I/EG A
INSTRUMENTATION
W O R K B O O K
F E A T U R E
by B. F. Dudenbostel, Jr., and Wm. Priestley, Jr., Esso Research & Engineering Co.
Process Control by Flow Colorimetry Flow colorimeters help to meet color specifications a n d control product quality by continuously monitoring plant streams
I T IS a time-honored privilege of chemists to identify compounds by removing the cork from the bottle and smelling the sample. Similarly, in the case of petroleum products it has been customary to hold up the sample to the light in order to estimate color and viscosity. In many cases, the customer judges the quality of a product by these tests. Thus, it has been the practice for many years to use a color specification as a guide to product quality. Actually, in many cases color is a good criterion of product quality. In the past, this specification was tested in the laboratory. Today, with the advent of many continuous processes and corresponding continuous analyzers, color specifications can be met by continuously monitoring the plant streams. These devices (flow colorimeters)
consist simply of a source, filter, sample cell, detector, and measuring system. The colorimeters normally are not spectrometers, but utilize filters to obtain narrow wave length band widths, and are tailored to the proposed application. The length of the cell, the filter to produce the proper wave length, and the proper detector for that wave length are chosen on the basis of the application. As Beer's absorption law holds only with instruments of narrow band path such as spectrometers, or ones using very good narrow band pass filters, process control colorimeters using wide band pass filters may be nonlinear. However, highly stable, reproducible, and photometrically accurate continuous instruments have been produced which will satisfactorily operate although the absorbance is nonlinear
Installation showing flow colorimeter monitoring turbidity of effluents in a paper mill
in nature. Zero drift, 100% reading, and wave length shifts must be negligible. In addition to single-beam colorimeters, such as those described above, ratio-recording flow colorimeters are also available. In these instruments, a single beam of light is passed through the sample. The transmitted light is split into two beams, one of which passes through a filter which transmits the measuring (absorbing) wave length to a phototube. The other beam passes through a reference (nonabsorbing) wave length filter to a reference phototube. The outputs from the two phototubes are then ratioed. The beauty of this system is that bubbles, turbidity, window deposits, etc., do not affect the ratio. Only color variations can change the instrument reading.
Long-path sensing element flow assembly and indicating amplifier
VOL. 48, NO. 12
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DECEMBER 1956
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I/EC
INSTRUMENTATION
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A Workbook
Fogging of the windows due to low temperature is eliminated in singlebeam colorimeters by separating the first compartment and the receiver compartment from the flow cell assembly by double glass windows separated by a dry air space. This problem does not arise in the case of double-beam instrumentation. These types of instruments are manufactured by Beckman Instru ments, Inc., Kaye Development Co., Buhler Brothers, Haalikaanen Co., and Manufacturers Engineering and Equipment Co. Applications
Numerous applications are cur rently being made in the field today. Control of Processes through Measurement of Turbidity by Con tinuous Flow Colorimetry. Filtra tion processes can be controlled by continuous turbidity determina tions. The colorimeter provides a continuous safeguard against product losses due to filter breakthrough or inefficient filtration when the de sired product is a filter cake. It also provides continuous quality control on solid impurities present and protects against filter break through in cases where the product is the filtrate. When precoating is used, the colorimeter speeds the operation by indicating when a clear effluent is coming through. In addi tion, a constant record of solids -^-
