In-line blending of multiple-ingredient liquid products
The in-line blending of liquids offers a variety of decisive advantages over the traditional batch mixing, especially if it can be followed directly by a further process (e. g. bottling) after the blending process. The combination of highly accurate measuring devices, digital control units and quality regulating devices with sophisticated monitoring makes in-line blending an ideal method for the manufacture of multiple-ingredient products as long as a minimum size of batches is existing. Since manual analysis of the bottled products can only be carried out on a random sampling basis, constant automatic monitoring of the process by means of on-line process analysers is recommendable. It must therefore be possible to ensure a maximum standard of reliability, especially where there are high quality requirements or where expensive products are being processed and product losses must be avoided at all costs.
Procedure
With in-line blending, liquid components are mixed continuously in constant proportions in the pipe, even when the total throughput quantity is variable. It allows accurate blending of a practically unlimited number of ingredients. The only limits are those of the desirable degree of complexity and the cost.
In the beverage production, high-volume components, e. g. water, sugar solution or syrup base are for the most part conveyed to the blending station by external pumps. In order to minimise product losses, especially in the case of expensive concentrates and base components, the container vessels should be positioned close to the unit.
Preferably, the ingredients then flow down to the plant by gravity. Before they enter the blending pipe they are deaerated and then pumped by centrifugal pumps or positive-displacement pumps integrated into the unit.
If any ingredients are used which carry a danger of air being trapped inside them, deaeration vessels are essential.
They have the following functions:
- expulsion of any air conveyed with the ingredient
- automatic switch-over of the product input when a container is empty
- alarm function and stopping of the plant in the case of product shortcomings
- regular automatic checking of the flow meter in the concentrate/base area.
The flow quantities of all the liquids are constantly recorded by the flow meters and reported to the plant control and regulation systems. This recording should be effected by the pulse transmitter of the flow meter (i. e. digitally). The advantage of this is that these outputs can be checked more accurately both at the supplier’s and on the customer’s premises than the analog outputs of the same devices. This also means that recalibration at the place of installation is simpler and more accurate.
The control unit compares the measured values with the blending proportions of the individual ingredients laid down in the recipe and regulates the control valves in such a way that the recipe values are precisely maintained.
The most varied proportions can be achieved if the right flow meter is selected. In the beverages industry flows of between 2 and 120,000 litres per hour are realistic and possible. Extreme ratio ranges can be covered.
Advantages of in-line blending
In-line blending offers the user a large number of advantages:
- direct processing of the raw products, resulting in
- rapid availability of the product,
- small stocks of the product in the plant,
- no large mixing tanks are necessary, leading to
- a low space requirement, so that the system can be realised in a restricted area,
- savings in the size of buildings,
- the possibility of reacting rapidly to changes in production planning.
Taken in isolation, the capital investment required for an in-line blending system is generally higher than that for a batch mixing system, but the relationship is in most cases reversed if all the above-mentioned points are taken into consideration.
Partial view of an in-line blending plant for 9 ingredients. In the foreground are the deaeration and testing vessels.
Components
The most important factor in highly accurate blending is the use of precise and reliable flow meters. The tolerance of the flow meter is directly reflected in the results of the blending. Where quality products are concerned, therefore, no compromises should be made. With very good flow meters it is possible to achieve measurement tolerances of between 0.1 per cent and 0.25 per cent of the measured value in a flow range of between 10 and 100 per cent. The figure for reproducibility is lower by a factor of about two.
Which type of equipment is the most suitable depends on the product characteristics and flow rates. In addition, the decision in favour of a volume or mass flow meter depends upon the process (e. g. are the recipes based on volume or mass?) and in some cases upon the additional functions that are required (e. g. density measurement).
Detail view of a plant for 8 ingredients with mass flow meter
Control valves are used if the product is conveyed by means of centrifugal pumps or by higher pressure in the supply tank. The control valves are equipped with pneumatic drives and I/P positioners. The flow can be set to the specified value by altering the variable cross-section of the control valve.
Positive pumps with frequency converters are primarily used with viscous products and products containing fruit, if it is necessary to convey it in a particularly gentle way. Since it is not permitted to throttle the flow when these pumps are used, the control unit and a frequency converter are used to change the speed of the pumps so that the desired blending proportions are achieved.
In all components it must be ensured that no backflow can occur.
For the homogeneous blending of products with very different of very high viscosities, dynamic mixers are installed downstream of the blending unit. The dynamic mixer is a motor-driven agitator installed in a wider section of pipe. In the case of low viscosities a static mixer is used, in which high levels of turbulence in the liquid can be created by deflector plates.
No mixer is required if there is downstream equipment (pump, heater etc.) or a long pipe that will ensure good blending.
It is advisable to have the whole plant set up on a base frame, since this makes it substantially easier to install the equipment. The control cabinet with the most important measurement and control devices should also be assembled and completely wired up in advance. This cuts the time required for installation and reduces the risk of product losses, since most of the functions can be tested at the manufacturer’s before delivery.
Control and regulation
Another important prerequisite for high product quality is a high-quality control unit. A digital control unit specially designed for in-line blending plants offers the following advantages:
- inputs for volume signals allow regulation that responds precisely to the pulses, without additional error arising through the signal conversion;
- error statuses of short duration are eliminated by regulation; i. e. a deviation in the positive direction caused for example by a sudden rise in pressure in a component will be compensated for within a short time by a correction in the negative direction.
