Novel process developed to re-refine and re-cycle used oils

Paul Boughton

Mineralöl-Raffinerie Dollbergen GmbH, located near Hannover, Germany, in an area with a distinctly rural character, has been involved in the processing and recycling of used oil since 1955.

During this period, processes had to be continually developed so as to keep pace with the ever changing requirements resulting from such interrelated factors as changes in used oil composition, recycling processes/technologies, tougher requirements in respect of base oil qualities.

Today, the refinery has a capacity allowing it to process 230000t/a of used oil and oil-containing liquids. About 120000t/a is used as feedstock for the production of 70000t/a of highest quality base oils.

The MRD Solvent Extraction Process is the latest development for refining vacuum distillates from used oils. This process has been developed from a technology which has proven its worth for many years and involves the following process stages:

  • Dewatering stage (atmospheric distillation for separating water and light ends).
  • Vacuum distillation with production of a gas oil fraction.
  • Gentle flash and film evaporation to obtain a vacuum distillate cut constituting the raw base oil fraction.
  •  Refining of this cut via clay treatment as well as fractional distillation into several viscosity grades.

To improve base oil qualities, a concept for substituting clay treatment with a hydrogenation process representing the state-of-the-art at that time was developed in the 1980s. In 1989, the regulatory authorities granted approval for the construction of a hydrogenation plant.

As a consequence of new quality criteria demanded in the market for state-of-the-art lubricants in the middle of the 1990s, new studies were initiated with a view to determining an optimum re-refining process. The studies focused, in particular, on such aspects as the future developments to be expected in the composition of used oils and on ways to overcome the negative quality characteristics of the hitherto produced re-raffinates in respect of colour and odour:

  • Extreme increase in the requirements to be met by state-of-the-art engine oils. These oils are the most significant, in terms of both quality and quantity play the greatest role in both qualitative and quantitative respect, accounting for about 50percent of the used oils. Longer oil change intervals as well as demands for saving fuel and reducing emissions necessitate the greater use of non-conventional base oils (XHVI, PAO). As these base oils offer significant quality benefits in comparison with conventional base oils (mineral oils), modern technologies must aim at recovering these partially and completely synthetic components in the re-raffinates to the greatest possible extent.
  • Longer oil change intervals. The content of polycyclic aromatic hydrocarbons (PAH) in used oils has increased as a result of the longer oil change intervals. Because of the undesirable toxicological characteristics of the PAHs, the recovery process must ensure that they will be largely eliminated.
    All of these aspects showed that the hydrogenation processes are not optimally suited for satisfying the requirements to be met by a modern processing technology. Extremely severe hydrogenation conditions are required for the quantitative removal of PAHs (OSHA requirement (USA)T>=427°C,p(H2)>=56bar).

However, these conditions lead to the cracking and, thus, destruction of the valuable synthetic base oil fractions (cf. B. Essiger, H. J. Oschmann, S. Brönn, Chem. Technik47 (1995)). On the other hand, mild hydrogenation conditions will not suffice for eliminating PAHs to a sufficient extent.

In view of this situation, MRD began to develop a new process based on the a liquid-liquid extraction technology. In three years of research, process parameters appropriate for the extraction of vacuum distillates from used oils were developed, using N-methyl pyrrolidone (NMP), a highly selective solvent for the removal of aromatics and heteroatomic compounds, which is also used in processes for the refining of lubricating oils from crude oils.

The novel process developed by MRD completely satisfies the requirements existing for a modern technology for producing from used oils high-quality base oils, the properties of which in some respects are even superior to those of the classical solvent raffinates produced from petroleum (eg viscosity index, low-temperature properties, aromatic hydrocarbon/PAH contents):

  • Quantitative elimination of polycyclic aromatic hydrocarbons (sum of PAHs after according to Grimmer <1mg/kg).
  • High-quality semi-synthetic and fully synthetic base oil fractions such as XHVI oils and PAOs and the associated positive properties are completely retained in the re-raffinate.
  • Customer requirements existing with respect to as regards colour and smell of the products are fulfilled.
  • The process does not produce wastes, because the solvent used is recovered and recycled to the process, while the extracts resulting from the process are used as a fuel oil component.
  • High product yields and excellent product quality provide for a high profitability.
  • Process ensures high flexibility, allowing users to respond to changing compositions of used oils and increasing quality requirements to be met by base oils.

