Transcript Kein Folientitel

Catalytic Tests I:
Flow systems and product analysis
Modern Methods in Heterogeneous Catalysis Research
Rolf Jentoft, November 15, 2002
Flow systems and product analysis
Product analysis
Gas flow control
Gas supply
Reactor
Flow Control
Flow Measurement
Pressure Measurement
Pressure Control
Outline
•Important parameters for reactor feed flow
•Gas flow measurement and control
Valves
Mass Flow controllers
•Pressure measurement and control
•Saturators and vaporizers
•Product analysis
•Gas chromatography
•Mass spectroscopy
•Infra Red spectroscopy
Parameters for reactor feed flow
Flow rate:
ml / minute or micromoles / minute
PV = nRT
use it!
Volumetric flow calibration must specify conditions
STP (0 °C, 760 mm Hg)
Correct for pressure:
1 ml in Berlin = 1.12 ml Denver Colorado
Non-deal gas mixtures
Pressure
Do you need to control the pressure
Pressure drop in reactor
Is your flow meter independent of pressure
Quartz reactor body: safety
Condensation (products or reactants)
Uncertainty of measurement
During the design of a gas flow system consider
the precision that you will need
Calculate uncertainty before you build
(Partial derivative analysis)
Laboratory reactors
Flow: 10 - 1000 sccm/min
5 mg to 5 grams catalyst
Pressures 1 - 200 bar
Minimize volume of tubing
Minimize “dead volume”
Flow measurement
Bubble flow meter
Not for in-line use
Pressure and temperature
correction necessary
Solubility of gas in bubble liquid
Inexpensive
Often used for calibration standard
Digital versions available
Dry piston flow meters
Flow measurement
Rotameter (variable area flow meters)
Can be installed in line
Pressure and temperature
correction necessary
Gas specific calibration
Flow measurement
Thermal mass flow meter (Electronic mass flow meters)
Measures temperature increase for a set energy input
Mass Flow 
Usually no temperature or pressure correction needed
Flow measurement
Thermal mass flow meter (continued)
Calibrated for specific gas and flow range
Conversion factor, K, can be used for measurement
of other gases
K  CP  
K N2
 N 2 CP ( N 2 )
QHe  QN2 
 QN2 

K He
 He CP( He)
Where Q  flow at standard conditions
  density at standard conditions
C P  heat capacity at standard conditions
Flow measurement
Thermal mass flow meter (continued)
More recently, flow meters which have polynomial
conversion factors are available
Gas must be filtered
Expensive
Flow Control
Control Valves
Valve constant: CV, KV, f
used to select valves
P2
P1
If 0.53 P1  P2 (P  P2 )
Q  30.8  KV
If 0.53 P1  P2 (P  P2 )
P  P2
 T
P1
Q  30.8  KV
2  T
Where   specific gravity (Kg/dm3)
T  absolute temperature (K)
P  absolute pressure (Kg/cm2)
Mass Flow Controller
Combines measurement of mass
flow with control valve
Check valves
He
Reactor
O2
C3H6
Add check valves to stop back diffusion
Minimize tube volume
No “dead” volume (gasses are not flushed away)
Pressure measurement and control
Bourden-tube gauge
Pressure transducers
Back pressure regulators
Tend to pulse at low flows
Specification should include minimum KV
Multiple control valves
Pressure
reduction
Mass flow
control
Check
valve
Back pressure
regulator
Reactor
Danger of interaction between control valves
1.90
1.85
Ion current m/e 43 x 10
10
Oscillations due to
interaction
between pressure reducer
and mass flow controller
1.80
1.75
1.70
1.65
1.60
20
25
Time, min
30
Multiple control valves
Pressure
reduction
Mass flow
control
Check
valve
Back pressure
regulator
Reactor
De-couple regulators by increasing volume if possible
Change control parameters of valves
Add restrictions to change response of the system to valves
Saturators and vaporizers
Reactants that are liquids at room temperature
and atmospheric pressure
Gas in
Saturators: carrier gas is bubbled through
a volume (height) of the liquid
to be evaporated
If design is correct, gas is in equilibrium
with liquid reagent and contains
Vapor content of outgoing gas function
of temperature and pressure
Need to control temperature and measure
pressure
Gas out
Saturators and Vaporizers
Carrier gas inert to liquid,
(saturates liquid at very low concentration)
Bubbles should be small (frit on bottom of inlet)
Height of liquid must be controlled
No foaming or formation of spray
Analyze to determine saturation and stability
Tubing down stream of saturator must be at
higher temperature than liquid
Saturators and Vaporizers
What is gas concentration in exit stream?
saturation of methanol in N2 at 21.2 °C and 1 bar
Flow N2 = QN2  20 sccm/min
Vapor pressure methanol =
pm= 0.132 bar
pN2  ptotal  pm  0.868 bar
Qtotal
 Ptotal
 QN 2 
 PN 2

