Environmental Engineering Reference
In-Depth Information
lift objects >40,000× its mass as it swells. Measurements to elucidate the structure (elec-
tron microscopy, N
2
porosity measurements, and infrared spectroscopy) indicate that the
morphology of the animated matrix consists of lexibly tethered silica particles that have
diameters of ~20 nm arranged in a complex and microscopically disorganized network
(Figure 8.3).
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The surface area of the dry, internally collapsed material is between 500 and
900 m
2
/g with internal pore volumes ranging from 0.5 to 2.8 mL/g. On the basis of the
morphology of the swollen state, it has been determined that swelling results from tensile
forces generated and stored by the capillary-induced collapse of the nanoporous matrix
upon drying. The multitude of interparticle interactions, resulting from the high surface
area, retain the compressed matrix in a state of tension. Tensile potential energy stored
within bond distortions are subsequently released if interparticle noncovalent interactions
are disrupted by absorbates. The high degree of interconnectivity within the pore struc-
ture allows the relaxation process to happen rapidly. The swelling process is slightly endo-
thermic (5.2 ± 1.2 J/g), indicating that expansion is likely entropically favorable.
Swelling is completely reversible if absorbates are removed by evaporation and/or
reverse mechanic pressure. Extremely high capillary forces within the nanoporous matrix
upon absorbate evaporation lead to the inward collapse of the matrix, which restores the
high degree of mechanical tension in the dry state after each regeneration cycle. Osorb
is functionally stable up to 225°C. At higher temperatures, sintering leads to additional
Si-O-Si bond formation in the collapsed network, leading to cross-linking and loss of
mechanical lexibility to swell. Above 300°C, Osorb begins to thermochemically degrade.
Given that the functional unit is the assembly of 20 nm nanoparticles, the material pos-
sesses all the same properties at scales from >2000 μm (bulk) down to 0.2 μm. Above the
size threshold of ~2 mm, the surface area to volume ratio is low and the outside region
of a particle predominantly swells, leading to potential delamination. Below 200 nm, the
material is likely too small to be considered an assembly. Sizes above and below the nor-
mal values of typical sorbent media particles have limitations in terms of process systems
engineering from a practical standpoint.
8.2.2 Removal of Dissolved Organics from Water
Produced water is composed of free oil and dissolved components. A focus is placed here
on the removal of dissolved organics from produced water since a variety of technologies
exist to physically separate free or dispersed oil in an eficient manner. It is more challeng-
ing to remove dissolved components to meet certain regulatory requirements. Toluene
and benzene are notable examples of formation hydrocarbons that have a nominal solubil-
ity in water (450 and 1200 ppm, respectively). Biocides, surfactants, and methane hydrate
dissolution chemicals may also be present in produced water. These additives often have
relatively high solubility in water and may not be removed by physical separation or mem-
brane technologies. Certain plays or reservoirs may have naturally occurring organics that
have higher solubility in water, including organic acids (e.g., aliphatics and phenols). Polar
species such as organic acids can also be more dificult to remove from produced water
using standard oil-water separation methods.
One of the distinguishing characteristics of swellable organosilica is the ability to extract
comparatively large amounts of dissolved organics from water. In laboratory tests (equilib-
rium binding studies, adsorption isotherms, breakthrough curve measurements), Osorb
has been demonstrated to have the ability to absorb a wide range of dissolved organic spe-
cies, including trichloroethylene, perchloroethylene, methyl
t
-butyl ether (MTBE), toluene,
naphthalene, acetone, phenol, 1,4-dioxane, and 1-butanol from water (Table 8.1).
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