Separation 101 ~ The Basics ~ Part I
The quality of marine fuels continues to decline as of a result of increasingly efficient refining techniques in attempts to obtain the of light end products from each barrel of crude such as gas, gasoline, and light distillates (JetA, MGO). In years past the primary blending agent for residual marine fuels was MGO or MDO. With increased regulations regarding sulfur limits, and the increased costs of MGO/MDO other cutter stocks are used in blending residual fuels these days to obtain the desired fuel specification with regards to density, viscosity, sulfur level. Unfortunately these cutter stocks often contain higher levels of fines or cat-fines as they are referred to. The first thing that comes to mind when a problematic fuel (high cat fines > 80ppm) is encountered leads many of the uninformed to blames the refiner. This is generally the last person to look towards! Catalyst is expensive and refiners go to extremes to reclaim and regenerate it during the refining process. Breakdowns do occur however, the majority of cat-fines come from the cutter stocks used in blending. From both an educational and operational perspective I have found that separators are often a mystery to many operators. I will attempt to outline some of the basics here.
Separator general limitations:
Conventional Separator (gravity disk) 991 kg/cm2 density - 600 cSt viscosity
985 kg/cm2 density - 600 cSt viscosity (Pre 1985)
Partial Discharge (Alcap/Unitrol/) 1010 kg/cm2 density - 700 cSt viscosity
Basic efficiency of separators is a function of:
Flow Rate, Process Temperature and Process Viscosity
Settling Area (size and number of disk stack/disks)
Settling Velocity (speed of machine)
The basic difference in densities between oil, water and particles to be separated
Separation efficiency and factors affecting efficiency:
Cat fines (Al + Si) 60-90%
Parameters of oil which are not affected by centrifugal separation:
Density, Viscosity, CCAI, Flash Point, Pour Point, Micro Carbon Residue, Sulfur/Vanadium and Asphaltenes
Parameters of oil significantly reduced by centrifugal separation:
Water, sodium, cat-fines (Al + Si), Iron, Magnesium, Ash and Calcium
To achieve the maximum efficiency of a centrifugal separator you must:
1. Use the correct gravity disk in conventional machine
2. Maintain a clean machine ~ in the event of serious off spec fuel - clean manually or by CIP
3. Maintain constant temperature at separator inlet 98 Deg.C
4. Maintain a constant minimum feed rate consistent with plant demand
5. Operate conventional separators in series (purifier/clarifier) if possible
6. Operate partial discharge or high density separators in parallel if possible
Points 3 & 4 are the most commonly overlooked. As to minimum feed rate, think of machining a piece on a lathe, the higher the speed and finer the feed results in a polished or fine finish. In essence we are polishing the oil. Operating high density separators in parallel results in a lower feed rate and longer residence time for the oil in the separator bowl.
Always consult your specific separator manufacturer with respect to operational recommendations.
How inlet temperature affects separation:
To achieve maximum efficiency you must maintain a continuous process viscosity in the bowl.
For 380 cSt oil a drop from 98 Deg C to 95 Deg C results in an increase in process viscosity from 26 cSt to 29 cSt.
Enough to digest today ~ Safe Voyage!