Oct-2004

Light crude oil treatment

A review of caustic treating technologies useful in the removal of volatile sulphur from sour light crudes

Felipe Suarez and Herbert Wizig, Merichem Chemicals & Refinery Services LLC
Liu Youchao, China National Petroleum Corporation
Jack Zhang, Jackson International Inc

Article Summary

The development of sour light crude oils or condensates in certain areas of the world, such as Kazakhstan and Qatar, has created a problem in storage and transportation to crude oil refining centres where these oils are fractionated and converted to fuel products and petrochemicals.

Crude oils contain all kinds of sulphur compounds, both inorganic as in the case of H2S, and organic as in the case of mercaptans and thiophenes. Both H2S and mercaptans are highly odorous and volatile sulphur compounds and, in the case of H2S, its presence raises major safety considerations since it is a poison to animal life as well as humans. When crude oils are light (above 40° API gravity) and contain volatile sulphur in sufficient quantities, these oils require either treatment to remove the noxious sulphur in order to allow their storage and transportation, or the use of more costly and sophisticated facilities along with special safety and environmental handling procedures.

The typical crude oil production facility employs a processing scheme that includes oil and gas separation followed by desalting of the oil, stabilisation, and finally storage in atmospheric tanks. In the case of light crude oils or condensate from sour fields, the final stabilised crude oil could contain several hundred ppm of H2S and several thousand ppm of mercaptans, forcing these facilities to further prepare the oil for safe and non-odorous storage and transportation that mitigate any releases of poisonous H2S and highly odorous mercaptans.

Many countries prohibit the storage or transportation of these sour crude oils within their territories unless certain specifications for H2S and mercaptans are met. Pipelines impose similar specifications as they not only transport these oils, but also provide intermediate storage at their terminals. Waterborne vessels also limit the level of volatile sulphur that they will permit while transporting crude oils.

Historically, oil producers have used various schemes to mitigate the problem. These schemes include processing facilities for stripping H2S and mercaptan, pressurised storage facilities and alternate higher-cost transportation options such as rail in lieu of lower cost pipeline transport. With ever-stricter safety and environmental regulations, storage facilities have become extremely costly. It was not until recently that producers began searching for a chemical treating process, such as caustic treating, for a more attractive option to solve their sour oil storage and transportation dilemma.

Project objectives
In March 2002, CNPC-AMG, in Aktyubinsk, Kazakhstan, approached Merichem with a need to treat a six million tons/year crude oil stream already in production. Up to that time, a portion of the crude oil had been transported to a refinery in Orsk, Russia, via pipeline at depressed pricing and the rest by rail to Russia and other Central Asian countries via rail tank cars at an average cost of US$23.4/ton. The new requirement to treat the oil was an increase in production coupled with the construction of a new oil pipeline that would reduce oil transportation costs to US$8.5/ton and open new markets for CNPC-AMG, provided that they could reduce the volatile sulphur to meet the following specifications:
—H2S (1ppm/wt)
—C1 and C2 mercaptans (<10ppm/wt)
—C1, C2 and C3 mercaptans (<30ppm/wt).

Design basis
The characteristics of the raw crude oil produced by CNPC are shown in Table 1. As the table shows, the oil not only has the sulphur impurities already mentioned but also contains two additional caustic extractable impurities, CO2 and naphthenic acids (the latter is measured as feed acidity in mg KOH/g).   

Carbon dioxide in itself does not present a major difficulty in treating the crude oil but its presence does affect chemical usage as this acid gas is also caustic extractable. Fortunately, for CNPC-AMG the CO2 levels of 40ppm/wt are minimal and do not result in large caustic usage. However, the reaction of carbon dioxide with caustic and the presence of magnesium and calcium chlorides along with H2S in the crude must be carefully considered in the design to avoid precipitation of salts, which could affect process performance and equipment reliability.

The naphthenic acid content of the crude, while relatively low, must also be carefully considered in the design because of several factors. First, naphthenic acids react with caustic to form soaps, which in conventional mixer settler treating systems will tend to form emulsions and cause foaming, making the process inoperable at times. Second, naphthenic acids also react with magnesium and calcium to form metal carboxylate salts, which can precipitate and/or plug the treating equipment.

Finally, naphthenic acids will interfere with the oxidation of mercaptans as they consume alkalinity needed for the mercaptan sweetening process. In order to remove this first set of impurities, which can be categorised as strong acids, Merichem licensed and supplied CNPC-AMG its proprietary Napfining process.

The removal of H2S, CO2 and naphthenic acids would only take the producer of the crude oil part of the way to preparing the crude oil for storage and transportation because mercaptans would still be present. Mercaptans are highly volatile organic sulphur compounds, which are only slightly acidic and very difficult to remove with caustic unless the caustic solution is maintained in a fresh condition (low spending and high alkalinity).

Mercaptans are also extremely odorous and removal to low levels is mandatory in most fuel products produced for industrial and consumer use. For example, in most industrialised countries the historical motor gasoline specifications for mercaptan is the qualitative “Doctor test” which almost always requires below 10ppm mercaptan and sometimes as low as 2ppm for passing as Doctor “negative”.

The Zhanazol crude oil contains approximately 659ppm/wt of C1–C5 mercaptan measured as sulphur. There are other heavier mercaptans in the crude oil but fortunately the lighter (lower molecular weight) C1–C3 mercaptan species, which represent 469ppm of the total, are the most odorous and thus the ones targeted for removal. In order to successfully remove these light mercaptans without creating very large chemical (caustic) usage and operating costs, the economics dictate that the process employed be regenerative, allowing the caustic solution to remain fairly fresh, as earlier stated. For mercaptan removal, Merichem designed and licensed to CNPC-AMG its proprietary Mericat technology.