Improving fractionation lowers butane sulfur level at Saudi gas plant

Lewis G. Harruff, Gary D. Martinie, Abdul Rahman
Saudi Arabian Oil Co.
Dhahran

Dimethyl sulfide contamination in a crude oil/natural gas stream from a gas-oil separation plant, such as the Haradh GOSP-1, led to high sulfur in a fractionation plant's butane product. Improving fractionation efficiency successfully addressed the problem. (Photograph courtesy of Aramco Services Co., Houston)

Increasing the debutanizer reflux/ feed ratio to improve fractionation at an eastern Saudi Arabian NGL plant reduced high sulfur in the butane product.

The sulfur resulted from dimethyl sulfide (DMS) contamination in the feed stream from an offshore crude-oil reservoir in the northern Arabian Gulf. The contamination is limited to two northeastern offshore gas-oil separation plants operated by Saudi Arabian Oil Co. (Saudi Aramco) and, therefore, cannot be transported to facilities outside the Eastern Province.

Two technically acceptable solutions for removing this contaminant were investigated: 13X molecular-sieve adsorption of the DMS and increased fractionation efficiency. The latter would force DMS into the debutanizer bottoms.

Increasing the reflux/feed ratio in the test debutanizer from 0.95 to 1.26 at a constant tray temperature reduced the DMS carryover from 40-50% to 13-16%.

It is important to note that the reflux/feed ratio is the determining factor in this effect, not the absolute reflux rate alone. In practical terms, increasing the reflux/feed ratio will allow this plant to tolerate up to about 100 ppm DMS as sulfur in the C₄+ feed to the debutanizer. Recently, the DMS concentration in that stream has been 30 ppm or less.

The recommendation was, therefore, that the reflux/feed ratio be increased slowly to a maximum 1.3 when the butane sulfur content rises to greater than 20 ppm. It should be noted that at high feed rates, the reflux/feed ratio may be limited somewhat by overhead condenser capacity in the very hot days of summer.

Also, the more efficient fractionation will drive more C5s into the natural gasoline. This has an economic impact on the butane product by lowering the volume of production and may necessitate operation of the Reid-vapor-pressure column to lower the vapor pressure of the natural-gasoline product. These factors need to be weighed against the plant's need to produce on-grade butane.

Use of 13X molecular-sieve adsorption remains technically viable to overcome this problem but would most likely require investment in capital equipment.

Product contamination

Saudi Aramco operates several large NGL fractionation and sweetening facilities in Saudi Arabia ( Fig. 1 [70,123 bytes] and Fig. 2 [64,534 bytes]). These plants typically see a mixture of sour C ₂+ and C ₃+ NGL from upstream production facilities and diglycol amine (DGA) gas-sweetening plants.

NGL is first fractionated into ethane, propane, butane, and C₅+ (natural gasoline). Afterwards, traces of hydrogen sulfide are removed from the propane by liquid/liquid extraction with di-isopropanolamine (DIPA).

Mercaptans are then removed from the propane and butane by caustic extraction in UOP Merox units. The sweet products are dried (4A molecular sieves), then sent to domestic customers and export markets.

For many years, this treatment has successfully produced propane and butane with 10 ppm or less total sulfur (30-ppm specification) from a sour NGL. In 1993, however, one of the plants began experiencing increased sulfur contamination in its butane product.

At times, total sulfur in the product approached the 30-ppm specification limit. The sulfur content of the propane produced in the same train remained low.

All of the operations at the plant were otherwise normal with no upsets; in short, there was no apparent reason for the increased sulfur content in the butane.

At that point, Saudi Aramco embarked on an investigation to identify the contaminant or contaminants and their sources.

Identification

One of the first things that comes to mind when the sulfur level of an LPG product from a Merox unit rises is disulfide carryover from the disulfide separator into the lean caustic (Fig. 2). If disulfide oil is entrained in the lean caustic from the regeneration unit, it will be extracted into the propane and butane and end up in the LPG product.

If this had been the case in the plant in question, the sulfur content of the propane would have increased along with the butane because they share caustic-regeneration facilities. Because the propane remained normal, investigators could quickly eliminate this possible cause.

Initial gas chromatographic (GC) examination of the contaminated butane suggested, based on retention time, that the high sulfur content was due largely to carbon disulfide (CS₂). Unfortunately, it can be difficult to distinguish among CS₂, ethyl mercaptan (EtSH), and dimethyl sulfide (DMS) by retention time alone.

