Process Gases and Mass Transfer Mass transfer between gas/liquid phases Oxygenation of wort Carbonation of beer

1. Process Gases and Mass Transfer • Mass transfer between gas/liquid phases • Oxygenation of wort • Carbonation of beer • Deaeration of dilution water (high gravity brewing) • Nitrogen for blanketing gas in beer storage, mixed gas dispense systems and dissolved into beers • Must control level of CO2 in beer • Must exclude O2 from beer • Principles: equilibrium, solubility, hydrodynamics of gas/liquid systems • Liquid/Gas mix in closed system at constant temperature – dynamic equilibrium • Rate of transfer liquid to gas = Rate of transfer gas to liquid

2. Process Gases and Mass Transfer • At dynamic equilibrium, amount of gas dissolved in liquid is proportional to the partial pressure in gas phase • p = H X or • Partial pressure = (Henry’s Constant) (Mole Fraction) • Henry’s constant increases with increasing temperature • Solubility of CO2 measured on vol/vol basis • That is, volume occupied by the dissolved gas at STP if it were removed from 1 m3 of beer • Conditions in fermenter determine amount of CO2 in green beer • Open square fermenter – 1 vol/vol • Cylindroconical fermentor – much greater • All brewery processes – must avoid gas breakout!

3. Process Gases and Mass Transfer Pasteurization – Solubility decreases, must raise pressure If gas breakout occurs, heating uneven, unreliable pasteurization Rate of dissolution of a gas in a liquid dm/dt = rate of mass transfer Kg and KL = overall mass transfer coefficients Pg and PE = gas and equilibrium partial pressures of the gas in the gas phase C and CE = liquid and equilibrium concentrations of the gas in the liquid phase A = interfacial surface area for mass transfer

4. Process Gases and Mass Transfer If partial pressure of CO2 above the beer in storage is greater than that required to keep CO2 in solution, up carbonation or pick-up occurs dC/dt = rate of CO2 concentration change w/ respect to time V = Volume of the vessel contents KL = overall liquid mass transfer coefficient C and CE = liquid and equilibrium concentrations of the gas in the liquid phase A = interfacial surface area for mass transfer

5. Process Gases and Mass Transfer • Decarbonation can occur in rough pipes • Bubbles form in pits • Serve as nucleation centers for more bubbles • Use smooth pipes and avoid constrictions • Can be problematic in beer dispensing systems • Oxygenation – similar mass transfer processes • Equilibrium O2 concentrations < CO2 concentrations • Nitrogen blanketing can prevent excessive O2 pick-up

6. Process Gases and Mass Transfer Beer containing 1.8 volumes of CO2 per volume of beer at stp is pasteurized at 73C. Assuming beer has the same molecular weight as water, calculate the mol fraction of carbon dioxide in the beer and the pressure required to maintain it in solution at pasteurization temperature. Explain why the pressure on the beer passing into the pasteurizer unit must be significantly greater than the pressure required at the pasteurization temperature Density of water = 1000 kg/m3 Gas constant for carbon dioxide = 0.189 kJ/kg.K. Molecular weigh of carbon dioxide = 44 Molecular weight of water = 18 Henry’s constant at 73 = 440 MPa/mol fraction

7. Process Gases and Mass Transfer A beer keg of 50 x 10-3 m3 capacity stored in a cellar at a temperature of 12C contains 40 x 10-3 m3 of beer whose CO2 concentration is 1.4 volumes per volume of beer at stp. The head space in the keg is filled with CO2 which is in equilibrium with the beer. Stating any assumptions made, calculate the pressure of the CO2 in the head space and the total mass of CO2 in the keg. Gas constant for carbon dioxide = 0.189 kJ/kg.K. Molecular weigh of carbon dioxide = 44 Molecular weight of water = 18 Henry’s constant at 12C = 120 MPa/mol fraction 1 kmol gas = 22.4 m3 at stp. Assume MW and density are same as that of water

8. Instrumentation Accuracy – “Freedom from Error” Process Industry – Accuracy = Inaccuracy…? 1% accuracy indicates that measured value should be within 1% of true value Accuracy vs. Precision Random Error (Precision) Systematic Error (Bias) Accuracy of instrument % of full scale % of measurement

9. Instrumentation • Selection and siting of remote sensors • Product composition, temperature, pressure • Cleaning and sterilization • Accuracy and repeatability • Reliability and maintenance • If instruments to not meet user requirements • Extra design/engineer effort for new plant • Delay in construction and start-up • Extra cost (man hours) for commissioning • Less than optimum performance • Adverse consequences on personnel, plant and/or environment

10. Pressure Measurement Manometer – Measure height of liquid (Calibrate)

11. Pressure Measurement Mechanical – Bourdon Tube

12. Pressure Measurement • Electrical – Strain, capacitive or piezoresistive • Strain – small deflection changes resistance • Capacitive – High freq. oscillator, plates vary gap • Piezoresistive – Monocrystalline silicon

13. Temperature Measurement • Thermometer • Liquid/gas • Bi-metallic • Resistance Temperature Detector • Thermocouple • Infrared Temperature Detector

