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A significantly higher number of centrifugal pumps than displacement pumps work in process engineering systems - they are more simply designed and accordingly, more attractively priced. However, centrifugal pumps reach their limits under critical conditions, as this case study at Puralube demonstrates.
The installed high-speed centrifugal pump produced vacuum gas oil and proved fairly susceptible to faults, but this task is now being fulfilled by a diaphragm piston pump. It performs the process with significantly higher energy efficiency and has been working without faults for more than 700 days.
Puralube Germany GmbH operates two refineries with 105 employees in the Zeitz Industrial Park, situated 40 km south of Leipzig in Germany. Systems at the sites extract high-quality base oils from used oils in a continuous process without residue, called the UOP-HyLube process. Both refineries have an overall annual throughput of 150,000 t, comprising of approx. 90,000 t base oils and 50,000 t additional mineral oil products. Due to their physical and chemical properties, the Puralube base oils meet the requirements of the API group II+ as well as the strict emission and performance demands made on modern lubricants.
The used oil market is highly competitive with regard to raw materials, as systems for thermal disposal are competing with re-refiners like Puralube. From an ecological point of view, the HyLube process provides a significant contribution to environmental protection by helping to conserve finite fossil resources. From one ton of used oil, Puralube extracts 750 kg base oil for lubricants.
The process developed by the technology company UOP is roughly divided into three steps:
1. Evaporation of the used oil Through intense mixing of the untreated raw material in almost 480°C hot hydrogen/circular gas stream, hydrocarbons up to approximately C40 are transferred into a gas phase and released from heavier compounds as well as other contaminations in a Flash Separator.
2. Catalytic hydroprocessing In multi-level high-pressure equipment, the gaseous hydrocarbons are first separated from any remaining contaminants and metal compounds (Guard Reactor) and desulphurised. Then a deep saturation (Conversion Reactor) of olefines and aromates is performed with hydrogenation catalysts, followed by hydro finishing under respectively high temperatures and pressures. This is combined with extensive desulphurisation and removal of other heteroatoms.
3. Product extraction The cleaned, liquid hydrocarbon mixture is then separated into gas oil and three base oil core slices through pressure relief in the hot separator in a 50 m high vacuum tower.
Pump replacement
The vacuum gas oil is stripped with steam in the residue column under vacuum and condensed in a hot separator at 180 mbar(a) and 120°C. In the last process step, the vacuum gas oil must be recirculated from the residue section to the reactor circuit using a pump.
For many years, a centrifugal pump was used in this part of the process with the following performance data: 132 kW drive, rotational speed approx. 19,000 min-1, discharge pressure 90 bar and output 6200 l/h. Due to the high rotational speed, the pump is very noisy and exceptionally susceptible to faults. Furthermore, the low efficiency was no longer acceptable in the current business climate and went against the fundamental philosophy at Puralube to use energy-efficient technology.
Based on previous positive experiences Puralube had with one of SPX’s pump brands, Bran+Luebbe, SPX was shortlisted as a supplier for a replacement pump. Puralube ultimately selected Bran+Luebbe’s three head design NOVADOS series 3H6 diaphragm pump with PTFE double diaphragm. This technology achieves the required flow rate and the necessary discharge pressure within a small footprint area. The pump selected is fitted with a 55 kW drive motor that produces approx. 4900 l/h vacuum gas oil from a stroking speed of 156 min-1 and has a discharge pressure of 90 bar.
Better choice
Compared to the previous centrifugal pump, the diaphragm plunger pump has been shown to be by far a better choice. The unit has proved to be trouble-free, is very quiet and impresses with very high efficiency. “The pump has been operating for more than 700 days without any fault,” says Puralube Operations Manager Michael Schlüter. The economy and availability predicted by the manufacturer have fully met expectations, as Schlüter confirms: “The investment for the new pump was completely returned within 14 months.” He was also impressed by the modern drive technology. “The level in the hot separator for the NOVADOS is set via a speed control by means of a frequency converter - this works very well and with a high level of control,” says Schlüter.
The big advantage of the selected NOVADOS H6 series pump (please see the short introduction at the end of the article) is its construction as a diaphragm plunger pump. This means the pump is completely leak-free without additional technical complexity, does not discharge emissions and thus meets the specifications of the Technical Guidelines on Air Quality Control (TA Luft). Schlüter is also impressed by the ease of maintenance of this pump and his employees in the maintenance department can perform many operations themselves on site.
Simplified system
From a technological perspective, the installation of the displacement pump significantly simplified the system as control valves and pipes previously required to adapt capacity to requirements were no longer needed. The design of the centrifugal pump meant it had to be set to a delivery volume which was far above the volume needed in order to meet the desired discharge pressure. Excess capacity was drained off and led back, leaving the efficiency of this system at a mere 20%. The new system with the displacement pump on the other hand, achieves an efficiency of more than 90%. Schlüter regards the simplification of the system technology as an additional advantage; removing unnecessary, superfluous complexity.
Five additional Bran+Luebbe brand pumps with the same design are already in operation at Puralube for different sub processes and media (used oil, demineralised water, soda solution and caustic soda solution). In addition to the NOVADOS 3H6, two further pumps have been converted on System 1. A NOVADOS 3H6 is also ready to be installed for the second refinery with installation planned for mid-2014.
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Author
Hans-Peter Schliephake, Field Sales Engineer, Pumps & Process Systems, Bran+Luebbe GmbH, Werkstraße 4, 22844 Norderstedt.
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