Light crude oil treatment

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 fraction-ated 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 transporta-tion, or the use of more costly and sophisticated facilities along with special safety and environ-mental 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 atmos-pheric 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 transpor-tation that mitigate any releases of poisonous H2S and highly odorous mercaptans.

Many countries prohibit the storage or trans-portation of these sour crude oils within their territories unless certain specifications for H2S and mercaptans are met. Pipelines impose simi-

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

lar 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 treat-ing process, such as caustic treating, for a more attractive option to solve their sour oil storage and transportation dilemma.

Project objectivesIn 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 refin-ery 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 require-ment to treat the oil was an increase in production coupled with the construction of a new oil pipeline that would reduce oil transpor-tation 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)

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

• C1, C2 and C3 mercaptans (<30ppm/wt).

Design basisThe 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 impuri-ties 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 emul-sions and cause foaming, making the process

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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 main-tained 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 coun-tries 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–C2 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 mercap-tan removal, Merichem designed and licensed to CNPC-AMG its proprietary Mericat technology.

Once treating is accomplished, the producer must then deal with the effluents produced in the treating unit, which in this case only consists of the spent caustic effluent leaving the Napfining and Mericat treating stages. This stream must be handled or disposed of in an environmentally acceptable manner. For treat-

Design basis

Chargestock Light crude oilFlow rate, bpsd Maximum 67800Design 56500Normal 56500Minimum 28250Molecular wt 110Specific gravity @ 15°C 0.81IBP-EP °C CH4-398Water content wt% 0.5

Inlet impurities Acid number, MgKOH/g 0.05H

2S, ppm (wt) 30

CO2, ppm (wt) 40

Total mercaptans as S, ppm (wt) 659C

1-C

3 Mercaptans as S, ppm (wt) 469

Methyl mercaptan as S 49Ethyl mercaptan as S 249Propyl mercaptan as S 171C

4-C

5 Mercaptans as S, ppm (wt) 190

Table 1

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ment of this stream, Merichem designed and licensed its proprietary Mericon process.

Process solutionIn the summer of 2002, CNPC-AMG issued a letter of intent to Merichem to license its tech-nologies and to purchase the proprietary equipment in order to have the facility in operation by the end of 2002, a project duration of less than six months. The fast track delivery and startup of the facility was necessary because it had to coincide with the completion of the new crude oil pipeline to Kenkiak, where the Zhanazol crude oil would ultimately be transported via another pipe-line to the Caspian Sea for ultimate shipping to the Black Sea and Mediterranean markets.

The contract involved supply-ing CNPC-AMG with the basic engineering design and propri-etary equipment consisting of the Fiber-Film Contactors, process strainers and other special equipment, the operat-ing and maintenance manuals and the startup services. In addition, Merichem also supplied the process vessels fabricated by its sub-contrac-tors in Ekaterinburg, Russia.

CNPC-AMG, using the East China Design Institute as its engineering contractor, was responsible for the detailed design of the plant, procurement of all other materials and equip-ment, installation, erection and interconnection of the facility. The facility applied a combination of US, Russian and Kazakhstan standards for design of equipment, materials of construction and methods of construction as well as safety and environmental standards.

Process descriptionThe Napfining system as illustrated in Figure 1 (following page) is designed to remove strong acids with a weak caustic solution (3-5wt% NaOH) in accordance to the following reactions:

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H2S + 2NaOH → Na

2S + 2H

2O

Hydrogen sulphideCO

2 + 2NaOH → Na

2CO

3 + 2H

2O

Carbon dioxideRCOOH + NaOH → RCOONa + H

2O

Naphthenic acids

The flashed and desalted crude oil supplied via pumps P-1 and 2 from the existing production facilities first passes through a pair of hydrocar-bon strainers (BS-1 and 2) to remove any scale, rust or other solid matter that could interfere with the proper functions of the system.

The crude oil then enters the Napfining stage Fiber-Film Contactor (FFC-1) where it comes into contact with a mixed fresh and recycled caustic solution. The total caustic rate is approx-imately equal to 10% by volume of the crude oil rate. The actual make up caustic rate includes

LC

PC

PC

AC

PC

PC

AC

PC

PC

V-2V-1LC

P-3&4

P-5&6

P-1&2

Water

Oxidation airTreated

lightcrude

Naphthenic/sulphidiccaustic to

spent caustic tank

Plant air

Untreatedlightcrude oil

Freshcaustic(continuous)

BS-3&4

BS-1&2

Naphthenic acid extraction Mercaptan oxidation

BS-5&6

Catalyst

Figure 1 Napfining/Mericat schematic flow diagram

fresh caustic being injected from the facility’s fresh caustic supply system and cascaded from the downstream Mericat mercaptan sweetening system.

The total fresh caustic makeup rate is set after a laboratory analysis of the circulating caustic solution to determine its “spending” level (or free alkalinity). Actual usage is directly propor-tional to the total sum of H2S, CO2 and naphthenic acids contaminants contained in the feed crude oil. The design caustic spending level of the Napfining stage is 60%. The spent caustic is removed from the bottom of the separator vessel (V-1) on level control to the spent caustic tank.

