PARTIAL OXIDATION OF

METHANE

FOR

METHANOL AND FISCHER

TROPSCH SYNTHESIS

AUTHORS

1. JOLOMI OLIVER AYAN . E.

GAS ENGINEERING

UNIVERSITY OF PORTNARCOUT

2. MATTHEW OKORO

GAS ENGINEERING

UNIVERSITY OF PORTNARCOUT

3. ANIOKE MODESTA GAS ENGINEERING UNIVERSITY OF PORTNARCOUT

ABSTRACT

This study involves the partial oxidation of methane for methanol and production

of syngas. It discuss the plant design for the synthesis of syngas from the partial

oxidation of methane and then using the syngas as a feedstock for the synthesis

of methanol and also to synthesized Fischer Tropsch products. Comparison is

then made with the partial oxidation process with other methods of synthesizing

syngas from methane. The mass balance, entropy and gibbs free energy

constraints were also discuss elaborately. Methanol and Fischer Tropsch synthesis

is discussed in relation to the partial oxidation of methane.

KEYWORDS

Partial Oxidation of methane, syngas, methanol, Fischer Tropsch, Gibbs free

energy, Steam methane reforming, dry reforming of methane, desulphurization,

reactor, zeolite, distillation, refractory lining, power density.

NOMENCLATURE

CH4 Methane O2 Oxygen POM Partial Oxidation of methane SMR Steam methane reforming DMR Dry methane reforming CO Carbon monoxide

H2 Hydrogen H2O Water Syngas Synthesis Gas -CH2- Hydrogen Carbon unit CE Carbon Efficiency HE Hydrogen Efficiency S1 Entropy for state 1 S2 Entropy for state 2 ∆SPROCESS Change in Entropy Q Amount of Heat H Enthalpy T Temperature SGEN Entropy Generated KJ kilojoules mol Moles

INTRODUCTION:

The partial oxidation of methane for methanol and Fischer Tropsch synthesis is a

process involving the production of synthesis gas (syngas) from the oxidation of

methane and then converting the syngas into methanol and for Fischer Tropsch

synthetic products. The syngas produced is simultaneously used as feed for both

the methanol reactor and Fischer Tropsch synthesis reactor.

The schematic in figure 1.1 below illustrates the process

CH4

Syngas Recycle loop

Fig 1.1. A flowsheet showing the Partial oxidation of methane for methanol and FT synthesis.

The plant shown in the flowchart above was designed to consist of three phases each describing a vital process employed in the chemical and process industry. In order to attain the objectives of this study the major process that made up each of the phases in this integrated plant will discussed. The constraints in each process, the thermodynamic properties, the economic advantages and the reactor compositions will be highlighted.

Air Separation

Desulphurization Partial Oxidation

Methanol Conversion Compression Distillation

FT Synthesis

Zeolite

Methanol Olefins Gasoline

O2

Olefins Gasoline Wax Diesel

Syngas

CH4 Syngas

PHASE 1 THE PARTIAL OXIDATION OF METHANE: Generally partial oxidation is used to describe a process in which the quantity of

the oxidizer is less than that required for the complete combustion of a

hydrocarbon fuel to produce synthesis gas also called syngas.

Syngas can be produced from different carbon and hydrocarbon sources. The

sources includes coal, biomass, methane etc.

This study makes use of methane as the hydrocarbon feed in the production of

syngas in a process called the partial oxidation of methane. In this process

methane is made to react with pure oxygen to produce syngas.

The methane is first desulphurized then passed into a reactor where it is made to

react with pure oxygen at elevated temperature and pressure to produce

synthesis also called syngas. The syngas produced is usually at a temperature of

1650K. This process is autothermal and requires no catalyst.

The overall reaction of the process is given below

CH4 + 0.5O2 → CO + 2H2 ∆H= -36 kj/mol

The enthalpy of the process tells us that the process is exothermic meaning that a

high pressure and low temperature will shift the equilibrium position to the right

favoring the formation of more syngas. Another factor that will increase the

formation of more syngas if the prompt removal of the syngas from the reactor

chamber as they are been produced. The schematic below shows the different

component of the partial oxidation of methane process.

Air Separation

Methane Desulphurization Partial Oxidation Reactor

O2

CH4 Syngas

Syngas

CH4

Fig 1.2 Partial Oxidation of Methane (POM)

1650 K

The illustration above shows a stream of methane passed through a

desulphurization chamber to remove every trace of sulphur. Then the methane is

passed into a reactor where it is combined with pure oxygen at elevated

temperature. The pure oxygen is produced from an air separation unit. A burner

inside the reactor chamber mixes the methane and pure oxygen causing them to

react in a turbulent manner to produce syngas. The reactor used in this partial

oxidation of methane process is shown in figure 1.3 below.

