Gas Syngas Reaction
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?
Steam Reforming
In early days most natural gas reforming were done by steam reforming. In steam reforming, steam and methane are preheated to a very high temperature. Then they are passed through a tubular reactor which consists of a bed of catalyst. The heat is provided from the burners around the tube for the reaction to carry out. The reaction taking place in the reactor is given below,
CH4 + H20 = CO + 3H2 (+ 206 KJ/mole)
This is followed by Water Gas Sift (WGS) Reaction, which takes place when mixture of CO and H2O are present. The WGS reaction is shown below,
CO + H2O = CO2 + H2
The main advantage for the process is the production of pure syngas. From the above reactions it can be seen that very little CO2, CO is produced compared to the amount of H2 produced.
The Disadvantage is:
1) Poor Heat Transfer
2) Endothermic reaction which puts the reaction temperature down
3) Energy Efficiency is less
H2/CO ratio obtained is 3:1 which aids in formation of lighter chain hydrocarbons in the downstream FT process. Therefore the role of WGS in this type of reforming is to knockout excess H2 and maintain the ratio of 2 and satisfy the optimum composition of FT process.
Carbon Dioxide Reforming
In the carbon dioxide reforming, the feedstock (natural gas) is sent along with Carbon dioxide to the reformer. The reforming reaction taking place is given below,
CH4 + CO2 2H2 +2CO (+ 247 KJ/mole)
The above reaction is a highly endothermic reaction and requires lot of heat for the reaction to take place. In addition, the H2/CO ratio obtained is 1 which causes problems in the downstream FT process as it aids in the formation of oxygenates.
Auto Thermal Reforming:
It is the dominant technology used due to its simplicity and less use of catalyst. The feedstock along with steam enters the autothermal reformer. Since, this is a highly endothermic reaction it is important to burn part of the natural gas as fuel, and also uses oxygen which helps in carrying out the combustion reaction. The reforming reaction for ATR is shown below,
CH4 + H2O CO + 3H2 ( +209 KJ/mole)
The combustion reaction carried out is given by,
CH4 + 2O2 CO2 + 2H2O (-519KJ/mole)
Schematic of ATR- Courtesy UWA Lecture Notes GTL Section
As seen, both endothermic and exothermic reaction exists within the reformer which leads to impure syngas. The presence of CO and steam in the reaction leads to water gas shift reaction as shown below,
CO + H2O CO2 + H2 (-41 KJ/mole) [Catalytic Zone]
This is an important reaction which helps to maintain the syngas ratio to 2 in contrast to 3 as obtained from the steam reforming reaction. It is important to keep the syngas ratio as close as possible to 2 as the optimum composition for F-T process requires it to be so.
There are lot of higher hydrocarbons formed which can be illustrated from the overall reaction as given below,
CnHm + nH2O nCO + (n +1/2*m)H2
Briefly discuss the role of water-gas shift reaction in gas to liquid conversion using Fischer Tropsch synthesis.
The water gas shift reaction is an important reaction, which playes the role of changing the H2/CO ratio in the syngas to suit the downstream Fischer Tropsch synthesis. This is a reversible reaction, the forward being exothermic and the backward being endothermic.
H2O + CO H2 + CO2 (-41 KJ/mole) for forward reaction
In most cases the H2/CO ratio required for downstream F-T process is 2, whereas in natural gas reforming we often get 3. Therefore, the water gas reaction at this stage helps to knockout CO if coal is used as feedstock, or converts CO2 back into CO, or H2 to water therefore maintaining the ratio of the syngas to 2.
This reaction also plays an important role in the CO2 capture business.
Through literature research list three catalysts for steam reforming of natural gas to produce syngas and discuss their relative performance in terms of advantage and disadvantage.
BASF The Chemical Company has devised a catalyst which is a Rare Earth (REO) promoted nickel catalyst. The composition of catalyst as given by the company is given below,
NiO = 16.5%
CaO = 6.0%
REO = 3.0%
Al2O3 = Balance
Si < 0.1 %
The advantages by using this composition and controlling the geometry of the catalyst results in, Low pressure drop, Long mechanical life time, Increased activity and low steam/CO ratio.
Disadvantage: Sintering effect, sulphur poising and loss of activity due to deposition of carbon.
A patented catalyst used in steam reforming comprising rhodium or nickel supported on lanthanum stabilized alumina or magnesium promoted lanthanum stabilized alumina.
- http://www.patentgenius.com/patent/4414140.html
The catalyst claims to be sulphur resistant and have high activity in the steam reforming.
The % composition of the Rhodium is 0.01% to about 6% by weight , whereas when nickel is on the lanthanum stabilized alumina support the composition of nickel varies from 1% to about 50% by weight.
However, Ni based catalyst tend to sinter therefore the surface area decreases which leads to reduced activity.
JB302 a cobalt molybdenum catalyst which is highly sulphur resistant , highly active, good surface area and active in wide range of temperature. The composition available from the source is Cao > 0.1%, and MoO3 > 8.0 %
Disadvantage: Very expensive
Through literature research list two Fischer - Tropsch synthesis catalysts and discuss their relative performance in terms of catalytic activity and product distribution..
