Methanol Stream Reforming For Hydrogen Production

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Abstract:

Methanol is extensively used in production of hydrogen through heterogeneous catalysis due to it high Hydrogen-Carbon ratio. Micro-reactors used for the methanol steam reforming has shown promising results and ignited extensive research in last 2 decades. This report discusses 3 novel ideas to develop a simplified model of Heterogeneous catalysis. It addresses the individual draw backs of a catalyst by multilayer deposition of two different kind of catalyst in the same reactor. And has explores the idea of increasing the yield from general combination of basic unit micro-reactor under safer limits of temperature and pressure. The novel ideas discussed includes the combination of two mirror imaged micro-reactor in to one single unit known as Micro R+. And subsequently stacking them together to form a Parallel reactor Mega M

Introduction:

Because of increasing demand of energy, decreasing resources of conventional Fuels and global warming, the demand for greener and high efficiency fuel has increased. Since last century a lot of ways has been presented by the scientist around the world for the solution but only few countable methods can be applied practically. Hydrogen fuel cell is one of the most effective technology which shown a high potential for handling all the energy requirement of the modern world. It was first developed in 1839 by sir William Grove and is now considered as the energy source for the future (Llinich, et al., 2008)

An extensive research has been initiated all around the world for development & optimization since last two decades for the production of hydrogen on miniature scale, so that it can be safe and easily incorporated in applications. However the technology has shown the promising results in laboratory scale models, but could not be successfully applied extensively on commercial scale. This is because of the difficulty in controlling not just the reactant and product but the reaction itself. Only very few models were successfully proposed and implemented commercially.

Methanol has been shown good results in both, in direct use of fuel cell as well as in the precursor for the hydrogen fuel cell. Methanol can be obtained from natural perishable hydrocarbon sources as well as from crude oil and natural gas. For commercial purposes, methanol is primarily produced from natural gas through a syngas route. Syngas is then converted to methanol over copper catalyst at around 200 °C. In 1960, a very active Cu-based catalysts was developed which revolutionized the catalytic process (Jiang, 1993). And since then there has been a lot of development in the field of Heterogeneous catalysis for hydrogen forming from methanol. Today, a finely tuned Cu/ZnO with Alumina (Al2O3) composition is employed for the same process.

Steam reforming of methane produces a mixture of CO2, CO, and H2 according to eq (1)&(2).

Methanol can be converted to hydrogen at low temperatures (150-350 °C) than most other fuels (>500 °C) because it has no carbon-carbon bonding. It can easily be activated at low temperatures than methane. Low-temperature conversion leads to low levels of CO formation, even if the catalyst provides no special mechanism for selectivity of CO2 over CO. But low temperature will lead to long residence time and high level of methane formation, without a catalyst. This will be a serious problem for the hydrogen production. But in Presence of a catalyst, the reforming of the methanol to hydrogen will be at higher rate and good selectivity.

Effectiveness of Cu-based catalysts in the production of methanol which occur naturally led to their investigation in the steam reforming of methanol, which can be seen as the reverse of reaction equation (3)

An excess of steam to the same reaction under various copper mixed catalyst can produce the equation (1) so as to keep the reaction forward.

Various catalysts were used for the purpose as converting methanol to hydrogen.

There are many catalyst proposed for the process, and all works with different selectivity and different rate conversion. It is generally seen that the higher the temperature more is the CO formation, which may lead to the catalyst poisoning. CO, opposite to CO2 has a very high adsorption rate to the catalyst, which can affect the overall rate of conversion, and degradation of the catalyst.

There are various catalysts which were proposed in varying combinations to tackle this problem but only tried and tested for small or micro-scale levels and also deriving the rate equation will be more complex than that for the individual catalysts.

Micro-reactors are micro-channel plotted on an inert or on an active substrate which may or may not take part in the reaction mechanism. These devices are devised to obtain the high efficiency yields, as they worked on a molecular level. They improve mass and heat transfer by reducing the effective transport distance and increasing interfacial area per unit reactor. They suppress heat accumulation areas and enabling safer operations. The Micro-reactors can operate under conditions that cannot be easily attained in conventional reactor systems because of fixed and well defined characteristics, high heat and mass transfer rates, spatial and temporal control over temperature, mixing and residence time.

The aim of the project is to develop a model to maximise the rate conversion and selectivity of the micro-reactor for hydrogen production by methanol steam reforming process. Simultaneously, it is to develop a high capacity model with the simplified equation of rate, within the safer possible process parameters.

