Diploma in Beverage Packaging (Beer) Unit 3.2 Fluid Mechanics
Qualifications
Diploma in Beverage Packaging (Beer) Unit 3.2 Fluid Mechanics
PROCESS GASES
Learning Material © Institute of Brewing and Distilling 2013 DIPLOMA IN PACKAGING (BEER) - MODULE 3
UNIT 3.2: Fluid Mechanics
ELEMENT 3.12.2: Process Gases
ABSTRACT: One of the objectives of this module is to 3.2.2.3 Gas solubility: enable candidates to understand the issues critical to enable effective engineering design and operation. This Concept of gas / liquid equilibrium, Henry’s law requires an understanding of the critical parameters and (and as applied their relationship to enable the design specification to be to gas mixtures) achieved. In order to achieve this we need to express the Dependence of gas / liquid solubility on relationships of the parameters mathematically in the form temperature of equations. To those not used to algebra these equations Molar fraction / molar ratio might seem daunting! This need not be so. Think of the Effects of hydrostatic head equations as a form of shorthand to describe a relationship. Definition of supersaturation (how it occurs in The equation can replace many lines of text to describe the beer, its effects on and threats to beer quality same relationship. in the packaging operation, in the retail outlet LEARNING OUTCOMES: On completion of this unit you will and to the consumer) be able to: Calculations of equilibrium conditions of gas solubility for single gas and mixed gas systems 1. A working knowledge of the role of process gases in the management and control of process 3.2.2.4 Gas dissolution: technology for packaging Principles of dissolving gases in liquids 2. A qualitative knowledge of identifying non Typical equipment for measurement and conformances control 3. An understanding of process gases is essential for Dissolved gas adjustment (see also 1.2.3.2) the successful packaging of beverages especially Effects of pressure and temperature on beer. The gases used not only affect the carbonation level during storage of beer appearance of the product both head and main product but also the taste. Realistic gas 3.2.2.5 Carbon Dioxide: specifications are important to enable product to be packaged meeting customer requirements. Carbon dioxide recovery and pre-treatment SYLLABUS. Liquid CO2 storage and vaporisation methods Carbon dioxide specifications 3.2.2.1 Gases used in packaging and dispense: For carbonation see 1.2.3.2 Health and safety aspects Typical applications: - Blanketing / oxygen exclusion 3.2.2.6 Nitrogen: - Carbonation adjustment - Nitrogenation and use of CO2 and N2 Nitrogen specifications mixtures Supply, storage and vaporisation Awareness of on-site generation methods 3.2.2.2 Gas laws: (pressure swing / membrane / cryogenic) Health and safety aspect Equations relating to pressure, temperature, volume and density using the perfect gas laws: - Boyle’s - Charles’ - Gay Lussac’s Concept of “Standard Conditions” (STP, NTP etc) Universal gas law and gas constant Concept of molar volume Dalton’s law of partial pressures Calculations involving gas laws
2 Diploma in Beverage Packaging (Beer) Element 3.2.2 Process Gases
Introduction Units are very important in engineering calculations. It is important to use units from the same system. IBD exam questions use only the SI (System International) system of units. Ensure you are familiar and confident with these.
An understanding of units enables a series of parameters to be expressed in their basic units. This is very useful as a series of dimensionless groups are used to describe a physical phenomenon. An example of this is Reynold’s number which describes the degree of turbulence of fluid flowing through a channel. In order to obtain the correct figure ALL the parameters MUST use the same system of units. This can be checked by dimensional analysis where the dimensions of each parameter are entered into the equation in place of the parameter. In the case of a dimensionless group these dimensions should cancel each other out. This is shown below:
Reynolds number (Re),
Where, = fluid density (kg/m3) u = mean fluid velocity (m/s) d = pipe internal diameter (m) = fluid dynamic viscosity (kg/ms) Carrying out a dimensional analysis gives the following:
Re = kg * m * m * ms m3 s kg
The dimensions cancel each other out giving a dimensionless answer.
The major gases involved include: 1. Carbon dioxide 2. Nitrogen 3. Compressed air
Gas Application Notes Carbon dioxide Addition to product to achieve quality Traditional gas used in breweries. specification. Can be recovered from Excluding oxygen from process plant fermentation and vessel downstream of fermentation including headspaces (rough and bright purging empty containers. tanks). Used for dispense of draught beer in Gas recovered from filter bowl can retail outlet. be used to neutralise effluent. Nitrogen Addition to the product to achieve Cheaper in terms of cost/volume quality specification. than CO2. Excluding oxygen from process plant Supplied as cryogenic bulk liquid including purging empty containers. but possible to generate on site. Can be used in conjunction with carbon dioxide for dispense in retail outlet. Use either to facilitate dispense with tight small bubbled head, promote head stability or facilitate top pressure dispense with reduced risk of beer carbon dioxide level pickup. Air Air use to operate pneumatic cylinders Air in contact with containers must on packaging plant, air knives for drying be oil-free and generally meet food plus control valves and instrumentation. grade specifications. Air used for operating equipment generally needs to be oil lubricated. Figure 1. Uses of Process Gases in Packaging
Dipl.Pack Revision Notes v2 October 2013 3 The objective of this section is to understand the relationships between gas pressure, temperature and volume for both pure gases plus gas mixtures. This understanding then enables the equilibrium between a gas and the quantity of that gas dissolved in the liquid to be understood and calculated. This understanding is important not only in the preparation of the product for packaging, during the filling operation plus heat based microstabilisation through to draught beer dispense.
Boyle’s law Boyle’s Law, formulated in 1662, governs the relationship between the gas pressure and its volume, for a given mass of gas. For a given mass of gas, the volume at a constant temperature will vary inversely according to the pressure.
Boyle’s Law states that:
This is expressed mathematically as:
V α 1/P Or V = K/P
Another way of expressing the law is:
P1/V1 = P2/V2 (at constant temperature!)
Expressed in words, Boyle’s law states that the product of pressure and volume is always constant. It is probably easier to remember that increasing the pressure by a factor of 2 will lead to a halving of the volume.
Note that in these gas laws, it is necessary to use absolute units and therefore Pressure is measured in either bara or pascals (P).
Charles’ Law / Gay-Lussac’s Law
Charles and later Gay-Lussac (1787 / 1802) investigated the behaviour of a gas at constant pressure but under different temperatures, again with respect to a given mass of gas. The result is the law, which may carry either attribute.
The volume of a gas is directly proportional to the absolute temperature
This may be expressed mathematically as:
V T Or V = K T Another way of expressing the law is:
V1 T2= V2 T1
As an ‘aide-memoire’, a two fold increase in temperature will cause a twofold increase in volume. Note that absolute temperature units must be used.
