TECHNOLOGIES AND METHODS AVAILABLE FOR RECYCLING USED OILS

Alan Gressel, Research Oil Company

I will approach my subject, Technologies and Methods Available for Recycling Waste Oil, from three perspectives this afternoon. They are: 1) where should the recycling take place?; 2) for what purpose should the waste oil be recycled?; and finally 3) what are the technologies that will best accomplish the desired level of recycling?

1. Where Should the Recycling Take Place?In general, the earlier in the life of a lubricant or metalworking fluid that it is exposed to recycling technology, the easier the job and the better the results. In the simplest case, a full flow or side arm filter attached to a machine sump provides for continuous removal of solid contaminants. The removal of these contaminants improves lubricant condition as well as reducing oxidation and emulsification, both of which may be catalyzed by small solid particles. Oxidation is more heavily influenced by metallic particles. Emulsification and the formation of rag layers are enhanced by the presence of organic fibers.

Filtration is rarely a complete recycling solution. Inevitably, moisture will accumulate in oil reservoirs, and "soluble oil" emulsion will deteriorate from the effects of tramp oil contamination and bacterial action. Portable oil recycling equipment, including a heat source, combined with vacuum, to lower boiling points, and pressure filtration can be brought to the factory floor. These units are employed to remove both water and dirt from the oil in recirculating systems, restoring these lubricants to a useful condition.

Even with the application of the best fluid maintenance practices, operating conditions will eventually result in oil deterioration. The spent fluids can be collected for a more thorough recycling in a central unit on the plant site. This type of unit can be permanently installed and operated by plant maintenance or plant engineering personnel, or complete recycling can be provided at the factory door by a truck-mounted, portable unit operated to recycle oil. These units should include, at the least, distillation and filtration components. In some instances, the addition of adsorption and/or chemical treatment to these systems, fixed or portable, will be necessary to insure lubricant quality substantially equivalent to virgin lubricants.

The maintenance and recycling of oil water emulsions follows much the same pattern as the in-house maintenance and recycling of oil-based lubricants. Filtration removes dirt from either, although the selection of the appropriate filter media may not always be the same. A centrifuge is often used to remove water and dirt from a petroleum product. It can also be configured to remove tramp oil, the light phase, and solids from soluble oil. Much of what actually takes place in the centrifuges operating at Research Oil is the separation of lubricating oil from oil-in-water emulsions with the actual emulsion breaking taking place in the next step.

Once it is determined that the waste oil must be sent off-site for recycling, the treatment of petroleum based and water based products diverge completely. As much water as possible is removed from the emulsions at the earliest point in the recycling process, at the factory or in the first step at the off-site recycling facility. From this point on, the aqueous phase is treated as wastewater. The petroleum phase is recycled, using one or more of the technologies described here. The recovered oil may be reformulated as a soluble oil.

In my 40 years in the oil recycling business, one thing, at least, has remained constant. The customer's expectationsfor the quality of recycled oil rises as the recycling process moves further from the factory. Fluid maintenance is fine for the factory floor, but much more is expected of oil recycled off site. When waste oils have suffered substantial dilution, contamination, and deterioration, effective recycling requires the application of a number of unit processes and is usually carried out in a substantial installation away from the factory. The customer, whether he is reusing oil refined from his own waste, or purchasing a re-refined base stock, expects virgin quality from a commercial re-refining facility. This complete recycling can also be done on site in transportable equipment.

2. For What Purpose Should the Waste Oil Be Recycled? Sufficient technology exists to re-refine quality base stocks from used oils that have been exposed to the most severe operating conditions during their service life. It is less obvious that waste oils which have been contaminated with other waste materials during or after their normal service life can be re-refined for their original purposes. However, even highly contaminated waste oils can be recycled for useful purposes, either as metalworking lubricants (neat or soluble) or as liquid fuels.

Crankcase drainings re-refined into engine oil base stocks represent the classic case of re-refining. The technology of this process is reviewed below. Modern engine oils are compounded with viscosity index improvers, pour depressants, detergent dispersants, and anti-wear additives. In service, they are exposed to flame and heat, contamination from fuel and moisture, and from combustion by-products, oil degradation products and airborne dirt. No wonder few have attempted to make a commercial success of engine oil re-refining.

Hydraulic oils, turbine oils, and other lubricants drained from central systems are exposed to far less heat and contaminants during their service life. They are much better candidates for cost-effective re-refining. In my own business experience, I have been involved in the successful recycling of such widely diverse industrial lubricants as transformer oil, quench oil, forging compounds, sulphurized cutting oils, highly chlorinated drawing oils, and synthetic lubricants made from glycols and phosphate esters. These programs to re-refine and reuse special compounds for their original use all depend on the exclusion of other lubricants as major contaminants of the used oils.

