Centrifuges, Especially in The Purification of Oil

General Having been involved for more than 30 years in rotating equipment and fluid separation technology, I have just as long been concerned about the many misapplications and misuse of centrifuges, especially in the so-called purification of oil. Properly used, in some cases, they allow us to separate substances to a considerable degree and do so quickly, although not inexpensively. However, misuse has become more common as centrifuge manufacturers seek to expand their markets where perceived applications seem suitable. Sadly, these practices often result is users acquiring centrifuges for applications that are actually detrimental to the oil and often produce inferior separation at great expense to the end-user. Economic justification and application must therefore be considered and knowledge of the rheological behavior of the fluid in a centrifuge is essential. Why Not a Centrifuge for Solids Removal? A centrifuge separates the liquid charge into two phases; the overflow consisting mostly of the lighter fluids and the fine solids and the underflow consisting of coarser, heavier solids and some of the interstitially retained liquid. Solids in a liquid such as rotating equipment lubrication and seal oil exist in a range of particle sizes from gross solids like corrosion products down to fine and ultra fine colloidal particles. Consider the behavior of solids in a liquid that is being centrifuged. With the centrifuge rotating at a constant speed the solid particles, which are of many different densities and shapes, respond to gravitational forces in accordance to Stokes' Law. The oil viscosity provides an opposing force on the solid particle. When the tow opposing forces balance one another, the particle in question stops moving. It has reached its terminal velocity. This illustrates the main disadvantage of a centrifuge. You have little control over the size and types of solids that are separated. Ultra-fine and colloidal solids will remain the overflow and are returned to the oil reservoir, where they accumulate and agglomerate. What about separating water from oil? Water exists in oil in three phases; free, emulsified and dissolved water, each with different densities. Free water readily settles out, while in an emulsion of water-in-oil the water particles are kept in a matrix of oil or vice versa. Emulsions are created by turbulent mixing of water and oil. An emulsion is a stable layer of tightly-held water particles and a centrifuge cannot separate an emulsion. As a matter of fact, the turbulence that normally exists in a centrifuge often creates an extremely tight emulsion that cannot be separated and is thus carried over to the oil reservoir. Mineral oil is essentially non-polar, while water is highly polar. Up to a certain limit, a small amount of water will dissolve in oil, depending on the temperature. The solubility of water in oil increases exponentially with the temperature. The water content directly affects the quality of the oil. In the separation of free, emulsified and dissolved water, a centrifuge is onlu capable of removing free water. It will not remove an emulsion nor will it remove dissolved water. The Maintenance will kill your budget Centrifuges have high power consumption. Experienced operators are required to operate and optimize performance as well as execute repairs. Performance is difficult to monitor because the operator's view of centrate and feed is obstructed Maintenance is intensive as with all high-speed rotating equipment, Special structural considerations must be taken into account. As with any piece of high speed rotary equipment, the base must be stationary and level due to dynamic loading, High noise levels while running, Spare parts are expensive and internal parts are subject to abrasive wear, Start-up and shut down may take an hour to gradually bring the centrifuge up to speed and slow it down for clean out prior to shut down, Power consumption is high, Electrical installation is expensive, Frequent starts and stops are not possible, Continuous vibration monitoring is essential, A large capital investment is required. Conclusion There are better, more economical and more effective systems for the purification of oil. These are based on proven solid chemical engineering principles and consistently produce clean separations of contaminants from oil. These systems are fully automatic, PLC-controlled and do not require operators to be in attendance. Source: allenfiltersinc.com