Standard Temperature and Pressure
In SI units, Standard Temperature and Pressure (STP) is 101325 Pa and 273.15K. Note that different industries, for example the aerospace and compressed air industries, use slightly different reference conditions.
1 kilogram mole of gas will occupy 22.414 m³ at STP
Note that 22.414 m3 is an approximation
Alternative units commonly used are litres and grams.
At STP, 1 gram mole of a gas will occupy 22.4 litres.
Figure 2 shows some alternative industry standard conditions.
Industry ISO Pressure Temperature Pneumatic power ISO 8778 1 bar = 20°C = & Compressed air 100000 Pa 293.15K Aerospace & ISO 2533 1013 bar = 15°C = Petroleum ISO 5024 101325 Pa 288.15K Figure 2. Alternative reference conditions
4 Diploma in Beverage Packaging (Beer) The compressed air industry uses the term ANR which is the abbreviation for ‘Atmosphère Normale de Réfèrence’.
Universal gas law and gas constant
Boyle’s, Charles/ Gay-Lussac’s Laws can be combined to form the Combined Gas Law.
P1 V1 / T1= P2 V2 / T2 = k
The value ‘k’ will be constant for a given mass of gas. In order to make the equation applicable for any mass of gas, it is usual to break this value down into:
n = the number of moles of gas R = the constant known as the Universal Gas Constant.
Note that in SI units, the molar quantity (n) is expressed in kg moles abbreviated to kmol. The gas equation then becomes:
P V = n R T
The value of ‘R’ in SI units is 8.3143 kJ kmol-1. K-1
This is known as the Universal Gas Equation and applies to ideal gases. The constant ‘R’ is the Universal Gas Constant.
The general rule is that gases at the pressures experienced in breweries, will obey the gas laws. At higher pressures, some deviation occurs. This will be shown by high pressure carbon dioxide (for example in a recovery plant) and ammonia in refrigeration plants.
Molar Volume An alternative view on the Universal Gas Equation is the concept of the Molar Volume. At standard conditions of temperature and pressure, (STP) 1 kmol of any gas will occupy the same volume. This volume is usually taken as 22.4 m³ but in fact is slightly higher at 22.41353 m³.
In SI units, the ‘mole’ is the kilogram mole, (kmol), which is the mass of the gas in kilograms divided by the relative molar mass.
Figure 3 gives the relative molar masses (RMM) of some gases used in a brewery
Gas Relative Molar Mass Air 29 Nitrogen 28 Oxygen 32 Carbon dioxide 44 Ammonia 17 Hydrogen 2 Figure 3. RMM of some typical gases
Note that the RMM of air is not exactly 29 since it is the result of a mixture of mainly nitrogen, oxygen and argon.
Example - Derivation of ‘R’ from Molar Volume
Consider 1 kmole of gas at 101325 Pa and 273.15°C.
The gas equation is: P V = n R T
Rearranging,
R = P V/ n T
Substituting values of P, V and T
R = 101325 * 22.414/273.15
R = 8314.5 N m kmol-1 K-1
Alternatively, Units of work are the joule, equal to 1 newton moved through 1 m, that is 1 J = 1 Nm.
R= 8314.5 J kmol-1 K-1 or 8.3145 kJ/kmol-1 K-1 Dipl.Pack Revision Notes v2 October 2013 5 The published value of ‘R’ is 8.3143 kJ/kmol-1 K-1 . The small difference arises because the approximation of 22.414 was taken as the molar volume.
Actual Gases The universal gas equation is valid for all gases at modest pressures. However at higher pressures, deviations start to occur, thought to be due to the interaction between molecules as the molecular density increases.
There are many equations which have been developed to describe these deviations. However the simplest incorporates a factor into the universal gas equation called the ‘Compressibility factor’ and usually denoted by the symbol ’z’. Note that the compressibility factor will depend upon pressure and temperature.
The Gas Equation then becomes:
PV = z n R T (where z is the Compressibility factor)
Figure 4 shows the compressibility’s for some commonly encountered gases over the range 0 to 20 bar a. The temperature is 300K, except for ammonia, where the saturated temperature was selected.
Gas compressibilities 1.05
1 Air Hydrogen 0.95 Ammonia Carbon dioxide
Compressibility 0.9
0.85 0 5 10 15 20 Pressure bar a Figure 4. Compressibility’s of some gases
Dalton’s law of partial pressures
The behaviour of mixtures of gases was studied by John Dalton around 1810.
Daltons law states that:
At a constant temperature, the total pressure exerted by a mixture of gases in a definite volume, is equal to the sum of the individual pressures which each gas would exert if it occupied the same total volume alone.
Or mathematically Pt = P1 + P2 + P3+ P4 +….
Where Pt = the total system pressure
By substituting P1 = n1 R T /V , P2 = n2 R T / V etc
The result is the relationships: Pt = Pt * n1/nt + Pt * n2/nt + etc.
Expressed in words:
The partial pressure is proportional to the molar fraction of each gas.
6 Diploma in Beverage Packaging (Beer) Calculations involving gas laws
Example - Find the number of kmoles and mass of air, and also carbon dioxide contained within a maturation vessel of volume 1000 hl under a pressure of 2 bar g and a temperature of 10°C.
Whatever method is used, the first operation is to convert all data to SI units. Volume = 1000 hl = 1000/10 m³ = 100m³ Pressure = 2 bar g = 3 bar a or 300000 Pa (In fact it is sufficient to convert to bar a. There is no need to convert to Pa.) Temperature = 10°C = 283.15K Then from P1*V1/T1 = P2*V2/T2 or V1 = V2 * P2/P1 * T1/T2 Volume at STP = 100 * 3/1 * 273.15/283.15 = 289.40 m³
Firstly using the Molar Volume method 1 kmol occupies 22.4 m³ at STP Number of kmols = 289.40/22.4 = 12.9196 kmol Whatever gas is involved, this represents the number of moles present Relative Molar mass of air = 29 Mass = 12.9196 * 29 = 374.67 kg Relative Molar mass of carbon dioxide = 44 Mass = 12.9196 * 44 = 568.46 kg
Secondly using the Universal Gas Constant: Here it is necessary to use pascals as the unit of pressure. PV = n R T n = P*V/R*T (where P=300000Pa, V=100m3, R=8314.5, T=283.15K n = 300000 x 100/(8314.5 * 283.15) n = 12.7432 Mass of air = 12.7432 * 29 = 369.553 kg Mass of carbon dioxide = 12.7432 * 44 = 560.700 kg
Note the slight difference arising from using 22.4m³ in place of the more accurate 22.41353 m³ for the Molar Volume.