Even as we accept reuse for the original purpose as the highest and best use of waste oil, we must recognize it is sometimes virtually impossible to avoid the dilutions that can make this approach impractical. A machine tool using a medium viscosity central system lubricant and a lower viscosity metalworking lubricant which mix during operations because of leakage may well result in a good quality used oil which cannot be re-compounded after re-refining for reuse in either service because the similar distillation curves of the two products resist their separation.

The example cited above is only one of the reasons that most used oil is recycled as fuel, not as lubricants. Economic issues play as great a role in the decisions to reuse waste oil as fuel as do the technical hurdles of re-refining to virgin oil quality. Many materials found in used oil and deemed unacceptable components of quality base stocks provide as many BTU's per pound as do the lubricants with which they are mixed. Gasoline, mineral spirits, diesel fuel, animal fats, and many additives made from hydrocarbons must be removed in lubricant re-refining but are acceptable components of liquid fuel.

re-refining waste industrial oil for reuse as lubricants can best be practiced by isolating the individual product types after use so that the re-refining process can address the specific nature of the contamination and degradation of each type of lubricant. Dirt, water, additive depletion, and partial oxidation can be addressed individually or in combination by the selection of the unit processes to be employed in re-refining. Crankcase oil has been demonstrated to be a quite uniform raw material and can be collected in sufficient volume to support the complex re-refineries necessary for its successful re-refining. Where generators are unable or unwilling to manage waste industrial lubricants as individual streams, the combined waste material offers potential as a raw material for both neat and soluble cutting oils. In many cases, the mixed used industrial oil can be re-refined into marketable base stocks for blended lubricants. Where the equipment or the interest in re-refining is lacking, waste industrial oil can be processed into marketable liquid fuels, generally after mixing with waste crankcase oil to reduce the percentage of sulfur and chlorine.

Whatever the reuse of waste oil, it replaces crude oil in an equivalent amount. Although the raw material and energy balance studies inspired by environmental concerns and the fear of another energy crisis are now aging, it is still reasonable to assume that a gallon of used oil burned for energy recovery replacing a gallon of virgin fuel oil provides a reasonable balance with the raw material and energy required to re-refine the same gallon of used oil into a replacement for virgin lubricant. re-refining results in the loss of up to one third of each gallon in the process and requires the application of considerably more energy to the re-refining process when compared to the much simpler process of recycling the used oil into fuel. This general observation is not applicable to the recovery of good quality waste industrial lubricants for reuse for their original purpose. It does apply, however, to the re-refining of crankcase oil even with the most modern technology.

3. What Are the Technologies That Will Best Accomplish the Desired Level of Recycling? In understanding re-refining technology, I have always found it convenient to conceive of a lubricant as a large body of individual molecules, an army of soldiers. Our molecular army has specialists, additives, and many foot soldiers, petroleum molecules. During the heat of the battle, our army is sometimes infiltrated by spies and enemy soldiers, contaminants. During the battle, some of our soldiers are wounded, partially oxidized, and some killed, oxidized into varnish or sludge. However, most of our soldiers survive the battle unscathed. As we design the re-refining process, it is clear that we must come up with a system which will remove the spies and the enemies as well as our own dead. We also need a process to heal the wounded so as to minimize our battle losses.

In discussing, in-plant recycling of used industrial oil and the possible end uses of recycled oil, we have already touched on simple recycling technology and equipment. At this point, I want to deal with the technology of re-refining crankcase oil in some detail because it encompasses all the available technology that can be used for any industrial or automotive oil recycling.

In the early years or re-refining, prior to 1970, the re-refining industry was satisfied to concentrate on removing oxidized and partially oxidized molecules as well as the contaminants before returning the recycling used oil to service. Modern re-refining technology is also capable of healing the partially oxidized molecules which once would have had to be removed. The simplest way to consider the technology available for re-refining used oil is to divide the re-refining process into three parts. The central part, a distillation or a distillation and fractionation step is the heart of crankcase oil re-refining operations. Clearly, this process may not be required to re-refine industrial lubricant from used industrial oil. However, its centrality to the design of the re-refining process requires beginning consideration of the unit processes that are available for oil re-refining with the distillation step.

