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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.