Challenges faced:

To develop a methanol-based system for the production of the hydrogen is not simple in any terms, though there are other technologies such as Partial Oxidation and Auto-thermal Reforming, Pyrolysis, etc. involves much more than simply the steam reforming and associated process operations. There are specific system challenges that have great bearing on type of system selected, how it is operated, how it is deployed, and ultimately how it performs practically.

The report discuss the technical challenges related to the system as a whole, and the focus remains on the portable power applications, which is where most of the methanol steam reforming work is directed.

Literature survey:

Hydrogen can be produced by various methods which are as follows:

Plasma Reforming.

Electricity is used to create a plasma which generates energy and forms the free radicals required for reforming. (Biniwale, 2004; Bromberg, 1999; O'Brien, 1996; Paulmier, 2005; Czernichowski, 2003; Sekiguchi, 2003). Usally steam is used to form free radicals, such as H+, OH-, and O- when injected with a fuel the electrons fascilitates redox reactions (Sekiguchi, 2003).

Plasma reforming has many advantages, such as lack of catalyst, smaller systems and lower operating temperature, high response time and elimination of poisoning factor. (Biniwale, 2004; Bromberg, 1999; O'Brien, 1996; Czernichowski, 2003)The main disadvantage of Plasma reforming is the requirement for electricity.

Also, the electrodes used in the process tends to erode during the operation which will add in to the maintenance cost

Pyrolysis.

Decomposition of hydrocarbons into hydrogen and carbon in a water and air less/free environment is known as pyrolysis and can be done with organic material. (Muradov, 2003) If no water or air is present, consequently no carbon oxides will formed. This process offers significant emissions reduction. Since no CO or CO2 is present, additional secondary reactors are not necessary for down stream reactors.

Pyrolysis can answers the question for the increasing concerns over CO2 emissions. It may play a significant role in environmental protection, since it can able to recover a significant amount of the carbon as a solid (Muradov, 2003; Guo, 2005).

Pyrolysis process requires vaporizers, a pyrolysis reactor, and recuperative heat exchangers in a typical setup. One of the major drawbacks is the deposition of carbon as a fouling agent for effective heat transfer, which is formed as a byproduct. (Guo, 2005)This can make the process limited to comparatively large installations only where carbon removal will be done easily.

Partial Oxidation and Auto-thermal Reforming.

Hydrocarbon is being used on widely for large scale hydrogen production, such as for automobile fuel (Trimm et al., 2001; Hohn et al., 2001; Krummenacher et al., 2003 & Pino et al., 2002). It employs a non-catalytic partial oxidation of hydrocarbons in the presence of oxygen and steam at temperature ranging from 1300-1500 °C to obtain a high conversion and reduce the carbon soot formation (Rostrup-Nielsen et.al., 2003). A catalyst is some time used to reduce the operating temperature, however, it is difficult to control because of coking and heat accumulatione (Trimm et.al., 2001; Song, 2002; Pietrogrande et al., 1993; Hohn, 2001; Krummenacher et al., 2003; Pino et al., 2002). (Krummenacher et al.,) it has shown good results in non-portable forms but it can not be suitable for light and portable devices.

Aqueous Phase Reforming.

When hydrogen is produced from oxygenated hydrocarbons and carbohydrates it is known as Aqueous Phase Reforming. (Cortright, 2006; Cortright, 2002; Davda, 2003)These reaction occur in a high pressures ( 25-30 MPa) and temperatures (220-750 °C) even with a catalyst.

This method is not suitable due to high heat and pressure requirements. Catalyst are being researched to overcome this problem

Ammonia Cracking.

Ammonia is one of the strongest competitor for the methanol steam reforming as it is inexpensive as a fuel and has been proposed for fuel cells in portable power applications.

Energy density of Amonia is 8.9 kW h kg-1, which is higher than that of methanol i.e.5.5 kW h kg-1 but it is less than that of diesel i.e.13.2 kW h kg-1.

Ammonia cracking takes place under endothermic conditions and is considered as the reverse reaction of synthesis.

But Ammonia is synthesised at very high temperature and pressure this made ammonia a relatively less investigated till now.

There are many techniques for hydrogen generation by reforming and from number of sources as well to the fuel cells But the choice always remain with the production of hydrogen instead of storage as it is more dangerous to handle in molecular form .Though the choice is dependent upon the requirement of the end user but in general the safer technology is most preferred over the otherwise. Therefore, Using micro-reactors is a viable option for steam reforming of methanol.

