Thermodynamicsin the production and purification of methanol from methane

Jacob Hebert, Michael McCutchen, Eric Powell, and Jacob Reinhart (Group 6)

Thermodynamics Review - Topics

• 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

HydrocarbonC5 or lighter

H2

present

Polar orHydrogenbonding

Sour Water

H2

present Electrolytes

T < 250 K

P < 200 bar

T < 250 K

γiexperimental

data

P < 350 bar

P < 4 barT < 150ºC

TwoLiq phases

Need moreexperimental

data

Select model thatgives best fit to

data

Use G-S

Use B-W-Ror L-K-P

Use P-Ror R-K-S

Use R-K-S

Use G-S

0<T<750KUse G-Sor P-R

Use NRTLor UNIQUAC

Use Wilson, NRTL or UNIQUAC

Useelectrolyte

Use sourwater system

Use UNIFAC toestimate

interactionparameters

Start

Y

Y

Y

Y

Y

Y

Y Y

Y

YY

N

N

N

N N

N

N N

N

N

N

Y

N

Y

N

Y

N

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/article/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

• Selectivity for different reactions depends on temperature and pressure of reaction• Water-Gas shift reaction favors formation of and high temperatures

http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf

Partial Oxidation: Temperature Effect• Calculated equilibrium distribution

for : 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 oxidation at increasing pressure

• Reduced conversion at high pressure can be compensated for by operating at higher O:C ratios

http://www.bjb.dicp.ac.cn/jngc/2004/2004-04-191.pdf

Methane conversion at different pressures and different O:C ratios

Water-Gas Shift

John Kitchin, using values from the NIST Webbook http://matlab.cheme.cmu.edu/2011/12/12/water-gas-shift-equilibria-via-the-nist-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.

• 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)

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/Publications/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

• 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 High-Temperature, 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-nist-webbook/

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/000506293-azeotropic.pdf