The problem is the solution

Deaeration of water in the beverage industry

All beverages react sensitively to the presence of oxygen in the product. It has a long-term impact on taste, colour and shelf life. In addition, oxygen dissolved in the product has a negative effect on the bottling or packaging process, as the beverage tends to foam up. Deaeration (in this case the removal of dissolved oxygen), combined with a greater focus on stability of taste and an increased unwillingness to add anti-oxidants (ascorbic acid or sulphur dioxide) has been gaining steadily in importance.

Whereas in the early days of water deaeration systems, it was considered to be adequate if a value of approx. 0.1 mg/l was achieved, nowadays residual oxygen content values of 0.02 mg/l or better are demanded. The theoretical basis for the deaeration of water is Dalton’s Law, which states that the total pressure of a gaseous mixture is the sum of the partial pressures of the component gases (p = pA + pB + pC).

This means that oxygen can dissolve in water only up to the point at which there is a balance between total pressure and partial pressures. To deaerate the water, the ratios of the partial pressures must be shifted in such a way that the partial pressure of oxygen is as small as possible. This can be done by reducing total pressure, increasing the steam pressure of the water or increasing the partial pressure of another component.

Deaeration processes

Fundamentally, deaeration processes fall into the following categories:

Vacuum deaeration
In the vacuum deaeration process, the water is sprayed into a vessel through fine nozzles. This fine distribution increases the area of the water/gas transfer surface. The vacuum causes the oxygen to be released, and it is then extracted from the system by suction. This effect is assisted by a strip gas. In this process, however, the size of the transfer surface and the contact time are limited by the design of the equipment. Generally, residual oxygen contents of approx. 0.07 to 0.1 mg/l are achieved. The use of a seal water pump to create a vacuum makes the equipment more expensive.

Hot deaeration
In this process, the water that is to be deaerated is heated to just below boiling point and the oxygen thus "boiled out". As this procedure is mainly used to deaerate boiler feed water, it is mentioned here only for the sake of completeness, and will not be considered further.

Column deaeration
In the column deaeration process, deaeration is achieved by having a large transfer surface and long contact times. A strip gas is introduced against the flow of the water, enhancing the release of oxygen and flushing it out of the column. The column is filled with packing which offers a large specific surface. Extremely good residual oxygen contents of approx. 0.01 mg/l can be achieved with this process. Column deaeration systems are simple in design and therefore largely maintenance-free. Their disadvantage is their great height of up to 8 m or more.

Membrane deaeration
Membrane deaeration exploits the physical effect of semi-permeable membranes, which are permeable only to certain substances. The membranes used are hollow fibre membranes, which offer a very large transfer surface. An additional vacuum on the gas side creates a large partial pressure difference. The use of a number of membranes in series allows a good deaeration performance with a residual oxygen content of approx. 0.02 mg/l to be achieved. However, the membranes themselves are relatively expensive and not very sturdy. Furthermore, in view of their structure with its many openings, membranes appear to be hygienically questionable, and their suitability for CIP is limited.

All these processes are generally assisted by a strip gas. The strip gas has the tasks of shifting the ratios of the partial pressures and of removing the oxygen from the system. A gas of lower reactivity such as nitrogen or carbon dioxide is required as the strip gas. In practice, carbon dioxide is generally used, as it is required in any case for carbonated drinks. Combinations of the processes described above are also often used.

 

Water deaeration systems

GEA Diessel’s deaeration systems were developed specially for the requirements of the brewing and beverage industries. They are completely CIP-compatible and simple to operate, and achieve residual oxygen content values of as little as 0.01 mg/l.

DIOX-2
In the first stage, vacuum deaeration is used to remove the greater part of the oxygen. The partially deaerated water is then transferred to the second stage, where it is enriched with carbon dioxide. The carbon dioxide brings about the release of additional oxygen. The carbon dioxide/oxygen mixture is sucked out of the vessel by the vacuum pump in the direction opposite to that of the spray jet, thus enhancing the deaeration effect of the first stage. The deaerated water is conveyed to the point of consumption by a pump.

