You are here: UK Essays » Custom Essays » Steam Reforming Gas

Free Essays - Essays

Steam Reforming Gas

Answer 1

Steam reforming

This process involves the catalytic reforming of natural gas (or light hydrocarbons) in the presence of steam to produce synthesis gas which is primarily a mixture of carbon monoxide and hydrogen. The synthesis gas is a major intermediary for the production of hydrogen which can be used as a fuel in itself or various other chemicals and is a major ingredient in the Fischer Tropsch synthesis which is the heart of the GTL (gas to liquid) technology which aims at converting gaseous hydrocarbons to liquid fuels. The steam reforming generally consists of the following two steps:

Reforming of natural gas: In this process steam reacts with natural at 800 C in the presence of Nickel, Cobalt or Copper as the catalyst to produce synthesis gas which is primarily CO and H2. The reaction (considering a natural gas stream predominantly methane with other lighter hydrocarbons) is shown below:

The above reaction is endothermic reaction and the heat associated with it is 206kJ/mol.

Water Gas Shit Reaction: This CO produced in the above reaction, reacts with the steam in the reactor to produce CO2 and H2 and in the forward direction it is exothermic reaction releasing 41kJ/mol.

This process generally gives a H2 to CO ratio equal to 3.

Carbon dioxide Reforming

Carbon dioxide reforming of methane or natural gas involves the production of synthesis gas (CO and H2) with H2 to CO ratio of 1 by reacting natural gas with CO2 at high temperatures over a suitable catalyst (such as Ni, Cu and other noble metals). This produced synthesis gas has its use in the manufacture of oxygenates and other liquid hydrocarbons such as acetic acid. The main reaction is depicted by:

This is an endothermic reaction with heat of the reaction being 247.1kJ/mol and the energy can be obtained by using the natural gas from the feed stream. The advantage of this process is that as both CO2 and CH4 (the main component of natural gas) are greenhouse gases this carbon dioxide reforming has found a superlative use for such environmentally unsafe gases. But the flipside of this process is that there are other undesirable reactions that along with the main reaction which include the cracking of methane which leads to the deposition of carbon as coke.

This process generally gives a H2 to CO ratio equal to 1.

Auto-thermal Reforming

Owing to its simplicity, lesser use and dependency of catalysts and lower capital investment auto-thermal reforming is the most widely used and dominant natural reforming technique. The reforming reactions takes place in a reactor which primarily consists of two distinctive zones as shown in the figure below:

Catalyst layer

Catalytic Zone

Combustion Zone: Here the feedstock (natural gas/methane) is mixed with steam and the natural gas is partially oxidised with oxygen to provide energy for the subsequent endothermic reactions. This combustion reaction is shown below:

This reaction releases a heat of about 516kJ/mol which aids in the following reactions.

Catalytic and Thermal zone: The un-reacted methane and the steam from the feed then passes over a catalyst layer (generally distinguishes the two zones and acts as an efficient method for distributing the catalyst) react to form synthesis gas. Additionally water gas shift reaction takes place in this zone.

ΔH= -216kJ/mol

This process generally gives a H2 to CO ratio equal to 3.

Answer 2

A typical gas to liquid conversion will involve the following processes namely:

A reforming reaction which coverts a basic feed (may be coal, biomass, natural gas etc.) to a mixture of carbon monoxide and hydrogen which is termed as synthesis gas. This is usually an endothermic process and the energy is provided externally.

A water gas shift reaction which monitors the ratio of the H2 to CO ratio of the synthesis gas.

Fischer Tropsch synthesis in which it takes the synthesis gas as the input and converts to a variety of products which depends on the H2 to CO ratio.

So as seen above a water gas shift reaction plays an important role in determining the nature of the liquids produced after the Fischer Tropsch process (the liquid products after the GTL process).

Synthesis gas may be produced using a variety of methods such as steam reforming, partial oxidation, Auto-thermal reforming etc. Considering an example of steam methane reforming reaction in which steam is reacted with methane at 800 C using Nickel as the catalyst to produce synthesis gas (reaction shown below) we have:

SYNGAS

The ratio of H2 to CO is 3. This is undesirable for a FT process which aims at producing long straight chain hydrocarbons. For an FT synthesis to produce heavy straight chain hydrocarbons similar to gas oil or petrol the H2 to CO ratio should be near about 2. Hence the water gas shift reaction finds its use in such a circumstance.

A WGS is a reversible reaction where in, the concentration of CO can be manipulated to suit the needs (of products after the FT synthesis) as desired by selecting the path of the equation given below and the operating conditions such as catalysts. The generic form of the WGS reaction is shown below:

For the example of steam reforming discussed above, the H2 to CO ratio desired is 2. The synthesis gas after the reforming reaction is 3. Hence, a reverse WGS reaction is preferred where in the excess H2 is converted to water.

