Oxygen – in breweries known as the greatest enemy of the beer. It leads to oxidation and changes the flavor profile of beer negatively. The shelf-life is shortened. Therefore it is important for all steps to prevent oxygen gets into the finished beer. This is only possible if the beer is in contact with oxygen-free water, especially for blending. For this reason, the water has always to be degassed reliable. The possibilities for this are varied and diverse according to the results. In order to bring light into this darkness, the different ways of water degassing for breweries are shown and evaluated for economic and financial considerations.
A lot of air is always released in the water. The oxygen of the air damages the quality, aging and stability of the beer. At various points the water comes in contact with beer. If the water contains dissolved oxygen, it damages the beer. Therefore, the water must be degassed.
How much oxygen may be in the water?
Before looking at the process, you have to make clear about the residual oxygen levels in water– and which values should be achieved and how they are generally accessible with the different methods. The oxygen content in water is 10 to 15 ° C and ambient pressure about 8 to 12 ppm.
According to the current demands of breweries, the beer after the high-gravity method has to have an amount of residual oxygen content in water from <0.01 to <0.02 ppm (<10 to <20 ppb). For example, if the water is used for the alluvium in the filtration or rinsing after cleaning. Oxygen values between 0.05 to 0.1 ppm are sufficent. Since each brewery has other demands on their water, they must decide for itself which residual oxygen levels they want to achieve.
Equilibrium curve / Concentration of oxygen
Physical division of the degassing process
For the generation of oxygen-poor water, you can use several methods of degassing. Leaving the chemical / catalytic processes in which is the addition of a reducing agent, the chemically bound oxygen should be ignored, since they are unsuitable for the production of food. It remains only the physical processes. These can be divided into three different methods:
The use of stripping gas is disturbing the balance of gases dissolved in the water. The concentration of oxygen in the gas phase is lowered by the stripping gas. The phase equilibrium shifts and oxygen passes from the water into the gas phase. Typically, CO 2 used as a stripping gas, which is the one in the beverage industry. It is almost always available and has on the other hand a very good water solubility and suppression effect on the dissolved oxygen. In addition to CO 2, nitrogen N2 can also be used. This gas is cheap and its emission has no negative effects on our climate.
2. Hot Water degassing
Before degassing the water is first heated. It takes advantage of that the gas solubility of water decreases (see figure) with increasing temperature of the water. The water can therefore absorb less oxygen at higher temperatures. With the exclusive application of this method, required oxygen levels can not be reached. Only in combination with the 1st Method requirements of the residual oxygen content can be produced with hot deaeration of water.
3. Vacuum degassing
Like the thermal degassing reduced, a lower pressure is almost proportional to gas solubility (see Figure). Under vacuum decreases the oxygen content of the gas surrounding the water. Oxygen comes out of the water in the gas phase until the restoration of the phase-
equal weight. Economically and with reasonable technical effort a vacuum of about 70 mbar absolute can be created with liquid ring pumps. The values of 1 ppm oxygen can be achieved. The required value is often less than 0.02 ppm, the lowering of the O2 concentration to this level is so not possible. Again, only in combination with the 1st Method degassed water is produced, which meets the requirements of the residual oxygen content.
This first observation leads to the situation that one can distinguish three processes by which useful results are obtained:
- Cold degassing under ambient pressure and use of a stripping gas
- Cold degassing under vacuum and using a stripping gas
- Hot degassing under ambient pressure and use of a stripping gas.
Alone on the classification of physical processes may not have any statement about the achievable residual oxygen levels and the required energy and strippgas consumption can be made. These parameters are mainly used by the degassers, its structure, dimensions and affects efficiency.
Comparison of degassers
The optimum contact apparatus must to have a continuous flow and the largest possible contact area between the degassed water to the stripping gas and produce. In this case the stripping gas and the water to be deaerated should preferably in countercurrent flow to each other. This ensures a maximum concentration gradient. For the transition of the oxygen concentration in the stripping or the compensation of course, a certain dwell time is required. At the same time the contact apparatus has to be fully CIP capable. The cost of the apparatus and the energy consumption must likewise be considered.
The known devices are examined below with respect to these properties, and evaluated. It will not be continuous processes such as A batch degassing tank is not considered.
Comparison of the different contact surfaces
1. Column degassing
A column with packing or structured material produces a large a specific surface. The area is sprinkled with water to be degassed. This flows downward through the packing elements and collected at the column base. Simultaneously, the CO2 flows countercurrent to the water upward through the column. Due to the great height of the column, the contact time between water and stripping gas is sufficiently long. These conditions can be a very good degassing of oxygen to reach values of <0.01 ppm.