14. Level Measurement Bubble Tube – Pressure required to inject air into a tank indicates height (bulk sugar tanks) Force balance – Measure pressure at bottom of tank, indicates height Ultrasonic – Sound waves reflect off of liquid/gas interface Float Single position

15. Flow Measurement • Magnetic – Faraday’s Law (Conductor moving through magnetic field, voltage produced) • 40:1 Rangability • 0.5% of f.s. accuracy • No obstructions • Fluid must conduct • No good for • Pure water • Gases • Hydrocarbon fuels • External elec/mag fields

16. Flow Measurement Non-Linear – Venturi, orifice, nozzle meters 4:1 Rangability 2% f.s. accuracy

17. Flow Measurement Turbine – magnetic pulse as turbine wheel spins 20:1 Rangability, 0.25% f.s. accuracy Easy to interface with control system

18. Gas Measurement • Sensor Performance Parameters: • Sensitivity (or signal magnitude) • Response time • Repeatability (or precision) • Linearity • Background current or voltage • Temperature, pressure effects • Stability of span and background • Expected lifetime • Interference

19. Gas Measurement • CO2 Infrared Sensor • Non-dispersive infrared (NDIR) • IR source at one end of tube • Variable filter (Fabry-Perot Interferometer) • IR detector • FPI varied to adsorption band of gas • Ratio of two signals indicates concentration • O2 Measurement in Solution • Chemical processes or electrochemical • Membrane separates fluid and electrode • Oxygen diffuses through membrane • Split into ions and electrons, goes to anode • Signal amplified

20. Drying and Psychrometrics Reasons for drying - Reduce mass of material - Reduce volume of material - Change handling characteristics - Material preservation Moisture in solids - Bound – water retained in capillaries, absorbed on surfaces or in solution in cell walls - Free – water in excess of equilibrium content

21. Drying and Psychrometrics Free water first Saturated surfaces Rate slows, diffusion Equilibrium reached T2 Temp (C) T2 T1 Material being dryed Time T1 Drying Rate Moisture Content

22. Drying and Psychrometrics Barley - Harvested (20%), Storage (10%) - Max. drying temp: 46C for 20%, 66C for 10% - Cascading barley with counterflow of warm air - Shaking perforated trays, warm through holes Malt - Drying to stop enzyme activity after malt modification is complete - Moisture content reduced from 45% to <4.5% - Stage 1: Tin = 65C, Tout = 30C, WC = 14% - Stage 2: Tin = 80C, Tout = 50C, WC = 5% - Stage 3: Tin = 100C, Tout = 100C, WC = 2% - Overall process – 18 to 48 hours

23. Drying and Psychrometrics Hops - Picked at 80% moisture, dryed to 10% - Tin 55 to 65C Yeast - Drying required after being autolysed - Sprayed onto rotating, steam heated drums - Removed with a “knife” after one rotation Spent grains - 75% moisture after being pressed - Must be dryed at low temperature - Air heated with steam or hot water

24. Drying and Psychrometrics Humidity ratio: mass of water vapor / mass of air Saturation humidity: saturation mass / mass of air at a given temperature Relative humidity: humidity / saturation humidity Dew point: Temperature at which a given mixture would become saturated at given humidity ratio Wet bulb temperature: equilibrium temperature at interface of water and air See psychrometric chart.

25. Drying and Psychrometrics Example 1: Determine the humidity ratio and relative humidity in the room today Example 2: Air at 50% relative humidity and 22C is heated to 35C. What is the new relative humidity, wet bulb temperature and dew point? How much energy was added per kg of dry air? Humidifying with water spray mw,in + mw,spray = mw,out Dehumidifying – 2 step process (cool, then heat) Evaporative cooling – Water spray into air, evaporation cools water (latent + sensible) Approaches wet bulb temperature (+3C)

26. What we’ve covered so far… Dimensions and Units Systems, Phases and Properties (Steam tables) Conservation of Mass and Energy Newtonian Fluids and Viscosity Reynolds Number, Laminar and Turbulent Flow Entrance Region, Velocity Profiles Friction Loss in Pipes and Fittings Flow Meter and Valve Types – Relative Merits Pumping Power, Types, Sizing, Cavitation/NPSH Filtration, Solids Settling Heat Transfer (Conservation of Energy) Conduction, Convection and Radiation

27. What we’ve covered so far… Heat Transfer Equipment LMTD and Overall Heat Transfer Coefficient Heat Exchanger Sizing, Heat Losses Combustion and Steam Generation Refrigeration Cycle Pasteurization – Flash and Tunnel Drying and Psychrometrics Primary and Secondary Refrigerant Applications Wort Boiling Process Gases and Mass Transfer Instrumentation and Control Materials and Corrosion

28. Analysis “Tools” • Mass and energy balance • Properties, phases, steam/refrig. tables • Pressure drop in pipework (Re, Moody) • Pump sizing technique • Heat transfer, resistance analysis • Heat exchanger sizing • Refrigeration cycle and tables • Psychrometric chart (for drying) • Gas-liquid mass transfer

29. Where we are Going The next 6 weeks – mornings only - Fluid Flow - Heat Transfer - Steam - Refrigeration - Materials (of Construction) - Process Control and Instrumentation - Sterile Filtration and Pasteurization Readings, Tests, Class Discussions