Aug-1998

Risks of accumulated sulphur in sulphur recovery unit

Follow-up data to an October 1997 article proves with refinery experience that caustic neutralisation does not improve system operations

Arthur L Cummings and Shade M Mecum, MPR Services

Article Summary

Caustic addition appears to be an inexpensive and easy way to deal with heat stable salt (HSS) problems in amine systems. However, refinery experience shows that operational problems persist when caustic is used to “neutralise” amine HSS. In contrast, when caustic addition is replaced by removing HSS and maintaining HSS anions at a low level, the refinery finds greatly improved operation of the amine system, lower maintenance costs and increased refinery throughput. This and other refinery experiences confirm that caustic addition is bad for an amine system.

It is well known in the refining and gas treating industries that accumulation of amine HSS has a very detrimental impact on amine system operations. Amine HSS reduce the effective capacity of the amine solution, contribute to corrosion and aggravate operational problems such as foaming, amine loss and fouling.

When HSS build-up becomes a problem, operators have a variety of options to manage the problem. The options include replacement of part or all of the solution, HSS removal or the commonly recommended practice of adding sodium or potassium caustic to neutralise the HSS. Many operators chose the neutralisation over replacement or salt removal, as it is perceived to be a more economical way to stop the problem. Unfortunately, neutralisation addresses only the capacity (available amine strength) issue. When other operational problems (corrosion, foaming, amine loss, fouling, filter plugging and upsets) persist following neutralisation, it may be erroneously assumed that HSS have no relationship to those problems.

However, in Part 1 we reported corrosion data, collected in a dynamic corrosion test apparatus, that show that caustic additions to amine solutions do little to improve the corrosivity of amine solutions. Thus, the problems aggravated by corrosion products will not go away. In some cases, corrosion rates are increased by caustic addition,1,2 which may lead to additional operating problems. In this part, we discuss the plant operating problems from plant experience and show how caustic addition has contributed to those problems. In contrast to caustic experience, operating performance improved greatly after HSS and sodium were reduced and maintained at low levels.

Plant case history
Experience with neutralisation
A Gulf Coast refiner upgrading heavy sour crudes had a large rate of HSS build-up, sometimes as much as 1000 lb per day (about 20 lb-mole per day) in a “formulated” MDEA solvent system. HSS anion (formate, acetate, thiocyanate, chloride, thiosulfate and sulphate) levels in the range of 3 to 5 wt% (equivalent to 7 to 12% MDEA or up to 25% of the total amine bound with HSS) were quite common. The refiner practised neutralisation as recommended by their amine supplier, usually adding caustic to reduce the amine HSS (bound amine) to less than 2 wt% MDEA. Caustic addition was advertised to significantly reduce corrosion, improve amine performance and return the bound amine to active amine. Only the last objective was ever achieved, and only to a limited extent.

Corrosion effects As HSS anion concentrations increased in the system, due to extensive efforts to control amine loss, corrosion rates all over the system increased tremendously. Regenerators corroded so rapidly that weld overlays were required almost yearly. An additional set of lean/rich exchangers was purchased so that fouling and corrosion failures in one set of exchanges would not shut down the regenerator. Reboiler fouling occurred so often that isolation valves had to be installed to clean the exchangers on line. Filter costs reached nearly $0.4 million per year. Eventually, a third regenerator train was installed to allow for one train to be down for repairs. Still, this did not prevent loss of hydrocarbon throughput due to corrosion/fouling problems.

Performance effects Amine operations at this level of HSS were poor at best. Contactors and regenerators were constantly foaming, leading to large losses and sulphur plant upsets. Liquid treaters lost large amounts of amine overhead, leading to many water treatment plant permit excursions. Amine system operation was a major bottleneck to hydrocarbon throughput.

Amine restoration effects Strengths were routinely reported to be in the range of 45% wt% MDEA. Caustic additions did not appear to change the strength. However, the heat stable amine salts would go down. It was expected that the strength would go up with caustic addition. It was determined that the major salt anions, formate (HCOO-) and acetate (H3CCOO-) built up to levels that actually interfered significantly with the amine strength titration.3 These ions would react with acid in the titration, as did the actual MDEA in the pH range of the titration end point colour indicator as with formate:

HCOO- + H+ ⇒ HCOOH

Formate at MW of 45 and acetate at MW of 59 were being counted at 2.6 and 2.0 wt% amine per % salt anion, respectively. Since caustic addition did not change the state of these ions, these were still counted in the total strength. Because of this error, the amine was severely undercirculated, rich loadings were excessively high, and this helped to aggravate the rate of corrosion over the entire system.