Sustainable Bioenergy REU: Summer 2014
Increasing Effectiveness of Membrane Reactors in Biofuel Production
This material is based upon work supported by National Science Foundation Grant: “REU Site: Summer Academy in Sustainable Bioenergy; NSF Award No.: SMA-1359082, awarded to Kansas State University. ”
A polymeric membrane is used in efforts to convert biomass based chemicals into liquid fuels and other useful products. The membrane serves as a selective supplier of hydrogen to catalytic sites where the biomass substrates then undergo hydrogenation. A successful membrane reactor requires a high flux of hydrogen, as well as a high durability of the membrane when exposed to the liquid fuel. Using a set up where one side of a Matrimid® membrane is exposed to a liquid biofuel and the other side to hydrogen gas, the durability of a membrane may be observed. Different techniques are used to enhance membrane stability, such as cross-linking the membranes with ethylenediamine vapors, backing the membranes with porous support, such as Tyvek® or Teflon®, as well as annealing the membranes to make the skin layer denser and improve the membrane performance quality. HPLC analysis is performed on samples taken throughout each trial run to determine if a reaction occurs. The specific hydrogenation reactions investigated are the hydrogenation of fructose and hydroxymethylfurfural (HMF). At a low reaction temperature of 110°C, it was found that a small amount of fructose was converted into HMF, and in a separate reactor run, a small amount of HMF was converted into other products.
During production, a biofuel has sugars such as fructose and hydroxymethylfurfural (HMF)contained in it. In order to be used, these sugars must be converted to dimethylfuran (DMF). In order to convert these sugars, the liquid fuel can be reacted with hydrogen. To do this, a polymeric membrane may be used to expose the hydrogen to the liquid reaction phase.
An anisotropic membrane is asymmetric. One side has many pores (this is the side that the hydrogen gas is exposed to). The other side is very dense. This is the side that a catalyst is coated on to and is exposed to the liquid reaction phase. Hydrogen travels through the pores towards the catalyst, where it is able to react with the oxygen.
Polymeric membranes easily degrade when exposed to the liquid biofuel. In order to prevent this, a technique known as cross-linking is employed. When treated with a chemical cross-linking agent, imide groups are converted into amide bonds, connecting (or cross-linking) two polymer chains together.
This technique yields a more stable membrane so it will not be broken down as easily by the liquid. It does, however, also reduce the amount of gas that is able to pass through the membrane and reach the liquid phase. The goal of my research is to determine whether or not the benefit of having the more stable, cross-linked membrane is worth having the amount of gas passing through the membrane to be reduced.
Annealing, or heating, the membrane also makes the membrane stronger. While it is being heated, the membrane's polymer chains have increased mobility. As the polymer chains move, more charge-transfer complexes are able to form between the ring structures. This interaction holds the membrane together, providing the structure with more durability. The chains stack themselves neatly on top of each other and hold the membrane together more tightly.
Making a Membrane
A polymer solution is created using a polymer, in this case Matrimid®, and solvents such as THF and NMP, or gamma-butyrolactone.
Membranes are cast onto a glass sheet and then submerged into a water bath.
After soaking in the bath for a few days, the sheets are air dried and then oven dried. Once the sheets are dry, two distinct sides are present.
The shiny side, which is the dense side exposed to the biofuel.
And the dull, porous side, which will be exposed to hydrogen gas.
Testing a Membrane
In order to test the effectiveness of a membrane, the time it takes both hydrogen and nitrogen gas to pass through the membrane are tested, using this machine.
Once a membrane is tested, the selectivity of hydrogen over nitrogen can be found. It is good if this is a large number, since a functional membrane will easily allow hydrogen to pass, but will not allow much nitrogen through.
After a membrane has been flux tested, it is tested in the reactor set up. This shows how structurally stable and durable a membrane actually is. In this set up, the dense side is exposed to pressurized ionic liquid, while the porous side is exposed to a hydrogen gas stream.
Treating A Membrane
A membrane can be cross-linked using a variety of methods. For our purposes we tried both ethylenediamine (EDA) vapor and 1,6-Hexanediamine. The 1,6-Hexanediamine required the membranes to be soaked in the solution over night. This caused the membranes to swell, requiring them to once again be trimmed down, resulting in a tedious process.
Exposing the membrane to EDA fumes resulted in the negative effect of causing the membrane to curl up around the edges.
In order to prevent the curling from happening, the membrane was encased in a testing cell, which has holes in it to still allow the vapor to reach the membrane surface. Since the membrane fits snug inside the cell, it is unable to curl.
