Process Gases and Mass Transfer

rangebeaverMécanique

22 févr. 2014 (il y a 3 années et 8 mois)

139 vue(s)

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

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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


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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,

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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

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-

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-

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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

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-

Stage 2: T
in

= 80

䌬⁔
out

= 50

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-

Stage 3: T
in

= 100

䌬CT
out

= 100

䌬C坃‽′%


-

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18 to 48 hours

Drying and Psychrometrics

Hops


-

Picked at 80% moisture, dryed to 10%


-

T
in

55 to 65




Y敡獴


-

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-

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


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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



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