Historically, refineries distilled predominately light feedstocks. Nowadays, however, real financial benefits can be realised by blending these lighter crudes with heavy or opportunity crudes. The delta between light and heavy crudes can vary dramatically and when this reduction in feed costs per barrel is applied to the refinery throughput, substantial savings can be seen. Feedstock costs are the major operational cost for a refinery (above 80%). If operators can reduce these costs, they can increase margins within the refinery, which ultimately leads to additional profit. However, a full understanding of how these blended crudes will impact the operation of the refinery needs to be understood to ensure safe and continuous operation of the plant.
From a material perspective, many refineries are not constructed to withstand heavy and corrosive crudes. Therefore, removal of these elements within the desalter vessel allows for these blended crudes to be refined with minimal infrastructure change within the overall refinery.
Desalting the feedstock
Blended feedstocks contain numerous contaminants that can cause issues within the process stream. Suspended solids, inorganic salt, sand, and drilling mud all need to be removed to ensure a high-quality refined product. These contaminants are washed from the crude oil by injecting water into the desalter vessel.
Blended crude containing contaminants is heated and mixed with washing water utilising either a mixing valve or static mixers to maximise the contact between the contaminated crude and the wash water. This mixed emulsion solution then enters the desalter vessel where it gets separated into organic and aqueous phases. This separation occurs by electrostatic coalescence, where electrical fields are generated to create water droplet coalescence within the mixed emulsion solution. As these water droplets increase in size, they fall out of the emulsion due to gravity. Dissolved within the water droplets are the contaminants previously suspended in the blended crude.
Control is key
Numerous measurement technologies are utilised for trying to identify and control what is happening in the desalter. Some external bridle mounted systems fail – not down to the measurement principle but its location. Formation of emulsion solution within the vessel is simply not replicated in an external bridle. This can lead to the emulsion band increasing in size (without the operator knowing), resulting in a shorting out of the electrical grids and a loss of production.
Some vessel mounted solutions also come with operational challenges. A conventional differential pressure measurement is based on the specific densities of the phases within the vessel. However, as blending is likely to change the overall density, this solution is fraught with constant maintenance issues. These issues may also be compounded by solid deposition within the vessel take-off points…
Written by Graham Barker, Tracerco, UK.
This article was originally published in the May 2020 issue of Hydrocarbon Engineering. To read the full article, and other great technical articles in this issue, view the full issue here. You can also register to receive a free regular copy of the magazine here.
Read the article online at: https://www.hydrocarbonengineering.com/refining/19052020/a-whole-new-level-of-understanding/