Rapid and accurate in-line measurement of alcohol is of importance in the distilling industry to ensure successful and profitable management of the process. This applies equally to modern and tradi-tional plants, particularly in the current economic climate states liquid concentration and density monitor DensiCheck.
Measurement and control is critical in determining the efficiency of the primary distillation process but is also required for efficient management of stocks and the final blending and reduction operations.
Ultrasound is an effective technique for measuring alcohol content in distilling appli-cations, where the main constituents are alcohol and water.
DensiCheck TX in-line liquid concen-tration analyser uses this technique by measur- ing the time of flight of a short-pulse of ultrasound at 1 MHz to 5 MHz.
Sound velocity in pure ethanol is less than the velocity of water, but a strong interaction occurs in mixtures, which results in an initial increase in the sound velocity until a peak is attained at 28% ethanol.
The peak velocity corresponds to a mini- mum intermolecular distance and com-pressibility, which is the prime cause of the increased sound velocity and is the result of increased hydrogen bonding.
There are two ethanol concentrations for velocity and temperature combinations. In practice, this should not be problematic, as it is generally clear from the process, which is the desired concentration. Ethanol concentration can generally be measured to 0.1% volume-volume (v/v) except in the region where there is little velocity change for a large change in concentration. This occurs in the range of 28% to 35% v/v when the error can be up to 0.5% v/v.
Densitech TX can be used in the distillery to measure the performance of the stills and control the diversion of the distillate to the appropriate collection vats. The ethanol concentration of the low wines from the wash can still be monitored and used to determine the cutoff point for distillation. In addition, if a flowmeter were integrated into the line, it would be possible to calculate the average ethanol concentration of the low wines.
The balance between foreshots, spirits and feints is critical to the correct operation of the spirit still. The ethanol concentration of the distillate can be monitored to determine the spirit cut points. The inclusion of a flowmeter would allow calculation of the average ethanol concen-tration and volume of each of the fractions, which could be used by the stillman to obtain a steady charge to the spirit still.
Reduction and Blending
The final reduction in spirit strength before bottling is a critical area of operation. It is important to ensure that no under-strength product is packaged as this will cost time and money in terms of rework required.
However, it is essential that the actual strength is tightly controlled and no over-strength product is packaged. In this situation, in-line measurement of alcohol can be used to control the blending operation, or to monitor the strength imme-diately before packaging.
When the velocity and temperature calib-ration is carried out over a narrow band of product strength, it has been shown in prac- tice that alcohol can be measured to 0.03% v/v. A more important criterion for continuous measurement and control is the repeatability on the in-line measurement. This can be defined as the standard devia-tion of 20 successive discrete measurements. The repeatability of the alcohol measure-ment is 0.0035%v/v, which represents the ability to discriminate a change in alcohol of 0.01% v/v.
However, as with any in-line sensing device, ultrasonic sound velocity measures only what is present in the pipe at the instant of that measurement. It will, therefore, respond to short-term fluctuation in the composition of the product, such as those introduced by blending systems, which lack good mixing characteristics. In the case of ultrasonics, this can lead to unstable signals, which cannot necessarily be smoothed mathematically.