Further investigation used another GC equipped with an atomic emission detector (AED), which allows the analyst separately and simultaneously to monitor compounds containing selected elements.¹ It also generates a signal that is linear with the amount of the selected element in each GC peak.

By simultaneously monitoring both the sulfur and carbon channels of the GC-AED trace, investigators were able to obtain a carbon/sulfur molar ratio of 2:1 for the peak in question. Clearly, the contaminant was not CS₂ that has a carbon/sulfur ratio of 1:2.

This left two candidates, ethyl mercaptan (EtSH) and dimethyl sulfide (DMS). Comparison with standards confirmed the identity of the culprit as DMS. Further confirmation came from gas chromatography-mass spectrometry.

Although DMS has been observed in NGL streams in other parts of the world,² this was the first time it was identified in hydrocarbons from fields in Saudi Arabia. Dimethyl sulfide is quite unreactive and cannot be removed from hydrocarbon streams by either DGA or Merox caustic extraction.

Therefore, it can be expected to pass through a desulfurization process and end up in the hydrocarbon product. The only problem associated with DMS contamination is increased total sulfur that can damage catalysts in a customer's petrochemical plant and increase sulfur dioxide emissions in butane-fuel applications.

It is a very stable compound that will not break down in shipping, nor is it corrosive. In fact, propane and butane spiked with 100-ppm DMS gives a 1A copper-strip test (ASTM D 1838).

Source

Once the contaminant had been identified, investigators began to work through the system to identify the source. The investigation was facilitated by the fact that DMS is a rather rare species in Saudi Aramco's NGL streams.

Some of the streams that needed monitoring, however, contained only very small concentrations of DMS. This meant that the sampling and analysis often had to be conducted to allow detection of DMS at the low to sub-part-per-million level. With careful sample cylinder preparation and GC-AED for analysis, investigators were able to accomplish this. Initial monitoring of the problem indicated that the sulfur level of the butane tended to rise when the plant received more C₃+ from a particular upstream facility-but not always.

This facility receives gas and condensate associated with crude-oil production from several fields in northeastern Saudi Arabia. The gas is sweetened (DGA) followed by liquids separation and dehydration.

The liquids from the sweetening process and the stripped condensate are combined, de-ethanized, and sent to the separate NGL fractionation plant as a C₃+ stream (Fig. 1).

An initial survey of DMS in the C₃+ from that plant was negative. At that time, the butane product was also very low in DMS.

Coincidentally, two northeastern offshore gas-oil separation plants in one field that normally feed into the sweetening plant were down for maintenance. When they came back on-line, the DMS reappeared in the suspect C₃+ and finally in the butane product. Investigators had their "smoking gun."

The source of the DMS contamination was confirmed when they found 7-10 ppm DMS in the sour gas from both suspect separation plants. Even though this level of contamination may seem small, it has the potential to be greatly magnified in the butane by the fractionation process. In fact, investigators have found 3-4 times this amount in butane product.

A thorough survey of the entire Saudi Aramco system fortunately confirmed that the DMS contamination was isolated to these two northeastern offshore separation plants. As such, it has affected only NGL plants in the Eastern Province and cannot be transported to the west because of configuration of Saudi Aramco's pipeline system.

Solutions

Saudi Aramco had originally hoped that by identifying the sulfur contaminant and finding its source, the company might be able to divert it so that it would not reach the Eastern Province NGL-fractionation facilities.

Unfortunately, the DMS was coming from wells that were part of an important crude-oil reservoir. Halting production from these wells was not an option. Therefore, another way to reduce or eliminate DMS from the butane product had to be found; several options were considered, including:

DMS removal from butane product by adsorption on 13X molecular sieves
Improved fractionation in the debutanizers to force the DMS into the C5+ bottoms
Extraction of DMS using alkylating agents to convert it to water-soluble sulfonium salts
Treating the effected wells with biocides if the cause of the DMS contamination were related to microbial action in the well bore.

DMS is known to be a metabolite of some marine algae ³ but has never, to investigators' knowledge, been found as a product of microbial action in oil wells.

The sulfonium salt approach was tested in the lab and proved technically feasible, but the cost and complexity of developing a commercial application of this technology were prohibitive.