In the early years of re-refining, 1920 to 1960, when multi-vis oils were not ubiquitous and the typical customer for a re-refined oil drove an automobile that burned a quart every thousand miles or less, the demand for re-refined motor oil was for an SAE 30 viscosity oil. An SAE 30 base stock, with a flash in excess of 400В°F, was obtainable by distilling the fuel contamination from the essentially non-detergent single grade used oil collected at the time. When this distillation was performed in pot stills using steam, it was possible to avoid the coking and fouling problems that have beset many modern re-refineries.

As the additive package in engine oil grew and the viscosity of the base stock into which the additives were blended dropped, a new re-refining process became necessary. Distillation of the base stock from the spent additive package was the ideal solution. Along with distillation for removal of moisture, fuels, and solvents, this process solution made distillation the central unit process in re-refining. Unfortunately, translating this simple theoretical solution to operating realities was far from simple. Although our healthy molecular soldiers can be distilled with relative ease, the enemies, the wounded and the dead turn to sludge and varnish when exposed to the temperatures required for distillation. The resulting sludge plugged heat exchangers, distillation towers, and piping, effectively shutting down these early plants.

Although it is over 30 years since my first experience with a plant designed to re-refine engine oil that included a distillation tower instead of a pot still, the shock upon opening the unit after only a few hours of operation is still vivid in my memory. The difficulties in distillation of used oil were addressed in two ways. The first solution was to design a distillation unit that can evaporate crankcase oil so quickly under such high vacuum and low temperature that the deposit of sludge and varnish can be minimized, if not eliminated. This quest led to the application of thin film evaporators, wiped film evaporators, the introduction of the crankcase oil through a tangential spray nozzle into a high vacuum distillation unit and, I am sure, solutions not known to me.

The second solution was to devise pretreatment systems that would separate healthy soldiers, the petroleum molecules from some, if not all, of the dead, the wounded, the spies, and the enemies. Early re-refining technology accomplished this pretreatment by adopting the acid treatment process from early oil refineries. This process had been applied to the production of motor oil from Pennsylvania grade crude and is still used in modern times in the production of white oils. As applied to used oil, both industrial and automotive, concentrated sulfuric acid acts as an oxidizing agent, preferentially attacking unsaturated and partially oxidized molecules. This process became more and more problematic as the detergent dispersant and acid neutralizing additive packages grew to be an ever increasing portion of the used oil feed stock.

Over the past 30 years, acid treatment has been virtually abandoned as a part of crankcase oil re-refining. It is still a useful treatment in many industrial oil recycling programs including ash reduction and emulsion breaking.

Intensive research by government and industry, both in the U.S. and Europe resulted in the development of automotive pretreatment methods for re-refining crankcase oil which can be broadly divided into two types.

The first, solvent extraction, involves the mixing of the used oil stream with another liquid in which it is partially soluble. A portion of the used oil dissolves into the solvent selected, leaving an insoluble residue. After the insoluble residue is separated from the bulk of the used oil and the solvent, a distillation step is used to recover the solvent. The used oil is now free of a substantial portion of the undesirable molecules, the metals, the polymers, and some of the oxidized material. In some processes, gravity settling is used to remove the undesirable insolubles. Centrifugation and filtration are other techniques available to enhance the separation.

Liquefied propane has been used as the extraction fluid in both Europe and Africa. Propane is also the basis of a pretreatment system developed by Interline Resources of Alpine, Utah. A process employing a mixture of three organic solvents was developed by a US Department of Energy laboratory then known as Bartlesville Energy Technology, and now known as the National Institute of Petroleum Energy Research. As far as I know, this process has never been commercialized, but it worked at pilot scale in our lab.

An alternate approach to pretreatment is based on the concept of moving the undesirable components of the used oil from their oil soluble state into a water soluble state. Often these contaminants are molecules that result from the reaction during service of a metal with an organic molecule. These processes generally involve the treatment of used oil with an inorganic chemical so that the organo-metals present in the used oil from heat, metalworking and from additives are induced to dissolve into the aqueous phase during treatment. After the undesirable portions of the used oil have moved into the water layer, the water layer and sludge can be removed by gravity settling, centrifugation, or pressure filtration, depending upon their physical state after treatment. Both inorganic pretreatment and solvent extraction are used in sequence in some re-refineries.