The previously mentioned Fiber-Film Contactor (Figure 2) is a proprietary mass trans-fer device licensed by Merichem consisting of a cylinder packed with metallic fibres that provide the surface area required for mass transfer to take place. The fibres, being hydrophilic, attract the aqueous (caustic) phase and provide the surface on which the hydrocarbon (crude oil) and aqueous (caustic) phases are in continuous contact as both phases flow co-currently down-ward though the contactor.

The caustic extractable impurities pass from the hydrocarbon to the caustic phase where they are removed from the system with the spent caustic. The mass transfer mechanism is

non-dispersive, which is distinctly different from the conventional mixing/settling approach, where caustic droplets are formed for mass transfer purposes. This results in a nearly perfect separation of phases and avoids the formation of emulsions, caustic carry-over with the crude oil and hydrocarbon carry-under with the caustic as would be experienced in mixer/settler processes.

The need for costly water washes to clean up the treated product for caustic entrainment is eliminated as well as losses of caustic with the hydrocarbon product or losses of hydrocarbon with the spent caustic leaving the system.

Mercaptans, being a weak acid, are not removed to any significant level in the Napfining system. Any mercaptans extracted when the caustic solution is at its highest level of free alkalinity are displaced back into the crude as the more acidic impurities are extracted into the caustic solution and the caustic spending increases. The product leaving the system will then require further treating in the second Mericat stage previously shown in Figure 1.

The partially treated crude oil enters the Mericat system where mercaptans are extracted and oxidised in accordance to the following reactions:

RSH + NaOH → RSNa + H2

Extraction

2RSNa+1 O2+H

2O catalyst RSSR+2NaOH

2 --------- → Oxidation

2RSH + 1 O2 + H

2O catalyst RSSR + H

2O

2 --------- → Combined

It should be noted that the combined reaction illustrates a 100% regeneration of the caustic, which fulfills the prior mentioned requirement that the caustic solution must not be signifi-cantly spent in order to extract the mercaptans with caustic. The RSSR (disulphide oil) by-prod-uct will re-enter the treated crude oil product and leave with the final product.

The crude oil entering the Mericat stage is first contacted with air in a specially designed air sparger (SP-1) to dissolve the oxygen needed for the oxidation reaction to take place. The crude oil containing the dissolved oxygen enters the Fiber-Film Contactor (FFC-2) where it comes into contact with a combined fresh caustic and re-cycled caustic solution containing a dissolved

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Clearhydrocarbon

phase in

Hydrocarbonphase in

Aqueousphase in

Clearaqueous

phase out

FIBER-FILM™Contactor

Figure 2 Fiber-Film Contactor

oxidation catalyst. The latter promotes the mercaptan oxidation reaction mentioned earlier. Catalyst additions are made on a batch or continuous basis depending on requirements and plant preference.

As the crude oil and caustic/catalyst solution flow co-currently downward through the contac-tor mercaptans are converted to disulphdes, which re-enter the crude oil phase. The treated crude oil, free of H2S, naphthenic acids and C1–C3 mercaptans, passes on to product tankage on pressure control. Since H2S is no longer present and mercaptans have been converted to much less volatile/odorous compounds, the treated crude oil can then be stored in conventional facilities and transported via truck, railcar and, more importantly, the lower cost pipeline to its final end user.

Operating performanceOn 17 February 2003 the Napfining/ Mericat system was started up by CNPC-AMG. Despite the extremely accelerated project schedule, which consisted of a total of eight months from contract execution to mechanical completion, the crude oil treating system successfully started up and has been running on-specification and onstream continuously ever since.

Some of the key results obtained since the startup are listed as follows:• The system has processed an average of 11500 tonnes/day of crude oil. Additional throughput will be processed once oil field production is expanded• The crude oil product has been successfully transferred via the new Kenkiak pipeline to its market destination, fulfilling one of the most important economic objectives of the project• The system has operated with a 100% onstream service factor since startup• Fresh caustic consumption as 100% NaOH has averaged 4.28 tonnes/day versus a design rate (adjusted to crude oil throughput) of 4.13 tonnes/day. This represents 104% of the design expectations• Spent caustic production has averaged 7.9m3/hr versus an adjusted design rate of 5.26m3/hr. This indicates that in addition to the H2S, CO2 and naphthenic acids, the caustic is also picking up free water contained in the desalted crude entering the caustic treating unitThe raw crude oil H2S content has ranged from

20~90ppm/wt, 300~2000 ppm/wt for C1 and C2 mercaptans respectively; and 400~2500ppm/wt C1, C2 and C3 mercaptans, figures which are waved far away from the design basis• Product H2S content has averaged 0ppm/wt, 0ppm/wt for C1 and C2 mercaptans respectively, and 10ppm/wt C1, C2 and C3 mercaptans, figures which are well below design requirements of 1ppm/wt, 10ppm/wt and 30ppm/wt, respectively.

