Saurabh Agarwala

A key component of refinery operations is the use of caustic solutions to sweeten Liquefied Petroleum Gas (LPG) streams. However, this process is essentially constrained by the equilibrium-bound nature of mercaptan removal and the irreversible degradation of the active sodium hydroxide (NaOH) into sodium carbonate (Na₂CO₃) as a result of CO₂ contamination. There are major OPEX and hazardous waste disposal issues as a result of current process designs that accept this degradation as an inevitable operational cost. This paper introduces a paradigm shift: This study introduces paradigm shift, a novel approach to process re-engineering that uses in-situ electrochemical regeneration to eradicate degradation's underlying cause rather than just managing it.

The fundamental chemical constraints of traditional caustic treaters were dissected using an analytical framework. A new process flowsheet with a side-stream Electrochemical Regeneration Cell (ERC) was created based on this analysis. In order to improve the ERC's architecture and confirm its performance against changing, real-world challenges, the main focus of this work was a multi-stage process design and validation analysis. 

1. Irreversible Degradation Stopped: The ERC, through the imposition of a controlled electric potential, splits stable sodium carbonate (Na₂CO₃) successfully into active NaOH and CO₂ gas, which is subsequently vented [4].
2. Shift in Equilibrium: The ERC always eliminates the sodium mercaptides (NaSR) by oxidizing them to disulfide oil (DSO), thus disrupting the reaction equilibrium and enabling the lean caustic to achieve near-complete elimination of mercaptans from the LPG.
3. 90%+ Caustic Consumption Reduction: Process modeling proved that this closed-loop system will lower fresh caustic purchases and spent caustic disposal by more than 90%.

The in-situ Electrochemical Regeneration Cell is an unprecedented advance in caustic treating technology and represents the most substantial new innovation in caustic treating technology in decades. By transitioning from simply managing degradation to eliminating it, this process re-engineering solution offers a pathway to not just a reduction of operating costs but a reduction of hazardous waste and an overall improvement in the efficiency and robustness of LPG sweetening operations.