What Are Critical Angle Refractometers?

Critical angle refractometersCritical angle refractometers use the principle of critical angle refractometry by which dissolved solids content is related to the refractive index of the solution. The basic refractometer unit unit consists of a sensing head and control console. The sensing head is mounted on the adapter installed in the process line and the process solution is usually viewed through a transparent window. The surface of the solution, which is in contact with the window, is optically scanned by an octagonal prism.

The refractive index is determined by a beam of light from the prism which repeatedly sweeps at a varying angle at the surface of the solution. At a point in the cycle, as the angle between the incident light beam and the surface decreases, the light beam instead of refracting into the solution is reflected back into the optical system. The point of change from refraction to reflection depends on the refractive index of the solution and is known as the critical angle. The prism scans the beam of light through the critical angle. A photodetector measures the alternating light and dark periods and its electrical output is used to provide a readout of refractive index in the required units (R. I., % concentration, degrees Brix., etc.).

Critical angle refractometers
Optical Path
Diagram - How They Work

Critical angle refractometers use an LED as a light source and utilizes state of the art CCD (charge coupled device) technology to accomplish scanning the reflected light returned from the prism.

Light radiated from the LED passes through the prism surface to be reflected off mirror 1 to the prism-to-process interface. The light reaching this interface intersects the same interface over a series of angles chosen to include critical angle for the process being measured. Light intersecting the interface at an angle greater than critical angle is refracted into the solution. Light intersecting the interface at less than critical angle is reflected up to mirror 2 and out of the prism up to the CCD linear array to be scanned.

A principal advantage of critical angle refractometry is that it measures the index of refraction at the surface of the process solution. Since the light beam does not penetrate into the solution, the instrument can be used for opaque as well as transparent materials.

Measuring Solids in Pulp & Paper Black Liquor

Pulp and paper plant
Pulp and paper plant.
Industrial paper manufacture involves a procedure known as the Kraft process, where wood is converted into wood pulp and then into paper. The process, however, produces a toxic byproduct referred to as black liquor. This primarily liquid mixture of pulping residues (such as lignin and hemicellulose) and inorganic chemicals from the Kraft process (such as sodium hydroxide and sodium sulfide) is toxic.

Until the invention of recovery boilers in the early 20th century, black liquor was often simply released into waterways. Black liquor recovery boilers allowed paper manufacturers to recover and reuse the inorganic chemicals and extract energy from the pulping residues.

Paper processingReliable, continuous measurement of black liquor solids content is a subject of considerable importance to the pulp industry. The solids content of liquor introduced into a recovery furnace can have a pronounced influence on firing behavior. Current trends toward better control of the recovery furnace for reasons of safety and reduced air emissions require a greater degree of control over incoming solids content and appropriate adjustment of operating conditions to handle variations in solids content. This, in turn, requires a reliable method for continuously monitoring solids content.

Instruments such as inline process refractometers successfully measuring black liquor solids concentration continuously. These instruments have the dependability, accuracy, and reliability to augment safe operation of recovery boilers. Refractive index-type instruments are more widely accepted and far more successful with less maintenance requirements than other types of instruments used for black liquor solids measurement, and have proven reliable and accurate for automatic monitoring of black liquor solids concentration.

Inline Process Refractometers for Fruit Juice Concentrate Production

Fruit Juice Concentrate
Just about every fruit harvested is processed to a concentrate. Fruit juice concentrate provides for easier transportation and longer storage life for both producers and consumers. Production technology for the juice concentration has become quite advanced, resulting in improved quality and consistency. Sweetness, color and solid components from the feedstock fruit juice carefully monitored and controlled.

Fruit juice concentrate production starts with dilute juice feedstock, the application of carefully controlled heat to evaporate off water, ultimately resulting in a uniform and consistent concentrated juice. The fruit juice stock is extracted from various fruits in a number of ways that are specifically adapted for the shape, size, and nature of the fruit. It is then purified and stored in primary holding tanks. Juice concentration will vary at this initial stage due to a number of natural factors and needs to be processed to desired quality standards.

One objective of the concentration process is to remove excess water in a consistent and uniform manner. Excess water removal is done through the use of specialized multi-stage evaporators that extract water without damaging the juice by applying improper amounts of heat. A closed-loop control system monitors a variety of process variables such as temperature, flow, and pressure from multiple process sensors. The readings from these sensors drive proportional outputs that modulate final control elements such as control valves.

Process refractometers are sensors used at strategic points to measure dissolved solids (sugar) concentration.  By monitoring and controlling percent solids and Brix, plant operators gain tighter control of product quality and more efficient use of equipment (possible energy savings).

For more information on the application of process refractometers in juice and juice concentrate, contact Electron Machine Corporation by visiting https://electronmachine.com or calling 352-669-3101.