In spite of the claimed advances in alternative energy sources, petroleum-based fuels are still, and will be for the foreseeable future, the main source of energy used to produce our electricity, heat our buildings, and power our vehicles. However, many governments are restricting the amounts of contaminants, sulfur, NOX, particulates, and/or aromatics and olefins, a fuel may have in order to assure the public a cleaner atmosphere when these fuels are burned.
Hydrogen treating has been the process of choice to remove the contaminants from almost all of the fuels manufactured today. Gasoline boiling components are often hydrogen pretreated using non-noble metal catalysts to remove sulfur, olefins, and nitrogen before they are catalytically reformed, using noble metal catalysts, to convert low octane materials into high octane components. Diesel Fuel has historically had the sulfur removed by using hydrogen in the presence of a catalyst, non-noble metal. This process is known as hydro-desulfurization (HDS), and is carried out at elevated temperatures, >700°F, and pressures, >150 psi. However, now that trace amounts of contaminants must be removed from the fuels, hydro-desulfurization alone becomes difficult, i.e. increased pressure and/or temperature.
In order to solve the problem, SK studied the contaminants in the feed that were causing the hydro-desulfurization to be so difficult. It has long been known that nitrogen compounds in the diesel feedstock inhibit the desirable catalytic reactions. They are considered temporary poisons. These compounds can be difficult to remove in a hydro-desulfurization unit, and require extremely high pressures and temperatures to accomplish their removal. Accordingly, the SK Corporation developed an adsorption type process to remove about 90% of the undesirable nitrogen compounds from the diesel feedstock which would normally go directly to a hydro-desulfurization unit for sulfur removal. The pretreated diesel feedstock, with nitrogen compounds removed, can easily produce lower sulfur in the refinery´s existing HDS unit. This is accomplished by using a special adsorbent at near room temperature and at modest pressures. When the nitrogen content of the feedstock is reduced before hydro-treating, the HDS process becomes more efficient and the final sulfur reduction is significantly greater, enabling the production of ultra-low sulfur diesel (ULSD). Process patents have been issued.
Nitrogen removal prior to hydro-desulfurization provides many benefits to the refiner. His final diesel product will have
In addition, the HDS unit will show the following:
- Little or no revamping
- Little or no downtime
- Less hydrogen consumption because of contaminant removal
- Same or longer stream time at lower sulfur levels
- Reduced poly-nuclear aromatics in the HDS product
The SK HDS Pretreating Process unit can be located any place within the refinery limits.
Here´s how it works:
The process uses identical twin adsorbers to permit continuous adsorption during the three major steps involved: adsorption, purging, and desorption. The raw diesel feedstock is pumped into the on-line adsorber where natural-polar compounds (NPC) are deposited on the adsorbent thus removing most of nitrogen-containing compounds from the feedstock. Meanwhile the other adsorber is being purged and desorbed. The de-nitrogenated product from the on-line adsorber goes to a solvent recovery section for separation and recovery of the pretreated gas oil and desorption solvent. Following the adsorption step, and prior to starting the desorption step, the adsorption bed is purged with previously-stored NPC to minimize the product yield loss. When purging is complete, the adsorber is desorbed with solvent, and ready to start another cycle. Each step is controlled by a group of automatic timer valves located in lines into and out of the adsorbers.
The solvent-NPC mixture from the adsorber has its own separate recovery system to separate the NPC and solvent.
The unique operation of this adsorption system is that both adsorbers are liquid-filled at all times. The adsorbent is not removed from the adsorber. SK´s tests show that the adsorbent should last for at least one year -- nearly 3000 3-hour cycles -- with no apparent loss in activity or selectivity.
The entire system is simple and cost-efficient. The solvents currently used are common, commercially available, reasonably priced compounds. The process units can be operated by one person per shift. A large demonstration plant is being designed and built in Korea and expected to be operational in mid-2002. more about this technology