Accurate Green Liquor Density Control with Reduced Maintenance

In pulp and paper processing, the term "green liquor" refers to a by-product of kraft pulping. It is the dissolved concentration of sodium sulfide, sodium carbonate, and other compounds in solution from the recovery boiler. The measurement and control of its concentration is important in the pulp and paper production cycle. 

Inline refractometers are used to provide continuous density measurement of green liquor in the pulp mill. A major issue in handling green liquor is scale build up and the resulting optical coating on the refractometer sensing head. If scale build-up is controlled for an acceptable amount of time during processing, the maximum accuracy of the refractometer will be achieved and the overall quality of the pulping process is supported. Understandably, it makes great sense that a system of measuring scale potential and a means to automatically clean the sensing head should be implemented.

The Electron Machine MPR EScan is used to measure the green liquor dissolved density, or TTA, at two stages in the process: after the green liquor dissolving tank and after the green liquor clarifier. The inline measurement, with the refractometer sensing head installed directly in the main process lines, allows real-time control of green liquor dilution to meet target TTA set-points. The measurement is also used to indicate (and prevent) excessive green liquor density and the resulting dangerous impending crystallization within the dissolving tank, and lower the potential for scaling.

Scaling issues, and therefore accuracy problems, are further reduced with the use of ancillary inline cleaning systems that use pressurized water, heated to the process temperature, to effectively clean the refractometer optical components. The Electron Machine HPC-2 High Pressure Cleaner is one example of a completely integrated system.

The compound result of using a refractometer to keep green liquor density within optimal levels, plus an inline cleaning system, provides an excellent payback both quality control and also with associated maintenance requirements.

Using a Spool Adapter with Your Inline Process Refractometer

Two versions of inline refractometer adapters.

Inline refractometers use process adapters as the mechanical connection between the refractometer sensing head and the process piping. The adapter, also referred to as a "spool piece", is designed specifically to accommodate the pipe size and application.

On applications where pipe sizes are 2" or larger, the use of a refractometer sensing head spool adapter is suggested. This ancillary piece of hardware provides a mounting point for the sensing head between two flanges directly in the process piping.

The spool adapter positions the sensing head so that it measures from the center of the process flow. The spool adapter design includes an internal deflector and a 5 degree (angular) sensing head mounting connection. The deflector and angled mounting connection reduces sensor head coating issues from the process media. A steam purge tube connection is standardly included to accommodate steam/fluid cleaning.

Spool adapter with sensing
head mounted.

Spool adapters recommended mounting orientation is horizontal, generally placed in a horizontal segment of process piping. Vertical mounting is also acceptable, as long as the process flow is in an upward direction.

Spool adapter with hand
cleaning option.

Inline refractometer spool adapters are available with option to facilitate cleaning, service and process protection. Examples are lined adapters, hand cleaner adapters, high pressure cleaner adapters, isolation valves, sanitary connection adapters, and weld-in adapters.