Last Week:CombustionPasteurizationProcess ControlMaterials

This Week:Gases and EquilibriumInstrumentationDrying and Psychrometrics

Process Gases and Mass TransferMass 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 beersMust control level of CO2 in beerMust exclude O2 from beerPrinciples: equilibrium, solubility, hydrodynamics of

gas/liquid systemsLiquid/Gas mix in closed system at constant temperature –

dynamic equilibriumRate of transfer liquid to gas = Rate of transfer gas to liquid

Process Gases and Mass TransferAt dynamic equilibrium, amount of gas dissolved in liquid

is proportional to the partial pressure in gas phasep = H X or

Partial pressure = (Henry’s Constant) (Mole Fraction)Henry’s constant increases with increasing temperatureSolubility of CO2 measured on vol/vol basisThat is, volume occupied by the dissolved gas at STP if it

were removed from 1 m3 of beerConditions 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!

Process Gases and Mass TransferPasteurization – Solubility decreases, must raise pressureIf gas breakout occurs, heating uneven, unreliable

pasteurizationRate of dissolution of a gas in a liquid

dm/dt = rate of mass transferKg and KL = overall mass transfer coefficientsPg and PE = gas and equilibrium partial pressures of the gas

in the gas phaseC and CE = liquid and equilibrium concentrations of the gas

in the liquid phaseA = interfacial surface area for mass transfer

)()( CCAKPPAKdtdm

ELEGG

Process Gases and Mass TransferIf 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 timeV = Volume of the vessel contentsKL = overall liquid mass transfer coefficientC and CE = liquid and equilibrium concentrations of the gas

in the liquid phaseA = interfacial surface area for mass transfer

)( CCAKdtdCV EL

Process Gases and Mass TransferDecarbonation 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 processesEquilibrium O2 concentrations < CO2 concentrationsNitrogen blanketing can prevent excessive O2 pick-up

Process Gases and Mass TransferBeer containing 1.8 volumes of CO2 per volume of beer at

stp is pasteurized at 73C. 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 = 44Molecular weight of water = 18Henry’s constant at 73 = 440 MPa/mol fraction

Process Gases and Mass TransferA beer keg of 50 x 10-3 m3 capacity stored in a cellar at a

temperature of 12C 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 = 44Molecular weight of water = 18Henry’s constant at 12C = 120 MPa/mol fraction1 kmol gas = 22.4 m3 at stp.Assume MW and density are same as that of water

InstrumentationAccuracy – “Freedom from Error”Process Industry – Accuracy = Inaccuracy…?1% accuracy indicates that measured value

should be within 1% of true valueAccuracy vs. PrecisionRandom Error (Precision)Systematic Error (Bias)Accuracy of instrument

% of full scale% of measurement

InstrumentationSelection 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

Pressure MeasurementManometer – Measure height of liquid (Calibrate)

Pressure MeasurementMechanical – Bourdon Tube

Pressure MeasurementElectrical – Strain, capacitive or piezoresistive

• Strain – small deflection changes resistance• Capacitive – High freq. oscillator, plates vary gap• Piezoresistive – Monocrystalline silicon

Temperature MeasurementThermometer

• Liquid/gas• Bi-metallic

Resistance Temperature DetectorThermocoupleInfrared Temperature Detector

Level MeasurementBubble Tube – Pressure required to inject air into

a tank indicates height (bulk sugar tanks)Force balance – Measure pressure at bottom of

tank, indicates heightUltrasonic – Sound waves reflect off of liquid/gas

interfaceFloatSingle position

Flow MeasurementMagnetic – Faraday’s Law (Conductor moving

through magnetic field, voltage produced)40:1 Rangability0.5% of f.s. accuracyNo obstructionsFluid must conductNo good for

• Pure water• Gases• Hydrocarbon fuels

External elec/mag fields

Flow MeasurementNon-Linear – Venturi, orifice, nozzle meters4:1 Rangability2% f.s. accuracy

Flow MeasurementTurbine – magnetic pulse as turbine wheel spins20:1 Rangability, 0.25% f.s. accuracyEasy to interface with control system

Gas MeasurementSensor 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

Gas MeasurementCO2 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

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

Drying and Psychrometrics

Free water firstSaturated surfacesRate slows, diffusionEquilibrium reached

T2

T1

Material being dryed

Tem

p (C

)T 2

T1

TimeD

ryin

g R

ate

Moisture Content

Drying and PsychrometricsBarley

- Harvested (20%), Storage (10%)- Max. drying temp: 46C for 20%, 66C 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 = 65C, Tout = 30C, WC = 14% - Stage 2: Tin = 80C, Tout = 50C, WC = 5%- Stage 3: Tin = 100C, Tout = 100C, WC = 2%- Overall process – 18 to 48 hours

Drying and PsychrometricsHops

- Picked at 80% moisture, dryed to 10%- Tin 55 to 65C

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

Drying and PsychrometricsHumidity ratio: mass of water vapor / mass of airSaturation humidity: saturation mass / mass of air

at a given temperatureRelative humidity: humidity / saturation humidityDew point: Temperature at which a given mixture

would become saturated at given humidity ratioWet bulb temperature: equilibrium temperature at

interface of water and air

See psychrometric chart.

Drying and PsychrometricsExample 1: Determine the humidity ratio and

relative humidity in the room todayExample 2: Air at 50% relative humidity and 22C is

heated to 35C. 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 spraymw,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 (+3C)

What we’ve covered so far…Dimensions and UnitsSystems, Phases and Properties (Steam tables)Conservation of Mass and EnergyNewtonian Fluids and ViscosityReynolds Number, Laminar and Turbulent FlowEntrance Region, Velocity ProfilesFriction Loss in Pipes and FittingsFlow Meter and Valve Types – Relative MeritsPumping Power, Types, Sizing, Cavitation/NPSHFiltration, Solids SettlingHeat Transfer (Conservation of Energy)Conduction, Convection and Radiation

What we’ve covered so far…Heat Transfer EquipmentLMTD and Overall Heat Transfer CoefficientHeat Exchanger Sizing, Heat LossesCombustion and Steam GenerationRefrigeration CyclePasteurization – Flash and TunnelDrying and PsychrometricsPrimary and Secondary Refrigerant ApplicationsWort BoilingProcess Gases and Mass TransferInstrumentation and ControlMaterials and Corrosion

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• Psychrometric chart (for drying)• Gas-liquid mass transfer

Where we are GoingThe 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