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Gas Reforming Reaction

1) Describe the three natural gas reforming reactions for syngas production. (1) steam reforming, (2) carbon dioxide reforming and (3) auto-thermal reforming. What are the heat of reaction and H2/CO ratio of these reforming reactions and their impact on the down-stream Fischer -Tropsch synthesis process?

(1) Steam reforming

Steam reforming or Steam methane reforming is a method in which we can produce hydrogen from natural gas. When steam reacts with methane it forms carbon monoxide and hydrogen (reforming reaction).In this reaction energy is consumed so it is an endothermic reaction. The heating value is -ΔHo298 = -206 kJ mol-1and the H2: CO ratio is 3 to 4(high).

CH4 + H2O = CO + 3H2

Carbon dioxide and hydrogen are formed from the reaction of steam with carbon monoxide. In this reaction energy is released, so it is an exothermic reaction. (Water-gas shift reaction). The heating value is -ΔHo298 = 41kJ mol-1and the H2: CO ratio is 1.8 to 2.1.

CO + H2O = CO2 + H2

CnHm represents the impurities in natural gas like C2, C3, hydrocarbons etc, In this reaction energy is consumed, so it is an endothermic reaction

.

CnHm + nH2O = n CO + (n + m) H2

Since there is exothermic and endothermic reaction in the same reactor, the output syngas will not be pure.

(2) Carbon dioxide reforming

Carbon dioxide reforming is also known as dry reforming. When carbon dioxide reacts with methane it forms carbon monoxide and hydrogen. This is an endothermic reaction in which energy is consumed. The heating value is -ΔHo298 = -247 kJ mol-1and the H2: CO ratio is 1(low).

CH4+CO2 = 2CO+2H2

(3) Auto-thermal reforming

In auto-thermal reforming, when steam reacts with methane it forms carbon monoxide and hydrogen (reforming reaction).This is an endothermic reaction in which energy is consumed. The heating value is -ΔHo298 = -206 kJ mol-1and the H2: CO ratio is 3 to 4(high).

CH4 + H2O = CO + 3H2

In order to keep the reaction going we need to burn a part of natural gas by the addition of oxygen. When oxygen reacts with methane it gives carbon dioxide and water. This is an exothermic reaction so it provides energy.

CH4 + 2O2 = CO2 + 2H2O

CnHm represents the impurities in natural gas like C2, C3, hydrocarbons etc.This reaction is endothermic reaction in which energy is consumed.

CnHm + n H2O = n CO + (n + m) H2

Since there is exothermic and endothermic reaction in the same reactor, the output syngas will not be pure.

2) Briefly discuss the role of water-gas shift reaction in gas to liquid conversion using Fischer -Tropsch synthesis.

The water - gas shift reaction is

H2O + CO → H2 + CO2

The water - gas shift reaction will be encountered in any gas to liquid or coal to liquid processes. The water - gas shift reaction makes sure that the H2: CO ratio is comparitable with the catalyst which we are using of the constituent synthesis reaction requires. The Fischer -Tropsch synthesis requires the H2: CO ratio to be 2. The water - gas shift reaction knocks out CO if we use coal as feedstock or converts CO2 into CO or converts H2 intoH2O to change the H2: CO ratio.

If syngas is produced from coal, the ratio tends to be less than 1 (0.7).In Fischer -Tropsch synthesis, the gas to liquid plant emits lot amount of CO2.For example a gas to liquid plant produces 7 tonnes of CO2 to get 1 tonne of liquid hydrocarbon product. We have the science supporting the technology to capture CO2 and that is water - gas shift reaction.

3) Through literature research list three catalysts for steam reforming of natural gas to produce syngas and discuss their relative performance in terms of advantages and disadvantages.

The three catalysts for steam reforming of natural gas to produce syngas are Nickel, Copper, and Cobalt.

Cobalt:

Advantages:

Cobalt is one of the most active catalysts

Cobalt catalyst has high sensitivity towards sulphur

Cobalt catalyst can operate at high temperature

Cobalt catalyst is more resistant to sintering

The inherent activity of cobalt catalyst is low

The conservative rate is high during reactions

The life time of cobalt catalyst is nearly 5 years.

Disadvantages:

Cobalt catalyst does not get activated quickly.

Nickel:

Advantages:

In the presence of hydrogen nickel catalyst can be more effective

Nickel catalyst can precipitate to any metal

In case of corrosion the nickel catalyst is extremely resistant.

The strength of nickel catalyst is excellent at low temperature.

Nickel catalyst is not tarnished by air.

Disadvantages:

When nickel is used as catalyst the reaction which is taking place is discontinuous

Nickel catalyst is of high cost

The internal resistance of nickel as catalyst is high.

Copper:

Advantages:

  • When copper is used as catalyst, the heterogeneous reaction can take place at normal temperature.

  • The solubility of the chemicals can be improved using copper as catalyst.

  • Copper catalyst can produce high yield.

    Disadvantages:

    Copper can be deactivated in the presence of chloride and sulphur.

    When copper reacts with zinc oxide, it is poisonous and hazardous.

    4) Through literature research list two Fischer -Tropsch synthesis catalysts and discuss their relative performance in terms of catalytic activity and product distribution.

    The two Fischer -Tropsch synthesis catalysts are

    Cobalt

    Iron

    Cobalt is one of the most active catalysts. Cobalt catalyst is used when natural gas is used as feedstock for the Fischer -Tropsch synthesis because of its high H2: CO ratio (1.8-2.1).Cobalt catalyst has high sensitivity towards sulphur.Cobalt catalyst is more selective and can last longer time. When we use cobalt catalyst, the water-gas shift is not necessary. Cobalt catalyst can operate at high temperature, more resistant to sintering and inherent activity is low.