Ruthenium chloride is spin coated onto the surface of a membrane and reduced under hydrogen gas at 150°C. The catalyst allows a reaction to occur in the reactor. Samples can be taken throughout the reactor run to determine whether a reaction is actually occurring. This is done using high performance liquid chromatography (HPLC).
Results and Discussion
The first membrane that was tested in the reactor was not treated with a cross-linking agent. After one hour in the reactor, the membrane began to get holes in it and crack, letting the liquid pass through.
A membrane cross-linked with EDA was tested in the reactor next. This membrane was placed into the cell along with a porous Teflon® backing to provide additional support. The membrane held the liquid apart from the gas for about two hours, before it too failed.
Next, the reactor was used to test a cross-linked and annealed thick film with [EMIM]oAc as the liquid phase. A Kalrez® o-ring was used to prevent the o-ring from breaking down and damaging the film. After heating to 80°C, the reactor ran for 4 hours with no apparent degradation of the film.
The next reactor set up used a cross-linked membrane that had been cast onto a Tyvek® backing. The reactor was stable with the [EMIM]oAc heated to 80°C for two hours until shutdown.
The next set-up was the same, but the membrane used was cast onto a PTFE backing instead of Tyvek®. This reactor was stable for the six hours that it was run and the membrane appeared unharmed with the exception of the backing peeling off after use.
The next run was done also using a cross-linked membrane on a PTFE back, but with water as the liquid phase to determine the amount of water that the membrane would allow through. The reaction was run at 80°C and approximately 10wt% of the water passed through the membrane in the four hours that it was used.
To determine if a reaction was occurring, the next test was done with a cross-linked and PTFE backed membrane that had also been coated with a Ruthenium chloride catalyst, which had been reduced by hydrogen at a temperature of 150°C. The liquid phase was a solution of fructose in water. The reactor was run at 90°C and stable until shutdown after eleven hours. Using HPLC analysis, it was determined that a reaction did occur, converting a very small amount of the fructose to HMF.
The following set up was the same, only using a solution of HMF in water. This set up was also stable until shut down 23 hours later. The permeate appeared to be clear and mostly contain water, while the retentate was darker in color, indicating that the membrane was able to hold back most of the HMF solution.
Next, two reactor control runs were done using coated membranes, with nitrogen on both sides of the reactor instead of hydrogen. One was done with fructose and water, the other with HMF and water. These were done to show what the HPLC reading looks like without any reaction occurring in the reactor.
The next experiment was done using HMF in water at 110°C and once again using hydrogen for the reaction. In order to attempt to increase the amount of reactant converted, the following reaction used the same set up, but the temperature was increased to 130°C.
In an attempt to better dissolve the products of the HMF reaction, a run was done at 130°C using HMF in the ionic liquid [EMIM]oAc. The membrane was completely deteriorated, however the PTFE backing remained in-tact. From this, it was determined that the ionic liquid is unable to be used with a Matrimid® membrane.
After it was determined that the ionic liquid could not be used, octanol was chosen as the new solvent in an attempt to better dissolve the products of the HMF reaction without harming the membrane.
The first reaction using octanol and HMF was done at 140°C. It appeared that no reaction occurred, but most of the solvent escaped through a valve leak in the reactor system. The membrane did remain unharmed in this trial.
The next run that used octanol was done at 150°C. The membrane remained unharmed. A large enough amount of permeate went through the membrane to change the concentrations in the retentate. In the future, the permeate will need to be recycled back into the system.
- Membranes perform better when backed
- Annealing improves the membrane durability
- Cross-linking may help reduce membrane degradation
- Water is unable to dissolve all products formed and the ionic liquid [EMIM]oAc degrades the membrane
- Permeate loss causes change in concentrations in the retentate
- A small amount of fructose was able to be converted to HMF, but to increase this value the temperature and pressure of the reaction need to be raised and more catalyst must be present
Future Areas of Focus
- Increase reaction temperature
- Increase reaction pressure
- Increase amount of catalyst on the membrane
- Use a solvent that is able to dissolve all products formed
- Recycle permeate back into system
- Eliminate need for membrane, and use only the porous support material
- School: West Virginia University
- Major: Chemical Engineering
- Class of 2016
- Hometown: Pittsburgh, PA
- Hobbies: Running, crafting, eating
- PhD Mentor: John Stanford
- Faculty Advisor: Dr. Mary Rezac
- This material is based upon work supported by National Science Foundation Grant: “REU Site: Summer Academy in Sustainable Bioenergy; NSF Award No.: SMA-1359082, awarded to Kansas State University. ”
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