Fig. 1.3. A typical Partial Oxidation of Methane reactor

Air Separation

The reactor is refractory lined to sustain the high temperature of the produced

syngas (1650 ⁰K).

The syngas produced is then passed through two different routs one to feed a

methanol synthesis reactor and the other a Fischer Tropsch reactor. See Fig 1.1

SOME OTHER METHODS OF PRODUCING SYNGAS FROM METHANE

In the chemical industry other methods exist in the synthesis of syngas from

methane. We shall study some of these methods here so we could compare them

with the partial oxidation method. This methods includes:

Steam Methane Reforming (SMR):

In this process Methane and steam (preheated to temperature of about 850K and

passed over a catalyst tube) are made to react to form a mixture of carbon

monoxide and hydrogen gas.

The process is represented by the equation below:

CH4 + H2O → CO + 3H2 ∆H = +206 kj/mol

CO + H2O → CO2 + H2 ∆H = -41 kj/mol

The equation above shows that the process is endothermic and thus the synthesis

process must be sustained by high temperature and heat will need to be supplied

to produce more syngas. This will amount to more design cost. There will also be

additional cost required to recycled and excess CO2 produced from the water gas

shift reaction.

Advantages of Partial Oxidation of Methane (POM) over Steam Methane

Reforming (SMR)

1. POM do not require catalyst while SMR does.

2. POM requires no additional heat to sustain the production syngas

3. POM has a compact design unlike SMR

4. The H2/CO for POM is about 2:1 where as that of SMR is between 3-5 to 1

5. POM do not require the water gas shift reaction unlike SMR which do.

6. POM has higher power density than SMR and as such are suitable for

electricity generation

7. POM is a simple but efficient process unlike SMR which is complex

8. POM is cheaper to design and operate than SMR

Dry Reforming of Methane (DRM):

This process involves the reaction of methane and carbon dioxide in the presence

of catalyst to produce syngas. This process require the water gas shift reaction

which helps to improve the H2/CO ratio.

The equation representing this process is:

0.5CH4 + 0.5CO2 → CO + H2 ∆H = +247 kj/mol

CO + H2O → CO2 + H2 ∆H = -41 kj/mol

The enthalpy of the process shows that its an endothermic process. Thus the

process must be supplied with heat from an external source in order to ensure

the continued production of syngas

Advantages of partial oxidation of methane (POM) over Dry Reforming of

Methane (DRM)

1, The POM process requires no additional heat in the formation of the syngas as

the process is favored by lower temperature since its an exothermic process,

unlike DRM which is endothermic and requires high temperature to maintain the

syngas formation.

2, POM is autothermal requiring no catalyst whereas DRM requires catalyst.

3, POM do not require the water gas shift reaction whereas DRM do.

4, POM has higher power density than DRM and as such are suitable for electricity

generation.

5, POM has a simple and compact design since it require no external heat

provider to enhance the production of since gas in the reactor unlike the DRM.

FACTORS THAT WILL INCREASE THE YIELD OF SYNGAS IN THE PARTIAL

OXIDATION OF METHANE

The exothermic nature of the POM process will allow certain constraints to

determine the yield of syngas from the process. They include.

1. Low Temperature: if the temperature of the reactor is lowered the

equilibrium position of the process will shift to the right favoring the

formation of more syngas.

2. High Pressure: A pressure will have the same effect as a lower temperature

in this process favoring the formation of syngas.

3. High Removal rate of the Syngas: the faster the rate at which the syngas

produced is been removed more syngas will be formed thereby increasing

the conversion rate of the feedstock.

4. Constant stream of the feedstock: the presence of sufficient feedstock in

the reactor increase the rate of formation of the syngas.

MASS BALANCE OF THE POM PROCESS:

The Mass balance process of the partial oxidation of methane is given by

CH4 + 0.5O2 → CO + 2H2

From the mass balance we can understand the minimum amount Of the reactant

or feed that will produce a given ratio of the product formed. If the amount of the

feed stock in increase then there will be byproduct which will amount to

environmental problem and also higher cost in recycling and treatments.

From the mass balance of the POM, to produce 1 mole of CO we will

Produce 2 moles of H2

Feed 0.5 moles of pure O2

Feed I mole of methane

When we increase the amount of O2 by 0.5 moles the following products will be

formed as shown in the mass balance below.

CH4 + O2 → CO2 + 2H2

It shows that no CO when 1 mole of O2 is used as feed stock. Lets look at a

situation when we used 2 moles of O2 in the process.

CH4 + 2O2 → CO2 + H2O + CO + H2

We have seen that when we used 2 moles of Oxygen in the process instead of 0.5

mole both CO2 and H2O will be produce as by products which we have to recycle

since we do not need then in this process. This will of cause amount to higher cost

in the operation of the plant.

