COMPARISON OF THE PROCESSES USED TODAY
In breweries and in the production of beverages in general, deaerated water – which actually means largely oxygen-free water – is used in numerous process steps for the kieselguhr preparation and the continuous KG dosing, for the precoating at the filtration, for pushing out the pipe to the bottle filler, for the last rinsing step after cleaning and last but not least for adjusting the original gravity or the alcohol content of the beer after filtration or high gravity blending. Especially in blending and the production of beer-mixed beverages, the use is absolutely necessary in order not to directly increase the oxygen content in the beer or finished product. This article describes the physical principles of water deaeration and compares the known processes for deaerating brewing water with regard to the possible residual oxygen content, consumption and operating costs, cleanability, space requirements and investment costs. Thus, it should help the potential user to select the most suitable process for his needs.
Especially in breweries, the requirements for degassing now go to very low residual oxygen contents of less than 0.02 ppm. Today, manufacturers offer degassing processes and plants that differ significantly in terms of process and equipment technology and the achievable residual oxygen contents. In general, as the residual oxygen content decreases, the process and apparatus engineering effort increases and thus also energy consumption and investment costs. The user should therefore clearly define from the outset what minimum plant capacity he needs and what residual oxygen content is effectively required for his manufacturing process or for maintaining his quality parameters.
In the brewing or spring water of breweries and beverage companies, gases and also oxygen are always dissolved, whose concentration depends on the respective environmental conditions. These environmental conditions are essentially the composition of the atmosphere, the temperature and the pressure, less so the humidity or the hardness of the water.
Figure 1: Equilibrium curves
The oxygen content in water, see Figure 1, is about 8 to 10 ppm at 10 to 15 °C and ambient pressure. In order to remove the oxygen from the water, the following process engineering measures are used individually or in combination:
– Reduction of the operating pressure (vacuum),
– Increase the water temperature,
– Partial pressure reduction by using a stripping gas, if possible in countercurrent operation,
– Creation of the largest possible contact or mass transfer surface between liquid and gas phase in the degassing apparatus,
– Ensuring the longest possible contact time between liquid and gas phase,
– Chemical bonding / reaction of oxygen in an oxygen-free atmosphere, also catalytic degassing.
These measures, with the exception of chemical/catalytic degassing, are examined in the following with regard to their effect and their effective combination possibilities.
Influence of pressure and temperature
In order to evaluate the influence of pressure and temperature, the physical principles of gas solubility in water and the possible factors influencing the oxygen content will be considered first. For this purpose the oxygen content for different operating pressures as a function of temperature at atmospheric composition is shown in figure 1.
It can be seen that the gas solubility decreases with increasing temperature and decreasing ambient pressure. The dependence on pressure is almost proportional, the dependence on temperature is comparatively low. Furthermore it is noticeable that the achievable O2 concentrations for the complete considered temperature and pressure range are in the ppm range. A reduction of the O2 concentration in water to the often required values of 0.02 ppm is therefore not possible by moderate pressure and temperature changes. Only through extreme conditions, i.e. a reduction of the operating pressure to below 2 mbar and temperatures of at least 20 °C, the 0.02 ppm O2 can be achieved – and this only under equilibrium conditions that are practically impossible to achieve. In addition, a plant capacity of 100 hl/h would already result in an uneconomically high exhaust gas volume flow of far more than 100 m2/h. For this reason, an additional component, the so-called stripping gas, is used in practice and thus in all processes considered here.
Partial pressure reduction by using a stripping gas
By using the stripping gas the oxygen content in the gas phase for the range of very low O2 concentrations shall be reduced and the phase equilibrium shall be shifted. Typically CO2 is used as a stripping gas, which is almost always available in the beverage industry and has a very good water solubility and displacement effect on the dissolved oxygen.
Figure 2: Henry law
The effect can be seen in figure 2. By using the additional component CO2, the oxygen content in the gas phase can be shifted to small values and – according to Henry’s law – the oxygen concentration in the water can be proportionally lowered down to small residual values in the water. By increasing the temperature, see red curve, and especially by vacuum, see blue curve, significantly lower residual oxygen values in the water can be achieved with comparable oxygen content in the gas phase.
However, figure 2 also shows that a very low oxygen content in the gas phase is necessary to achieve the required oxygen values of less than 0.02 mg/l. In order to avoid the very large and thus uneconomical stripping gas quantities required for this purpose, all modern and efficient degassing processes use countercurrent flow.
Fig. 3: Concentration profile in gas and liquid phase along a countercurrent degassing system
Therefore (as shown in the diagram in figure 3 using the example of a column degassing) the stripping gas, shown in red, is fed in countercurrent to the water flow to be degassed, shown in blue. The advantageous concentration curve is shown in the diagram: The water is fed with about 10 ppm at the top of the column, flows into the sump and is continuously degassed by the inflowing CO2. The water, which is already largely oxygen-free at the bottom of the column, is stripped with the oxygen-free CO2 that has just been fed into the column – thus maximizing the concentration difference and thus the degassing. As a result, the stripping gas consumption and at the same time the residual oxygen content in the water can be significantly reduced.
