Oct-2024

Desalter optimisation strategies: Part I

Analysis and optimisation strategies implemented for a two-stage desalter processing light to medium API crude blends at a Southeast Asian refinery.

Venkatesan Mani
Veolia Water Technologies and Solutions

Article Summary

Crude oil desalting plays a pivotal role in refinery operations by removing salts and impurities that can wreak havoc on downstream equipment through corrosion, fouling, catalyst poisoning, and product quality degradation. Despite significant technological advancements in desalter design, optimising desalter performance remains an intricate challenge due to the complex interplay of multiple factors, including crude oil composition, operational conditions, equipment design, and chemical treatment. This interdependence among factors often hinders achieving overall performance goals through isolated optimisation efforts.

Insights gained from a Southeast Asian refinery desalter optimisation detail how the interdependence of key parameters was validated and addressed at each step of the optimisation process, enabling the refinery to consistently achieve and sustain desalter performance indicators, including desalted crude salt levels below 0.5 pound per thousand barrels (PTB), overhead chloride levels below 30 ppm without caustic dosage, and sodium levels in atmospheric residue below 1 ppm to prevent downstream catalyst poisoning.

Interdependent factors
Electrostatic desalter technology gained traction in the oil refining industry during the mid-20th century for oil-water separation processes. Despite significant advancements in desalter design and technology, as well as the introduction of novel demulsifiers and best practice strategies for optimisation, crude oil desalting remains a formidable challenge even in the 21st century. This complexity stems from the multitude of interdependent factors, perpetuating the belief that desalter optimisation is more an art than a science.

Inconsistencies in crude oil quality and associated impurities impact on refinery units, as captured in NACE Technical Committee Publication 21415-SG,1,² coupled with variability in oil field additives during production, often necessitate desalter optimisation even within the same crude supply. Tracking the quality of each crude oil parcel is an intense and often daily challenge for refineries, prompting many to relinquish desalter optimisation responsibilities to local process additive suppliers. These suppliers not only provide desalting chemicals but also monitor most other aspects of desalter operations.

While unit operators and refinery laboratories still carry out routine analytical tests and adjust desalter operations on a shift or daily basis as needed, major changes are often based on prior testing and recommendations provided by the process additive supplier. While relying on process additive suppliers can make sense, failures in desalter optimisation often stem from selecting suppliers solely based on the lowest-cost chemical supply, without expertise in desalter optimisation, lack of periodic review, narrow focus on key performance indicators (KPIs) with deviation action plans, lack of long-term optimisation goals, and the mindset that a single demulsifier chemistry will work for all crude types, even with operational limits on temperature, residence time, and electric grid. These factors often relegate desalters to the proverbial stepchild of crude unit operations.

A poor desalting operation incurs significant expenses (see Figure 1). It is crucial for each refinery to benchmark the cost of desalting, as monitoring overall costs and reporting regularly will drive intense optimisation plans. While refineries often desire to build large desalter vessels capable of processing all types of crude oil, capital expenditure (Capex) and operational expenditure (Opex) limitations hinder the existence of such universally effective designs.

Even large, long-residence desalters with double-stage desalting may fail to meet KPIs due to variability in crude quality, poor control over performance chemical management, and lack of a holistic optimisation approach. Thus, understanding the design and operation of desalters is the first step in troubleshooting, followed by analysing data on interdependent factors, debottlenecking operational limits if necessary, and finally establishing a desired optimisation range for each influencing parameter, which will help work towards achieving the desired performance indicator goals.

Desalter performance optimisation
For effective electrostatic desalter performance optimisation, it is essential to recap and understand the following steps:
- Desalter performance influencing factors
- Data analysis and troubleshooting discrepancy parameters
- Optimisation of key parameters – Southeast Asian refinery case study
- Desalter performance influencing factors.

The four key factors that drive desalter performance are fluid quality, operational parameters, chemicals, and mechanical aspects (see Figure 2). These factors govern emulsion formation and water-oil separation phenomena in desalters, with operational and chemical factors being critical and primary control parameters, while mechanical and crude oil fluid quality are independent and fixed variables.

From the desalter survey (see Figure 3), it is evident that mix valve differential pressure (DP) and demulsifier feed rates are the top variables influencing desalter performance. The survey also identifies common causes of desalter upsets, including crude tank switching, crude quality changes, slop reprocessing, wash water quality, demulsifier and adjunct chemical feed rates, and interface level control, which are the most common and practical control parameters to focus on for any desalter optimisation exercise.

Typically, a single-stage desalter achieves 85-90% desalting efficiency and 90-95% dehydration efficiency. A double- stage desalting process can achieve 90-98% desalting efficiency and 95-99.5% dehydration efficiency in the long-term optimisation, with an increasing number of desalting stages increasing desalter performance efficiency. Generally, a desalter achieving less than 1 PTB salt with basic sediment and water (BS&W) less than 0.2 vol% in desalted crude has been the predominant refinery target for decades. However, due to growing concerns about downstream catalyst poisoning and the integrated design with atmospheric residue hydrotreating (ARHDT) and residue fluidic catalytic crackers (RFCC), the focus is now on achieving crude desalting targets of less than 0.5  PTB to enhance plant operation reliability.

The critical aspect of desalter troubleshooting and optimisation is establishing safe operating boundary ranges for each key control parameter. These boundaries are determined based on desalter design, historical data analysis, and current baseline process operating conditions (see Figure 4 for key optimisation parameter boundary limits established for the Southeast Asian refinery). It is essential to review KPIs at each stage of desalter operating parameter optimisation, fine-tune the safe operating boundary ranges based on analytical and operational data analysis, and perform periodic instrument calibrations and troubleshooting to address any offsets observed in critical monitoring parameters.