1 May 2002

Refractive index measurements perk up petrol profits

By John Groetsch

Traditionally, lab-based analysis has moved into the process environment.

In the mass production of refined oil products, the measurement and control of concentration is an important quality parameter. Continuous monitoring enables uniform product quality and minimum waste.

Unlike periodical sampling, continuous monitoring can provide instant feedback on changes in the process. Instant feedback can then lead to real-time process control.

Light incident on a solid object, liquid, or gas can reflect, partially reflect, or penetrate that substance. The degree of each is a characteristic property of the medium.

In addition to varying with the medium, the degree changes with the concentration of dissolved solids. The study of this property of light has developed into the science of refractometry.

One simple example of this property of light shows up when one puts a pencil in a glass of water. It appears the pencil is bending when in fact the light is bending. The rays distort by passing through the water, glass, and air, not because the pencil itself bends.

The amount of the apparent bending is characteristic of the water and the amount of dissolved solids. The presence of bubbles and suspended solids in the water does not affect the bending.

Light travels at different speeds in different media. The more dense a medium, the slower the speed of light in that medium. When light passes from one medium to another at any angle other than 90°, it changes not only speed but also direction at the boundary between the two media.

The refractive index of a medium is the speed of light in air divided by the speed of light in the medium. This calculation method is the transmission method of determining refractive index. It is useful in the laboratory but not for process measurements.

When light passes from one medium to a more optically dense medium, there is both reflection and refraction for all angles of incidence. Starting with a small angle of incidence, there will be a weak internally reflected ray and a strong refracted ray.

As the angle increases, the angle of refraction also increases. At the same time, the intensity of the reflected ray gets stronger than that of the refracted ray. Finally, at the critical angle of incidence, the angle of refraction becomes 90°.

It is impossible to have an angle of refraction greater than 90°. It follows that for all angles of incidence greater than the critical angle, the light will experience total internal reflection.

In order to utilize this principle to make a useful process measurement, the generated reflected/refracted image must focus on and project onto a measuring element. This focusing element is a prism.

The light passes into a prism, which directly contacts with the process fluid. The light contacts the prism/process interface at various angles. Some rays are totally reflected, and some rays refract. The transition point between total reflection and refraction defines the refractive index of the process fluid.

As the concentration of dissolved solids in the process fluid changes, the refractive index changes. Increasing concentrations of dissolved solids increases the amount of refraction and reduces the amount of reflection. As the amount of dissolved solids decreases, the amount of refraction decreases, and the amount of reflection increases. From these changes in the optical image, the measurement occurs.

The refractometer determines the concentration of a solution by measuring the optical refractive index. Ernst Abbe invented the refractometer in 1874 when he published a description of an apparatus for determining the refractive index (RI) of solids and liquids.

The RI is a lab technique that has made the transition into process measurement. This transition required that laboratory sensors get much tougher. In-line sensor construction must withstand harsh conditions.

The internal in-line sensor construction has three basic components: a light source, an optical prism, and an image analyzer.

An all-digital in-line process refractometer uses a charge coupled device (CCD) microchip to determine the position of the borderline and relates its position to refractive index and concentration. The CCD then converts this optical image into a digital signal. This conversion eliminates drift, increases the stability, and eliminates human error.

An instrument that measures by digitizing the optical image truly determines the liquid’s refractive index and concentration. The interference, due to back scattering from suspended solids and bubbles, does not register because it occurs outside the critical angle region.

This advancement to digital signal processing yields a high accuracy and stable dissolved solids concentration measurement unaffected by typical interference.

The RI is temperature dependent. To generate a stable, accurate, and reliable temperature measurement is important.

Due to the effect of temperature on RI, mounting and location of the prism is important to the measurement. Locating the sensor in turbulent flow region away from pipe walls keeps the prism at the same temperature as the process fluid.

A precise temperature measurement ensures the proper concentration indication.

If process residue fouls the sensor’s prism, an accurate in-line measurement can’t happen. Typically, either a water or steam wash is used to wash the prism. Steam works when the monitored liquid is particularly sticky or tenacious. In applications where steam may bake the process liquid onto the prism, pressurized water works as the prism wash medium.

A process refractometer needs to measure refractive indices over a wide range and with high accuracy. The typical range of a refractive index is between 1.3 and 1.6. Suspended solids or bubbles do not influence the refractive index, whereas the solution’s total density is affected. An RI will remain constant for a saturated or supersaturated solution.

This analyzer species works in refinery processes such as sulfuric acid alkylation, amine gas treaters, caustic scrubbers gasoline interface, hydrotreater, and lube oils production.

The ability to make this measurement has eliminated sampling, improved product consistency, and reduced waste. IT

Behind the byline

John Groetsch Jr. has a degree in chemical engineering and more than 14 years’ experience in optical and acoustical-based in-line instrumentation. He is a member of ISA and AIChE. He is the general manager of K-Patents, Inc.