In many production processes of the chemical, petrochemical and pharmaceutical industry, color is an important parameter for the quality of the product or the production process. Might be the exact color value, might be the requirement of an ideally colorless product, might be the color range in a specific production step of a product. Examples are decoloration processes as in phthalic anhydride, paraffin or naphta or color measurement in hydrocyanic acid, oils, and fuels. Moreover, color or absorption measurement can be used to determine the concentration of specific substances by their characteristic absorption, e.g. in copper or chromium solutions for plating processes or ozone concentration in air.
With 60 years of experience in process measurement, SIGRIST now offers the ColorPlus as the state-of-the-art in-line color monitor. The well-proven dual-beam technology with measuring and reference beam enables a rather accurate and stable measurement by eliminating all influence of temperature, aging, etc. Using up to three different wavelengths out of a range from 254 nm UV through the visible up to 700 nm red light all standard color and UV absorption can be covered. Turbidity compensation is available as an option. The range is from lowest colors (<1 Hazen or 30 Saybolt) up to 6000 E/m. For the flow cell we can offer customer specific in-line and bypass designs out of a variety of materials as stainless steel, PVDF, Hastelloy, etc. also including special options as heating or cooling jackets. The ColorPlus comes in a compact and rugged stainless steel housing, optionally available in an ATEX Eex-p version for use in hazardous areas.
We want to give here two specific application examples, which have been realized in various installations:
ASTM color in gasoil
Gasoil is a preproduct in the refining of diesel fuel and light fuel oil, partly also used directly as a fuel. The color of the gasoil normally might not exceed 2.5 to 3 ASTM color units. Higher values are a hint for a malfunction of the process and so this measurement is crucial for the quality of the gasoil or subsequent refining steps. At a value of 5 ASTM, the process is running off and has to be interrupted. The measurement is done at a wavelength of 520 nm. The temperature is around 110°C.
Hazen-color in phthalic anhydride
Phthalic anhydride is widely used in industry in the production of dyes (rhodamine, anthraquinone derivatives), insecticides, plasticizers, in pharmacy, in analytic chemistry, etc. The colorness of the material is an important quality issue and thus has to be measured continuously. The typical range for the measurement is 0..20 Hazen, a range, where the human eye can no longer detect any coloration. The measurement is done at a wavelength of 390 nm. Another important fact to be taken into account is maintaining the product temperature at least at 160°C to prevent crystallization of the anhydride. For this reason an in-line flow cell with steam heating jacket is used.
Other applications have been realized for color measurement in Hydrocyanic acid (0..600 Hazen), color in Acrylic acid (0..10 Hazen), concentration of copper sulphate solution for electroplating (0..120 g/l), chlorine dioxide concentration in desinfection or bleaching solutions (0..15 g/l), residual ozone concentration in ultrapure water (0..50 ppb) or ozone concentration after the generators (0..300 g/m3).