Low-sulfur specifications cause refiners to look at hydrotreating options

Lawrence L. Upson, Mark W. Schnaith
UOP Des Plaines, Ill.

Future environmental regulations may require refiners to produce fluid catalytic cracking (FCC) gasoline with less than 100 ppm sulfur.

To comply, refiners can choose to hydrotreat the feed or desulfurize the gasoline. Process options to desulfurize gasoline include:

• Undercutting the FCC gasoline
• Hydrotreating the full-range FCC gasoline
• Hydrotreating the heavy FCC gasoline fraction
• Using the extractive Merox process to treat the light FCC gasoline
• Using liquid-liquid extraction to remove sulfur compounds from the heavy FCC gasoline
• Recracking the heavy-gasoline
• Selectively hydrotreating with the ISAL process developed by Intevep SA, Venezuela
• Using sulfur adsorption for the full-range FCC gasoline.

Feed hydrotreating is the only option that provides a positive return on investment. Because the required capital is large, however, many refiners choose gasoline desulfurization.

The process of sulfur adsorption is currently under development. If this development is successful, the adsorption process will be an attractive gasoline desulfurization option.

U.S. environment

In 1995, the U.S. Environmental Protection Agency (EPA) required gasoline sold in areas with poor air quality to satisfy reformulated gasoline (RFG) standards defined by the simple model. These standards required a reduction in volatile organic compound (VOC) and toxic compound emissions by a minimum of 15% relative to 1990 reference levels. In addition, the standards ensured that NOx levels would not increase. In these regulations, sulfur in gasoline was capped at the 1990 refinery level. ¹

On Jan. 1, 1998, the quality of RFG will be defined by the complex model, which will result in a 1.5% reduction in NOx emissions and, in some areas, a reduction in VOC and toxic emissions.

Using the complex model, refiners may meet the pollutant-emission requirement by any combination of gasoline parameters. Gasoline sulfur content is one of these parameters.

In the year 2000, RFG quality requirements will be tightened even more to reduce toxic emissions and NOx emissions to 6.8% less than 1990 levels.

In the complex model, reducing gasoline's sulfur content reduces toxic emissions and NO_x. Fig. 1 [35,995 bytes] shows that, for the average U.S. refinery, reducing sulfur content from 340 to about 200 ppm reduces NO_x emissions about 5%. When combined with the reduction in aromatics content anticipated to occur when 15% oxygenates are blended into the gasoline pool, decreasing the sulfur content to 200 ppm achieves the 6.8% reduction in NO_x required by the EPA.

Whether the mandated sulfur level in gasoline in the U.S. will be reduced is uncertain. California already requires that gasoline sold in areas with severe air-quality problems contain no more than 30 ppm sulfur on average,² a level that is one tenth the sulfur level currently required by EPA regulations. Political pressures could drive other communities to match the California restriction.Also, car manufacturers are lobbying for a 49-state car, which would run on the same fuel and have the same emission characteristics anywhere in the U.S.³ To run properly, the engine would require a sulfur level in gasoline of about 50 ppm.⁴

European environment

For gasoline-fueled cars, the most recent limits, based on a 1994 European Union (EU) directive (94/12/EC), became effective in 1996. This directive reduced the allowable hydrocarbon andNO _x emissions for new cars nearly 50% to a level of 0.50 g/km. ⁵

In June 1996, the European Auto/Oil Program commission recommended thatNO_x be further reduced to 0.15 g/km in 2000. If achieved, these reductions will put the averageNO_x level in Europe at a lower level than that required by the EPA in the U.S.

For gasoline-quality improvements, the commission found that only sulfur reduction had a consistent effect on decreasing pollutants. It recommended that a maximum sulfur level of 200 ppm be established and that a target of average gasoline-sulfur level of 150 ppm be set throughout the European Community.⁶ This target would be a 50% reduction of 1996 levels.

In April 1997, the European Parliament voted to establish even lower sulfur limits: 100 ppm by 2000 and 30 ppm by 2005. Action by the European Council of Ministers is now needed to finalize these lower limits.

FCCU role

Sulfur in gasoline comes from three sources: FCC gasoline, coker naphtha, and straight-run naphtha. More than 90% of the sulfur in the gasoline pool comes from FCC gasoline. To achieve lower sulfur levels in the finished gasoline product, the refiner must produce a lower-sulfur FCC gasoline.

For a fluid catalytic cracking unit (FCCU) that processes a typical vacuum gas oil (VGO) feed, the sulfur content in the FCC gasoline is about one tenth of the sulfur in the feed. For example, an FCCU feed with 1 wt % sulfur would produce FCC gasoline containing about 1,000 ppm (wt) sulfur. For a typical FCCU gasoline yield of 50 wt % of the feed, the absolute quantity of sulfur in the FCC gasoline is then about 5 wt % of the feed sulfur.