Influence of mass transfer surface and contact time
The effects considered were based on the equilibrium state between gas and liquid phase. In fact, the equilibrium state can only approximately be reached due to the temporally limited contact time between gas and liquid phase. In order to come as close as possible to the state of equilibrium, a mass transfer as extensive as possible is required.
The factors influencing the mass transfer m between two streams are described in Fick’s 1st law:
(1) m = DA0 ∫ T (∂c/∂x) dt
The diffusion coefficient D is very unfavorable for oxygen/water with D20 °C = 2,1·10-9 m2 and can be influenced only slightly. For example, an increase in temperature from 20 to 80 °C only increases the diffusion coefficient by about 20 percent. Therefore, the largest possible mass transfer area A and contact time T must be ensured in order to achieve an extensive mass transfer. This is produced in different ways in the various processes.
The greatest possible concentration gradient (dc/dx) can in turn be achieved most efficiently by a consistent countercurrent flow of stripping gas to the water and a turbulent, surface-regenerating process control.
Additional design influencing parameters
Kl* is the liquid-side mass transfer coefficient and a is the specific liquid surface area in m2/m3. In addition, the mass transfer Kl*a can be individually intensified by design measures for the individual processes. For a modern stripping column, according to investigations by Zuiderweg (F.R.I. Report 92):
(2) Kl*a ~ wGwl 0,58*ReG -0,4*ScG-2/3
In contrast to the gas-side mass transfer coefficient KG*, Kl* is transport determining and can therefore be used alone for the estimation of the mass transfer. For the calculation of the mass transfer according to (2), wG and wl are the gas and liquid velocity and ReG and ScG are the Reynolds (turbulence measure) and Schmidt number (viscosity diffusion ratio) of the gas phase, which can be significantly influenced by design and process engineering measures and only this enables an economic degassing to the values required today.
Comparison of the processes
All common degassing processes like
– membrane degassing,
– Spray degassing,
– Column degassing cold under ambient pressure,
– Column degassing hot under ambient pressure,
– Column degassing cold under vacuum
are based on the use of these influencing parameters.
The design and dimensioning of the degassing apparatuses may well affect the influencing parameters described above in the opposite direction. For example, a high flow velocity and thus high turbulence usually means a shorter contact time. It is up to the plant manufacturer to find the optimum compromise based on experience, calculations etc.
In membrane degassing, the water to be degassed is guided along hollow fiber membrane bundles through which gas can easily diffuse but no water can permeate. On the other side of the membrane inside the hollow fiber, a strong driving concentration gradient of the oxygen towards the stripping gas side and thus an efficient degassing is achieved by using CO2 and applying vacuum. Due to the countercurrent flow, the high concentration gradient is maintained even with only a low oxygen content in the water. Thus, with appropriate dimensioning, i.e. series connection of membrane modules, residual oxygen values below 0.02 ppm can be achieved.
Membrane degassing is a compact and efficient degassing method for small degassing capacities with very low CO2 consumption and energy requirements. Towards higher degassing capacities, however, the number of membrane modules required increases – since these have to be connected in parallel to achieve the throughput – and thus the investment costs increase relatively strongly, so that other degassing methods such as column degassing are generally preferable. The CIP cleaning used in the beverage and food industry is only possible in a limited temperature range and not with all common cleaning agents and additives.
In spray degassing, water is cold atomized with CO2 in one or more evacuated containers. Due to vacuum operation and the use of CO2, good degassing with relatively low CO2 consumption can be achieved initially. Single-stage spray degassing is often sufficient for the oxygen values required in the soft drinks industry and is therefore widely used in this field. However, very small residual oxygen values can only be achieved by a multi-stage and therefore expensive arrangement with significantly higher CO2 consumption due to the lack of countercurrent effect and the comparatively small gas/liquid contact time and exchange surface (droplets approx. 100 µm).
Therefore, in existing spray degassing plants it can often be useful to add a membrane stage with CO2 coupling to the existing plant in order to achieve the oxygen values and CO2 consumption required today. According to a patent of corosys GmbH this can be done without additional CO2 consumption. The spray degassing can be cleaned without restrictions with all common cleaning agents and temperatures.
Column degassing cold, hot and vacuum are all performed according to the same principle. A column with packing or structured packing to create the largest possible surface area is sprinkled with the water to be degassed. The water flows along the packing into the lower part of the column and is stripped in countercurrent with CO2, which is fed into the lower end of the column. This creates a high driving concentration difference for the oxygen from the water phase to the gas phase along the entire sprinkled packing.
