(242e) Impact of Opportunity Crudes on Desalter Operation and Wastewater Treatment Performance in a Refinery

Abstract

Refiners are increasingly depending on “Opportunity Crudes” to stay competitive and enhance margin in the face of uncertain nature of business at the present times. Opportunity crude is a general term to describe newer types of crudes that have not been traditionally processed in the past. These are crudes with undesirable properties, e.g. low API gravity, high viscosity, high acidity (as expressed by the Total Acid No. (TAN) in terms of mg of KOH/g of crude), high metals concentration, high filterable solids, etc. These include crudes from a wide range of sources – light tight oil (LTO) from hydrofracturing operation at various shale plays in North America, especially from Bakken and Eagle Ford; heavy crudes from Canadian Rockeis; Doba crude from West Africa, etc.

Although LTO are light crudes with high API Gravity (typically >40) it presents several operating challenges, primarily high filterable solids, entrained H₂S that requires addition of amines for transportation, high concentration of paraffin wax leading to fouling of equipment at low temperatures, etc. On the other hand, the heavy crudes have low API gravities (<22) and have poor hydrodynamic properties, high concentrations of naphthenic acids, presence of heavy metals, etc.

Although these crudes offer the refiners an opportunity for margin enhancement, their abundance has created major challenges to the refinery operation globally. Each of the characteristics described above is undesirable. High H₂S concentration leads to corrosion of metals and odor nuisance, high filterable solids create the problem of solids deposition in the process equipment, heavy metals result in several problems in downstream processes. Refiners are compelled to take special measures to overcome these difficulties. Therefore, refiners resort to blending the opportunity crudes with the traditional ones. However, in many cases such crudes are not compatible, and lead to formation of solid or semi-solid materials that tend to precipitate in the process equipment, particularly desalters.

Desalter is the first equipment to handle and process crudes in a refinery. Its primary function is to make intimate contact between the crude and water by thorough mixing to transfer the salt contained in the crude to the water and remove it as a brine stream. Oil and water phases are present in two separate layers with a distinct interface. Desalted oil is fed to the atmospheric distilling column and the effluent brine stream is led to the wastewater treatment plant.

High performance of desalter is essential to ensure minimum entrainment of oil in the effluent brine stream, and water in the desalted crude. Carryover of excessive oil in effluent brine due to emulsion formation and poor water-oil separation in desalter leads to excessive oil and grease (O&G) load to primary (API) and secondary (DGF) oil-water separation steps and large COD load to biological treatment system. Introduction of opportunity crudes disturbs the operation of desalters, which are not designed to handle the widely fluctuating constituents that were not originally designed to be processed by the refinery. As a result, the performance of wastewater treatment also suffers significantly, including the quality of treated effluent. For example, large concentrations of naphthenic acids in crudes carry over to the desalter brine, which are not sufficiently broken down by biological treatment. The residual COD in the treated effluent contributes to toxicity to the aquatic life in the receiving stream.

High concentration of filterable solids in the crude transfers to the desalter brine. The solids in this stream deposit in all the process equipment in the wastewater treatment plant, starting from the API separator all the way to the secondary clarifier. As a result, the active volume, and hydraulic retention time of these equipment reduce drastically, affecting their performance.

Amines used in the production process (tramp amines) as H₂S scavenger are associated with crude, which partition into the water phase in the desalter, also lead to the wastewater treatment plant. This leads to a very large COD and nitrification (TKN) loads, and dissolved oxygen (DO) demand in the biotreater. If the aeration system is not adequate to handle excess loads arising from tramp amines, then there is a possibility of part of the COD leaving the wastewater plant untreated.

The objective of this paper is to demonstrate the challenges faced by desalters and wastewater treatment plants in refineries processing of a variety of opportunity crudes. A brief review of the underlying principles of desalter chemistry will be provided along with advances in wastewater treatment processes that are better equipped to handle the adverse impact of opportunity crudes on their performance. Case studies from two refineries – one from Asia and a second from the US Gulf Coast will be presented.