Concept of gas/ liquid equilibrium
It is important to understand the driving forces for movement of gases in and out of liquids. This understanding enables not only for controlled changes to take place but also to help prevent unplanned changes which would move a product out of specification.
Dalton’s Law describes how, under equilibrium conditions, a gas will exert a partial pressure above the liquid proportional to its molar fraction.
If the gas is in contact with beer at a pressure above its equilibrium pressure, gas will pass from the gas phase (headspace) into the liquid phase at a rate described by the equation:
dc/dt = kL * A * (CE – C) / V
Dipl.Pack Revision Notes v2 October 2013 7 where: kL is the mass transfer coefficient A is the interfacial gas: liquid area CE is the equilibrium concentration C is the concentration of the gas in solution at time t V is the volume of the liquid (beer) and dC/dt is the rate of change of concentration with time
The rate of gas solution into liquids has a number of implications with respect to beer, notably carbonation, but also dispense of keg beer and avoiding air pickup.
So what are the factors that will lead to rapid gas pickup into beer?
Or, what can make dC/dt large?
Interfacial Area (A) - if the size of the bubbles injected is very small, then the interfacial area will be large and the rate of transfer high. We will see later how this can be achieved in practice by the choice of carbonation equipment.
If the beer is in a tank or a keg, then it is the surface area of the beer that will have an effect on gas pickup.
Mass Transfer Coefficient (kL) - this can be increased by having highly turbulent flow at the gas injection point. Units distance per unit time eg m/s
Volume of Beer (V) - if the volume of beer is small, the rate of increase of dissolved gas concentration will be faster. This is seen in dispense of keg beer. As the volume of beer in the keg decreases, there is a rapid increase in the ratio A/V and carbonation can quickly reach the equilibrium level determined by the applied dispense pressure.
Concentration Gradient (CE/C) - the further the carbonation level is from equilibrium, the faster will be the transfer rate. We will see later how this can also be affected by temperature and pressure.
Henry’s law
The equilibrium conditions of a gas between the solution and the gas phase is defined by Henry’s Law.
Henry’s Law states that:
At a given temperature, the amount of a gas dissolved in a liquid is proportional to the partial pressure of the gas above the liquid.
Expressed mathematically:
Henry’s Law is: P = H * x
P is the partial pressure of the gas in absolute units. H is the Henry’s Constant. x is the molar fraction of the gas in the solution.
The solubility of carbon dioxide in beer is very close to the solubility in water. Henrys law is illustrated in Figure 5
Figure 5. Henrys Law Amount Dissolved in Liquid Proportional To Pressure in Gas Phase Above Liquid 8 Diploma in Beverage Packaging (Beer) Henry’s Constant
Henry’s constant changes with temperature. As temperature rises, the solubility of the gas, at constant pressure, decreases.
The ultimate condition occurs where the boiling point of the liquid is reached. The pressure above the liquid is solely the result of the vapour pressure of the liquid. Since the partial pressure of the dissolved gas will be zero, the concentration of the gas in solution will also be zero.
This is the principal used in one of the de-aeration processes for water where the water is boiled under a slight vacuum to remove dissolved oxygen. This de-oxygenated water can then be cooled and used for post-fermentation dilution of high gravity – brewed beers.
The variation of gas solubility with temperature and pressure is illustrated in Figure 6.
Figure 6. Solubility of Gas in Beer
Dependence of gas/ liquid solubility on temperature
Figure 7 shows the variation of Henry’s constant with temperature. It should be noted that the relationship is a curve and not a straight line.
The solubility of carbon dioxide also varies with concentration within the liquid. This figure is valid for carbonation levels in the range 2 to 3 volumes per volume; there is some variation outside this range. A possible explanation for this is the chemical combination of carbon dioxide in the liquid in addition to the physical solution.
800
700
600
500
400
Thousands 300 Henrys constant 200
100
0 0 20 40 60 80 100 Temperature °C Figure 7. Variation of Henry’s constant (kPa/ mole fraction) with temperature for carbon dioxide (at 2.0 v/v)
To illustrate this point further, Figure 8 gives the value of the Henry’s constant for carbon dioxide for a carbonation level of 2.5 vol/vol.
Dipl.Pack Revision Notes v2 October 2013 9 T H (ºC) (kPa/mole fraction) 0 78916 5 96799 10 115943 15 136350 20 158017 25 180947 30 205138 35 230591 40 257305 45 285281 50 314519 55 345018 60 376779 62 389837 64 403096 66 416558 68 430221 70 444086 72 458153 74 472422 76 486893 78 501565 80 516440 Figure 8. Henry’s law constants
Molar fraction/Molar ratio
Calculations using Henry’s law are based around using the molar fraction of the gas in the liquid phase, which must not be confused with molar ratio.
The mole fraction of the gas in the liquid phase is the number of moles of gas in the liquid divided by the sum of the number of moles of gas in the liquid plus the number of moles of liquid. Often for calculations concerning beer we assume that the liquid is water with a molecular weight of 18 to estimate the number of moles of liquid. Molar ratio is the number of moles of gas in the liquid phase divided by the number of moles of liquid and should not be used in Henry’s law calculations.
Example – What is the mole fraction of CO2 in beer carbonated to 2.2 vols? Assume beer is at STP and molecular weight water = 18 and CO2 is 44
1 gm mole will occupy 22.4 litres at STP Therefore no gm moles CO2 = 2.2 = 0.098214 22.4
No gm moles water = 1000 = 55.5555 18
Mole fraction CO2 = moles CO2 = 0.098214 = 0.00715 moles CO2+moles water 0.098214+55.5555
Henry’s law as applied to mixed gases
Henry’s law can be applied to mixed gases using Dalton’s law of partial pressures to understand the behaviour of the gases in the gas phase. In equilibrium conditions the total pressure in the gas phase will the sum of the partial pressures of the gases as calculated using the Henry’s constant for each gas at that temperature.