Column degassing is characterized by its simple structure of plant technology. It is virtually maintenance free and can be cleaned with all the usual cleaning and temperatures. The required space of such a system is low. Depending on the procedure and required oxygen content and throughput but requires a ceiling height of four to six meters for the preparation of the stripping column.
2. Spray degassing
In the spray-water CO2 in one or more evacuated container is sprayed cold. By operating the vacuum and the use of CO2 by a good degassing initially be achieved, but at relatively high CO2 consumption. Single-stage Spray degassing sufficient for the required oxygen levels in the soft drinks industry often sufficient.
Very low residual oxygen levels, as they are needed in breweries, cannot be achieved. Due to the lack of counter-current effect , the comparatively small contact time and exchange surface is needed values are not reached. It is only through a multi-level and expensive arrangement with a significantly higher CO2 consumption possible.
3. Membrane degassing
Flows in specific membrane modules, the water flows around the outside of the hydrophobic hollow fibers in countercurrent to inside the CO2. The hollow fibers produce a very large contact surface between the water to be degassed and the stripping gas. Only gas and no water can pass. The membrane degassing at low degassing is a compact and efficient degassing of residual oxygen levels below 0.01 ppm and very low CO2-consumption and low energy consumption.
For larger point, however, the number of required membrane modules degassing increase and the linear investment costs, too. The CIP cleaning is possible only in a limited temperature range and not with all usual detergents. A membrane degassing has high demands on the water quality. Particles can clog the membrane modules form or dissolved water constituents a deposit on the membrane. Both lead to a deterioration of the degassing.
Comparison of the contact surfaces
The summary of this comparison is that the degassing column is best on investment costs, operating costs and efficiency. In most cases the most appropriate contact apparatus for degassing. In special cases, e.g. in the absence or low headroom installation services can also membrane degassing (cold, under vacuum) represent the most appropriate method (see Table 1).
With the column as a contact system, all degassing methods can be applied to all early described. All are widely used in practice and hereafter considered (see Table 2).
classification of procedures for column degassing
1. Column degassing – Cold
When used alone cold column degassing is the concentration difference for countercurrent flow and the very large mass transfer area of the pack along for gassing. Column temperature and pressure correspond to environmental conditions. This residual oxygen values <0.05 ppm O2 can be reached in the water. The water is fully saturated with CO2, so that it contains after degassing more than 2.0 g / l CO2. In addition, a slight excess of CO2 is required, resulting in a total consumption of 2.4 g / l CO2 results.
Advantage is that no additional resources as needed, for example steam. The cost of such a plant are low. If more cooling is needed there is an additional cooling with glycol followed.
2 Column degassing – Hot
The water in this process is usually heated to a temperature of 74 ° C and then degassed. As already described, the increase in temperature lowers the gas solubility of the water and the mass transfer is improved. In this way, the CO2 consumption is reduced and the oxygen removal are enhanced – all the way to values well below 0.01 ppm. A positive side effect is pasteurization of the water and the low CO2 consumption of 0.8 g / l to 0.5 g / l CO2 can be dissolved in the water .
The disadvantage are the increased consumption of steam and glycol, and the high cost of equipment associated with high investment costs. Microbiological safety and lowest oxygen values, combined with a low CO 2 consumption, however, justify the use of this facility.
3 Column degassing– Vacuum
In contrast to the hot deaeration degassing vacuum is used to lower the pressure for reducing the gas solubility. This increases the turbulence in the column and removal of O2. Residual oxygen levels of <0.02 ppm (<20 ppb) can be achieved easily and at a very low CO2 consumption of only 0.4 g / l, 0.2 g / l CO2 are dissolved in the water.
There is no need for steam, only slightly higher investment costs and electricity costs for operating the vacuum pump will be added over a cold degassing. The costs are amortized relatively quickly by the significantly lower CO2 consumption. The pressure drop is thus the most efficient and economical way to measure very low oxygen levels to reach the water. In order to ensure microbiological safety, the degassing of a UV system be installed downstream.
When deciding on an appropriate method, it is important to consider the following factors:
- Required oxygen content
- Required throughput
- investment costs
- availability of supplies steam, glycol and CO2
- Specific costs for fuel and energy
- space and available space height
- CIP capability and cleanability.
In summary, it should be noted that none of the previously studied degassing is optimally suited for specific conditions. Rather, in dialogue with the brewery’s individual requirements and circumstances should be analyzed. After that, the most appropriate method column degassing as cold, heat, vacuum and membrane degassing under vacuum can be selected.
Project manager, corosys Prozeßsysteme und Sensoren GmbH (www.corosys.com)
Managing director, corosys Prozesssysteme und Sensoren GmbH (www.corosys.com)
Head of Production and technical sales, Flensburger Brauerei Emil Petersen GmbH & Co. KG (www.flens.de)