PTQ Q1 2023 Issue

Figure 2 New monitoring technique for three examples of desalter trylines

 Metals contamination, including organic calcium, par- ticulate 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 trans - fer. 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 siz- ing, 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/ammo- nia, 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 exam- ples 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 mid - dle picture containing a less severe rag layer. The qualita- tive 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 fil - terable solids and bottom sediment at the oil-water inter- face, 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 calcula- tion 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

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Figure 3 Material balance on filterable solids. Theoretical minimum FS at boundary layer (5% WW)

PTQ Q1 2023