Effects of hydrostatic head
In a large vessel, the pressure at the top of the vessel will differ from that at the base by virtue of the hydrostatic head. In terms of pressure, the effect of the height of liquid is given by :
10 Diploma in Beverage Packaging (Beer) P = h g
where h is the height of liquid (m) is the density (kg/m³) g is the gravitational constant (9.81 m/s²)
Example Find the equilibrium carbonation level at the top and base of a maturation tank 10 m high and under 1 bar g pressure and a temperature of 0 °C. Henrys constant (H) = 78. 916 x 103 kPa/ mole fraction
Pressure at top (P) = 1 bar = 1 + 1.01325 bar a = 201.325 kPa Mole fraction = P/H = 201.325/78.916 x 103 = 2.55113 x 10-3 Mole fraction = mol gas/(mole gas + mole beer) Mole beer = 55.55555 2.55113 x 10-3 = mol gas / (mole gas + 55.55555) =
2.55113 x 10-3 x mole gas + 2.55113 x 55.5555 x 10-3 = mol gas mole gas x (1- 2.55113 x 10-3 ) = 2.55113 x 10-3 x 55.5555 mole gas = 0.14209 Molar volume = 22.414 litres/mol Volume = 3.185 vol/vol
Pressure at base of tank = gas pressure + hydrostatic head Hydrostatic head = 10 x 1000 x 9.81 = 98100 Pa Total pressure = 201325 + 98100 = 299425 Pa = 299.425 kPa Mole fraction = 299.425 / 78.916 x 103 = 3.794224 x 10-3 Moles gas x (1 – 3.79422 x 10-3) = 3.79422 x 10-3 x 55.55555 Moles gas = 0.21159 Molar volume = 22.414 litres/mol Volume = 4.743 vol/vol
Calculations of gas (including mixed gas) equilibrium conditions in process vessels and pasteurisers
In calculations involving Henry’s Law, it is necessary to work to at least 6 decimal places in order to achieve the required accuracy.
Beer with a carbonation level of 2.2 vol/vol is to be pasteurised at 73°C. Find the pressure necessary in the holding tube to ensure the dissolved gas remains in solution.
Assume that 1 bar in excess of equilibrium pressure is necessary to ensure stable operation of the process.
Step 1 In 1 m³ of beer, there will be: 2.2 m³ of CO2 Molar volume = 22.4135 m³/kmol No of mols = 2.2/22.4135 = 0.098155 kmols
Step 2 In 1 m³ of beer there are:1000/18) = 55.555555 kmol of beer Total number of kmols = 55.555555 + 0.098155 = 55.653711 kmol -3 Mol fraction CO2; 0.098155 / 55.653711 = 1.763674 * 10
Step 3 The Henry’s constant for 73°C is 454.26387 x 103 kPa/mole franc
The equilibrium pressure is given by the basic Henry’s law equation P = H xP = 454.26387 * 103 * 1.763674 * 10-3 Pa = 801.173 kPa = 8.01173 bar abs Dipl.Pack Revision Notes v2 October 2013 11 To ensure that stable liquid conditions are maintained, an additional 1 bar is usually added, making a total of 9.01173 bar abs
Beer is filled into kegs to give a product containing 4.0 g L of carbon dioxide at s.t.p. The kegs are stored at 12.5 °C and dispensed with an overpressure of 0.7 bar above the pressure required to maintain the carbon dioxide in solution. What will be the equilibrium concentration of carbon dioxide under the dispense conditions?
If the beer were dispensed under the same conditions but using a mixed gas (60% vol/vol carbon dioxide; 40% vol/vol nitrogen) what would now be the equilibrium concentration of carbon dioxide in the beer?
Data: Relative Molecular Mass (RMM) of carbon dioxide is 44. RMM of water is 18. Henry’s constant for carbon dioxide in beer/ water at 12.5 °C is 1230 bar(a) mole fraction-1 .
Notes: s.t.p. (standard temperature and pressure) is 0 °C, 1.013 bar(a). Under these conditions 1 mol of gas occupies 22.4 L. Beer can be treated as water for calculation purposes (i.e. RMM = 18, density = 1000 g L-1 )
Answer:
Henry’s law calculates the partial pressure in the gas phase (p) based on the mol fraction in the liquid phase (mf) times the Henry’s constant (H) at that temperature for that gas. i.e. p = H * mf
Thus we need to work out the mole fraction of the carbon dioxide in the liquid phase:
Moles carbon dioxide in 1 Litre beer = 4/44 = 0.090909 g moles
Moles beer in 1 litre beer = 1000/18 = 55.55556 g moles
Total number moles in 1 litre carbonated beer = 0.090909 + 55.55556 = 55.64646 g moles
Mole fraction carbon dioxide in beer = 0.090909/ 55.64646 = 0.001634
Using Henry’s law the partial pressure of the carbon dioxide in the gas phase pco2 can be calculated:
pco2 = H * mf = 1230 * 0.001634 = 2.0094 bar(a)
If carbon dioxide over pressure in keg during dispense is 0.7 bar then the new equilibrium carbon dioxide concentration in the beer can be calculated using Henry’s law:
mf = pco2 /H = 2.7094 / 1230 = 0.00203
No moles carbon dioxide in 1 litre beer (x):
0.00203 = x/ (x + 55.55556)
Solving for x:
x = (0.00203 * 55.55556) / (1-0.00203 = 0.122648 g moles
New carbon dioxide concentration in beer = 44 * 0.122648 = 5.3965 = 5.4 g L-1
If mixed gas (60% carbon dioxide) used rather than carbon dioxide by itself?
Partial pressure of carbon dioxide in gas phase can be calculated using Dalton’s law of partial pressures i.e.:
New carbon dioxide partial pressure = 2.7094 * 0.6 = 1.62663 bar(a)
The new equilibrium carbon dioxide concentration in the beer can be calculated using Henry’s law:
mf = pco2 /H = 1.62663 / 1230 = 0.001322
No moles carbon dioxide in 1 litre beer (x): 12 Diploma in Beverage Packaging (Beer) 0.001322 = x/ (x + 55.55556)
Solving for x:
x = (0.001322 * 55.55556) / (1-0.001322) = 0.073524 g moles
New carbon dioxide concentration in beer = 44 * 0.73524 = 3.235 = 3.2 g L-1
From this example it can be seen that if the dispense system is relying on keg top pressure to dispense the beer rather than a pump if carbon dioxide only is used as a top pressure gas with time the carbon dioxide concentration in the beer could increase from 4.0 g L-1 to 5.4 g L-1 . If mixed gas (60% carbon dioxide/ 40% nitrogen) is used with the same pressure conditions with time the carbon dioxide concentration in the beer could drop from 4.0 g L-1 to 3.2 g L-1.
In order to avoid the carbon dioxide in the beer changing during dispense mixed gas should be used with the carbon dioxide partial pressure increased to the carbon dioxide equilibrium pressure for 4.0 g L-1 i.e. 2.00 bar(a) which would mean that the total mixed gas pressure on the keg would be = 2.00 / 0/6 = 3.33 bar(a). This pressure might produce stable dispense otherwise a pump would need to be considered.
