thermodynamics (theory, Chemcad)

Report
Thermodynamics
in the production and purification of methanol
from methane
Jacob Hebert, Michael McCutchen, Eric Powell, and Jacob Reinhart
(Group 6)
Thermodynamics Review - Topics
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•
•
•
•
•
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Important Terms & Concepts
Models in CHEMCAD
Partial Oxidation
Steam Reforming
Water-Gas Shift
Methanol Synthesis
Methanol Purification
Important Concepts
• Gibbs energy (G, ΔG): denotes what happens
spontaneously, and to what degree
• Enthalpy (H, ΔH): relates to sensible heat
• Entropy (S, ΔS): commonly referred to as
“disorder”, universal entropy always increases
• Chemical equilibrium (matters most at long time
scales and/or fast reactions!)
Significance of the Thermodynamic Model
• There are many models allowing for good
approximations in a variety of situations
• These models account for the non ideality
which arise when chemical interactions cause
unexpected effects.
The Selection Process
• Factors to be considered include:
– Intermolecular bonding
– Pressure and Temperature of the reaction
– Phase separation
– Molecular weights (for hydrocarbons)
Choosing our model
N
Y
Use G-S
Start
T < 250 K
Y
Y
H2
present
Use P-R
or R-K-S
Hydrocarbon
C5 or lighter
N
Use B-W-R
or L-K-P
Y
Y
Use G-S
Y
N
H2
present
Y
P < 200 bar
N
Use R-K-S
N
Y
P < 350 bar
N
Need more
experimental
data
Y
Use
electrolyte
Electrolytes
N
N
0<T<750K
Use sour
water system
N
N
Y
Y
Sour Water
N
T < 250 K
Use G-S
or P-R
Polar or
Hydrogen
bonding
N
P < 4 bar
T < 150ºC
Y
Use UNIFAC to
estimate
interaction
parameters
N
γi
experimental
data
Y
Two
Liq phases
N
Use Wilson, NRTL
or UNIQUAC
Y
Use NRTL
or UNIQUAC
Select model that
gives best fit to
data
Towler, G., & Sinnott, R. (2008). Chemical engineering design principles, practice and economics of plant and process design. Amsterdam: Elsevier/Butterworth-Heinemann.
Steam Reforming
• CH4 + H2O -> CO + 3H2
• Affected by temperature (higher temperature favors the
reaction)
• Affected by pressure (lower pressure favors the reaction)
• Very endothermic reaction
Ali, M. S., Zahangir, S. M., Badruddoza A. Z.
M., Haque M. R. (n.d.) A Study of Effect of
Pressure, Temperature, and Steam/Natural
Gas Ratio on Reforming Process for
Ammonia Production. Journal of Chemical
Engineering 23 1995-2005
http://www.banglajol.info/index.php/JCE/ar
ticle/view/5565
Partial Oxidation
– Methane and oxygen feed to reactor at 2:1 for methanol
synthesis preparation
– Three different reactions can take place during partial
oxidation
A) CH4 + 0.5O2  CO + 2H2 ΔH = -38kJ/mol
B) CH4 + 2O2  CO + 2H2O ΔH = -803kJ/mol
C) CO + H2O ↔ CO2 + H2
ΔH = -41kJ/mol
– Choose reaction conditions to maximize Reaction A and
minimize Reactions B and C
Partial Oxidation: Reaction Enthalpies
http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf
1
kJ
• CH4 + 2 O2 ↔ CO + H2
∆H = −38 mol
• Selectivity for different reactions depends on temperature and pressure of reaction
• Water-Gas shift reaction favors formation of CO and H2 and high temperatures
Partial Oxidation: Temperature Effect
• Calculated equilibrium distribution
for CH4 : O2 molar feed ratio of 2:1
• Partial oxidation favored over
complete oxidation at higher
temperatures
• Above 1000K, methane conversion
and syngas selectivity >90% can be
achieved
• Without catalyst, operating
temperature of 1400K is necessary
for reaction to occur
http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf
Partial Oxidation: Pressure Effect
• Despite excess feed oxygen, all
oxygen was consumed,
indicating a lack of inhibition
for total oxidation
• Increase in moles of gas
molecules inhibit partial
Methane conversion at different pressures and different O:C ratios
oxidation at increasing pressure
http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf
• Reduced conversion at high
pressure can be compensated
for by operating at higher O:C
ratios
Water-Gas Shift
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CO + H2O -> CO2 + H2
Slightly exothermic, but G and H vary with temperature
Low temperatures favor forward reaction
Pressure affects the reaction (even if it appears it should not)
John Kitchin, using values from the NIST Webbook http://matlab.cheme.cmu.edu/2011/12/12/water-gas-shift-equilibria-via-thenist-webbook/
Chang, T., Rosseau, R. W., Kilpatrick, P. K., (1986).
Methanol Synthesis Reaction: Calculations of
Equilibrim Conversions Using Equations of State. Ind.
Eng. Chem. Process Des. Dev. 25 477-481.
