-chemical (M-O-C) programme approach, a refiner will never be able to capitalise on the potential benefits this technology brings. Without stable operations in the desalter that control emulsion band growth and effluent quality, regardless of crude slate processed, making process changes to mix valve dP, level set point, mud wash practices and so on will be unlikely. However, when you can provide a ‘tried and true’ best practice desalting programme that addresses industry concerns, only then can you optimise the process.

The discussion will begin with two recent case studies that reflect how critical desalting programmes can be for solving the immediate processing challenge such that further optimisations can be made and sustained.

These sites run a challenging mix that often reaches 100% of a Heavy Canadian crude blend, the main reason for this being potential improvement in operating margin. As Figure 1 shows, the average discount on the WCS benchmark compared to a light basket of crudes is still $10-15/bbl, and there are other Heavy Canadian crudes that offer further discounts.

Case study 1
A North American refiner had been working with an incumbent process treatment provider for many years. Over time, performance had deteriorated markedly as the refinery introduced new and challenging feedstocks to improve operating margins. Significant capital expenditures had been made to improve perceived equipment shortfalls at the waste treatment plant, but oil under-carry remained excessive, and became worse with the cost advantaged crude slate the refinery needed to process. Oil under-carry was such a consistent problem that approximately 30% of crude charge was bypassed around the first stage of a double stage desalter.

When Nalco Champion was invited to trial, the immediate goal was to show that the treatment programme proposed – based on comprehensive pre-trial site audits and research evaluations of the crude feed – would eliminate the continual oil under-carry problem. With the main operational constraint removed, the aim shifts to optimising the desalting process by following established Nalco Champion M-O-C best practice protocols. From there, agreed-upon trial protocols are followed to restore the expected overall unit performance. At that point, the programme’s robustness can be fully tested with increasing amounts of the site’s main heavy Canadian opportunity crude. Table 1 shows an overview of the unit.

Solution
Nalco Champion first conducted thorough site audits to gain insight into current operating practices, review baseline performance, and evaluate tank farm and desalting equipment to try to understand whether there were any design flaws or mechanical limitations that could be responsible for poor performance. These audits also included understanding impacts to wastewater treatment operations.

Audit findings were then coupled to the research evaluations and the company’s experience of successfully processing heavy, high solids-laden crude slates. Our recommendations then established an effective desalter monitoring and sampling trial protocol to allow for continuous evaluation of the correct areas. Along with this, best practice injection locations and safe staging areas for product storage in the unit were also identified.

The Resolv treatment programme chosen utilised a current best practice primary emulsion breaker (EC2472A) in conjunction with an enhanced solids removal aid (EC2600A). Nalco Champion Research initially developed the EC2600A to meet the growing challenges of heavy, high solids crudes, focused on Canadian dilbits. Today, this combination programme has an extensive treating library for reference cases like this.

The primary goal from the start was to achieve a new baseline performance (new system steady state) with no new upsets. The first trial involved EC2472A being used with the opportunity crude in the slate at the historical maximum rate processed – only 2-3%. Oil under-carry immediately ceased (see Figure 2, ‘NC Trial’ legend). The incumbent provider was allowed a second test run, but the old performance returned. As was mentioned earlier, a key component of these tests was to determine robustness in the treatment programme. Consequently, during a second test the amount of advantaged opportunity crude significantly increased as part of the crude slate to test robustness. During this period, the opportunity crude level reached maximum rates above 40% due to hydraulic limits on the unit. Since Nalco Champion started to treat the unit permanently, the amount of advantaged crude has averaged more than 30%, while maintaining or exceeding past desalting and solids removal KPIs.

Figures 2a-e highlight some of the performance gains in terms of significant reductions to desalter effluent oil and grease levels (COD in the total influent to wastewater also dropped as a result), increased solids removal and desalting efficiencies, higher crude charge with more advantaged crude being processed. It is also important to note that these performance improvements are statistically significant events, shown by P-values of less than 0.05.

Value
Bringing in higher levels of opportunity crude can greatly elevate refinery margins if the potential downsides can be mitigated to deliver consistent performance. The integrated team approach and systematic protocols described above allowed for sound mitigation strategies to be developed and utilised while reducing the operating risks. To make a step change while maintaining high levels of availability requires this work and planning to be done in a consultative manner.

There are chemical treatments and monitoring tools and techniques that can be employed to do this safely and hold the gains when running these kinds of advantaged crudes.