  23.0 sccm/min

Saturators and Vaporizers
Vaporizors: liquid flow is controlled and liquid is
mixed with carrier gas and evaporated
Liquid
Pump
Mass flow
controller
?
Reactor
Heated
Liquid flow are very low (5 microliters methanol / minute)
Configuration of vaporizer is not trivial
The vaporization process tends to oscillate at low flows
Commercial products are available
Heating (tracing) tubes and valves
Product
analysis
Pressure
reduction Mass flow
control
Reactor
With saturator or vaporizer, tubes must be heated
Products tubes should also be heated
Tubes need only be heated to stop condensation
Reactor Bypass
Pressure
reduction
Mass flow Check
control
valve
Reactor
Check for reactant stream purity / stability
Particularly useful for systems with saturators
Product analysis
Products are usually not unknowns
Quantification is necessary
Rapid analysis may be necessary
Describe in more detail:
Gas chromatography
Mass spectroscopy
Infrared spectroscopy
Product analysis: GC
Best for quantification and stability
Slower than MS or IR
Not as flexible as MS
GLC and GSC
Product analysis: GC
Product analysis: GC
Injector system:
Liquid injector usually included
Useful for product identification
and troubleshooting
Not good for calibration
Split / splitless
Direct on-column
Paralyzing
Product analysis: GC
Injector system:
Gas sampling valves
Sample loop adjusted to sample concentration
and detection limit
Disadvantage: can increase pressure upstream
Product analysis: GC
Columns:
Capillary vs. Packed
Separation of complex mixtures may require
more than one column
Vendors can be sent a product list and asked
for solutions
Product analysis: GC
Detectors:
Product analysis: GC
Detectors:
Flame Ionization Detector
High sensitivity for hydrocarbons
Does not detect water or many
permanent gases
Destructive
Product analysis: GC
Detectors:
Thermionic Emission Detector
High sensitivity for nitrogen
and phosphorus
Product analysis: GC
Detectors:
Thermal Conductivity Detector
Changes in conductivity due to
product in the carrier stream
Non-destructive, often combined
with FID
Product analysis: MS
Fast scanning speeds
Less stable and more difficult to quantify
Not suitable for complex mixtures
Difficult or impossible to separate isomers
Flexible and portable
Inlet
Ionization
Separation
Detection
Product analysis: MS
Inlet:
Usually a capillary in the product stream takes
continuously samples products
About 2 ml / min required
Heating is necessary
Capillary leads to a pinhole or to a
concentration device
Product analysis: MS
Ionization:
Electron or hard Ionization
Fragments molecules
Positive ions are directed to analyzer
Product analysis: MS
Ionization:
Electron or hard Ionization
Product analysis: MS
Ionization:
Soft ionization (Chemical ionization)
Reagent gases are ionized (Hard ionization)
Ionized reagents (CH4) are mixed with product molecules
Products MH+ for amines or ethers
Saturated hydrocarbons often give (M-1)+
Product analysis: MS
Ionization:
Product analysis: MS
Ionization:
Product analysis: MS
Seperation:
Quadrupol mass spectrometer
Product analysis: MS
Ionization:
Time of flight
Product analysis: MS
Detectors:
Faraday cup
Stabile and reliable
Product analysis: MS
Detectors:
Multichannel electron multiplier
Signal gain of 105
Product analysis:
Analysis:
After identification of products:
Selected ions are monitored with time
For simple mixtures a calibration matrix can be measured
and concentrations can be calculated
Product analysis:
IR spectroscopy
A more specific IR analyzer
Detector contains analyte, responds
only to radiation absorbed by analyte
References:
F.W. McLafferty and F. Turecek, Interpretation of Mass Spectra,
Fourth Edition, University Science Books, 1993
H. H. Willard, L. L. Merritt, Jr., J. A. Dean, and F. A. Settle, Jr.,
Instrumental Methods of Analysis, Seventh Edition,
Wadsworth, 1988