The three processes differ significantly in the use of physical effects for efficient degassing:
Column degassing cold
In cold column degassing, only the concentration difference with countercurrent flow and the very large mass transfer surface along the packing is used for degassing. Temperature and column pressure correspond to ambient conditions. This allows residual oxygen values < 0.05 ppm O2 in the water with a relatively high CO2 consumption of 2 to 3 g/l, of which, however, by far the largest part is dissolved in water and is not lost.
Column degasssing hot
In hot column degassing, the water is heated to usually 72 °C (pasteurization temperature) and is degassed at this temperature. The increased temperature reduces solubility and improves mass transfer, thus increasing O2 reduction and reducing CO2 consumption. Residual oxygen values ≤ 0.02 ppm O2 in the water are achieved. Additionally, the water is pasteurized by hot stripping. However, the investment costs increase significantly due to the heating and cooling process required for this. Also the operating costs increase especially at small capacities due to the additional heating energy and heat losses, so that this process is almost only interesting for large to very large capacities.
Column degassing vacuum
Finally, the column degassing vacuum developed by corosys GmbH – in contrast to hot stripping – uses the lowering of the operating pressure to increase the partial pressure gradient and thus for O2 reduction. This pressure reduction is – as we now know – the most efficient method to achieve very low residual oxygen values in the water. On the one hand, the comparatively lowest partial pressures for small residual oxygen quantities are achieved by the vacuum and on the other hand, significantly higher gas velocities and thus improved turbulence and mass transfer according to formula (2) are achieved. Residual oxygen values < 0.02 ppm O2 in water are reached.
In addition, the gas solubility and consequently the CO2 solubility in water is greatly reduced, resulting in significantly lower CO2 consumption.
The investment costs increase slightly compared to pure cold stripping. However, these somewhat higher investments pay for themselves relatively quickly due to the much lower CO2 consumption. Vacuum stripping is the most efficient and economical process for degassing medium and large capacities to residual oxygen levels below 0.02 ppm.
All strip degassing units can be cleaned without restriction with all common cleaning agents and temperatures. The space requirement is low. However, depending on the process and the required residual oxygen content and throughput, a room height of 5 to 13 meters is required for the installation of the stripping column.
Factors in the decision for the most suitable process
When deciding on the most suitable process, the following factors must also be taken into account:
– Available media and energies and their specific costs,
– Cleanability/CIP capability,
– Maintenance requirements,
– Operating costs,
– investment costs,
– Required residual oxygen content,
– Required flow rate.
In addition, customer-specific decision criteria such as space requirements, water temperature, CO2 purity, etc. must be taken into account. Under these boundary conditions, the optimum process for the respective operation must be found out.
Typical applications and use cases
A smaller 50 000 hl/a brewery needs degassed water only for the precoating of the filters and for flushing out. An existing tank can be used to buffer degassed water. This is sufficient to cover the short-term high demand for degassed water. For this task a very small membrane degassing is ideally suited. This can run continuously and fill the tank. The investment costs and also the operating costs of a membrane degassing system are minimal in this case.
Column degassing vacuum or hot
A large foreign 2 500 000 hl/a brewery needs the degassed water not only for the precoating of the filters, but also mainly for blending of high-gravity beer. The purchase quantities can be up to 450 hl/h if all filters are operated simultaneously. The oxygen value must be as low as possible to avoid an increase of the oxygen value in the blended beer. For this task a column degassing vacuum or hot is suitable. Both systems achieve oxygen values of 20 ppb or less. Hot stripping is more expensive to purchase and operate than vacuum stripping due to the steam and glycol consumption. For this application the vacuum stripping must be equipped with a UV disinfection system. With hot stripping, this is not necessary due to the simultaneous pasteurization of the water at 72 °C.
Column degassing cold
A medium-sized 200 000 hl/a private brewery requires degassed water in addition to the usual purposes in the brewery for the production of beer-mixed drinks, soda water and the adjustment of the original gravity after filtration. A high CO2 content in the water is definitely an advantage here. For this purpose a simple cold column degassing is optimal with regard to investment and operating costs. The plant reliably supplies water with significantly less than 0.05 ppm O2 and approx. 2.0 g/l CO2. It has a simple design, is inexpensive to purchase and nearly maintenance-free. The plant was realized as a unit together with the fine blending and carbonation plant. A central control system and the connection of the filtration, the buffer tank for deaerated water and the subsystems via a panel allow not only fine blending and carbonation after filtration but also cooling of the deaerated water and production of soda water. The control of the buffer tank and the supply of deaerated water to the various consumption points within the brewery is also fully automatic through the integrated Simatic S7 300 control system.
Dr.-Ing. Matthias Stumpf
Technical manager, corosys Prozeßsysteme und Sensoren GmbH, Hofheim am Taunus
Dipl.-Ing. Stephan Dittrich
Managing director, corosys Prozeßsysteme und Sensoren GmbH, Hofheim am Taunus
Technical manager, Brauerei Zwettl Karl Schwarz GmbH