PROCESS EFFICIENCY OF THE PARTIAL OXIDATION OF METHANE

The process efficiency of this process tells us if there will be any form of waste in the process. The carbon efficiency, CE. and hydrogen efficiency HE is given below.

CE=

HE =

From the process mass balance we had

CH4 (g) + 0.5O2 (g) → CO(g) + 2H2 (g)

CE = =1 and HE = = 1

.

THE ENTROPY BALANCE PROCESS:

The entropy balance of this process must be greater than or equal to zero. The

partial Oxidation of methane is considered to be one of such process. In this

process the following holds.

The second law of thermodynamics which state that all real processes occur so as to increase the entropy of the universe.

The change in entropy of any system can represented by

∆S = ………………………………………………………. (1)

S1 S2

CH4 O2 Syngas

Fig 1.4 Showing the entropy of the process

To maintain a spontaneous process , from equation (1) above

∆S ≥ ≥ 0

Therefore the entropy balance across the methane reforming process is

S2 – S1 = ∆SPROCESS =Q/T + SGEN ………………………………………..(2)

In designing the partial oxidation of methane process we must put conditions to

Table 1.1 Showing CE and HE

PARTIAL OXIATION OF

METHANE REACTOR

FEED PRODUCT CE HE

CH4 (g) + 0.5O2 (g) CO(g) + 2H2 (g)

1 1

ensure that the entropy of the system is constant so that we could attain a change in entropy value ∆S, equal to zero. i.e ∆S = 0. The refractory lining on the reactor serves this purpose.

GIBBS FREE ENERGY AND ENTROPY FOR THIS PROCESS

The enthalpy ∆H, associated with this process is -36kjmol-1 Also the required temperature for this process is 1650 ⁰K

Therefore the Gibbs free energy is given by

∆G = H - T∆S ………………………………………………………………(3)

For the partial oxidation of methane the following parameters holds

H = -36 kj/mol

T = 1650 ⁰K

Thus by imputing values of T and H above into equation (4), we have

∆G = -36 - 1650∆S ……………………………………………………………(4)

From the model (4) above we see that ∆G will be negative provided that the

values of ∆S is equal to or greater than 0. Thus the partial oxidation of methane

process will be spontaneous at this values of ∆S which is very feasible from our

process design.

The table 1.2 below shows values of ∆G that is obtained for this process for

different vales of ∆S

Table 1.2

∆S (kj/mol) 0 1 2 3 4

∆G (kj/mol) -36 -1686 -3336 -4886 --6636

CALCULATIONS:

When ∆S = 0 kj/mol

From equ (5)

∆G = -36 - 1650∆S =247- (1223 *0) = -36 – 0 = -36

For ∆S = 1 kj/mol

∆G = -36 - 1650∆S =247- (1650 *1) = -1686

For ∆S = 3 kj/mol

∆G = -36 - 1650∆S =-36- (1650 *2) = -3336

For ∆S = 3 kj/mol

∆G = -36 - 1650∆S = -36- (1650 *3) = -4886

For ∆S = 3 kj/mol

∆G = -36 - 1650∆S = -36- (1650 *4) = -6636

PHASE 2:

THE SYNTHESIS OF METHANOL FROM SYNGAS

Syngas is the building block of several chemical processes one of them is the

synthesis of methanol. The syngas used in this study to produce methanol is from

the partial oxidation of methane. The flowsheet of methanol synthesis from

syngas is shown below for better understanding of the process.

Fig 1.5 Methanol Synthesis

Compression Methanol Conversion Distillation Methanol

Olefine Gasoline

Zeolite

Syngas Recycle Loop

Syngas

Cu/ZnO

METHANOL SYSTHESIS FROM SYNGAS

The production of methanol from syngas is a very efficient process and widely

used worldwide. This process involves subjecting syngas to elevated temperature

and pressure inside a reactor in the presence of a catalyst. The process is

illustrated in the figure 1.5 above. The process is exothermic and occurs together

with the water gas shift reaction. The equation below shows the process.

CO + 2H2 → CH3OH ∆H = - 90.84kj/mol

CO + H2O → CO2 + H2 ∆H = -41 kj/mol

The process is exothermic thus lower temperatures will increase the yield of

methanol.

EFFECT OF CATALYST IN THE METHANOL REACTOR

The surface of the catalyst (Cu/ZnO) in the reactor at high absorb the molecules of

CO and H2. Thereafter there is dissociation of the H2 molecules to H atoms. The H

atoms then combines with the CO bond on the surface of the catalyst at high

temperature to form methanol which then desorbs from the catalyst surface.

