News - Variable speed drive pumps for food & beverage industry
August 2023
Variable speed drive pumps for reverse osmosis units for food & beverage industry – example: dairy
Technical paper by Andreas Pentke M.Eng. and Dr. rer. nat. Sven Fleischer
Water is an essential component for the production of Food & Beverages. It is required for products as product water and for other processes as process water. Since its composition as a raw material is usually not suitable for direct use, the water must be treated to meet the specification of its intended use. The following example illustrates how this can be done in a particularly efficient and resource-saving way e.g., two reverse osmosis plants (RO) with an integrated concentrate recycling stage (KS stage) in a dairy factory in Scandinavia.
The raw water used in the dairy is surface water that comes from the immediate vicinity of the production site. During water treatment, the raw water is first treated with gravel and sand filters to separate undissolved turbidity and suspended materials. This procedure prevents deposits in the pumps and pipelines. In the following step, the salt concentration of the pre-treated water is adjusted to the requirements of the process water using the HERCO UO-S7 50.000 AS/KS/FU reverse osmosis unit.
The principle of reverse osmosis in water treatment
To understand what reverse osmosis is, this process should be briefly explained. Osmotic pressure is created by the presence of two liquids with different chemical potentials (e.g., salt concentration) separated by a semi-permeable (selectively permeable) membrane (Fig. 1 left).
In nature, this causes water to flow from the fluid state with lower chemical potential to the fluid state with higher chemical potential to balance the two potentials. The gradient that causes this is named as osmotic pressure (Fig. 1 right).
Reverse osmosis counteracts this natural process. With the help of multi-stage high-pressure booster pumps, the reverse osmosis system transports the water from the side with the higher concentration towards the lower concentration, i.e., the osmotic pressure is overcome, so that the substances dissolved in the water are separated, except for a small proportion.
The liquid that comes out purified through the membranes is called permeate. The liquid that remains is called concentrate (wastewater). To evaluate the system in terms of its efficiency of the material flows, the term yield (or recovery) is used. In practice, reverse osmosis systems are operated at pressures from about 10 bar up to 80 bar or even 120 bar (high-pressure RO), depending on the raw water quality.
The permeate yield of a RO system depends on the composition of the raw water (salt content in μS/cm), the pressure applied by the pump, the media temperature and the type of membrane used in the process.
Figure 2 illustrates how raw conductivity affects the amount of pressure required for the pump to overcome the osmotic pressure. On the other hand it depicts, how much power consumption of the VSD pump is required for this (including frequency converter).
The pressure requirement and power consumption increases along with the conductivity in the raw water. To be able to react to different raw water qualities, the use of a frequency converter is recommended to ensure only the necessary energy is used. Designing the pump for a maximum value would mean that the system would have to be throttled in the feed to the membranes, e.g., by a control valve, so that the correct pressure prevails transmembrane, and the membranes are not unnecessarily loaded. This would be tantamount to unnecessary energy dissipation.
Technical data of the reverse osmosis system for water treatment:
In the following, the two reverse osmosis plants planned and built for the project in Finland will be described in detail. The project scope includes two individual plants, each providing a maximum process water output capacity of 50 m³/h (Fig. 3).
Figure 3: UO-S7 50.000 AS/KS/FU
Technical data (per unit):
• Operating pressure: approx. 17 bar
• Dimensions: Length x height x depth: approx. 4,000 x 2,500 x 1,800 mm
• Weight: approx. 4,500 kg
• Number of pressure vessels: 18 pieces
The units are manufactured with a low dead zone (EHEDG-/FDA-compliant hygienic design) and made of high-quality stainless steel (AISI 316L) using orbital welding. The low dead space design ensures that the flow through the unit is complete and that no germs can multiply in flow dead zones.
As shown in Figure 4, a CIP tank (Cleaning in place) is provided in the inlet of the unit, in which a cleaning medium for cleaning the RO units is fed if required.
Figure 4: Process diagram of the UO-S7 50.000 AS/KS/FU
Dosing pumps are provided for dosing the chemicals (cleaning agent and antiscalant (hardness stabilizer)) (Fig. 5).
Two multistage regulated VSD centrifugal pumps (Fig. 6) are installed at the reverse osmosis system. Furthermore, they can be adapted to the required throughputs during production or cleaning for optimal energy-efficiency.
Figure 5: Dosing pumps for cleaning chemicals and antiscalant
In addition, by using the KS stage, approx. 50 % - 60 % of the accumulating concentrate volume can be fed back into the feed of the reverse osmosis plants to the raw water inlet (Figure 4). The term KS stage (German abbreviation for Konzentrat-Recycling-Stufe) is explained by the fact that the concentrate of the first reverse osmosis stage is processed in a second nanofiltration stage for higher salt contents. On one hand, the concentrate is increased in its salt content, but on the other hand, water as a valuable raw material is reduced in its salt content and recovered. This can be recycled to the raw water intake after further purification.
An integrated Siemens S7-1.200 control unit enables for Profinet Interface communication with the plants higher level control room.
Figure 6 Frequency-controlled high-pressure pump
Savings and efficient continuous operation of the water treatment plant:
By using energy-efficient, frequency-controlled high-pressure pumps, energy savings of 30 % - 50 % are possible in the reverse osmosis system compared to the systems without a frequency controller. In addition, VSD pumps allow the systems to start up smoothly, which protects the connections in the systems and the membranes and is reflected in particularly quiet operation.
The KS stage integrated in the reverse osmosis systems increases the total yield of the systems to up to 90 %. Without this second stage, the yield is between 75 % and 80 %. As a result of the higher efficiency, less raw water is required, which also conserves resources and produces less wastewater.
Product portfolio for water treatment
Herco has been active in the field of industrial water treatment for over 75 years and stands for its “made in Germany” quality. The focus is on the production of pure and ultrapure water systems. In addition to reverse osmosis systems, softeners and pressure boosting systems are also produced at the site in Freiberg am Neckar (Baden-Württemberg). Furthermore, to standard sizes, Herco develops water treatment solutions tailored to the customer's application. In the food sector, not only dairies but also breweries or wine producers are customers of the reverse osmosis systems described.
Systems with hot sanitization (up to 85 °C), blending devices (e.g., for adjusting the carbonate hardness in the brewing industry) or UV treatment of product water can also be integrated into the treatment lines.
Moreover, the above-mentioned system provided to our customer included an integrated KS stage. Additional KS stages can also be retrofitted separately as supplements for existing systems.
The advantage of a downstream stage is the possibility of treating the concentrate independently of the existing plant. This means that the existing production process is not affected. Before a water treatment plant is delivered, it is tested in the company's own test field.
You can also find the complete article here (in German).