Desalting chemistry and monitoring methods to help expand crude basket
Widening the crude basket increases the challenges of maintaining asset reliability and plant performance, including formation of rag layers.
Mahesh Subramaniyam, Debjit Chandra, Vivek Srinivasan, Ajay Gupta and Hiten Makwana
Dorf Ketal Chemicals
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Geopolitical tensions and economic pressures have increased the need for refiners to expand their crude basket. India is an excellent point of reference for this global trend, as crude oil production in India is only about one-fifth of the refinery capacity of 5 MMBPD.
Data from India1 comparing the top 10 oil exporters to India by country over the last three years demonstrate the speed and magnitude of change in crude supplies (see Figure 1).
Russia and Brazil are new to the top 10 list, and imports from the US increased in 2021-2022. Although not in the top 10 of India, Canadian exports to Asia are now at record levels.
This example of the dynamic nature of sources of crude in India is not unique. At the microeconomic level of a single refinery that can receive crude oils by ship, every refinery faces an increased pace of change in crude selection. The future of assigning carbon efficiency to each crude will further increase the complexity of crude selection, increasing the importance of crude flexibility.
Widening the crude basket increases the challenges of maintaining asset reliability and plant performance. This article discusses Dorf Ketal's experience in supporting refiners with the management of change, including innovative monitoring methods and desalting chemistry to assist with the impact of solids and amine contamination that are a leading root cause of unsolved problems associated with the management of change in crude supply.
Refining challenges with changing crude supply
Introducing new crudes in the blend requires a holistic approach towards measuring and mitigating the risk of change. The biggest challenge with new crudes is often seen in the tank farm and desalter, which eventually becomes the bottleneck to the use of new crudes. Challenges include tank farm management, asphaltene precipitation, solids-induced rag layer, oil under-carry, amine carryover, increased water in desalted crude, brine circuit fouling, increased preheat fouling, increased slop formation, and downstream impact on waste water treatment plant (WWTP) and refinery process units.
The challenges can be further classified by the source of the oil. Light sweet crudes from the US are highly paraffinic and often contain particulate iron, soluble iron, and tramp amines. Canadian oils often contain high solids, organic calcium, and are viscous and low API. Pre-salt crudes from Brazil introduce high salt and chloride loading and can reduce brine pH, increasing the risk of corrosion in the brine circuit.
Generic solutions for improving salt removal and dealing with compatibility issues are well documented and not repeated here. However, these generic solutions have proven to be insufficient for:
1: Handling the challenges of solids contamination that often result in the formation of a rag layer at the oil-water interface in the desalter.
2: Amine contamination.
3: Metals contamination, including organic calcium, particulate iron, and soluble iron.
Challenges of rag layer formation on expanding crude basket
Rag layer is a mixture of water, oil, solids, and interfacial active components with a density close to water. The rag layer mixture floats on the water layer in the desalter, impeding mass transfer across the interface. Solids can also be floating in the oil layer, which impedes mass transfer. The accumulation of emulsion reduces the effective desalter capacity and performance.
The degree to which the desalter performance is reduced is a function of the size of the rag layer, the effectiveness of mud washing to sustain water layer residence time, and the extent to which the desalter is oversized relative to the fundamental challenge of oil-water separation as described by Stokes' law.
Like the concept of fouling factor in heat exchanger sizing, it is standard practice for desalter suppliers to increase the size of the desalter over that required by Stokes’ law. This recognises the likely potential for accumulation of an emulsion in the desalter, along with other factors affecting the capacity needs of the refinery.
Rag layer stability and size are promoted by solids in crude (organic/inorganic), alkaline pH, tramp amine/ammonia, metals, and viscosity. The stability is also affected by the particle size distribution of the solids and whether they are oil coated, as is frequently the case. The holistic approach of rag layer treatment by Dorf Ketal begins by assessing qualitatively and quantitatively the extent of the rag layer.
Qualitative and quantitative assessment of rag layer
Figure 2 depicts the monitoring technique for three examples of desalter trylines. The top picture demonstrates a desalter without any rag layer of significance. The bottom picture of a desalter is a significant rag layer, with the middle picture containing a less severe rag layer. The qualitative assessment is the visual assessment of the trylines to see the presence of visible oil and solids in the water layer.
The quantitative assessment is the measurement of filterable solids and bottom sediment at the oil-water interface, tryline 3 in this picture. As the accumulation of solids increases, the amount of filterable solids (FS) and bottom sediment relative to the theoretical minimum increases.
The theoretical minimum FS at the boundary layer is a material balance based on solids in the raw crude, % solids removal, and % wash water. Figure 3 shows this calculation for 5% wash water. For example, if the FS in the raw crude is 150 ppm and FS removal is 50%, the theoretical minimum FS level in the water phase is 1500 ppm.
When the solids in the sample at the oil-water interface are near this level, there is no rag layer. As the rag layer grows, the level of solids at the interface increases above the theoretical minimum, as depicted in Figure 2.
It has been demonstrated by Exxon in US patent 1026000722 that the rag layer emulsion can be resolved by injecting emulsion breaker directly in the rag layer. Dorf Ketal laboratory studies of the rag layer have confirmed that there are multiple ways to chemically resolve this emulsion in the lab. The challenge is to accomplish this in the field. One obvious solution is to modify the desalter internals to allow the emulsion to be withdrawn and externally processed. As with the issue of injecting emulsion breaker into the rag layer, these options have intellectual property complications. There is a new option, in-situ resolution of the rag layer emulsion.
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