If in case the desired products of the FT synthesis were methanol and other oxygenates the H2 to CO ratio should be 1 and hence the above reaction can be manipulated to achieve the same (H2 to CO ratio to be 1) by controlling the mechanism of the WGS reaction based on the nature of the synthesis gas formed at the end of the reforming reaction. As a thumb rule in all the GTL processes using natural gas as the feed stream the reverse reaction is preferred this is an endothermic reaction (ΔH=41kJ/mol) where as if in a coal to liquid conversion the forward WGS is preferred.

Answer 5

Considering a FT process:

Considering the Anderson-Schulz-Flory distribution for the products:

Where,

Wn is the weight of the hydrocarbon with carbon number n

n is the carbon number,

α is the chain growth probability for this catalyst the value of α =0.5.

The graph of Wn/n Vs n and the Wn Vs n is shown below:

Sensitivity analysis is performed on methane and this is achieved in varying the value of α and checking the response on its distribution:

The graph of this distribution is shown below:

From the above two equations its clear that methane is ever present as a by product of FT process, and the quantity of methane being a product of FT process can be controlled by the value of α, i.e. the higher α is the lesser is the quantity of methane produced.

Answer 4

The catalysts used in the Fischer Tropsch process are basically Iron based or Cobalt based. The choices of these catalysts are based mainly on 2 factors:

Quality of synthesis gas produced after reforming i.e. the H2 to CO ratio.

Desired products after the Fischer Tropsch process.

Iron based catalysts: Iron based catalysts are the most widely used catalysts in the Fischer Tropsch process. The advantages of using Iron as the catalysts are:

It is relatively inexpensive.

It remains an efficient in its catalysis for a wide range of H2 and CO ratios so in effect it is independent of the type of the reforming reaction used.

The use of Iron as a catalyst solves issues with process integration.

The disadvantages of Iron based catalyst include:

Iron sinters at high temperatures hence this decreases the activation sites. This increases the frequency of their regeneration.

The selectivity of products is poor for an Iron based catalyst. Hence wide variety products are obtained form an FT process using iron based catalysts.

The other widely used catalyst group used in the FT process are the Cobalt based catalysts. The advantage of Cobalt catalysts over the iron based catalyst are:

• Cobalt can be used at higher temperature as compared to Iron based catalysts.

• Cobalt has higher product selectivity as supposed to the iron based catalysts.

• They have more longevity as compared to the Iron based catalysts.

The disadvantages of Cobalt based catalysts are that:

• They are slightly more expensive than the iron based ones.

• Their use is limited by the H2 to CO (cannot be used if ratio is less than 2) ratio meaning to say that they have lower range of use.

Answers 6

The product reaction:

The Synthesis gas reaction

WGS reaction:

H2 to CO ratio after the synthesis reaction is evidently 1. This has to be around 2 hence the WGS is employed to keep this ratio to 2.

In order to find the amount of CO2 released during the production of 1 tonne of gas oil:

Step one:

Assume 'x' moles of C reacting in the Synthesis gas reaction to produce x moles of H2 and x moles of CO hence the ratio of H2 to CO is 1:

Step two:

'y' moles of this CO participates in the WGS to produce y moles of CO2 and y moles of H2.

Step three:

The net CO from the two equations = (x-y)

The net H2 from the two equations = (x+y)

Therefore the new H2:CO ratio is =

This H2 to CO ratio should be equal to 2. Hence

Step four:

The number of moles reacting to for the formation of diesel is 16. This means that form the synthesis gas equation and the WGS reaction we have

Step five:

Solving the two equations simultaneously; it will yield values of x as 24 and y as 8. Hence, x=24 and y=8. Hence the synthesis gas reaction and the WGS reaction will reduce to:

This reaction gives us the ideal H2 to CO ratio of 2.

Step six:

Form the above equations it is clear that 16 moles of CO produces about 8 moles of CO2 as 16 moles of CO produces one mole of diesel; it can be stated that the 1 mole of diesel produces 8 moles of CO2.

The molecular weight of diesel is calculated as 226g which is the weight of 1 mole of diesel. Further the molecular weight of CO2 is 32 grams. Therefore 32X8 moles of CO2 is produced for every 226 grams of diesel and hence, mass of CO2 released for 1 tonne of diesel can be calculated.

The total amount of CO2 released is therefore equal to 1.62 tonnes which is the amount of CO2 released for production of 1 tonne of diesel.

All of the essays in the free essays section were written by students and then submitted to us to display and help others. Thanks to all the students who have submitted their essays to us. You should not hand in our essays as your own. We do not condone plagiarism! If you need custom essay help, then have a look at our essay writing services.

Sign up and be the first to receive our latest offers: