Removal of Sulfur from a Light FCC Gasoline Stream

Herb Wizig, Merichem Company (USA)
Ralph Maple, Merichem Company (USA)

1. Introduction

2. Presentation Overview

3. Sulfur Distribution in Light FCC Gasoline and Removal Possibilities with Caustic

3.1 Advantages of Extractive Caustic Treating Over Hydrotreating Light FCC Gasoline to RemoveSulfur

4. Advantages of Merichem's FIBER-FILM^TM Contactor Treating Systems

5. The FIBER-FILM^TM Contactor Advantage

6. The Typical Extractive Caustic Treating System

7. Process Description

7.1 Mercaptan Extractors (THIOLEX^SM)

7.2 Caustic REGENeration (REGEN^SM)

7.3 Improved Sulfur Extraction

7.4 Maximum Sulfur Extraction

8. Annual Operating Cost Estimation Factors

9. Composition and Flow Rate of Effluent Streams

10. Commercial Experience

11. Recombinant Mercaptans from Heavy FCC Gasoline Hydrotreating

12. Conclusions

1. Introduction

Refiners throughout the world are evaluating sulfur removal technologies and strategies for meeting future government dictated gasoline sulfur requirements. These requirements will vary depending upon the location of the refinery's gasoline market and the sulfur reduction schedule mandated by the various governments of the market. For Europe and North America, the sulfur content of finished gasoline must be reduced by about 90% down to an average level of 30 wppm.

The major finished gasoline component in Europe and North America is full boiling range fluid catalytic cracked (FCC) gasoline. It is documented that FCC gasoline contributes about 98% of the sulfur that is contained in today's finished gasoline. The octane of full boiling range (C5 - 400 ° F) FCC gasoline comes from the olefins in the light FCC portion (C5 - 158 ° F) and the aromatics in the heavy FCC portion (158 - 400 ° F). Therefore, hydrotreating or EXOMER^SM extraction technology can be used to reduce the sulfur of the heavy FCC gasoline. Since olefins are destroyed by hydrotreating, it is not an economically acceptable process for the light FCC gasoline portion. Fortunately, for the light FCC gasoline, the sulfur can be removed via caustic treating, which does not affect the olefins in any way. For the heavy FCC gasoline, EXOMER^SM extraction technology removes the sulfur without destroying the olefins and significantly reducing the octane of the finished product.

Our presentation will describe the types of sulfur compounds present in light FCC gasoline, the sulfur removal possibilities with caustic and a process description of a light FCC gasoline extractive caustic treating system. Commercial experience of seven actual sulfur reduction operating units will also be described. Methods of caustic treating recombinant mercaptans produced by hydrotreating heavy FCC gasoline streams will also be discussed.

2. Presentation Overview

This presentation will cover the following items:

• Sulfur distribution in light FCC gasoline and removal possibilities with caustic.

• Advantages of caustic treating over hydrotreating light FCC gasoline to remove sulfur.

• Advantages of Merichem¨s treating technologies in this service.

• The FIBER-FILM^TM Contactor advantage.

• Process description of a light FCC gasoline treating system.

• Annual operating cost factors for the light FCC gasoline treating system.

• Composition of effluent streams.

• Commercial experience.

• Recombinant mercaptans in hydrotreated heavy FCC gasoline.

• Conclusions.

3. Sulfur Distribution in Light FCC Gasoline and Removal Possibilities with Caustic

The distillation cut point between light and heavy FCC gasolines varies between refineries since it is set by each refiner to meet specific pool gasoline specifications. The sulfur compounds in the initial boiling range of light FCC gasoline are primarily mercaptans which are caustic extractable. As it¨s final boiling point increases, thiophenic sulfur begins to appear. Thiophene boils at 183.2 Î F. Unfortunately, thiophenes are not caustic extractable. Therefore, a well designed FCC gasoline splitter to keep thiophenes out of light FCC gasoline will be very important to the refinery of the future. Analysis of an actual light FCC gasoline with a 135 Î F end point indicate that about 97% of the total sulfur is caustic extractable as methyl, ethyl, propyl and butyl mercaptans. The predominant sulfur compounds in the heavy FCC gasoline are thiophenes and as of now must be removed by some type of hydrotreating.

Table 1 and Figure 1 show what extractive caustic treating can be expected to do as the light FCC gasoline end point varies. They also show the relative amounts of caustic extractable and unextractable sulfur in light FCC gasoline. The results show that up to 95% of the sulfur in the light FCC gasoline stream can be removed by two-stage caustic treating.

3.1 Advantages of extractive caustic treating over hydrotreating light FCC gasoline to remove sulfur

• Extractive caustic treating requires significantly less capital (90% less than hydrotreating), no hydrogen, and minimal operating costs (95% less than hydrotreating).

• Extractive caustic treating does not remove or saturate olefins. Valuable octane is maintained while in the case of hydrotreating the octane loss can be as high as 12 numbers.

• Caustic discarded from extractive treating is small in volume and is not classified as hazardous waste. It can either be neutralized on site and then processed in the refinery wastewater treating system or sent to companies specializing in sulfidic caustic disposal.

• Extractive caustic treating can make use of existing FCC gasoline sweetening equipment, which would otherwise be idled in most cases.