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 CO
2
in beer
Must exclude O
2
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
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 CO
2
measured on vol/vol basis
That is, volume occupied by the dissolved gas at STP if it
were removed from 1 m
3
of beer
Conditions in fermenter determine amount of CO
2
in green
beer
•
Open square fermenter
–
1 vol/vol
•
Cylindroconical fermentor
–
much greater
All brewery processes
–
must avoid gas breakout!
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
K
g
and K
L
= overall mass transfer coefficients
P
g
and P
E
= gas and equilibrium partial pressures of the
gas in the gas phase
C and C
E
= liquid and equilibrium concentrations of the gas
in the liquid phase
A = interfacial surface area for mass transfer
)
(
)
(
C
C
A
K
P
P
A
K
dt
dm
E
L
E
G
G
Process Gases and Mass Transfer
If partial pressure of CO
2
above the beer in storage is
greater than that required to keep CO
2
in solution, up
carbonation or pick
-
up occurs
dC/dt = rate of CO
2
concentration change w/ respect to time
V = Volume of the vessel contents
K
L
= overall liquid mass transfer coefficient
C and C
E
= liquid and equilibrium concentrations of the gas
in the liquid phase
A = interfacial surface area for mass transfer
)
(
C
C
A
K
dt
dC
V
E
L
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 O
2
concentrations < CO
2
concentrations
Nitrogen blanketing can prevent excessive O
2
pick
-
up
Process Gases and Mass Transfer
Beer containing 1.8 volumes of CO
2
per volume of beer at
stp is pasteurized at 73
䌮C䅳獵浩湧s扥b爠桡猠桥h獡浥s
浯汥捵污爠m敩杨e慳⁷慴敲Ⱐ捡汣畬慴攠瑨攠浯氠晲慣瑩潮映
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/m
3
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
Process Gases and Mass Transfer
A beer keg of 50 x 10
-
3
m
3
capacity stored in a cellar at a
temperature of 12
䌠捯C瑡楮猠㐰砠
-
3
m
3
of beer whose
CO
2
concentration is 1.4 volumes per volume of beer at
stp. The head space in the keg is filled with CO
2
which is
in equilibrium with the beer. Stating any assumptions
made, calculate the pressure of the CO
2
in the head
space and the total mass of CO
2
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 m
3
at stp.
Assume MW and density are same as that of water
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
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
Pressure Measurement
Manometer
–
Measure height of liquid (Calibrate)
Pressure Measurement
Mechanical
–
Bourdon Tube
Pressure Measurement
Electrical
–
Strain, capacitive or piezoresistive
•
Strain
–
small deflection changes resistance
•
Capacitive
–
High freq. oscillator, plates vary gap
•
Piezoresistive
–
Monocrystalline silicon
Temperature Measurement
Thermometer
•
Liquid/gas
•
Bi
-
metallic
Resistance Temperature Detector
Thermocouple
Infrared Temperature Detector
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
Flow Measurement
Magnetic
–
Faraday’s Law (Conductor moving
through magnetic field, voltage produced)
40:1 Rangability
〮0%潦o献s慣捵牡捹
乯扳N牵捴楯湳
Fluid must conduct
No good for
•
Pure water
•
Gases
•
Hydrocarbon fuels
External elec/mag fields
Flow Measurement
Non
-
Linear
–
Venturi, orifice, nozzle meters
4:1 Rangability
2% f.s. accuracy
Flow Measurement
Turbine
–
magnetic pulse as turbine wheel spins
20:1 Rangability,
〮05%昮f⸠慣.畲慣y
Easy to interface with control system
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
Gas Measurement
CO
2
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
O
2
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 first
Saturated surfaces
Rate slows, diffusion
Equilibrium reached
T
2
T
1
Material being
dryed
Temp (
C)
T
2
T
1
Time
Drying Rate
Moisture Content
Drying and Psychrometrics
Barley
-
Harvested (20%), Storage (10%)
-
Max. drying temp: 46
䌠景爠f0%Ⱐ㘶
䌠景爠C0%
-
䍡獣慤楮朠扡牬by睩瑨潵湴敲晬潷f潦⁷慲洠慩
-
卨慫楮朠灥牦潲慴敤e瑲慹猬s睡牭⁴桲潵杨桯汥l
䵡汴
-
Drying to stop enzyme activity after malt
modification is complete
-
Moisture content reduced from 45% to <4.5%
-
Stage 1: T
in
= 65
䌬⁔
out
= 30
䌬⁗䌠㴠C4%
-
Stage 2: T
in
= 80
䌬⁔
out
= 50
䌬†坃‽‵%
-
Stage 3: T
in
= 100
䌬CT
out
= 100
䌬C坃‽′%
-
佶敲慬a灲潣敳猠
–
18 to 48 hours
Drying and Psychrometrics
Hops
-
Picked at 80% moisture, dryed to 10%
-
T
in
55 to 65
䌠
Y敡獴
-
䑲D楮朠牥煵楲i搠慦瑥爠扥湧慵瑯汹獥d
-
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 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.
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
䌠
楳敡i敤e瑯t㌵
䌮†坨慴楳⁴桥i湥眠牥污l楶i
桵浩摩瑹Ⱐ睥琠扵汢l瑥浰敲m瑵牥慮搠摥眠灯楮琿i
䡯H浵捨m敮敲杹睡猠慤摥搠灥爠歧映摲k慩a?
Humidifying with water spray
m
w,in
+ m
w,spray
= m
w,out
Dehumidifying
–
2 step process (cool, then heat)
Evaporative cooling
–
Water spray into air,
evaporation cools water (latent + sensible)
Approaches wet bulb temperature (+3
䌩
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
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
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
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
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