    Iron catalyst is used when coal or biomass is used as feedstock for the Fischer -Tropsch synthesis because of its low H2: CO ratio (<1). Iron catalyst is susceptible to sulphur poisoning. Iron catalyst tends to sinter (very soft). Iron catalyst can operate at lower temperature. Iron catalyst is less selective and cheaper. Iron catalyst tries to increase the H2: CO ratio by catalyzing water-gas shift reaction.

    5) Consider Fischer -Tropsch Reaction

    (2n+1)H2 + nCO → CnH(2n+2) + nH2O, and

    Anderson-Schulz-Flory distribution

    Wn/n = (1-α)2αn-1

    If α has a value of 0.5 for a given catalyst and operating conditions, what is the product distribution like?

    Perform a sensitivity analysis on the dependence of methane formation on α.

    The Fischer -Tropsch Reaction is

    (2n+1)H2 + n CO → CnH(2n+2) + nH2O

    The Anderson-Schulz-Flory distribution is

    Wn/n = (1-α)2αn-1

    If α has the value of 0.5 for the catalyst, the product distribution curves are

    n

    α

    Wn/n

    Wn

    n

    α

    Wn/n

    Wn

    1

    0.5

    0.25

    0.25

    21

    0.5

    2E-07

    5E-06

    2

    0.5

    0.125

    0.25

    22

    0.5

    1E-07

    3E-06

    3

    0.5

    0.0625

    0.1875

    23

    0.5

    6E-08

    1E-06

    4

    0.5

    0.03125

    0.125

    24

    0.5

    3E-08

    7E-07

    5

    0.5

    0.01563

    0.07813

    25

    0.5

    1E-08

    4E-07

    6

    0.5

    0.00781

    0.04688

    26

    0.5

    7E-09

    2E-07

    7

    0.5

    0.00391

    0.02734

    27

    0.5

    4E-09

    1E-07

    8

    0.5

    0.00195

    0.01563

    28

    0.5

    2E-09

    5E-08

    9

    0.5

    0.00098

    0.00879

    29

    0.5

    9E-10

    3E-08

    10

    0.5

    0.00049

    0.00488

    30

    0.5

    5E-10

    1E-08

    11

    0.5

    0.00024

    0.00269

    31

    0.5

    2E-10

    7E-09

    12

    0.5

    0.00012

    0.00146

    32

    0.5

    1E-10

    4E-09

    13

    0.5

    6.1E-05

    0.00079

    33

    0.5

    6E-11

    2E-09

    14

    0.5

    3.1E-05

    0.00043

    34

    0.5

    3E-11

    1E-09

    15

    0.5

    1.5E-05

    0.00023

    35

    0.5

    1E-11

    5E-10

    16

    0.5

    7.6E-06

    0.00012

    36

    0.5

    7E-12

    3E-10

    17

    0.5

    3.8E-06

    6.5E-05

    37

    0.5

    4E-12

    1E-10

    18

    0.5

    1.9E-06

    3.4E-05

    38

    0.5

    2E-12

    7E-11

    19

    0.5

    9.5E-07

    1.8E-05

    39

    0.5

    9E-13

    4E-11

    20

    0.5

    4.8E-07

    9.5E-06

    40

    0.5

    5E-13

    2E-11


    Performing a sensitivity analysis on the dependence of methane formation on α

    n

    α

    Wn/n

    1

    0.01

    0.9801

    1

    0.1

    0.81

    1

    0.2

    0.64

    1

    0.3

    0.49

    1

    0.4

    0.36

    1

    0.5

    0.25

    1

    0.6

    0.16

    1

    0.7

    0.09

    1

    0.8

    0.04

    1

    0.9

    0.01

    1

    1

    0

    6) In hydrocarbon synthesis the ideal H2:CO ratio is close to 2: 1 as seen from a simplified synthesis reaction for producing diesel as below:

    16CO + 33H2 C16H34 + 16H2O

    If the syngas for the above synthesis process is made from steam gasification of pure carbon:

    C + H2O → H2 + CO

    The resulting syngas has H2: CO ratio of 1.0. Then, water gas shift reaction

    H2O + CO → H2 + CO2

    has to be operated to adjust the H2: CO ratio to the desired value. Estimate how much carbon dioxide will be generated for every tonne of such diesel produced.

    The Molecular weight of C16H34 =12* 16 + 1*34

    = 192 + 34

    = 226

    Therefore the Molecular weight of C16H34 = 226

    Number of moles of C16H34, n = m/M

    = (1000*1000) / (226)

    = 4424.778876

    H2O + CO → H2 + CO2

    From the above equation we know that

    1 mole of CO2 = 1 mol of CO

    16CO + 33H2 C16H34 + 16H2O

    From the above equation we know that

    16 mole of CO = 1 mol of C16H34

    By comparing the above two equations we know that

    1 mole of C16H34 = 16 mol of CO2

    So number of moles of CO2 = Number of moles of C16H34 * 16

    = 4424.778876 * 16

    = 70796.4602

    Mass of CO2 = n * M

    = 70796.4602 * 44 (Molecular weight of CO2 = 44)

    = 3115044.25 gm

    = 3115.04425 kg

    = 3.11504425 Tonne

    Therefore 3.11504425 Tonne of carbon dioxide will be generated for every tonne of such diesel produced.

    Reference

    1) http://webct.uwa.edu.au/webct/urw/lc103130001.tp0/cobaltMainFrame.dowebct

    2) http://en.wikipedia.org/wiki/Fischer-Tropsch_process

    3) http://en.wikipedia.org/wiki/Steam_reforming

    4) http://en.wikipedia.org/wiki/Water_gas_shift_reaction

    5) Course materials and Lectopia.

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