Methanol Synthesis
• temperature dependent, much
like most other reactions
CO + 2H -> CH3OH
• exothermic reaction
• strongly pressure dependent due
to the reduction in total moles
(high pressure favors the forward
reaction)
Chang, T., Rosseau, R. W., Kilpatrick, P. K., (1986). Methanol
Synthesis Reaction: Calculations of Equilibrim Conversions Using
Equations of State. Ind. Eng. Chem. Process Des. Dev. 25 477-481.
Final Step: Separation
• One of the most important steps where
thermodynamics plays a role.
• The goal is to minimize energy loss
• These losses are due to mixing as well as heat and
mass transfer
• Thermodynamic properties and phase equilibrium
are notoriously hard to predict in methanol, water,
and hydrocarbon mixtures.
Distillation
• Differing boiling points among the species allows for a flash drum
followed by distillation
• Two distillation columns are used to separate water and methanol
• In a perfect system the thermodynamics would be controlled by
heaters and coolers with appropriate duties at each stage of the
columns, main issue is the cooling of mixture exiting at high
temperature
• Thermodynamically distillation is a good separation process but
care must be taken to specify proper column specifications in order
to avoid unnecessary complications.
Simple Example
http://www.scielo.br/scielo.php?pid=S0104-66322008000100021&script=sci_arttext
Hydrate formation
• At specific temperatures and pressures hydrate
formation can occur within the stream
• Hydrates are solid crystalline compounds that are
created by natural gas compounds occupying the
empty lattice positions in a water structure
http://www.esrf.eu/UsersAndScience/Publicati
ons/Highlights/2009/materials/mat09
Azeotropes
• No Azeotropes in data for methanol, water,
and hydrocarbons
• The closest azeotropic mixture is ethanol and
water
• Care must always be taken when dealing with
separating a mixture that no azeotropes exist.
Pressure Swing Adsorption
• An easier separation process that deals with gases,
thermodynamically favored
• Separates a gaseous species based off of the molecular
affinity for absorbent materials
• High pressure adheres the gas to solid surface and low
pressure it is released
• Very useful cleaning catalysts for hydrocarbons are zeolites
http://www.gazcon.com/sw13931.asp
References - Models
Towler, G., & Sinnott, R. (2008). Chemical engineering design
principles, practice and economics of plant and process
design. Amsterdam: Elsevier/Butterworth-Heinemann.
References – Water Gas Shift and
Methanol Synthesis
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Bustamante, F., Enick, R., Rothenberger, K., Howard, B., Cugini, A., Ciocco, M.,
Morreale, B. (2002). Kinetic Study of the Reverse Water Gas Shift Reaction in HighTemperature, HIgh-Pressure Homogeneous Systems. Fuel Chemistry Division
Preprints 47(2), p. 663-664
Daza, Y. A. , Kent, R. A., Yung, M. M., Kuhn, J. N. (2014). Carbon Dioxide Conversion
by Reverse Water-Gas Shift Chemical Looping on Pervoskite-Type Oxides. Ind. Eng.
Chem. Res. 53, 5828-5837.
Park, S., Joo, O., Jung, K., Kim, H., Han, S. (2001). Development of ZnO/Al2O3
catalyst for reverse-water-gas-shift reaction of CAMERE (carbon dioxide hydrogenation
to form methanol via a reverse-water-gas-shift reaction) process. Applied Catalysis A:
General 211 p. 81-90
Chang, T., Rosseau, R. W., Kilpatrick, P. K., (1986). Methanol Synthesis Reaction:
Calculations of Equilibrim Conversions Using Equations of State. Ind. Eng. Chem.
Process Des. Dev. 25 477-481.
Kitchin, J. (2011) “Water Gas Shit Via the NIST Webbook”.
http://matlab.cheme.cmu.edu/2011/12/12/water-gas-shift-equilibria-via-the-nistwebbook/
References POX and Steam Reforming
• Ali, M. S., Zahangir, S. M., Badruddoza A. Z. M., Haque M. R.
(n.d.) A Study of Effect of Pressure, Temperature, and
Steam/Natural Gas Ratio on Reforming Process for Ammonia
Production. Journal of Chemical Engineering 23 1995-2005
http://www.banglajol.info/index.php/JCE/article/view/5565
• Lyubovsky, M., Roychoudhury, S., & LaPierre, R. (2005). Catalytic
partial “oxidation of methane to syngas” at elevated
pressures. Catalysis Letters. doi:10.1007/s10562-005-2103-y
• Zhu, Q., Zhao, X., & Deng, Y. (2004). Advances in the Partial
Oxidation of Methane to Synthesis Gas. Journal of Natural Gas
Chemistry, 13, 191-203. Retrieved from
http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf
References - Separations
• Demirel, Dr Y., "Retrofit of Distillation Columns Using
Thermodynamic Analysis" (2006). Papers in Physical
Properties. Paper 6.
http://digitalcommons.unl.edu/chemengphysprop/6
• Lide, D.R., and Kehiaian, H.V., CRC Handbook of
Thermophysical and Thermochemical Data, CRC Press, Boca
Raton, FL, 1994.
http://chemistry.mdma.ch/hiveboard/picproxie_docs/000506
293-azeotropic.pdf

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