New Kerosene Mercaptan Oxidation Process

This article describes the new kerosene mercaptan oxidation process recently installed at the Elf, Feyzin Refinery in France. The system employs a patented non-dispersive FIBER-FILM™ Contactor and a carbon bed impregnated with oxidation catalyst licensed by Merichem Co. as MERICAT II Technology. Operating and design data are also presented.

Guy Francoise, Elf France and Tom Varadi, Merichem Company
Reprinted from Hydrocarbon Technology International 1993

This project was undertaken to increase jet fuel sweetening capacity while minimizing capital costs and required plot area. Merichem designed the 38 m³/Hr (5736 BPSD) jet fuel/kerosene mercaptan oxidation unit in December of 1990. Elf's subcontractor, Inter G Solytec, developed the detailed design and built the system. The unit was started up in December, 1991.

The system design employs Merichem's proprietary MERICAT II technology to remove any naphthenic acid breakthrough from an upstream conventional mixer settler prewash and to oxidize mercaptans contained in the kerosene stream.

Elf's Kerosene Treating System

Raw kerosene from the crude unit passes through a conventional caustic prewash to remove most of the naphthenic acids before going to an intermediate storage tank. The kerosene is then pumped on flow control to both the new Merichem MERICAT II system and an existing UOP Merox system after preheating. The two systems operate in parallel. The Merichem system consists of a single vertical vessel with a dual-purpose Film-Film contactor that removes final traces of naphthenic acids and oxidizes part of the mercaptans. The remaining mercaptans are oxidized by a catalyst-impregnated bed of activated carbon.

The sweetened kerosene from the MERICAT II and Merox systems is split into three streams for final clean up after cooling. Each clean up train includes a conventional vertical water wash followed by a salt drier and clay filter. The treated kerosene is then pressured into a jet fuel storage tank.

Figure 1 is a schematic flow diagram of the MERICAT II process at Elf. It uses sophisticated non-dispersive liquid/liquid contact technology to remove trace naphthenic acids and oxidize part of the mercaptans. Contact is achieved by cocurrent flow of the two phases along ultra-fine metallic fibers packed in the Film-Film contactor.

Specially Designed

Water-saturated preheated kerosene enters the MERICAT II system through one side of parallel 150-micron basket strainers, BS-1 and BS-2, which remove any solids. Oxidation air is added to the kerosene through a specially-designed air sparger, SP-1, that helps dissolve the air in the hydrocarbon. The kerosene is then introduced to the top of the Film-Film contactor, FFC-1, in the MERICAT II separator vessel, V- 1. In the Film-Film contactor, the kerosene contacts a recycled caustic-oxidation catalyst solution constrained on a myriad of ultra fine proprietary metallic fibers.

The air-bearing kerosene and recycled caustic-oxidation catalyst solution flow downward through the contactor concurrently. In the contactor naphthenic acids, mercaptans and oxygen in the kerosene diffuse across the minute distances between caustic-wetted fibers and enter the caustic-catalyst interface. The caustic extracts some of the light primary mercaptans which are then oxidized by air and catalyst to the corresponding disulphide compounds (butyl-mercaptan to dibutyl-disulphide and so on) which are relatively insoluble in caustic and immediately re-enter the kerosene. Thus, the caustic is free of mercaptans (regenerated) as it leaves the Film-Film contactor.

The kerosene leaving the Film-Film contactor (liquid/liquid) section of the MERICAT II system contains traces of caustic, which helps maintain the alkalinity of the downstream carbon bed. Upon separating from the caustic in the outlet of the Film-Film contactor, kerosene flows upward and enters the top section of V-1 through a Johnson screen grid which supports the carbon bed. The kerosene then flows up through the catalyst impregnated carbon bed for final sweetening of the heavier and branched mercaptans. Treated kerosene exits the top of V-1 and flows to the water wash, salt drier and clay filter finishing processes which insure that the jet fuel meets product specifications for water separation (WSIM) and filterability.

The formulas in Table 1 depict the mechanisms taking place in the MERICAT II system:

Table 1.

The caustic solution leaving the Film-Film contactor adheres to and flows down the fibers until it reaches the caustic layer in the bottom of V-1. The caustic solution is then recycled by P-1 or P-2 from the bottom of V- 1 to the top of FFC-1. The caustic solution is reused at a rate approximately equal to 20 volume me per cent of the kerosene rate.

Efficient Treating

Fresh 3-5°Be caustic is continuously charged to V-1 by metering pumps, through basket strainers, BS-3 and BS-4. Spent caustic is continuously removed from V-1 on level control and reused in the existing prewash for final spending. This continual addition of fresh caustic to the MERICAT II system maintains a fresh recycle caustic quality to insure efficient treating.

The complete removal of naphthenic acids from the kerosene before it enters the MERICAT II carbon bed prolongs the sweetening efficiency of the carbon bed and minimizes downtime. In systems employing a conventional prewash followed by a fixed-bed sweetener, the quantity of naphthenic acids still present in the feed to the fixed-bed plays a major role in how often the bed needs to be recausticized, hot water washed, and reimpregnated with catalyst.

Sweetening

Oxidation catalyst is added to the system manually via the catalyst addition pipe, SP-2, to maintain an active catalyst concentration in the recycle caustic solution. This insures efficient sweetening in the Film-Film contactor section of the MERICAT II.

As mentioned earlier, the carbon bed section of the unit continually receives some minimal alkalinity from the kerosene. This small amount of caustic is usually enough to promote sweetening; however, the system is also designed so that the carbon bed can be recausticised while still on stream to maintain an efficient level of alkalinity. The Film-Film contactor prewashes the kerosene so well that the carbon bed section treats kerosene containing little or no hydrogen sulphide and naphthenic acids. Therefore, the carbon bed seldom requires recausticizing, hot-water washing or catalyst reimpregnation. Catalyst reimpregnation is necessary no more often than once a year, while the life of the carbon is projected to be five years.

Reversed

When the carbon bed is recausticised, kerosene flow to V-1 is reversed on-line. This process is performed monthly. With kerosene flowing downward, the recausticization solution is routed to the caustic distributor above the carbon bed. The circulation rate is set at 9 m³/hr. By recycling in this manner with 21°Be caustic, the alkalinity of the carbon bed is restored without interrupting the treating operation. After causticizing, the kerosene flow is reversed again to its normal direction, upward through the carbon bed.

The carbon bed can be reimpregnated with catalyst during the annual turnaround. A mixture of monoethanolamine and water is used to distribute the catalyst through the carbon bed. The carbon bed first must be thoroughly washed with hot water. After the bed is washed, the vessel is filled with the MEA/water mixture, which then is recycled while catalyst is added to the recycle line via the catalyst addition pipe, SP-2.

Merichem's treating systems are virtually maintenance-free and have a much higher on-stream factor than the units providing their feedstocks. The only moving parts are the pumps, which are 100 per cent spared. Film-Film contactors are normally inspected and water-washed during turnarounds.

Basket strainers require cleaning very infrequently. Elf has cleaned the kerosene strainers once in four months and the feed caustic strainers once in four months. Quick opening strainers allow baskets to be removed, water-washed and reinstalled in a few minutes.

The specially-designed air sparger is protected by an air filter (F-I or F-2). If necessary, the air sparger can be cleaned by backflushing with the stream being treated.