Definition of super-saturation
Super saturation is where the quantity of gas dissolved in a liquid is above the equilibrium conditions for that temperature and pressure.
Super-saturation is not a stable condition. Over time, gas will leave the solution phase until the equilibrium is achieved. Any disturbance or presence of a suitable nucleus will increase the rate of dissolution. Opening a bottle of carbonated beverage is likely to create a supersaturated solution as is the snifting down to atmospheric pressure of a bottle or can on the filler. Shake a bottle of carbonated drink, release the cap and the results can be spectacular, if not messy. Nevertheless, this is a good example of the effects of super-saturation.
Super-saturation is the property on which the beer, soft drinks and sparkling wine industries depend. The bubbles of carbon dioxide released from the drink as it poured, stands in the glass and finally passed through the mouth, feature strongly in the attraction of the product, both from appearance and mouth-feel.
Principle of dissolving gases in liquids and the factors upon which they depend under static and dynamic conditions
Our understanding of the above laws can be used to enable us to design equipment to dissolve gases in liquids. The operation can be broken down into two stages.
Stage 1 – to introduce the gas into the liquid. This can be facilitated by either dropping the pressure of the liquid as in a venturi carbonator or raising the pressure of the gas.
Stage 2 – to dissolve the gas in the liquid. To facilitate the mass transfer operation from gas to liquid phase the gas area needs to be as high as possible. Small bubbles as created by a sintered metal carbonator facilitate this. Reducing the temperature also facilitates dissolution.
Dynamic conditions with good mixing facilitate gas dissolution into liquid.
Under static conditions for example a tank top pressure above the equilibrium conditions for that gas at that temperature dissolution will take place but this will be slow with uneven results as there will not be adequate mixing.
Typical equipment including measurement and control
The process of dissolving a gas in a liquid is helped by a number of factors, as defined in equation 3.6.10 (above) namely:
1) Solubility is favoured by low temperatures. 2) Solubility is favoured by high pressure. 3) Solubility is favoured by a fine dispersal of gas as microbubbles.
Generally all 3 factors are observed when designing, for example a carbonation facility.
Venturi Carbonator
Figure 9 shows the external view of the venturi carbonator. Figure 10 shows a diagram showing the changes in velocity and pressure through the device.
Dipl.Pack Revision Notes v2 October 2013 13 Figure 9. External view of venturi carbonator (Witterman)
CO2
Venturi formed by flattening a section of 100 mm dia pipe. Low pressure zone created at point of maximum velocity. Gas injected into this low pressure zone. Absorption occurs as the pressure is recovered as the area increases and velocity decreases.
Figure 10. Diagram of a section through a venturi carbonator The venturi is made, for example, from a section of 100 mm diameter pipe that has been ‘flattened’ so giving a section having a reduced flow area. As beer passes through the venturi, the velocity increases and there is a corresponding decrease in pressure (see Bernoulli principle). Carbon dioxide is injected into the beer at the low pressure zone. Absorption is aided by the high degree of turbulence in this zone and by the increase in pressure, which occurs in the diffuser section. In this section, the area increases, consequently there is a reduction in velocity and an increase in pressure.
Sintered diffusers Sintering is a process where metal or ceramic particles are pressed into a shape then heated to just below their fusion point. Partial fusion takes place to give a mechanically strong yet porous material. For gas diffusion, the sintered material is made into a cylindrical shape, often referred to as a ‘candle’. The effect of passing gas through the candle immersed in a liquid is the formation of a very many fine bubbles. Fine bubbles are absorbed readily by the liquid. Figure 11 shows a range of sintered metal and ceramic components.
14 Diploma in Beverage Packaging (Beer) Figure 11. Sintered metal components
Nozzles Many breweries still prefer the simplicity of a simple injection nozzle. The gas is forced through the fine hole at the end of a nozzle and the result is a fine dispersal of bubbles in the liquid. Nozzles have an advantage over sintered diffusers in that they are more easily cleaned.
Location of Gas Absorption Points In selecting the point in the process at which to inject gas, consideration is given to the factors that promote gas absorption; that is, low temperature and high pressure.
For example, a wort aeration point would be located immediately after the wort cooler where the temperature is low (lower than hot wort!) and the pressure can be kept high. Often a static mixer is incorporated after the injection point to create turbulence and so aid solution.
For carbonating beer, the selection of an injection point as a compromise between temperature and turbulence/ mixing is often the choice. For example, if injection is made immediately before a beer chiller, the turbulence through the chiller will increase the transfer rate and the drop in temperature that occurs through the chiller will also help gas solution.
Figure 12 shows the arrangement for a typical aeration or carbonation point. This particular arrangement is fitted with a sterile filter and a steam sterilisation/CIP connection. This is necessary for aeration points using air from the brewery compressed air system but not necessary for gas derived from cryogenic storage or of certified quality. Note that a sight glass has been included. This gives a visual check on the gas injection.
Sight glass
Flow Flow
Non return valve Gas Flow indicator regulator Gas flow FI Sterile flter
Steam/CIP connection
Figure 12. A manually controlled gas absorption system
Figure 13 shows a system for carbonation where the admission of gas is controlled automatically by means of a CO2 in-line sensor, the signal from which is used to vary the gas flow by a standard process control loop. This system also shows a static mixer. This may be included to accelerate the absorption of gas into the beer.
Dipl.Pack Revision Notes v2 October 2013 15 Sight glass Static mixer Flow Flow
Non return valve S
CO2 sensor Control & controller
FI Gas flow
Flow indicator Figure 13. Carbonation with process control
Dissolved gas adjustment
Dissolved gas adjustment can either be done dynamically through a trim carbonator or in tank by purging gas through the beer. Gas reduction is normally done in tank be reducing the pressure below the current equilibrium pressure then destabilising the super-saturated solution by purging either with the gas to be adjusted or with another gas. Care needs to be taken not to over adjust. Gas increase is done by raising the tank pressure above the current equilibrium pressure then purging the beer with the gas which needs to be dissolved. With both adjustments the beer should be allowed to stabilise for at least two hours before a reliable dissolved gas reading is taken.
