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Materials Modification

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Supercritical Fluid Extraction

Supercritical carbon dioxide’s unusual combination of physical properties can be used to produce desirable modifications to materials. These modifications constitute some of the most exciting potential applications of supercritical fluids. We now describe three such applications from Craig Taylor's thesis work.

Concrete

The Supercritical CO2 treatment we use reacts with the concrete to form calcite, but the water by-product is dissolved in the CO2 and is carried away. Since the CO2 is supercritical, it has no capillary forces and no surface tension, allowing it to reach every pore space and convert the concrete into calcite. Benefits of this process include a 200-300% increase in compressive strength, a 75% increase in tensile strength, and increased density.

We have also shown remarkable success substituting high amounts (80-90%) of fly ash for Portland cement. After treatment we are left with a lightweight, waterproof, high strength cement product suitable for applications from consumer products such as roof tiles and pavers to building products like cement blocks. View poster on supercritical CO2 and cements.

Polymers

The use of supercritical carbon dioxide for the modification and processing of plastics is being explored for specific applications.

  • Many polymer properties can be varied over wide limits and with a high degree of control.
  • The surface properties of virtually all polymers can be modified to produce better adhesion or to promote specific surface chemistries.
  • With appropriate treatments, the volume of a polymer fiber can be expanded to produce better insulating properties.

We are presently exploiting these discoveries to modify fiber and textiles for specific applications, and to develop novel means of recovering plastic wastes for reuse and recycling.

Enhanced Oil Recovery and CO2 Sequestration

Carbon dioxide is commonly found in very pure form as a byproduct from oil and gas wells. In fact most of the carbon dioxide marketed today is “mined” in this way.

  • Oil reservoirs are normally found with an overhead gas “cap” that provides the pressure for the primary recovery from the well.
  • Secondary recovery usually involves flooding the reservoir with an oil immiscible fluid such as brine water to displace the oil thus enhancing the recovery. Typical recoveries are on the order of 20-30% using this technique.
  • Tertiary oil recovery involves flooding the reservoir with an oil miscible fluid. This fluid would need to be inexpensive and readily available at the injection well.

The above procedure necessitates the use of compressed “cap” gasses such as liquified petroleum gas (LPG), or carbon dioxide normally found co-located with oil reservoirs. The efficiency of enhanced oil recovery by dense CO2 and LPG is often marred by an unfavorable mobility ratio, which results from a viscous fluid being displaced by a non-viscous fluid. This is unfavorable since the frontal instability usually results in the growth of “viscous fingers” and early breakthrough of the displacing fluid. Chapter 6 of Craig's thesis examines a novel approach to this problem as it looks at the solid-state and solution structure of a polymeric additive used to increase the viscosity of dense carbon dioxide.

Similar experimental and theoretical approaches are also being usaed to study the sequestration of CO2 in geological formations. This process includes characterization of core and grout materials in flow and static systems.

Chemical Synthesis

The majority of organic synthesis reactions are carried out in a liquid because the collision of reactant molecules to form products is more favorable in this phase. The unusual combination of physical properties of the supercritical state makes it an excellent medium in which to carry out some of these reactions.

  • The high diffusivity of SCF carbon dioxide suggests that there are many more collisions between molecules, per unit time, than occur within the liquid state.
  • Because the reaction of dissolved molecules is promoted by these collisions, the reaction rate will be much faster in the supercritical fluid.
  • By adjusting temperature and pressure we can, via the changes in diffusivity, control the collision rates.
  • Thus, we can control the reaction rate over a much broader range than is possible with the simple temperature control available with conventional liquid solvents.

We know that temperature and pressure also determine the density of a supercritical fluid, and, since the density largely determines the solubility of a given solute, one can imagine the selective addition and removal of reactants and products from a single synthesis chamber, eliminating the traditional, multiple-stage synthesis techniques. Reactions in SCFs can also be used to promote specific synthetic pathways, producing a higher fraction of a specific molecule.


May 21/2008


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