The Sticky Problem of Viscous Additives
We all know that the entire lubricantindustry is moving toward lower viscosity fluids, right? Wrong. There is one area – additives packages, mainly for crankcase lubricants – where an almost perfect storm of requirements has pushed viscosities higher. This is causing headaches for some.Trevor Gauntlett investigates.
Lower-viscosity crankcase lubricants improve engine efficiency, lowering greenhouse gas emissions and reducing local air pollution. It would be logical to assume that the viscosities of all the major ingredients in a liquid lubricant would also be trending in the same direction, but this is not the case.
While base fluid viscosities are certainly dropping as automotive lubricants move to lower viscosities, the same is not true for additive packages, namely detergent-inhibitor, or DI, additive packages. The consequences can be significant, particularly for plant engineers at older lube oil blending plants that do not have the requisite equipment to cope with this change.
In many cases it is possible to blend lubricants at ambient temperatures, Chris Cowell, an independent lubricant industry consultant from the United Kingdom, told LubesnGreases. However, there is a trend to supply performance additive packages to lube oil blending plants that are increasingly viscous.
Although higher-viscosity additive packages lower overall supply chain costs by reducing the amount of base oil diluent that they are shipped in, this can present significant problems at the blending plant.
Some performance additives are so viscous that attempting to blend them at ambient temperature would result in unacceptably long blend cycle times. Consequently, particularly viscous performance additives are heated to above ambient temperature, either in a drum oven or in a bulk storage tank to assist in the mixing process, said Cowell. The blend cycle time is the time between blends in the same piece of equipment.
This situation is exacerbated in colder climates, according to another U.K.-based independent consultant, Charles Djaelani, who told LubesnGreases that, where temperatures dip well below zero, additives become more viscous, resulting in increased product transfer times.
As they comprise the biggest molecules, dispersants usually account for the greatest viscosity lift, or increase, among the components of the DI package. Obviously, a requirement for good dispersancy in a lubricant means the package contains a lot of dispersant. But that is not all. Sometimes, chemical interactions between the components of the package – usually the detergent and dispersant – are so strong that aggregates form, further adding to viscosity.
Nowadays, the major additive suppliers have a much better understanding of those issues, according to Phil Reeve of ADLU Consultancy, a lubricant and additive consulting company based in the U.K.
The key factor to minimize detergent-dispersant interactions in a package is to keep them well apart in the blending process. This is usually achieved by adding the detergent with sufficient base oil at the start of blending and adding the dispersant towards the end, or vice versa, Reeve told LubesnGreases.
Additives companies can therefore produce concentrated DI packages that are stable (they have no haze or sediment) and do not have strong interactions between ingredients.
A second factor comes into play, and that is lower recommended handling temperatures. This has come about partly due to the risk of accidental emission of toxic gases when blending plants heat sulfur-based additives, as well as changing additive chemistry.
It has been known for many years that overheating sulfur-containing additives, such as some extreme pressure and antiwear compounds, can result in bad smells. However, an incident at a blending plant occurred when heating one common additive. The suppliers safety data sheet stated the additive was stable up to 70 degrees Celsius, but it was found to release lethal hydrogen sulfide when heated to less than 40 C.
The lower recommended handling temperatures have affected storage practices but have not extended blend cycle times, since the higher viscosities of the extreme pressure and antiwear additives or packages at lower temperatures were still comfortably within the operating ranges of the blending equipment, mainly due to the absence of dispersant.
The big impact came when the additives suppliers reviewed their recommendations for handling DI packages. Maximum handling temperatures dropped to 70 C from 90 C or even higher. Common perceptions were that this was due to the thermal stability of zinc dialkyldithiophosphate.
The additives companies are very safety conscious, so dont want to risk being seen to be the cause of an incident, said Reeve.
However, David Whitby of Pathmaster Marketing, a downstream energy industry consulting company based in the U.K., told LubesnGreases that these recommendations were not solely due to the thermal stability of ZDDP.
Some newer friction modifiers are heat sensitive, and newer antioxidants are designed to work at lower temperatures, Whitby said.
Changes in the additives required to enhance performance, such as faster formation of antiwear films or better oxidative stability of the lubricant overall, meant that the components of a new-generation lubricant had to be handled at lower temperatures than the previous generations.
The lower recommended temperatures meant that electrical trace heating was less desirable, as this could lead to localized hot-spots in transfer lines and blending vessels. Trace heating uses a resistant element that runs alongside the piping or vessel to control fluid temperature. If pumping stops or stirring is inefficient, the fluid does not move away from hot spots and can therefore degrade. Whitby said that he never recommends electrical trace heating because of this issue.
Control systems should prevent overheating of fluids in those circumstances, but as Cowell pointed out, there have been reports of accidental hydrogen sulfide emissions from plants, either as a consequence of the failure of a temperature cut-out system leading to uncontrolled heating of the additive in a bulk storage tank or overheating in a drum oven.
Extending Blend Cycle Times
Higher-viscosity additives have a double-negative effect on blend cycle times. Firstly, it can take longer to pump the additives into the blending vessel or through the in-line blender. According to Djaelani, fluid transfer systems are typically designed for a given fluid, operating temperature, available pressure at the suction head and required pumping pressure in order to deliver the fluid to the required location. If the pumps are required to move different and more viscous additive packages, operational issues could arise including inefficient pumping and pump cavitation, whereby the liquid does not flow in the tank, so air passes through the pump. These can lead to shorter maintenance cycles or even pump failures.
The second effect is on the time it takes to achieve a well-dispersed blend. A cooler additive brings less energy into the blending vessel, and energy must be added to break up the brown, sticky package and disperse it at the molecular level in the same time as before. Newer blending plants can achieve this by heating the base oil before it reaches the blending vessel or heating the blending vessel or introducing greater shear into the mixing process. However, older plants may not have the facilities carry out any of these options safely.
Heating, of course, adds to plant costs and can also be potentially hazardous if not controlled, said Cowell.
While a plant may be able to raise the temperature of the base oil storage tanks, they run the risk of gradually oxidizing the base oil before they blend the product.
Of course, given an infinite budget these issues can be overcome at the plant, but it can be difficult to gain approval for capital expenditure. What else can be done?
A relatively simple operating solution is to introduce a recirculation line on the blending vessels. A simple branch allows the high shear pump used to empty the tank to instead return fluid to the tank. This will assist the dispersion of the additive package.
A few plants have the capability to cut back, or dilute, an additive on receipt, but this requires them to have a tank available and the necessary transfer lines to get the high-viscosity additive from the truck or railcar into that tank. But the extra dilution stage adds one more potential weak link in a plants quality control process.
All those interviewed say that more consultation is necessary. According to Djaelani, closer cooperation with additive suppliers to ensure … additives meet the design viscosity specified for existing equipment in the plant is required, and the cost-saving in treat rates should be balanced against any expenditure to upgrade existing equipment. This requires a discussion between product management, plant operations and engineering.
Reeve accepts that many customers may feel they cannot have such a dialogue with a supplier selling an off-the-shelf additive, but said, It is essential for the customer to ensure the additive supplier knows what their blending plant capabilities are. The additives suppliers not only want to help their customers , they [also] want to be seen to be going that bit further. It helps them retain business.