Effects of pressure and temperature on carbonation level during storage of beers, e.g. bright beer tanks and pub cellars
There are four reasons for applying carbon dioxide/ nitrogen or mixed gas top pressure to a tank or draught beer container: a. To maintain the dissolved gases in solution to a within specification concentration. In this instance the top pressure to be applied needs to provide a partial pressure of the gas concerned equal to the equilibrium pressure at that temperature for the desired gas content. This can be calculated using Henry’s law along with Dalton’s law of partial pressures for mixed gas systems. b. To move the product out of the container with or without the aid of a pump. In this instance the pressure needs to be at least the pressure calculated in (a) along with sufficient pressure to enable the beer to move through the system overcoming head losses and achieving the desired flowrate. If a gas pressure above the equilibrium pressure (a) remains after the transfer then gas will start to dissolve in the beer until a new equilibrium is reached. This could move the beer gas content out of specification possibly leading to foaming problems when the beer is next moved. This is applicable in the brewery plus in the pub cellar. Good practise is post transfer adjust the gas pressure to the equilibrium pressure calculated in (a). In cellars the gas supply is usually turned off when the dispense session finishes. c. The use of carbon dioxide or nitrogen for top pressure excludes oxygen thus preventing the beer from becoming oxidised and developing papery/ cardboard flavours. d. The exclusion of oxygen is also beneficial as this creates conditions where aerobic bacteria do not grow thus limiting beer spoilage organism risks to anaerobic bacteria. This procedure is sometimes used for the dispense of cask beer where a demand valve is used to supply carbon dioxide to a cask to replace the beer drawn out during dispense. Typical pressures are around 0.5 psi.
Recovery collection and pre-treatment
The brewing process produces recoverable carbon dioxide and many breweries have found that it is economic to install recovery equipment, especially in areas where carbon dioxide is an expensive commodity. It is possible to recover gas from the process for either sale from the site or for use by an associated plant on-site, for example a soft drinks plant.
Specification The specified oxygen content in recovered carbon dioxide is normally 50 ppm. Some breweries may enforce a tighter specification if the gas is to be used in critical stages of the process, for example, packaging. It is also important that there is no risk of taint or contamination. If the gas is to be sold then the specification will need to be tighter and agreed with the customer be it a gas company or end user. Each batch of gas sold will need to be analysed with a certificate of conformity. 16 Diploma in Beverage Packaging (Beer) Availability The maximum amount of carbon dioxide available from the fermentation is 0.11kg/h l°Sac or 0.43kg/hl °P. However it is difficult to recover this quantity.
Most ‘simple’ plants achieve the purity requirements by rejecting gas at the start of fermentation. The specified purity limit is usually 99.5%. Consequently, recovery is poor at between 0.04 and 0.06 kg/hl° Sac (0.16 and 0.24 kg/hl °P). Recently, plants incorporating liquid gas distillation have become available. These plants enable gas with a purity as low as 80% to be recovered, liquefied then distilled to give the required purity.
When only high purity gas is to be recovered, several methods are used to ensure the purity: 1) Time from pitching, eg 36 hours. 2) Gravity change, eg 1°Pl or 4 °Sacc. 3) Gas analysis by either on-line analysers or manual sampling.
Recovery Plants The recovery process comprises of the following major items of plant (Figure 14): 1) Collection from the fermenters 2) ‘Fob traps’ to collect any liquid carryover. 3) Buffer storage in the form of a ‘balloon’ to provide a steady flow of gas to the recovery plant. 4) Gas washing to remove the majority of the volatiles and taint producing compounds. 5) Compression to increase the pressure to liquefaction pressures, (approx 18 bar). 6) Drying to reduce the moisture content 7) Deodorising to reduce the level of taint producing materials 8) Liquefaction to condense the carbon dioxide and to act as a single stage purification process, (approx –20°C). 9) Optional distillation to increase the purity of the recovered gas. 10) Storage
Figure 14. Flow diagram of a conventional CO2 recovery plant.
Figure 15 shows a model of a typical CO2 recovery plant without distillation. The large balloon can be seen to the rear of the model and the sloping shell and tube liquefier is towards the front. The liquefier is supplied with low temperature ammonia from a small refrigeration plant, the compressor of which is shown on the centre right of the model. The high pressure CO2 compressor is in the centre foreground of the model.
Dipl.Pack Revision Notes v2 October 2013 17 Figure 15. Model of CO2 recovery plant
Figure 16 gives an approximate list of the contaminants in recovered CO2
. Contaminant mg/litre Ethanol 1054 Ethyl Acetate 30 Di methyl ketone 25 Iso amyl acetate 17 Acetaldehyde 5 Amyl alcohol 9 Hydrogen sulphide 12 Di methyl sulphide 2 Figure 16. Table of contaminants in recovered CO2
Figure 17 shows the operation of the scrubbing column. In this column, recovered CO2 is washed with water as it passes over a packing in the tower, in order to remove potential taint producing volatiles.
Figure 17. Scrubbing column for removal of volatiles
18 Diploma in Beverage Packaging (Beer) Figure 18 shows the operation of the drying unit. This unit dries the recovered carbon dioxide, after the compressor, by contact with active alumina desiccant. Regeneration is achieved by passing a stream of low pressure dry carbon dioxide over the desiccant: the process being aided by the application of heat.
Figure 18. CO2 drying plant
A similar arrangement is used to remove any final traces of odour producing compounds such as sulphides. Activated carbon is used as an absorbant. Figure 19 shows a diagram of the deodoriser unit and regeneration facilities.
Figure 19. Deodoriser unit
Figure 20 shows the arrangement of the liquefier. The carbon dioxide condenses on the surface of the tubes in which a refrigerant such as ammonia evaporates. A gas release valve on the evaporator, removes part of the gas from the liquefier in order to maintain the purity of the liquefied product.
Figure 20. Liquefier Dipl.Pack Revision Notes v2 October 2013 19 Figure 21 shows the arrangement when a distillation column is incorporated into the system. Impure liquid CO2 runs down the column and comes into contact with gas passing up the column. Oxygen and other gases pass from the liquid to the vapour phase. As the liquid passes down the column it becomes progressively weaker in non-condensables. At the base of the column, the liquid is high purity CO2 . Part of this is withdrawn into storage and part is heated to form the vapour returning up the column. At the top of the column, some of the vapour is rejected and part re-liquefied for returning to the column. By employing gas distillation, it is possible to recover gas where the initial purity is as low as 80%.
Figure 21. CO2 distillation
Liquid CO2 is stored in well insulated storage vessels under conditions of 18 bar pressure and –20°C. Vacuum insulation is often used to achieve the high degree of insulation required to minimise gas losses resulting from heat in leak.
CO2 is recovery sources
Fermenters 1.8 to 2.1 kg CO2/hl.
Maturation and Bright beer tanks during filling 2.1 to 2.5 kg CO2/hl.
Bottle Fillers this CO2 can be used to neutralise bottle washer effluent in the case of returnable bottling. This will be discussed later.