The principal sulfur compounds in FCC gasoline are benzothiophene and methylbenzothiophene, found in heavy-gasoline; alkylated thiophenes, found in the middle boiling range; and mercaptans, mainly found in light-gasoline. Table 1 [6,348 bytes] shows the approximate distribution.

Hydrotreating feed

Because sulfur in the FCC gasoline so strongly depends on the sulfur content of the FCC feed, moderately severe feed hydrotreating results in less feed sulfur in the gasoline than other desulfurization methods. ⁷

For example, a light Arabian VGO with a sulfur content of 2.5 wt % typically produces an FCC gasoline with 2,500 ppm (wt) sulfur. After hydrotreating, the same feed has a sulfur content of 0.125 wt % (95% desulfurization) and produces an FCC gasoline with 60 ppm (wt) sulfur. The ratio for gasoline sulfur to feed sulfur is typically one twentieth for hydrotreated feeds, rather than one tenth.

Feed hydrotreating has other benefits besides meeting refiner sulfur targets. It also improves feed quality by reducing metal content, nitrogen content, and Conradson carbon.

When the proper operating conditions are combined with specifically designed catalyst systems, as is done in UOP's VGO Unionfining process, ⁸ some of the polynuclear aromatic compounds become saturated. Such an improvement in feed quality can result in an increase in FCC gasoline by 7-9 wt % of fresh feed at constant riser outlet temperature, a 10% relative decrease in coke yield, or a 90% relative decrease in SO_x emissions from the regenerator.⁹

Unfortunately, the investment required for an FCC feed hydrotreating system is four to five times more expensive than the other options. Because of the high capital cost, refiners will look to gasoline desulfurization options to meet gasoline-sulfur requirements.

Undercutting FCC gasoline

Because most of the sulfur in FCC gasoline resides in the high boiling fraction of the gasoline, the simplest method to reduce gasoline-sulfur content is to cut the gasoline so that the heaviest 10-15% is transferred from the gasoline to the distillate fraction. This approach solves the gasoline-sulfur problem when FCC gasoline requirements are not too severe (400-600 ppm) and the sulfur in the full-range FCC gasoline is not too high (700-1,500 ppm).

In most cases, this option is insufficient to allow the refiner to achieve a low sulfur level (<50 ppm) in the gasoline pool. this option also results in a significant loss in gasoline yield and puts additional sulfur and aromatic compounds into the distillate pool. In some cases, it also results in octane loss.

Hydrotreating FCC Gasoline

Hydrotreating the fcc gasoline is more costly than undercutting, but it eliminates the disadvantages of undercutting.

Although a 50-ppm sulfur level in the gasoline pool could be achieved by this approach, in most cases, the entire gasoline fraction would have to be hydrotreated. Such approach would lead to a substantial octane loss, which results from the saturation of olefins in the light-gasoline fraction. A ^7-10 loss in research octane number (RON) or 3-4 loss in motor octane number (MON) could be expected.^{8 10}

Desulfurizing the heavy fraction of the FCC gasoline is a more practical approach. Because most of the sulfur compounds are found in this fraction, and the olefins are mainly found in the lighter fraction, hydrotreating the heavy fraction results in a major decrease in sulfur content and a minimum loss of octane. The amount of back-end fraction to be hydrotreated depends on the sulfur specification and the sulfur content of the untreated full-range gasoline.

Fig. 2 [82,022 bytes] shows the FCC gasoline split so that 60% goes into the heavy fraction sent to the hydrotreater. After hydrotreating, this heavy fraction is blended back with the light fraction, resulting in a 92% reduction in sulfur in the full-range gasoline, a 3.5 loss in RON, and a 1.6 loss in MON.

Extractive Merox process

Combining sulfur removal from the light-gasoline fraction with hydrotreating of the heavy-gasoline fraction reduces the amount of heavy fraction that is treated and thus reduces the octane loss. UOP's extractive Merox process is a light-gasoline desulfurization process used for this application.

Because the sulfur compounds in the light-gasoline are mainly mercaptans, they can be removed using an extraction process based on Merox technology. The technology involves a reaction with caustic to form water-soluble sodium mercaptans followed by catalytic oxidation to form an insoluble disulfide oil.

Fig. 3 [76,277 bytes] illustrates the extractive Merox process, and Fig. 4 [88,648 bytes] shows the coupling of this process with the hydrotreating of the heavy-gasoline fraction. In this case, the heavy fraction going to the hydrotreater is only 40% of the total gasoline. The remaining 60% goes to the extractive Merox unit.

Compared to hydrotreating of FCC gasoline, the same degree of desulfurization is achieved but with only a 1.0 loss in RON and a 0.6 loss in MON.

Liquid-liquid extraction

The previously described approaches rely upon distillation to achieve a sulfur-rich stream that can then be hydrotreated to remove sulfur. Liquid-liquid extraction is an alternative separation technique to segregate a sulfur-rich stream.