The points listed above are essential criteria for the decision to install a digital regulator, especially when a high degree of quality consistency is required or when expensive products are being blended.
Nowadays it goes without saying that recipes can be stored in the control unit. But these should not only contain the blending proportions, but also the target values for analysis, limit values for monitoring and, where relevant, regulating parameters (important when the mixing proportions are very varied), in order to be able to react flexibly to any changes. The entry of quantities per „unit” and ingredient in the recipe memory simplifies operations and the subsequent verification of the recipes. It is from this data that the control system automatically calculates the blending proportions.
In addition to the regulating function proper in respect of the proportions and the control functions, the plant control can where required also perform the following tasks:
- automatic correction of the blending proportions as between sugar solution and water, if the Brix value of the sugar solution fluctuates;
- continuous correction calculations, e. g. for temperature compensation when ingredients with high coefficients of expansion are blended (not necessary if mass flow meters are used);
- calculation of blending proportions from product-specific values;
- monitoring of the plant;
- logging of the process and of the process data;
- prevention of unauthorised interference with target values, regulating parameters etc.;
- control of the forward propulsion and expulsion of the product with as little loss as possible, and of the cleaning of the plant as a whole or in part.
Monitoring
Particular attention must be paid to a high level of safety in the manufacture of products. The following measures are recommended:
Deaeration: Deaeration vessels prevent air from remaining trapped in the product, being measured with the product and thus leading to dosage errors.
Product shortcoming: Sensors in the deaerating vessels stop the conveyance of the product if there are any shortcomings in the product. The outage of one sensor must not lead to any malfunctioning, and must be registered by the control system.
Raw material monitoring: The measurement of the conductivity or density of the concentrates or basic materials makes it possible to check that there has been no inadvertent substitution of one ingredient for another and/or that the blend complies with the specifications. This is an important measure especially when different products are to be blended successively or different containers attached.
Correct functioning of the flow meters: Limit values are stored for the throughput of every ingredient. As soon as any malfunction occurs (e. g. outage of a flow meter, non-opening or only partial opening of a control wave), the plant is switched off.
Monitoring of regulating deviations: Deviations must be compensated for within the shortest possible time. If this is not possible for any reason (a defective control valve or pump, excessive water pressure etc.) the plant gives an alarm.
Testing vessel: The accuracy of the flow meters is automatically tested at regular intervals. This is done by comparing the known content of the deaeration and testing vessel with the measured quantity. If the deviation is too high, an alarm is given.
Exceeding of limit values: Measured values and setting quantities must remain inside predetermined limits.
Monitoring of agitators in the component tanks and containers, in cases where the products tend to sediment. Agitators must be activated some time before the start of the blending process. Products in the deaeration tanks must also be blended before production if a homogeneous blend is to be achieved even after a longish outage.
Functional monitoring of valves and pumps.
Control and analysis devices
An analysis of the blended product is advisable for quality control purposes. This can be effected in a buffer tank downstream of the blending station, or in the pipe. It can only be performed if the components have already been well blended. The following tests may be used, depending on the product:
Brix measurement: In the case of sweetened beverages this is done by means of density or refractometer measurement. Even small deviations in the liquid sugar and water doses are identified. But errors in the dosages of higher density concentrates also lead to changes in the measured value.
Concentrate measurement: For diet drinks both of the measurement principles described above may be used. Since neither the density nor the refraction index differs essentially from that of water, it is necessary to calibrate the device specifically for each beverage and to store the calibration in the recipe. Deviations from the target value of more than 1.5 to 2 per cent trigger an alarm.
Conductivity measurement: This serves as a check on the degree of acidity.
CO2 measurement: For carbonated soft drinks, beer and beer mix, e. g. by means of a DI-TRACO device with a measuring tolerance < 0.1 g/l.
Alcohol and original wort measurement: For beer mix and alcopops, e. g. by means of a DI-CHECK ultrasonic meter, and if need be, combined with a density meter.
Other physical parameters such as pH value, viscosity, turbidity and colour are used to check the product.
The measurement values are used to trigger alarms and to stop the plant if the tolerances are too great, or else to correct the blending proportions.
Summary
Improved measurement techniques and enhanced reliability, especially of the analysis devices, make the in-line blending technique highly suitable even for complex tasks, as it provides outstanding and reliable results while taking into account the safety functions that have been mentioned. From a financial point of view as well, in-line blending equipment is thoroughly worthwhile, if the investment in buildings and tanks that would otherwise be required is also taken into account. The rapid availability of the product and short reaction times to changes in production planning are other important criteria when deciding in favour of this blending technology.
In-line blending systems are successfully applied for the production of many multi-ingredient liquid products such as soft drinks, juices, spirits, beer mix, but also for liquid cleansing agents, shampoos, detergents etc.
The author:
Gerhard Krüger, born 1944 in Hameln/Germany; 1967 – 1971 studied electrical engineering in Hanover; 1971 – 1975 sales engineer for measuring and process technology at Siemens; since 1975 project manager responsible for design and sales in the food and beverage section of GEA Diessel GmbH in Hildesheim