PROCESS CONSTRIANTS FOR METHANOL SYNTHESIS

Process Mass Balance:

The Mass balance process of the methanol synthesis is

CO + 2H2 → CH3OH

From the mass balance , to produce 1 mole of methanol we will

Feed 1 moles of CO

Feed 2 mole of H2

When we increase the amount of any of the feeds byproducts will be formed

which will amount to extra cost to deal also may lead to waste build up.

If the amount of each feed is increase we will obtain additional products along

with the methanol, see the equation below.

2CO + 3H2 → CH3OH + C + H2O

The implication of this is the production water and carbon soot. The water need

to be looped back into the reactor will the carbon be removed through any means

chosen to be the best feasible. This will impact on the design of the plant, amount

to higher cost of design, lead to wastage of the feedstocks, a bigger plant and

most seriously lead to environment problem.

PROCESS EFFICIENCY OF METHANOL SYNTHESIS

The process efficiency of the methanol synthesis is important because it tells us if the process will be clean or if there will be by products

The CE and HE of this process as gotten from the process mass balance shows that

all the feeds used are converted to methanol and no waste is formed in the

process.

USES OF METHANOL: methanol can be used as transportation fuel directly or

blended with other petroleum fuel, olefins, gasoline etc. Methanol can be

converted to a wide range of chemicals used in the chemical industries e.g.

ethers, esters, alcohols, alkanals, etc. Methanol can be converted to gasoline in

the presence of zeolite as catalyst. Liquid fuels from methanol has low carbon

emissions compare to petroleum fuels.

Table 1.3 Showing CE and HE foe Methanol synthesis

FEED PRODUCT CE HE

CO (g) + 2H2 (g) CH3OH

1 1

PHASE 3

FISCHER TROPSCH SYNTHESIS FROM SYNGAS

Fischer Tropsch synthesis was first built and applied in Germany in 1935. It was

later then applied be countries with no known reserve of crude oil thereby

providing liquid fuel from syngas. Fischer Tropsch synthesis is based on GTL

technology which provides syngas from available carbonated fossil substances

and then converts them to diesel. The fuel produced is sulphur free and are also

called syncrude. In this process syngas in the presence of catalyst is converted to

straight chain hydrocarbons in a reactor. A flowsheet for this process is shown

below

Fig. 1.6. A Fischer Tropsch Flowsheet The Fischer Tropsch (FT) Process is represented the mass balance below CO + 2H2 → -CH2- + H2O The general equation of the FT process is given by

(2n + 1) H2 + nCO → CnH2n + 2 + nH2O

Where n varies from 10-25 or above.

the catalyst that are used in are Iron which favours the formation of C1 – C4

hydrocarbons that are mainly gas. Cobalt catalyst favours the formation liquid

hydrocarbon of C5 – C9 (gasoline) and C10 – C18 (Diesel).

CONCLUSION

The partial oxidation of methane has been shown in this study to be a very

efficient process that process that produce syngas of H2/CO equal 2:1. This makes

the partial oxidation of methane process been widely applied as a feed process in

several chemical processes including methanol and Fischer Tropsch synthesis as

FT Synthesis Reactor Syngas Gasoline, Diesel

Hydrocracking of Wax Gasoline, Diesel

which this study focused on. The partial oxidation of methane is an exothermic

process and as such requires no heat and work supply, thus a low temperature

and high pressure favours the process. No catalyst is required for this process as it

is autothermal. When compared to other methods of producing syngas from

methane the partial oxidation of methane has the following advantages.

1. It has a compact design

2. It has a H2/CO equal 2:1 making it very suitable for several chemical

process including methanol production

3. It has higher power density making it most suitable for electricity

generation

4. Its CE and HE shows it produces minimal by products.

5. Its cheaper to design and run since it requires no heat and work supplied to

it and its exothermal

6. It requires no catalyst because its autothermal

7. Its very practicable to attain a negative gibbs free energy making the

process spontaneous.

The methanol synthesis process is an exothermic process. The process is very

vital because of the important uses of methanol which have been highlighted

in this study. The partial oxidation of methane produces syngas that are best

applied the production of methanol and the Fischer Tropsch synthesis as

shown in this study.

Reference

1. Kok J.B.W., Albrecht B.A., Dijkstra N. and van der Meer Th.H., Generation of synthesis gas and power by partial oxidation of natural gas in a gas turbine, Final Report, Laboratory of Thermal Engineering, University of Twente, 2004 http://www.thw.ctw.utwente.nl/.

2 Christensen T.S., Dybkjaer I., Hansen L. and Primdahl I.I., Design and performance of secondary and autothermal reforming burners, AIChE Safety Meeting, Vancouver, USA, paper No. 39, pp. 205−215, October 1994.