Typical use rates are: Bottling 0.3 to 0.5 kg/hl Kegging 0.5 to 0.8 kg/hl Overall use 1.8 to 2.0 kg CO2/hl beer
Recovery Liquid CO2 storage conditions and vaporisation methods including secondary refrigerants for energy saving Liquid CO2 is stored in horizontal and vertical tanks. These are well insulated to hold contents at –20°C and 18 bar. A small fridge set condenses vapour to keep pressure.
Liquid CO2 is used to fill gas cylinders for beer dispense of draught beer at retail outlets. Otherwise CO2 needs to be vaporised prior to use around the brewery and packaging plant. A low temperature sensor downstream of the vaporiser is good practice to ensure that liquid carbon dioxide does not enter the distribution system. Vaporisation can be done with steam in cold climates and warm air in warm climates. Another alternative is to use the liquid CO2 to cool secondary refrigerant thus reducing the refrigeration load. Payback on the plate heat exchanger used needs to be checked prior to implementing this to ensure project viability. Figure 22 shows a steam vaporiser and Figure 1.23 shows an air vaporiser. 20 Diploma in Beverage Packaging (Beer) Figure 22. Steam Vaporiser (courtesy of Yara)
Figure 23. Air Vaporiser (courtesy of Yara)
CO2 Hazards
CO2 is toxic and an asphyxiant.
Short term exposure limit is 1.5% (15000 ppm) Long term exposure limit is 0.5% (5000 ppm)
At concentrations above 3% there is the risk from oxygen depletion in addition to the toxicity. Oxygen levels below 18% are hazardous.
CO2 is heavy, therefore can accumulate in ‘low-points such as cellars.
Gas from liquid storage and the equipment systems will be cold (below –20°C). Risk of ‘Frostbite’ or ‘Cold burns’.
Dipl.Pack Revision Notes v2 October 2013 21 Other Operational Risks
CO2 reacts readily with alkaline detergents causing a loss of detergent and possible vacuum damage to vessels.
Maturation and BBT are often acid cleaned without loss of CO2 or detergent strength.
Fermenting vessels need alkali detergent for protein removal so need adequate venting
A working knowledge of the management and control of Process Technology for Packaging
Supply/storage/vaporisation
Supply
Carbon dioxide in breweries is sourced from; the brewing process, generation onsite by burning fuel or is delivered by an outside supplier. Outside suppliers generally obtain their carbon dioxide as a by product from ammonia manufacture for fertiliser.
Nitrogen is either generated on-site using membrane or cryogenic methods or is delivered by an outside supplier. Generally an outside supplier will use a cryogenic method of separating the nitrogen from air and liquefying it.
Compressed air is often viewed as being free within the brewery and packaging plant. This is far from the case as not only is there the capital cost but also the revenue costs from the energy to run the compressor plus the system which dries the air and ensures that it is free from contamination. Compressors tend to be either reciprocal (figure 24), double tooth (figures 25 and 26) or screw (figures 27 and 28) the later two can be fitted with a variable speed drives to improve energy efficiency.
Figure 24. Reciprocal Compressor (courtesy of Atlas Copco)
Figure 25. Double Tooth Compression Element (courtesy of Atlas Copco)
22 Diploma in Beverage Packaging (Beer) Figure 26. Double Tooth Oil Free Compressor Flow Diagram (courtesy of Atlas Copco)
Figure 27. Screw Compressor Oil Free Flow Diagram (courtesy of Atlas Copco)
Dipl.Pack Revision Notes v2 October 2013 23 Figure 28. Screw Compressor Compression Element (courtesy of Atlas Copco)
If the air is to come anywhere near to the product the it must be free from oil (figure 29), taint, microbiological contamination plus moisture. Compressed air used for instrumentation must be free from moisture to avoid corrosion. This is achieved by drying the air. Dryers remove water vapour from the air, which lowers its dew point. This is the temperature to which air can be cooled before water vapour begins to condense. There are four basic types of industrial compressed air dryers: deliquescent, regenerative desiccant (figure 30), refrigeration (figure 31), and membrane (figure 32).
Figure 29. Compressed Air Oil Removal (courtesy of Atlas Copco)
Figure 30. Regenerative Desiccant Drier (courtesy of Atlas Copco)
24 Diploma in Beverage Packaging (Beer) Dipl.Pack Revision Notes v2 October 2013 25 Figure 31. Refrigerant Drier (courtesy of Atlas Copco)
Figure 32. Membrane Drier (courtesy of Atlas Copco)
Storage
As explained earlier carbon dioxide is normally stored in liquid from prior to use. Gaseous carbon dioxide is often collected and held in flexible balloons as illustrated in figure 33.
Figure 33. Balloon Gas Storage
26 Diploma in Beverage Packaging (Beer) Figure 34 shows a bulk liquid carbon dioxide tank along with the refrigeration set to keep the contents cool and prevent the pressure rising and eventually the pressure relief valve lifting. The tank is well insulated to minimise heat pick up.
Figure 34. Carbon Dioxide Bulk Tank
Liquid nitrogen is normally stored in vertical tanks. These are usually vertical tanks constructed of a pressure vessel with another tank constructed outside to create an air gap which is then evacuated. The evacuation creates as near to vacuum conditions as possible which provides excellent insulation which is required because of the low boiling point of nitrogen. A nitrogen cryogenic installation is shown in Figure 35.
Figure 35. Bulk Nitrogen Installation
Vaporisation Vaporisation of nitrogen is mainly achieved using atmospheric vaporisers. The reason for this is that the boiling point is considerably lower than that for carbon dioxide and thus there is enough temperature difference to achieve this. The vaporiser tubes have fins on to improve heat transfer as shown in figure 36.
Figure 36. Nitrogen Vaporiser
Dipl.Pack Revision Notes v2 October 2013 27 On-site generation, pressure swing/membrane/cryogenic methods
Both carbon dioxide and nitrogen can be generated on site. The most common method with carbon dioxide is to collect the gas from fermentation and process tanks as described earlier. Another alternative is to burn fuel and collect carbon dioxide from the combustion gases. Figure 37 shows a schematic for the generation of carbon dioxide from fuel with the plant shown in figures 38 and 39.