The microbial contamination hypothesis would require a lengthy microbiological study to prove. The economics and probability of technical success of this approach were too risky.

There remained the molecular sieve and fractionation options.

Adsorption

A major domestic customer for Saudi Aramco's butane manufactures methyl tertiary butyl ether (MTBE). Isomerization of the normal butane to isobutane is an early step in this manufacturing process.

The catalyst used for the isomerization is sensitive to poisoning and requires a feed nearly free of water (<1 ppm) and sulfur (<0.5 ppm). as such, it is necessary for this customer to remove the last traces of water and sulfur from the butane being supplied from the fractionation and sweetening plant.

This is accomplished by passing the raw butane feed through a 13X molecular sieve drier/adsorber. The cycle time between regenerations for this drier/adsorber varies with the amount of sulfur in the incoming raw butane: 500 hr with 10-ppm total sulfur, 170 hr with 30-ppm total sulfur.

Regeneration is accomplished by back flushing the column with hot, clean isobutane (approximately 60 tons) which must then be flared.

Because the butane being supplied to this customer from Saudi Aramco's plant contained 5-30 ppm sulfur, at times largely as DMS, the operation and performance of the drier/adsorber was of some interest.

Analysis of samples of the raw butane feed and effluent from the columns indicated that the 13X mole-sieve adsorbent removed all of the sulfur-containing compounds including the DMS ( Table 1 [50,078 bytes]). It should be noted that the 4A molecular sieve used to dry the butane produced at the fractionation plant does not remove DMS.

Even though these samples from the MTBE plant were taken on a day with low sulfur contamination, this customer had been able to provide sulfur-free butane to his isomerization units even when the DMS contamination was as high as 20 ppm. Clearly, the 13X molecular sieves effectively remove DMS and other sulfur compounds from butane.

This approach remains a viable option, if necessary, to make on-specification butane from DMS-contaminated NGL. It could be applied by either replacing the 4A molecular sieve currently being used in the dryers at Saudi Aramco's plants or by placing a "polisher" column containing the 13X downstream of the existing dryers.

The polisher approach is attractive because the column could be activated when the contamination rises and left off-line when it is not needed.

Improved fractionation option

In principle, most of the DMS should be able to be diverted to the debutanizer bottoms by increasing the efficiency of fractionation in the debutanizer. This seemed logical because of the relative boiling points of the species involved ( Table 2 [21,578 bytes]).

Increasing the reflux/feed ratio is one simple way to increase the fractionation efficiency of a distillation column. This approach seemed attractive because the plant had spare reflux pump, reboiler, and overhead condenser capacity in the effected modules.

Throughput could be maintained at an increased reflux/feed ratio by increasing reboiler steam to maintain a constant Tray 9 temperature ( Fig. 3 [48,154 bytes]). Investigators carried out a plant trial in early 1995 to test this hypothesis.

Sets of sample analyses from the debutanizer C₄+ feed, overhead, and bottoms were rejected if the DMS balance (feed = overhead + bottoms) was outside 615% of 100% and/or the overall liquid balance was outside 610%. Average values for the DMS and liquid balances were 99% and 104%, respectively.

Phase 1: Normal C₄+ feed rate. The trial began on the morning of May 27, 1995, with the collection of baseline data. The feed was set to give a C₄+ debutanizer feed of about 2,800 bbl/hr. The reflux rate to the debutanizer was set at 2,650 bbl/hr to give a reflux/feed ratio of 0.95.
These conditions correspond to normal operating conditions. During this period, the C₄+ debutanizer feed contained 17-30 ppm DMS. The overhead contained 13-29 ppm that corresponds to a 40-50% carryover of DMS from the feed (Fig. 4 [81,249 bytes]).
During the evening of May 29, the reflux rate was increased to 3,350 bbl/hr, keeping the C₄+ feed to the column near 2,950 bbl/hr. This gave a reflux/feed ratio of 1.14.
This change caused an immediate drop in the DMS carryover to <20% of the feed or less than half of what was observed with a 0.95 reflux/feed ratio (fig. 4).
Phase 2: Maximum C₄+ feed rate. In the afternoon of May 30, the rates were adjusted to reflux = 3,100 bbl/hr and C₄+ feed = 3,200 bbl/hr for a reflux/feed ratio of 0.97. This represents a "normal" ratio for the maximum feed-rate condition. It should be noted that the debutanizer feed rate is currently limited by the capacity on the downstream Merox treating unit. This set of conditions resulted in increased DMS carryover to 29-36% of the feed contamination (Fig. 4). On the morning of May 31, the reflux rate was increased to 4,040 bbl/hr with the C₄+ feed rate held at 3,200 bbl/hr for a reflux/feed ratio of 1.26. This represents a practical operating maximum feed and reflux condition for the unit. The DMS carryover dropped to 13-16% of the feed-the lowest value of the test.
Phase 3: Intermediate conditions. In order further to confirm these results, in the early morning of June 1, the C₄+ feed was lowered to 2,900 bbl/hr and the reflux to 3,120 bbl/hr for a reflux/feed ratio of 1.08. This caused the DMS carryover to increase into the 25-30% range (Fig. 4).
Overall relationship of the reflux/feed ratio to DMS carryover. When all of the experimental conditions that were used in this test were taken into consideration, a rough linear relationship emerged between the reflux/feed ratio and the percent DMS carryover.