Various catalysts used in the micro-reactors:

  1. Cu/ZnO
  2. Cu/ZnO/Alumina (Al2O3)
  3. Pd/ZnO
  4. Pd/ZnO/ Alumina (Al2O3)
  5. Mixtures of different Catalyst in varing %ages.

There are many challenges faced in both forms of catalyst (individually and in combinatipon) in micro-reactors. Especially in deriving rate equations, Poisoning etc.

Methodology:

There are three broad steps involved in the fabrication of new micro-reactor. These are

  1. Writing the micro-reactors mirror images
  2. Joining the two mirror imaged micro-reactor as MicroR +
  3. Stacking the single unit of MicroR+ to form Mega-M 1

Writing the micro-reactors mirrors:

There are two basic steps in the fabrication of the Primary unit. The first step is the basic unit of the Mega-M1 is the basic micro-reactor fabricated most commonly in today's time, starting from lithographic writing of PMMA +ve template which will produce the parent -ve Template. The second step is by using Electron beam vapour deposition technique for writing the catalyst on to the surface of the channels.

a. The first step is the foundations for the new reactor in which even numbers of exactly same micro-reactors of ceramic substrate will be fabricated using a PMMA templates fabricated from Parent ceramic template itself. The only change in the design will be leaving a small hole in all the reactors except for two which will have hole only on one side on the substrate plate in such a way so that they lie on the alternate route.

The Production cycle is being given in fig.1

(Fig.1) Process flow diagram of Ceramic Micro-reactor production

In the second step the layers of cu and Pd catalyst along with ZnO and Alumina will be coated. But will be in separate zones in micro-reactors. The zones were divided under the philosophy that the micro-reactor will be resistant to high temperature and poisoning as well as high in conversion rate. As shown in figure 2

Joining of the two mirror imaged micro-reactor as MicroR +.

The two micro-reactors can be joined by carefully using the same ceramic which has been used for the substrate of the micro-reactor. The process steps are briefly described in the process fig 4. below.

Fig 4. Cementing of two microreactors.

Stacking the single unit of MicroR+ to form Mega -M1:

The single unit of microR+ is placed on the top of the each other very carefully so as to obtain a sandwiched layered structure of the micro-reactor. The stacking should have the inlet at one face and the outlet on the other face of the reactor but at the bottom. This structure will allow the equal distribution of reactants over the catalytic surface inside the channels.

As shown in the diagram.The stacking can be hold together by a Plate and frame type heat exchanger arrangement in fig 5. between the two supports.

Fig 5 . Plate Heat Ex. Type Assembly of MicroR+: Mega M

Comparison to the conventional methods

Conventionally the micro-reactors were used as a single unit and homogenously coated with the catalyst or mixture of catalysts

Mega M has the following comparison with the conventionally used micro-reactor:

Mega M is a multiple layer micro-reactor which can able to produces more volume through them the. It can also encourage the scale up design of the micro-reactor. it can be calculated by employing one of the most simple way to calculate rate as each catalyst as function of surface area of the micro-reactors from the Langmuir- Hinshelwood kinetics. The effect of temperature in this reactor will be minimum. The effect of CO Poisoning on this reactor will be minimum as the initial conversion will be Catalysed by Pd which show very less production of CO.

Novelty:

The process can be considered Novel in Various ways:

  1. Layered coating for micro-reactor channels can provide the better way of reforming
  2. MicroR+ can be considered as an safer way to increase the volume of yield without affecting the rate conversion
  3. Stacking the MicroR+ in to Mega M1 can also be considered as a novel way to produce a high volume yield but the % conversion needs to be studied experimentally.
  4. All the method described above can taken up as a broader approach to develop more specific models for further study.

Conclusion

The New Reactor Micro-R+ and MegaM1 shows a promising advancement in the fields of micro-reactors as providing the new and novel method of keeping the Rate equation simple as well as increasing the yield volume of the reactor.

There may be a chance of heat accumulation in the MegaM1 due to low heat transfer rate between the ceramic substrate. But this problem can be overcome by introducing thermocouples between the two MicroR+ units.

Reference

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  11. Rostrup-Nielsen J., 2003, In Encyclopedia of Catalysis, Horvath, I. T., Ed., Wiley-Interscience, Vol. 4.
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