It is well known that carbon dioxide is a major contributor to greenhouse gas emissions and global warming, and energy consumption can be directly related to the manmade contribution of this gas. Manufacturing lithium grease-which accounted for 72 percent of global grease production last year, according to the National Lubricating Grease Institutes annual production survey-is an energy-intensive operation. Both conventional and complex lithium greases require a maximum cooking temperature of 200 to 210 degrees Celsius, compared to 175 C for calcium sulfonate complex grease or just above ambient temperatures for bentonite grease. The higher temperatures also translate into longer production time, which translates into even more energy consumption.

The study by Nynas, Stratco and Eldons measured energy consumption of industrial scale production of lithium grease using a pressurized grease reactor and then compared it to the energy used in an open (atmospheric) kettle reactor. The Stratco Contactor pressurized reactor is a jacketed vessel within another jacketed vessel, and the outer vessel contains an axial pump that circulates process fluid between the nested vessels. Turnover volume of the reactants, or the materials used to make the grease, is seven to 10 times per minute. This high mixing velocity, coupled with three times the heating surface compared to a conventional jacketed vessel, results in greater heating efficiency. Pressurized operation also helps control foaming and retain heat from vented vapors.

All process parameters were kept constant, as well as the viscosity of the base oils used to formulate the greases. The selected base oils were naphthenic oils and API Group I oils typically used to prepare lubricating greases. The naphthenic oils were hydrotreated and covered a wide viscosity range (22 to 600 centistokes), and the paraffinic oils included viscosities of 103 cSt and 218 cSt.

The total energy consumed during production, including electricity for mechanical operations such as pumping, mixing and homogenizing plus fuel for heating, was recorded for all production stages: vessel charging, cooking, cooling or diluting and homogenizing. The energy consumption for each batch was converted to normalized CO2 emissions, and savings in utilities for each of the batches were evaluated. In order to make this comparative study more accurate, the performance characteristics required by end users were also evaluated for each of the NLGI Grade 2 finished greases.

A total of eight batches of 8 metric tons each were evaluated. The base oil ISO viscosity grades 100 and 220 were chosen, since these are typical grades for multipurpose lithium greases. The production stage, time, temperature and consumption of electricity and liquefied petroleum gas were recorded throughout each batch.

The final grease product was loaded with an additive package at a typical treat rate. Throughout this study, it was critical to demonstrate that the produced greases not only met the production parameters specified in the above paragraph, but also met the required performance characteristics of a commercial grease, as required for full-scale production.

Grease Comparison

Comparing the impact of manufacturing grease in an open kettle to the Contactor for the two paraffinic based batches (Batch 2 and Batch 3), it seems that the pressurized reactor contributes to a reduction of the soap content and less risk of oxidation during the cooking stage.

Looking at Batches 5, 6 and 7, which were cooked in the Contactor, the significantly lower thickener content in the case of Batches 5 and 7 can only be related to the use of the naphthenic oils with a higher degree of solvency and higher viscosity.

Surprisingly, when the average viscosity of the base oil was increased from 100 cSt to 220 cSt, a number of the performance characteristics were improved. (See Table 1.)

Production time in the atmospheric vessel increased by a total of 225 minutes over the pressurized vessel (Batches 7 and 8).

Looking at the results, it should be noted that all grease batches produced in an ISO VG 220 base oil blend, starting with a high-viscosity naphthenic base oil in the cooking stage and finishing with a lighter naphthenic or paraffinic base oil, showed an improved yield. This means that the energy requirement, on a kilowatt per kilogram basis, will be lower. This was true irrespective of the manufacturing procedure.

When formulating the grease on a paraffinic base, this effect was not observed. In fact, there was a small energy penalty for the ISO VG 220 oil over the VG 100, which required an additional 4 percent in the cooking stage and 7 percent in the finishing stage.

Energy and Emissions

From the results obtained in this study, the electricity consumed during grease production only represents a small amount of the total system energy, ranging from 12 to 16 percent. The majority of energy is required for heating.

The supply of high-pressure steam as a utility, where available, presents a unique energy calculation, but, in this case, the source of steam generation also needs to be evaluated.

For fuel-fired systems, the CO2 emissions can easily be accounted for using a greenhouse gas protocol from the World Business Council for Sustainable Development, applicable for the energy source used by the local utility supplier. For most of the grease plants, fuel for the heating source is some sort of fossil fuel. This could be natural gas, liquefied petroleum gas, diesel, a heavier distillate or an alternative fossil fuel. Depending on the region and supplier, these sources can incorporate a sustainability factor, such as biofuel for diesel.

In order to evaluate the overall carbon footprint of the production stage, the different test batches were compared. The production in the pressurized kettle using a blended naphthenic-paraffinic base stock of ISO 220 was used as a benchmark, as this grease demanded the least energy to produce.

Average CO2 emission values were calculated using emissions per kilowatt hour for the LPG fuel used, as well as the electricity production fuel mix during the batch production period (43 percent lignite, 37 percent diesel, 12 percent liquid natural gas and 8 percent renewable).

The different test batches were compared in order to evaluate the overall carbon footprint of the production stage. Production in the pressurized vessel using a blended naphthenic-paraffinic base stock of ISO 220 (Batch 7) was used as a benchmark, as this gave the overall lowest results in terms of energy demands.

Comparing batches with the same base oil viscosity, it can be seen that, for the grease with an ISO VG 220 oil, an overall reduction in CO2 emissions of 35.2 percent can be achieved by switching from a paraffinic base oil in an open kettle to a paraffinic-naphthenic base oil mixture in a pressurized reactor.

Similarly, for ISO VG 100 base stock, the combination of a paraffinic-naphthenic base oil blend in a pressurized reactor produced a 41.6 percent reduction in CO2 emissions.

Looking at the absolute values per ton for every 1,000 tons per year of production, the annual reduction in CO2 emissions between open and pressurized kettle processes is estimated at 18 to 20 tons compared to paraffinic base oil and 23 to 25 tons compared to a naphthenic base oil.

For perspective, comparing an 8 ton batch of ISO VG 100 paraffinic base grease cooked in an open kettle against an ISO VG 220 naphthenic-paraffinic blend made in a pressurized vessel, the equivalent emissions of 1,800 kilometers of modern passenger car travel could be saved. Producing 80 180-kg drums of grease to be shipped in a container would offset the sea freight journey from Rotterdam to Singapore.

The obtained results highlight that the grease made with an ISO VG 100 base oil in an open kettle was the worst performer out of the 8 different combinations considered. Energy consumption, as well as CO2 emissions, were more than 30 percent higher than some of the more efficient options. This is partly due to the actual improvement of the yield that can be obtained by using a more polar, higher-viscosity base oil, but also to the improved heat and mass transfer rates that can be obtained in the pressurized process.

In many of the cases, this 30 percent energy savings results in a reduction in overall production costs and up to 50 percent reduction in production time. An additional benefit that manufacturers can take advantage of is the increase in production capacity and reduction of production costs that can be found by optimizing parameters that relate to the type of base oil used.

Mehdi Fathi-Najafi is senior technical advisor and group specialist with Nynas AB. He has more than 20 years of experience in the base oil and lubricating grease industry. Contact him at mefa@nynas.com.

Andreas Dodos and George Dodos of Eldons SA and John Kay of Stratco contributed to this article.

Further Reading

Fathi-Najafi M., Kay J.; Moving Forward…can lubricating grease be produced in a more responsible way? NLGI Spokesman Vol 82, No. 6 Jan/Feb 2019, Page 22-28.