Certain polyalkylene glycol compounds are efficient solvents for extracting sulfur-containing hydrocarbons from FCC gasoline. Liquid-liquid extraction does a better job of concentrating the sulfur compounds into an extract stream than distillation.¹¹

The use of such an extraction process results in a smaller hydrotreater and less olefin and octane loss to achieve the same degree of desulfurization.

Fig. 5 shows a liquid-liquid extraction process where the full-range FCC gasoline is fed to an extraction unit and the subsequent high-sulfur content extract is hydrotreated. A comparison of Fig. 5 [79,908 bytes] with the distillation approach in Fig. 2 shows that the extraction approach reduces the stream to be hydrotreated about 40%.

Recracking Heavy-gasoline

All of the previously described sulfur-reduction approaches result in some octane loss in FCC gasoline. Recracking the heavy FCC gasoline by recycling it to the catalytic cracker reduces the sulfur in FCC gasoline while actually increasing RON and MON. UOP's pilot plant studies show that this approach can reduce the heavy-gasoline sulfur content about 50% and can increase the RON and MON of this fraction by about 4.0. Similar results have been reported. ¹²

Gasoline produced from recracking is highly aromatic, i.e., Virtually no olefins remain in the recracked material. If this material is recycled to a separate reaction zone, as is possible with today's highly contained disengaging systems, such as UOP's VSS design or a direct-connected cyclone system, the olefin-free gasoline from this recycle can be separately hydrotreated with no loss in octane.

Recracking the heavy-gasoline results in a significant loss in gasoline. For the severity of the recracking described, about 6% of the full-range gasoline is lost. Under certain market conditions, however, the increased octane and slight increase in LPG make the overall product slate attractive.

A diagram illustrating the recracking concept coupled with the extractive Merox treatment of light-gasoline is shown in Fig. 6 [72,017 bytes].

ISAL process

The ISAL process is a two-reactor, selective hydrotreating process for treating a wide range of naphtha feedstocks. ¹³ the process, developed by Intevep SA and licensed by UOP, uses a bifunctional catalyst system to desulfurize FCC gasoline and to achieve a controlled transformation of the gasoline hydrocarbons.

Evidence indicates that the isomerization and cracking of gasoline molecules, along with fragment recombination, are all facilitated by this catalyst system.¹⁴

When treating FCC gasoline, the ISAL process reduces the sulfur content to very low levels, saturates almost all of the olefins, and causes little, if any, loss in octane or yield. This contrasts with the considerable octane loss that occurs with olefin saturation in a conventional FCC naphtha hydrotreater.

Whereas hydrocarbon rearrangement occurs in conventional naphtha reformers, in the ISAL process, the gasoline aromatic content remains essentially unchanged. Also, unlike in reforming, the ISAL process results in no volume loss.

The operating conditions and process equipment used in the ISAL process are similar to those used in a conventional naphtha hydrotreater. Thus, no special operating procedures are required.

If the refiner needs to produce gasoline with a low olefin content and high octane, the ISAL process can treat the full-range FCC gasoline. More commonly, however, the process is used to treat the heavy fraction to minimize hydrogen consumption.

The light-gasoline fraction can be treated with the extractive Merox process.¹⁵ For this combined process, virtually no octane or yield loss occurs, and a 92% reduction in sulfur content of the FCC gasoline is achieved (Fig. 7 [82,747 bytes]).

Adsorption

An emerging technology for FCC gasoline desulfurization is the adsorption of sulfur compounds from the gasoline stream. This technology is actively under development at UOP at present.

Various adsorbents have the ability to remove polar organic compounds containing sulfur, oxygen, or nitrogen atoms from naphtha-type streams.¹⁶ in particular, various zeolites^{17 18} and solid solutions, such as hydrotalcite,¹⁹ can selectively remove a wide spectrum of sulfur compounds, including organic sulfides, mercaptans, and thiophenic compounds.

A commercial system using adsorbents of these types could process the full-range FCC gasoline stream through a system of swing-bed adsorbers that provide repetitive adsorption and desorption cycles. In this adsorption process, sulfur would be totally removed from the FCC gasoline with little loss in gasoline yield and no loss in octane.

Development work indicates that the adsorption capacity of the preferred adsorbent is sufficient to support practical commercial design, provided that the adsorption capacity can be maintained over a reasonable bed life. Current development work is directed toward establishing whether an adequate life cycle can be achieved with the preferred adsorbent.

If an adsorbent life of 1 year can be achieved, the economics of the adsorption process is quite attractive. Compared to a relatively simple system (Fig. 2), in which a naphtha hydrotreater desulfurizes a heavy-gasoline fraction and a refiner must install a splitter to obtain the heavy-gasoline cut and a hydrotreater, the adsorption process requires half the capital cost and half the operating cost.

References

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The Authors