Since 1998, an online XL7 viscometer has been in use at  one of Scotland’s largest grain distilleries, the North British Distillery Company Ltd in Edinburgh, to monitor the viscosity of a by-product from the distillation called “spent wash”, ultimately used in the making of Distillers Dark Grains animal feed.

“North British”, now owned by Lothian Distillers, was established in 1885 by a group of independent blenders including Andrew Usher, one of the pioneers of commercial whisky blending.  Its main product is Scotch grain whisky, the base spirit for many of today’s leading blended whiskies such as J&B, Famous Grouse, Chivas Regal and Cutty Sark.  With three Coffey stills in continuous operation, North British now produces over 1.4 million litres of spirit every week.

In the distillery, maize and malted barley are mixed with water, yeast is added and the mixture allowed to ferment.  The resulting liquid (called “wash”), containing about 8% alcohol, is transferred to the still, where it is heated by low-pressure steam.  The alcoholic liquor distils off and is collected for maturation, storage and bottling.  The spent wash condensate is centrifuged and passed through a series of evaporation stages.  The resulting syrup is mixed with dried residues from the mashing process (“draff”), pelletized and sold on for cattle feed.

During the evaporation process, production staff monitor the viscosity of the condensate in order to ensure that the syrup will have the right consistency.  Installed around two-thirds of the way down the line, the XL7 viscometer readings give an instant reflection of the degree of evaporation and thus the progress of the process. All the process controllers have to do is watch the readings and adjust the control valve position changing the ratio of flow forward to recycled flow to the final stage of evaporation. Traditional craft still plays its part, however: while the Hydramotion viscometer gives a vital early warning of change, controllers call on their time-honoured skills to interpret the viscometer readings.

“One great advantage of the XL7 is its simplicity.  It is extremely easy to use,” comments Instrumentation Supervisor Brian Morris.  With a single solid sensor and no delicate or expensive accessories, the XL7 viscometer is quick to install, needs no calibration on site and is completely clean-in-place compatible.  Morris adds: “Running costs have been negligible.  The instrument was installed in 1998 and is used every day in the 24/7 process, but the only maintenance it has ever had is the cleaning it gets as part of the biannual maintenance shutdown of the plant.”