The summary of the operating cost variables based on design light crude flow rate and impu-rity levels are contained in Table 2.

Some of the problems faced during the opera-tion can be summarised as follows:• The hydraulic throughput of the facilities has been restricted by some bottle-necks in the pipe-lines feeding the Napfining/Mericat system. CNPC-AMG is in the process of debottlenecking these pipelines in order to achieve the full processing potential of the facilities as the oil production in the field continues to increase• The spent caustic currently being produced has been plugging existing disposal facilities due to its high salt content. The proprietary Mericon system, which will be used for treatment of this effluent, is expected to eliminate this problem.

Future plansCNPC-AMG is planning to increase crude oil production in the near future and will be consid-ering additional processing facilities that will allow them to transport and market the Zhanazol crude oil to far away places like mainland China. A new 1300km pipeline is planned for comple-tion by 2006.

In order to improve its environmental abate-ment programmes and eliminate the problems

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Operating cost variables

Crude oil Crude oil Napfining MericatFresh NaOH, kg 2.2 million* –Spent caustic, m3 65821* –Oxidation air, m3/h – 562Oxidation catalyst, kg – 1134Electricity, kWh 231779 296162Operating labour, MH 300 300Maintenance labour, MH 150 150

*Combined caustic use for Napfining and Mericat

Table 2

associated with spent caustic disposal, CNPC-AMG is planning the implementation of the previously mentioned Mericon spent caustic neutralisation process in the near future (Figure 3). The basic engineering and licensing of this technology has already been completed.

The processing scheme consists of a low pH neutralisation of the spent caustic followed by H2S stripping and then a re-neutralisation of the neutralised aqueous material to a neutral pH.

The goal is to achieve near total removal of reactive sulphur compounds; sodium sulphide (Na2S), sodium hydrosulphide (NaHS) and sodium mercaptide (RSNa). This will allow the safe and non-odorous handling of the material in the plant effluent treating systems.

In addition, the spent caustic neutralisation system will also provide for removal of organic acids, primarily naphthenic acids that are extracted by the caustic solution. The system’s spent caustic feed and treated spent caustic specifications are shown in

Table 3.

Investment justificationThe investment made by CNPC-AMG for caustic treating the Zhanazol crude oil has been a very successful and profitable project. The caustic treatment of light crude oil for removal of light volatile sulphur compounds is not only techni-cally feasible with the appropriate technology, but also extremely attractive from an economic standpoint and environmentally sound when a

well-designed effluent caustic treating unit is made part of the project.

Oil producers should avoid the use of conventional mixing/settling caustic treating processes that will create emul-sions as crude oils have a strong propensity to emulsify when contacted with a caustic solution.

Napfining, Mericat, Mericon, Fiber-Film Contactor and Mericon are trademarked technologies owned by Merichem Chemicals and Refinery Services LLC.

Felipe Suarez is vice president of Merichem Chemicals & Refinery Services LLC, responsible for the process technology and caustic management services business units. He holds a BS in chemical engineering from Louisiana State University and is a registered professional engineer in Louisiana. E-mail: fsuarez@merichem.com

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Fuel gas

Solvent

Sulphuricacid

Water

Spentcausticfeed

PC

FC

FC

LC

FC

FC

FC

LC

LC

Offgas tosulphur plant

Freshcaustic

Neutralisedbrine toWWTP

Acid oils tostorage tank

NC

Cooler

Figure 3 Mericon schematic flow diagram

Spent caustic data Caustic type Mixed naphthenic and sulphidic Standard density or gravity @ 15°C 1.01Operating density or gravity @ 45°C 1014kg/m3

Flow rates @ 15°C Design 20.0m3/h Normal 18.0m3/hSpent caustic specifications Composition, wt%Water 97.76NaSH –Na

2S 0.31

NaSR –Free NaOH 1.26NaOR –NaOOCR 0.46Na

2CO

3 0.22

Hydrocarbon oil TraceTreated brine specifications, max: Product specificationspH 6.0–8.5Na

2S as sulphur <10ppm wt

Mericon spent caustic specifications

Table 3

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Herbert Wizig is assistant director of marketing for the process technology division of Merichem Chemicals & Refinery Services LLC. He holds a BS in chemical engineering from the University of Texas and MS in chemical engineering from Georgia Institute of Technology, Atlanta.E-mail: hwizig@merichem.com Liu Youchao is chief process engineer for CNPC-AMG in Kazakhstan, in charge of project management and new process application. He is a member of the China Petroleum Society with more than 20 years experience in the petroleum industry.Jack Zhang is vice president of Jackson International Inc, Beijing office. Jackson International is the sole representative of

Merichem technology and service in China. He graduated from the chemical engineering department of Dalian University of Science and Technology in 1987. Email: jack.zhang@jackson-international.com

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