Both national and international patents have meanwhile now been granted for the process.

The Upgraded Selective Refining process uses the liquid-liquid extraction principle. Vacuum distillates from the flash distillation unit arranged upstream of the Enhanced Selective Refining Unit in the lube oil production line are used as feed. These distillates are processed in a production cycle which can be adjusted to the quantity to be processed. Before the distillate enters the extraction column proper, any residues of dissolved oxygen contained therein are removed in an absorber using steam. Then the distillate is sent to the bottom part of the extraction column. As the distillate rises, undesirable aromatic hydrocarbons and other contaminants are separated out by the counter-flowing heavier solvent, N-methyl pyrrolidone, which is fed in at the top of the extraction column. The solvent containing raffinate phase leaves the extraction column at the top and is routed to the downstream raffinate recovery section consisting of a distillation and a stripping column where the solvent is removed. The Kernsolvat (high quality base oil) obtained is routed for further disposition to the offplot product tanks.

What is known as the ‘extract phase’ is continuously withdrawn from the bottom of the extraction column, cooled down to a defined temperature and separated in a separation drum from the separated secondary raffinate. The latter is returned to the extraction column in order to optimise the process yield. The extract phase from the secondary separation drum is sent to the extract recovery section where the solvent is eliminated.

The extract recovery section also consists of a distillation and a stripping column. The resulting extract is routed to the offplot intermediate storage tank for use within the refinery as a fuel oil or mixing component for heavy oil.

The dry solvent separated in the distillation columns of the raffinate and extract recovery sections is returned to the solvent tank. The humid solvent separated in the stripping columns of the raffinate and extract recovery sections is returned to the solvent drying column, where excess water is removed. The ‘dry’ solvent which now is free from water is also returned to the solvent tank. From there, the dry solvent can again be used for extraction purposes in the extraction column.

Together with a leading engineering company, a leader in the field of refining technology, MRD translated the results of its process development activities into preliminary basic engineering, forming an essential basis for all subsequent activities undertaken in preparation of for a project for the planning and erection construction of the MRD Selective Refining Unit.

Since the manpower resources of MRD were not sufficient for speedily implementing the project within the 12months available for planning, building and starting the unit, support by an outside engineering company with expertise and experience in the field of refinery technology was required. Chemieanlagenbau Chemnitz GmbH (CAC) was chosen.

Planning of the unit was carried out by CAC in Chemnitz under the technical and commercial direction of MRD. To ensure that the project targets will would be achieved on schedule and within budget, a joint project team composed of MRD and CAC staff was formed. Another partner, Prolynx Project-Management & Support, was called in involved to provide project controlling assistance in the area of project controlling. As the contract with CAC was entered into on a reimbursable basis, so that exclusively MRD had to bear the resultant risk exclusively, MRD was in overall charge of project management.

The detail engineering was completed within a very short time thanks to the use of the state-of-the-art planning tools (PDS, Design Review, static structural loading programme for equipment, etc.) that CAC has at its disposal for planning such types of unit.

This was demonstrated by the overlapping, extremely short project implementation stages for the detail engineering and procurement which were completed after just six months as well as by the fact that supply and assembly erection were completed after only eight months.

Since the beginning of assembly erection works for the Upgraded Selective Refining Unit, about 1500cubic metres of earth have been moved and 100tons of reinforcing bars and 650cubic metres of concrete used in the construction of the secondary containment structure for the unit. A total of 135tons of steel elements have been used for erecting the structural steelwork. Next, 88 items of equipment and machines were installed. The total weight of the piping, comprised of 325 individual items including brackets, was 89 tons.

The electrical as well as the instrumentation and control equipment required for the fully automatic control of the unit (about 520 field devices and 44 drives) had to be installed, wired and tested during winter.

Commissioning works were expected to be completed by the end of June 2005.

Jörg Engelmann is Dputy CEO with Chemieanlagenbau Chemnitz GmbH, Chemnitz, Germany. www.cac-chem.de

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