Dual-Frequency Desalting Technology Breaks Bottlenecks, Lowers Cost of Ownership
Authors: Carpenter Chris 
Published: January 2023

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This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper IPTC 22365,“No More Inefficient Crude Desalting: Breaking Bottlenecks With Dual-Frequency Technology Lowering Total Cost of Ownership,” by Prabhu Elumalai, Gary Sams, and Umanath Subramani, Schlumberger, et al. The paper has not been peer reviewed. Copyright 2022 International Petroleum Technology Conference. Reproduced by permission.

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A major independent crude oil refinery in Asia is processing crude oil in two crude distillation units (CDUs) using an alternating-current (AC) desalting system. Because of inefficient desalting, only 60% desalting efficiency was achieved for each train with subsequent low dehydration efficiency. After a holistic review of the entire desalting operation, the service company recommended upgrading the CDU 1 desalter vessel to dual-frequency technology and adding a new dual-frequency desalter at the second stage of the CDU 2 vessel. The complete paper examines the successful project from early engagement through project execution leading.

Refinery Desalters
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Overview of Case-Study Refinery.
The refinery was operated with a design capacity of 206,000 B/D through CDUs 1 and 2. CDU 1 included two trains of two-stage legacy AC desalters (Trains 1 and 2). Train 1 operated with 40% of CDU 1’s total flow (60,000 BOPD), while Train 2 operated with 60% of CDU 1’s total flow (90,000 BOPD). The AC desalting system desalted and dehydrated crude oil before feeding the crude to preheated trains. The desalters in this refinery operated for 20 years with the AC technology.

Dual-Frequency AC/Direct Current (DC) Desalter Configuration.
AC crude dehydration technology is over a century old. The complete paper recaps different types of AC desalter configurations, but this synopsis concentrates on dual-frequency AC/DC desalter configurations.
The vessel design for the dual-frequency technology is identical to that of the AC/DC vessel, with similar spreaders, electrodes, and collectors. What differentiates the dual-frequency technology is the power unit, which functions with two frequencies for dehydration and desalting. The base frequency typically is in the range of 800–1,600 Hz, and the modulation frequency is in the range of 0.5–4 Hz.
The computer-based controller controls the output of the variable voltage/frequency power unit and produces a nearly infinite number of waveform configurations. Also, these units facilitate online changing of maximum voltage, minimum voltage, base frequency, and modulation frequency applied to the electrode.
Conventional power units cannot be operated beyond 40% of their rating. The dual-frequency power unit, however, can operate successfully beyond 80% of its rating.
The combination of AC/DC fields with dual-frequency performance provides the high water tolerance of the AC field, the high efficiency of the DC field, and the improved desalting offered by the dual-frequency voltage pattern. This combined electrostatic technology offers more-efficient bulk water and small-water-droplet removal from the crude oil. Highly efficient dehydration means smaller treaters, better performance, lower operating temperatures, and fewer demulsifier chemicals. Further improvements include optimized electrode configurations and enhanced fluid distribution inside the electrostatic treaters.