Demulsifier control system and method for AC/DC modulated dehydrating and desalting coalescers

1. A method of controlling a demulsifier dosage to prevent growth of a rag layer comprising brine and suspended solids emulsified with crude oil, in an AC/DC modulated coalescer, comprising:

monitoring a characteristic of an electric current drawn by an electrical component of the AC/DC modulated coalescer;

determining a stable range of the characteristic, wherein the characteristic lying within the stable range indicates absence of the rag layer and wherein the characteristic lying outside the stable range indicates presence of the rag layer;

setting the demulsifier dosage to a stable demulsifier dosage when the characteristic lies within the stable range;

setting the demulsifier dosage to a disruption demulsifier dosage when the characteristic lies outside the stable range; and

controlling the demulsifer dosage, comprising:

raising the demulsifier dosage to the disruption demulsifier dosage or

reducing the demulsifier dosage to the stable demulsifier dosage.

2. The method of claim 1 , further comprising setting a target separation efficiency for the AC/DC modulated coalescer based, at least in part, on the characteristic.

3. The method of claim 1 , wherein setting the demulsifier dosage to a disruption demulsifier dosage is based, at least in part, on a combination of the characteristic, an integral of the characteristic, and a derivative of the characteristic.

4. The method of claim 1 , wherein the characteristic comprises a rate of change of magnitude of the electric current.

5. The method of claim 1 , wherein the characteristic comprises a magnitude of the electric current.

6. The method of claim 2 , wherein setting the target separation efficiency comprises determining the stable demulsifier dosage.

7. The method of claim 6 , wherein determining the stable demulsifier dosage comprises determining a target demulsifier dosage required to achieve a stable value of the characteristic.

8. The method of claim 2 , wherein setting the target separation efficiency comprises determining an operating temperature of the AC/DC modulated coalescer.

9. The method of claim 1 , wherein the AC/DC modulated coalescer comprises a dehydrator or a desalter.

10. A non-transitory computer readable medium storing instructions executable by a computer processor for controlling a demulsifier dosage to prevent growth of a rag layer comprising brine and suspended solids emulsified with crude oil, the instructions comprising functionality for:

monitoring a characteristic of an electric current drawn by an electrical component of an AC/DC modulated coalescer;

determining a stable range of the characteristic, wherein the characteristic lying within the stable range indicates absence of a rag layer and wherein the characteristic lying outside the stable range indicates presence of a rag layer;

setting a demulsifier dosage to a stable demulsifier dosage when the characteristic lies within the stable range;

setting the demulsifier dosage to a disruption demulsifier dosage when the characteristic lies outside the stable range; and

controlling the demulsifer dosage, comprising:

raising the demulsifier dosage to the disruption demulsifier dosage or

reducing the demulsifier dosage to the stable demulsifier dosage.

11. The non-transitory computer readable medium of claim 10 , the instructions further comprising functionality for setting a target separation efficiency for the AC/DC modulated coalescer based, at least in part, on the characteristic.

12. A system for controlling a demulsifier dosage to prevent growth of a rag layer comprising brine and suspended solids emulsified with crude oil, in an AC/DC modulated coalescer having an adjustable dosing mixing valve, the system comprising:

a current sensor arranged to monitor an electric current drawn by an electrical component of the AC/DC modulated coalescer; and

a processor configured to:

determine a characteristic of the electric current;

determine a stable range of the characteristic, wherein the characteristic lying within the stable range indicates absence of the rag layer and wherein the characteristic lying outside the stable range indicates presence of the rag layer;

set the demulsifier dosage to a stable demulsifier dosage when the characteristic lies within the stable range;

set the demulsifier dosage to a disruption demulsifier dosage when the characteristic lies outside the stable range; and

control the demulsifier dosage, comprising:

instructing the adjustable dosing mixing valve to increase the demulsifier dosage to the disruption demulsifier dosage or

instructing the adjustable dosing mixing valve to reduce the demulsifier dosage to the stable demulsifier dosage.

13. The system of claim 12 , wherein the stable range is determined based on a target separation efficiency for the AC/DC modulated coalescer.

14. The system of claim 12 , wherein the characteristic comprises a rate of change of magnitude of the electric current.

15. The system of claim 12 , wherein the characteristic comprises a magnitude of the electric current.

16. The system of claim 12 , wherein the stable demulsifier dosage is determined based on the demulsifier dosage required to achieve a desired value of the characteristic.

17. The system of claim 13 , wherein the target separation efficiency is based, at least in part, on an operating temperature of the AC/DC modulated coalescer.

18. The system of claim 12 , wherein the AC/DC modulated coalescer comprises a dehydrator or a desalter.