The relationship follows the formula, 1.02-0.69(reflux/feed ratio) = (%DMS carryover) with R ² = 0.73 for the debutanizer ( Fig. 5 [71,446 bytes]).

Because of the scatter in the plot in Fig. 5, it can only be used as a rough guide within the reflux/feed ratio range of 0.95-1.30. It can now be said for sure, however, that increasing the reflux/feed ratio of this debutanizer is an effective tool that can be used with current equipment to combat DMS contamination of butane product at this plant.

Acknowledgments

The authors wish to thank the Ministry of Petroleum & Minerals of the Kingdom of Saudi Arabia and the Saudi Arabian Oil Co. for granting permission to publish this work and managements of the lab research and development center, the process and control systems department, and Ju'aymah gas plant for their support.

Acknowledgment is also due the teams of scientists and engineers who participated, especially Tom Strauss, Anwar Al-Khawaljah, Stephen Bushkuhl, and Lance Evanson.

References

Sullivan, J.J., and Quimby, B.D., "Characterization of a Computerized Photodiode Array Spectrometer for Gas Chromatography-Atomic Emission Spectrometry," Analytical Chemistry, Vol. 62, pp. 1035-43, 1990.
Harryman, J.M., and Smith, B., "Update on Sulfur Compound Distribution in LNG: Plant Test Data, GPA Section A Committee, Plant Design," 75th Annual Convention of the Gas Processors Association, Mar. 11-13, 1996, Denver.
Balows, A., et al., The Prokaryotes, 2nd Ed., Vol. 1, p. 437, Springer-Verlag, 1992.

The Authors

Lewis G. Harruff is an independent consultant specializing in DGA and Merox treating, NGL recovery, Claus sulfur recovery, and molecular- sieve dehydration technologies. For 13 years, he served as a gas processing and sulfur-recovery specialist for Saudi Aramco. Harruff earned a BS with honors (1972) in chemistry from Wayne State University and a PhD (1977) in organic chemistry from the University of Oregon. He is a member of the American Chemical Society, the American Association for the Advancement of Science, and the New York Academy of Sciences

Gary D. Martinie since 1995 has held the position of science specialist for Saudi Armco, having joined the company in 1992 in the advanced instruments unit as a senior laboratory scientist. Before joining Saudi Aramco, he was manager of analytical research, product quality, process chemistry, regulatory affairs, and product technical service for High Point Chemical, a Division of Kao Corp. International. Martinie worked 8 years as a staff member in spectroscopic research at Northern Illinois University, Dekalb, 5 years as a senior scientist in analytical and process chemistry research and development at Uniroyal Chemical Co., 12 years as a senior research advisor and manager of analytical and process chemistry research and development for ARCO Chemical, as well as manager of manufacturing support at Atlantic Richfield International. He holds a BS (1966) in chemistry from Eastern Illinois University, Charleston, and a PhD (1975) in analytical chemistry from Northern Illinois University.

Abdul Rahman has worked for Saudi Aramco since 1981 as a laboratory scientist and specializes in gas chromatography. Previously, he worked for North Eastern Gas (a part of British Gas Corp.) from 1970 to 1981. He holds HND from England and is a Licentiate of Royal Society of Chemistry and an associate member 'T-ENG Grade' of Institution of Gas Engineers.