In a Fischer Tropsch synthesis, the catalyst is broadly classified into two types, Iron based catalyst and Cobalt based catalyst. The choice of catalyst depends primarily upon the quality of syngas and type of products to be achieved, though former is the driving force for selection. The H2/CO ratio plays an important role in deciding the type of catalyst used. From the figure below, it is seen that if the H2/CO ratio is 2 or greater then Co catalyst is used else, Iron catalyst is preferred.
Courtesy Lecture Notes Form GTL Section
The iron catalyst compared to the cobalt catalyst is more active, produces lot wax, and operates at a low temperature. The main disadvantage of iron catalyst is that it's more susceptible to sintering. It is relatively soft as compared to the Co catalyst, which operates at a higher temperature and is more resistant to sintering, but the inherent activity is low. At high temperature lots of methane as well as higher hydrocarbons are also formed, which is another drawback for Co based catalyst.
Even though, the Fe based catalyst seems to be more favourable than Co (from the figure), the former looses its activity and therefore is required to be changed every 15-18 months in contrast to Co which can remain active up to 5 years (Regeneration of catalyst takes place whist their respective periods, until uneconomical to do so).
Consider Fischer Tropsch Reaction
(2n+1)H2 + nCO CnH(2n+2) + nH2O
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.
For α = 0.5
Product Distribution
|
n |
W/n |
W |
|
1 |
0.25 |
0.25 |
|
2 |
0.125 |
0.25 |
|
3 |
0.0625 |
0.1875 |
|
4 |
0.03125 |
0.125 |
|
5 |
0.015625 |
0.078125 |
|
6 |
0.007813 |
0.046875 |
|
8 |
0.001953 |
0.015625 |
|
9 |
0.000977 |
0.008789 |
|
10 |
0.000488 |
0.004883 |
|
11 |
0.000244 |
0.002686 |
|
12 |
0.000122 |
0.001465 |
|
14 |
3.05E-05 |
0.000427 |
|
15 |
1.53E-05 |
0.000229 |
|
16 |
7.63E-06 |
0.000122 |
|
17 |
3.81E-06 |
6.48E-05 |
The product distribution curve shows that for a given a value of α = 0.5 the weight fraction of methane is the highest as compared to the higher hydrocarbons. It is important to increase the value of α as close to 1 a possible to avoid methane formation, which is the primary component of feed and therefore tremendously reduces the efficiency of the process.
Sensitivity Analysis
For n = 1.
|
α |
W |
|
0.1 |
0.81 |
|
0.2 |
0.64 |
|
0.3 |
0.49 |
|
0.4 |
0.36 |
|
0.5 |
0.25 |
|
0.6 |
0.16 |
|
0.7 |
0.09 |
|
0.8 |
0.04 |
|
0.9 |
0.01 |
α, chain growth probability
From the sensitivity analysis it is shown that as the value of α increase from 0.1 to 0.9 the probability of methane formation decreases, and therefore more of higher chain hydrocarbons will be formed. At α =1, the probability of formation of methane is '0' which is the desired outcome from the reformer for it run on maximum efficiency.
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 + 33 H2C16H34 + 16H2O ------ (3)
If the syngas for the above synthesis process is made from steam gasification of pure carbon:
C + H2O H2 + CO -------------------- (1)
The resulting syngas has a H2:CO ratio of 1.0. Then, water gas shift reaction
H2O + CO ß H2 + CO2 ------------------ (2)
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 first step is to find the no. of moles of pure carbon needed to form diesel.
Therefore, if we assume x moles of C reacting in the equation (1) and y moles of H2O reacting in equation (2), and we also know that 16 moles of CO and 33 moles of H2 are required to form diesel, then we need to obtain the optimum number of moles of carbon reaction which maintains the H2/CO ratio as 2 (which is done by water gas shift reaction (2)) and takes into account the formation of CO2
Therefore applying this concept in equation (1) & (2), we get
For CO,
x - y = 16
For H2
x + y = 33
Therefore solving above two equations, we get
x = 24.5 , y = 8.5
Therefore the equation (1) & (2) becomes
24.5C + 24.5H2O 24.5H2 + 24.5CO ----------- (4)
8.5H2O + 8.5CO ß 8.5H2 + 8.5CO2 ---------- (5)
The equation (5) and (6), gives us the exact number of moles of C required, to produce 1 mole of diesel.
We also conclude that to produce 1 mole of diesel , we need to reject 8.5 moles of CO2.
1 mole of diesel = 226g, produces 238g of CO2.
Therefore 1 tonne of diesel produces 1.65 tonne of CO2.
Reference
Lecture and Course Notes
www.catalysts.basf.com/Main/download.axd/a7d4e9dd240c4782bdb630578ddc1249.pdf?d=BF-8
http://www.patentgenius.com/patent/4414140.html
http://www.jjjjch.com/template/proe10.htm
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