Although the pretreatment steps currently commercially available have made recycling more efficient and substantially lengthened times between turnarounds for the distillation units which they protect, there is still room for improvement. Research on better methods continues. This work spans a broad spectrum of potential solutions. On one end of the spectrum, companies, both very large and very small, have approached used oil as a refinery crude stock that can be beneficially recovered avariety of fuels through the application of heat. A typical process employs distillation, followed by thermal and catalytic cracking and ending with a solid coke-like residue in the process unit. These destructive distillations are carried out in small package units designed to be installed in an automotive garage, in mini refineries, designed to process a few thousands barrels a day, as well as in the delayed cokers of large refineries. At the other end of the spectrum are enzymes extracted from bacteria which preferentially consume organo-metals, effectively removing ash from used oil.

Our industry is fortunate in that the level of research into technology to improve used oil re-refining is probably out of proportion to the economic benefits that could possibly accrue to even the most successful inventors.

Even when one combines the best pretreatments available with the most sophisticated distillation techniques, the resulting oil is still not up to the requirements for reuse as a base stock in modern blended lubricants. Small amounts of metals are often still present. Some molecules are still present that are too unstable to survive the expected service life. If not removed, some of these molecules will catalyze further oxidation and deterioration. Nitrogen, phosphorus, chlorine, and sulfur may also be present in amounts in excess of acceptable limits. The partially refined oil will probably be too dark in color to meet consumer acceptance standards.

Ail in all, more refining is still necessary to finish the process, even after pretreatment, distillation, and fractionation. re-refining practice followed early refining technology and adopted the use of fuller's earth as a finishing step. Percolation and clay contacting have both been used to remove undesirable color bodies and extraneous elements from re-refined base stocks. Although I am unaware of any facility which still uses percolation, clay contacting is still a widely used technology in re-refining of both engine and industrial lubricants. Over the years, other types of clay have been developed to supplement and replace fuller's earth. In my own experience, acid activated bentonite clay has proved versatile, although it is far better for color improvement than for reduction of odor or acidity.

In modern re-refining practice, hydrotreating has replaced clay as the ideal finishing technology. Hydrotreatment involves the replacement with hydrogen of the oxygen, nitrogen, phosphorus, sulfur, and chlorine atoms in petroleum molecules. This process requires heat, pressure, and a catalyst. The replacement of clay adsorption with hydrotreating demands effective pretreatment to eliminate the molecules that tend to poison the catalyst. Long catalyst life is a key to effective cost control in re-refining. Although guard beds to protect the catalyst are designed into the re-refining process, care must be taken in preparing hydrotreating feel stock.

In re-refining engine oil, the three components of the re-refining process must be carefully designed as an integrated whole. The more effective the pretreatment and distillation steps, the less demands will be made on the finishing step and the longer catalyst life will be extended. Specific parts of the idealized re-refining process are, or can be, employed in re-refining spent industrial lubricants.

Current state-of-the-art re-refining is not a perfect solution to the problem of recovering all the potential value in used motor oil. In a perfect world, the re-refining process would recover the expensive additive package as well as the base stock. This would require the separation of the additives from the contaminants and the by-products of oil degradation that are held in suspension in the oil when it is drained. I know of some development work that points in this direction.

For instance, UOP has developed a re-refining process in which used oil is exposed only to a hydrogen atmosphere from the beginning and throughout the re-refining processes. This technology is effective in dechlorination, including breaking down PCB's. The products of the UOP process maintain the high-viscosity index of the used lubricant. Before you rush out to order this system, please remember that recovering the viscosity index improvers in tact does not necessarily imply oxygen stability and detergent quality. It does, however, point to a direction that one day could produce a re-refined product worth twice as much as base stock. The new high viscosity index base oils made by hydrocracking will eventually lower the additive package, ease re-refining, and increase re-refining base stock value.

Fractionation of re-refined oil is carried out in modern re-refineries near the end of the process producing two, three, or even four base stocks of different viscosities. Unfortunately, the ratio of these base stocks is limited by the nature of the original engine oil blends. Viscosities in excess of 100 seconds are hard to come by.

Why spend so much time on the re-refining of engine oil in front of ILMA, whose prime business is providing specialty industrial lubricants? Crankcase oil has survived the most severe use and contains the widest range of contaminants. A complete understanding of what can be done to recover value from motor oil helps one to select the right sequence of unit processes to most cost effectively recover quality lubricants from other used oils that have not suffered the full range of stress to which motor oil is exposed. Neutralization, filtration, centrifugation, liquid/liquid extraction, distillation, fractionation, adsorption, and hydrogenation represent the unit process choices. Each can play a role, often the sole required role, in the recovery of a quality lubricant from used oil. Approached in this manner, the re-refining process depends on the contaminants. Of course, economies of scale and the cost-effectiveness of the unit process will determine the viability and profitability of any re-refining venture.