For years lubricant blenders have been scrounging for bright stock, thanks to closings of API Group I plants, which left the industry with a shortage of those heavy fluids.
Since earlier this year, a new stream of bright stock is flowing from a surprising source: Shanxi Luan Groups coal-to-liquids Group III plant in Changzhi, China.
The coal giant and nascent base oil producer said recently that it is making a Group III+ bright stock and has capacity to make 50,000 metric tons of the fluid per year. Industry analysts said Luans oil may be the first Group III bright stock to be produced on a commercial scale. They offered varying opinions about how popular it will be.
I see it as a novel and interesting product, said Amy Claxton, principal of U.S.-based My Energy Consulting & Training. The industry is thirsty for more bright stock, but there is no need for the high viscosity index that you would have with a Group III bright stock.
Luan is one of Chinas largest coal suppliers, and it operates one of several CTL facilities that have opened in the country the past few years. The Changzhi complex includes a low-viscosity polyalphaolefin plant that opened in 2018. In March, after several months of delays, the company opened a separate plant with capacity to make 300,000 t/y of Group III+ base stocks.
The bright stock produced by that plant has a kinematic viscosity of 23.5 centiStokes at 100 degrees C, Luan Group General Manager Liu Junyi said during an email exchange. Thats lighter than many Group I bright stocks - which can range between 30 and 33 cSt at 100 C - but still within the range of viscosities for bright stock. Luans bright stock also has a pour point of minus 24 C, a cloud point of 30 C, a flash point of 306 C and Noack volatility of less than 1 percent. The viscosity index of Luans bright stock is 158, far above Group I bright stocks, which typically have VIs of 95 or less.
Bright stock is a product of most Group I base stock plants and is far heavier than other base oil grades. Group I solvent neutral 500 oils, referred to as heavy grades, typically have kinematic viscosity of around 10 cSt at 100 C. Numerous Group I plants have closed the past few decades as the global base oil industry shifted to Group II and III oils. Group II and III plants do not produce bright stock because they use more severe refining processes - usually hydrocracking - which convert the heavier molecules that would be used to make bright stock into lighter fractions.
Analysts agree there is now a shortage of bright stocks, which have been popular base stocks for applications such as gear oils and marine lubricants, so there has been frequent discussion in recent years about where the industry might turn for alternatives. A few refiners have claimed to develop the ability to make Group II bright stock, but actual production has been minimal. A few naphthenic refiners have developed and are producing naphthenic base oils that are as heavy as bright stock. Analysts say polyisobutylenes have been the most popular replacement.
It is possible that Luans Group III+ bright stock will remain a novel product, according to sources contacted for this article. Several agreed that it is not practical for mineral oil Group III plants, which constitute the vast majority of Group III facilities, to produce bright stock.
Group III bright stock could only be gas-to-liquids or coal-to-liquids as you cant hydrocrack crude oil sufficiently to get a crude-derived Group III bright stock, said David Wedlock, an independent consultant based in Chester, United Kingdom. "The more you hydrocrack crude, the lower the viscosity goes, so its a diminishing returns proposition."
Other CTL or GTL base oil plants would be capable of making Group III bright stock but might not.
Any CTL base oil plant could also produce a bright stock because by nature of the Fischer-Tropsch process, there is significant wax produced that is heavy enough to make bright stock, said Kenny J. Peinado, technology marketing manager for new business development at Chevron Technology Marketing, which supplied its Isodewaxing wax isomerization technology for Luans plant.
He noted, though, that dewaxing units for CTL and GTL plants can be designed to so as not to yield bright stock. He pointed out that Chevron Technology Marketing is also working with G-TiBase Oil Technology, a joint venture between Yitai Coal and Jiyang Petrochemical Group, which is developing a CTL Group III plant in Ordos, China. That facility is being designed to produce 2, 4 and 8 cSt oils but no bright stock.
There is only one commercial scale GTL Group III plant in operation today - the Pearl joint venture between Shell and Qatar Petroleum in Ras Laffan, Qatar. It makes wax but not bright stock. Luans Changzhi complex has the only CTL Group III plant, but a few more are under development.
Claxton expressed skepticism that CTL producers would want to make bright stocks, contending that they would have a cost disadvantage compared to Group I bright stock.
Creation of waxy synthetic liquids via Fischer-Tropsch technology - whether starting with coal, natural gas or biomass - is a much higher investment and operating cost platform than traditional crude refining, she said. She also claimed there would be no advantage from the high viscosity index of Luans fluid, the characteristic most distinguishing it from Group I versions.
Another source suggested that lubricant formulators may find applications that take advantage of the characteristics of a Group III+ bright stock.
Liu said Luans lubricant business, Luan Taihang Lubricants Co., is currently using all of the bright stock produced in Changzhi in-house for its production of gear oils and hydraulic fluids. In the future, if the product can provide good benefits, I think we should be able to balance production capacity with 8 cSt oils and conduct external sales, he said.