The use of seal water is minimised by the integrated temperature control. The equipment is controlled and monitored by sensors for pressure, level, and if required oxygen.

The product line is also used for cleaning the water deaeration equipment.

Special features:

  • Residual oxygen content approx. 0.1 or (0.07) mg O2/l
  • Low operating costs
  • Low CO2 consumption
  • Compact design
  • Factory-tested unit
  • Can be supplied for flow rates of between 5,000 and 65,000 l/h
  • Option: vacuum pump without seal water
  • Option: monitoring of O2 measurement

VARIDOX-C
This water deaeration system works on the principle of trickle deaeration. The water to be deaerated is fed in at the top of the deaeration column. A strip gas of low reactivity (carbon dioxide, nitrogen) is introduced at the foot of the column. The water trickles, in the opposite direction to the flow of the strip gas, over the packing of the column, which has a large surface area in order to produce a brisk transfer of substances. The column is filled with structured high-performance stainless steel packing which is fully CIP-compatible. The strip gas brings about a reduction in the partial pressure of the oxygen dissolved in the water, so that the oxygen is released. The strip gas also has the task of blowing the released oxygen out of the system.

The water is enriched with carbon dioxide or nitrogen as it leaves the system. It is easy and inexpensive to enhance the degree of carbonation of the water leaving the equipment to a particular set value. The proper functioning of the system is monitored by sensors for flow rate, level and optionally also residual oxygen. The product line is also used for cleaning the system.

This process enables a residual oxygen content of approx. 0.03 mg O2/l to be achieved.

Special features:

  • Residual oxygen content approx. 0.03 mg O2/l
  • Low operating costs
  • Low wear and tear
  • Simple to operate
  • Factory-tested unit
  • Can be supplied for flow rates of between 5,000 and 80,000 l/h
  • Twin-column version can be supplied if there is insufficient height at the place of installation
  • Option: pre-carbonation
  • Option: monitoring of O2 measurement

 

 

 

 

VARIDOX-H in the twin-column version

 

 

 

 

 

VARIDOX-H
This water deaeration system works on the same principle as the VARIDOX-C device just described, except that in this case the water is heated in a heat exchanger to approx. 72° C before being fed into the deaeration column.

The increased temperature sharply reduces the solubility of oxygen in the water. Thus a further enhancement of the deaeration performance is achieved, while little strip gas is required. A further advantage is the simultaneous pasteurisation of the water. In the regeneration stage of the heat exchanger, around 90 per cent of the energy is recuperated during cooling. In a further stage, the water is cooled to the desired temperature. The cooling circuit is equipped with protection against freezing. The proper functioning of the system is monitored by sensors for flow rate, level and optionally also oxygen. The product line is also used for cleaning the system.

This process enables a residual oxygen content of approx. 0.01 mg O2/l to be achieved.

Special features:

  • Residual oxygen content approx. 0.01 mg O2/l
  • Low operating costs
  • Low wear and tear
  • Simple to operate
  • Factory-tested unit
  • Can be supplied for flow rates of between 5,000 and 80,000 l/h
  • Twin-column version can be supplied if there is insufficient height at the place of installation
  • Option: pre-carbonation
  • Option: monitoring of O2 measurement.

Packing and column
The most important component of a column deaeration system is the packing inside the column. The packing used by GEA Diessel has a very high specific surface area, which directly influences the deaeration performance. The structured design leads to optimum flow patterns.

Outlook
With today’s technology, residual oxygen content values can be achieved that fully satisfy the requirements of the food and beverage industry. In some cases, the values achieved are so low that analysis technology is being pushed to its limits (the limit of demonstrability of oxygen dissolved in water is around 0.001 mg/l). From this point of view, it hardly makes sense to further optimise the equipment in respect of residual oxygen content values. It would be more useful to continue to improve the equipment in respect of its availability and operating costs.

 

The author:
Paul Günther, born 1967; trained as a toolmaker, studied process engineering at Fachhochschule Hannover and graduated as Dipl.-Ing. (FH). Since 2002 a project engineer at GEA Diessel GmbH in Hildesheim, Food & Beverage division.