Figure 37. Carbon Dioxide Generation from Fuel Schematic (courtesy of Union Engineering)
CO2 Generating Plants (CBU) are based on combustion of various fossil fuels such as diesel oil, heavy oil, kerosene or natural gas. The plant is based on combustion in the MEA heater unit. Flue gas is washed with soda lye in a flue gas scrubber. The flue gas is cooled and any sulphur dioxide is removed. From the flue gas scrubber, the flue gas is led through the absorber by means of an exhauster where the CO2 content of the flue gas is absorbed in a MEA (Monoethanolamine) solution. The gas residue is exhausted. The CO2 rich MEA solution is preheated in the rich/lean solution heat exchanger before it is pumped to the stripper where by further heating CO2 is released. The stripped lean MEA solution is led from the stripper through the rich/lean solution heat exchanger and the MEA cooler back to the absorber. The CO2 gas is cooled in a gas cooler cleaned in a potassium permanganate (PPM) scrubber and led to the CO2 compressor, which compresses the gas in two stages to approx. 16 bar(g). To allow condensation the CO2 gas is dried in a dehydrator to a dew point of abt. -60°C. The dehydrator is electrically heated and automatically regenerated by inert gas from the CO2 condenser and thereby acts as a purging system for non-condensable gases. The CO2 is then passed through a carbon filter for removal of odours. The purified and dried gas is condensed in a multitube heat exchanger at a temperature of about -30°C. The matching refrigeration plant is equipped with an ammonia or freon compressor. The refrigeration plant maintains the correct operating pressure in the CO2 condenser. The liquid deep- cooled CO2 is led to an insulated CO2 storage tank.
Figure 38. Carbon Dioxide Generator (courtesy of Union Engineering)
28 Diploma in Beverage Packaging (Beer) Figure 39. Carbon Dioxide Generator (courtesy of Union Engineering)
Nitrogen can be generated on site using various methods depending on the purity required. Commercially available liquid nitrogen is 99.999% nitrogen. These methods are in increasing order of purity:
1. Membrane separation (95 to 99.9%) Figure 40 2. Absorption separation (> 5ppm oxygen) Figures 41 and 42 3. Absorption separation plus distillation (< 1ppm oxygen) Figures 43 and 44
Figure 40. Membrane Nitrogen Separation (courtesy of Air Products)
Figure 41. Absorption Nitrogen Separation (courtesy of Air Products) Dipl.Pack Revision Notes v2 October 2013 29 Figure 42. Absorption Nitrogen Separation Flow Diagram (courtesy of Air Products)
Figure 43 Absorption plus Distillation Nitrogen Separation (courtesy of Air Products)
Figure 44. Absorption plus Distillation Nitrogen Separation Flow Diagram (courtesy of Air Products)
30 Diploma in Beverage Packaging (Beer) Nitrogen generation is used in retail outlets to produce the blends of nitrogen and carbon dioxide required for different beers and dispense conditions. For smaller retailers the blending is carried out using two types of gas cylinder carbon dioxide and a 30% CO2/70% nitrogen mixture. The system supplies the 30/70 mix direct from the cylinders, but it also blends this with CO2 to produce a 50/50 and a 60/40 mix as well (Figure 45).
Figure 45. Cellar Gas Blending No Onsite Generation Figure 46. Cellar Gas Blending Onsite Generation (courtesy of (courtesy of BOC) BOC)
For larger retailers on site nitrogen generation can be carried out using a single stage absorption system with larger nitrogen cylinders used to provide a buffer (Figure 46).
Quality Specifications
It is important that not only is a specification agreed with the supplier of bought in gases but also procedures are put in place to ensure that the gas is to this specification. Specimen specifications for carbon dioxide (figure 47) and nitrogen (figure 48) are detailed below. There have been several incidents in recent years where contaminated carbon dioxide has caused possible taints in carbonated beverages. Most gas suppliers analyse the gas despatched and send a certificate of conformity. Users have been known to develop techniques of bubbling gas through water and tasting each batch to try to detect any possible off flavour which could cause tainting. Where the gas in generated in house then procedures should be in hand to ensure that there is no risk if taint from this gas as well.
Figure 47 Liquid Carbon Dioxide Specimen Specification (courtesy of BOC)
Dipl.Pack Revision Notes v2 October 2013 31 Figure 48 Liquid Nitrogen Specimen Specification (courtesy of BOC)
The physical properties of carbon dioxide and nitrogen are presented in Figures 49 and 50.
Figure 49. Carbon Dioxide Physical and Chemical Properties (courtesy of BOC)
Figure 50. Nitrogen Physical and Chemical Properties (courtesy of BOC) 32 Diploma in Beverage Packaging (Beer) Typical Contaminants
Oxygen is the most typical contaminant of both gases if they are generated on site. Oxygen in carbon dioxide typically comes from collecting the gas when it is too impure. Oxygen in nitrogen is generally related to the method of generation. The absorption processes need several (normally three) stages to produce nitrogen of a similar oxygen content to that in purchased cryogenically distilled nitrogen. It is important that the plant is specified appropriately for the use of the gas to be generated. For de-aerating water, purging and nitrogenating beer the higher purity is required i.e. 99.999%. Gas with more oxygen in it would result in oxidised flavours in the beer. For top pressure gas for beer dispense because of the short contact time between the gas and the product prior to consumption then a lower level of purity is required. Thus nitrogen generation plants supplying gas for brewery use are either multistage absorption or absorption followed by distillation.
The next category of contaminants are those that could taint the product. These tend to be hydrocarbons. This is controlled by analysing the gas plus also bubbling it through water and then tasting the water.
The last category of typical contaminants are microbiological. These tend to be anaerobic bacteria and are normally controlled through good housekeeping procedures i.e. gas filtration along with downstream CIP or steaming.
Air Specifications
When setting the specification for compressed air it is important to define the use, this will define the requirements. The three major impurities in compressed air are solid particles, water and oil. Additional contaminants are microbiological plus other gases. Atmospheric air always contains water vapour. When atmospheric air is compressed the partial pressure of the water vapour increases but, because of the increase in temperature caused by the compression, no water precipitates. When air subsequently cools (e.g. in an intercooler or aftercooler, in the distribution pipework or during the expansion process in a pneumatic operation) water will condense to liquid, but the air will be fully saturated with water vapour. In order to overcome this, the air is dried pre distribution. A method of specifying compressed air has been developed and is published in ISO 8573. This standard also details the methods of analysis for each type of contaminant. The standard enables compressed air to be specified using the designation ISO 8573-1 A B C where:
A – refers to the figure for solids (figure 51) B – refers to the figure for humidity or liquid water (figure 52 and 53) C – refers to the figure for total oil classes (figure 54)
When a class for any particular contaminant A, B or C is not specified, the designation is replaced by a hyphen.
Figure 51. Solids Classification (courtesy of BSI)
Dipl.Pack Revision Notes v2 October 2013 33 Figure 52. Humidity Classification (courtesy of BSI)
Figure 53. Liquid Water Classification (courtesy of BSI)
Figure 54. Oil Classification (courtesy of BSI)
34 Diploma in Beverage Packaging (Beer)