01 September 2004
Sulfur in oil X-ray
By David Liu and Bela Lipták
Online analysis of sulfur in oil products is imperative because environmental authorities are tightening the limits on sulfur levels in petroleum products.
Limitations of 300 parts per million (ppm) in blended gasoline and 500 ppm in diesel are not uncommon.
Even with excellent hydrotreating capability these limits are often difficult to meet, so careful process monitoring is important. The timely information provided by continuous online monitoring is necessary to consistently meet sulfur content limits, especially when blending the various product or feed streams.
The X-ray absorption technique has worked for many years in the laboratory and online to measure total sulfur content in liquid petroleum products. The intensity of X-rays after they pass through a liquid hydrocarbon sample depends on the density and elemental composition of that sample.
For X-ray radiation in the region of 10-20 keV (kiloelectron volt-a unit of energy), sulfur has a much greater mass absorption coefficient than does carbon and hydrogen. At 20 keV, the mass absorption coefficients of carbon (C) and hydrogen (H) are about equal, so that changes in X-ray absorption due to the C:H ratio are negligible.
Thus, the changes in X-ray absorption depend primarily on sample density and sulfur content.
Sulfur in oil analyzers: the facts
- Type of sample: Crude and fuel oils, diesel, gasoline, middle distillates, kerosene, gas oil, jet fuel, and lubricating oils
- Method of analysis: X-ray absorption
- Range: 0-0.5-0-5% sulfur; 0.1-1 gram per milliliter density
- Inaccuracy: 0.5% of full scale or 0.005 weight % sulfur
- Sensitivity: 0.5% of full scale or 0.0005 gram per cubic centimeter
- Cost: $40,000-$80,000
The X-rays emitted from an americium-241 source bombard a target, producing secondary X-rays at 20 keV. These penetrate a beryllium 5-mm-thick backplate and a thin Teflon plate, pass through the sample cell, and enter the ionization chamber.
The current from the ionization chamber detector is the input to the sulfur converter. This input compares with the zero set signal from a standard current source. The difference between these signal currents is the input to the picodiode electrometer, where the differential current ramps up to provide an output signal to the S-set sulfur converter unit.
At the S-set unit, the signal undergoes linearization, compensates for density at the zero point using the signal from the density converter, and then morphs to a voltage signal that is proportional to the sulfur concentration and density. After span adjustment the signal transmits to the divider unit. This unit divides the sulfur concentration signal from the S-set unit by the density signal (calibrated at the sample temperature of the sulfur detector). The divider unit produces a 4-20 mA DC output signal proportional to total sulfur content.
The most sensitive range of the system is 0-0.5 weight percentage (wt %) sulfur, with a repeatability of 0.01 wt %. The density measurement has a sensitivity of 0.0005 grams per cubic centimeter (g/cm3).
Behind the byline
Bela Lipták, P.E., has an M.M.E. and is an ISA Fellow. David Liu has a B.S., an M.S., and a Ph.D. This piece appears in Instrument Engineers' Handbook, ISA, 2003. Nicholas Sheble (firstname.lastname@example.org) edits the Control Fundamentals department.