Hydrate-based decontamination of toxic gases

Details Hydrate-based decontamination of toxic gases Hydrate-based decontamination of toxic gases

2002-10-15

2004-07-06

Hendrickson, Stuart L. (Department: 1754)

Hazardous or toxic waste destruction or containment

Containment

Solidification, vitrification, or cementation

C588S253000, C588S249000

Reexamination Certificate

active

06759564

ABSTRACT:

1. FIELD OF THE INVENTION
The invention relates to decontamination using gas hydrate to capture toxic gas molecules and render them inert within solid gas hydrate crystals.
2. BACKGROUND OF THE INVENTION
The United States now faces increased terrorist threats. Of concern is attack using toxic gases and/or liquids.
Beside terrorist attacks, other dangers involving toxic gases exist. For example, certain gases and liquids that are available for industrial use may be highly toxic. These and other gases that are commercially and widely available are likely to be spilled in quantities on the scale of large tanker trucks, either accidentally or deliberately, well away from the industrial sites at which they are normally handled (and where such spills ordinarily are attended to).
Decontamination of major spills of toxic gases is generally difficult due to the chemical hazard presented (both to innocent bystanders as well as to decontamination personnel) and because the toxic materials often are highly persistent and difficult to capture. Typically, it is believed, “clean-up” involves little more than dilution of the toxic material, which is often simply washed or flushed away rather than collected and disposed of safely because collecting toxic material and concentrating it is difficult and dangerous.
In general, gas hydrate is a non-stochiometric mineral species that forms from water and gas molecules. The crystalline structure is an open network of water molecules, with voids containing the gas molecules. The presence of the gas molecules stabilizes the water molecule meshwork via van der Waals electrostatic bonding. The voids in gas hydrate usually are of different sizes, depending on the particular type of gas hydrate species. Table 1 below shows examples of the void sizes of the three most common types of naturally occurring hydrate, from which the size of the gas molecule that can form a particular type of hydrate can be estimated. (Other types of hydrate species may have different (i.e., smaller or larger) ranges of void size and/or different numbers of voids.) To determine the diameter of a gas molecule that may be accommodated in a particular void, it may be necessary to subtract the van der Waals radius of the water molecule (1.4 Å) from the average cavity radius.
TABLE 1
Hydrate Crystal
Structure
I (sI)
II (sII)
H (sH)
Void Size
Small
Large
Small
Large
Small
Medium
Large
Description
5
12
5
12
6
2
5
12
5
12
6
4
5
12
4
3
5
6
6
3
5
12
6
8
Number of Cavities/
2
6
16
8
3
2
1
Unit Cell
Average Cavity
7.90
8.66
7.82
9.46
7.82
8.12
11.42
Diameter, Å
In addition to the geometric information shown in Table 1, pressure/temperature fields of stability for a wide variety of gas hydrates are known, with the fields of stability for hydrate of some gases being more completely known than the fields of stability for hydrate of other gases. In general, the most information is known about sI gas hydrates, which include the relatively common methane hydrate (very widely distributed naturally on Earth) and carbon dioxide hydrate. Often (but not always) sII type hydrate will include more than one type of gas.
Pressure/temperature fields of stability for various sH type gas hydrates are also known, albeit comparatively imperfectly. (The term “sH” is used at the present time to describe gas hydrates in which the dimensions of the voids that house the gas molecules are strongly different; it is likely that many species of hydrate that have yet to be described will fall within this sH category.) For a given gas or mixture of gases, however, it is known that sH gas hydrates will form under certain pressure and temperature conditions, and the ability of a given gas or mixture of gases to form hydrate is widely and strongly believed to be a function of the size of the gas molecule. Table 2 below shows examples of relatively large-molecule gases that are known to occupy the large void spaces of sH type gas hydrate.
TABLE 2
Large Guest
Diameter, Å
Pressure, MPa
Temperature, ° C.
2-Methylbutane
7.98
1.974
0
2,3-Dimethylbutane
7.99
1.439
0
2,2-Dimethylbutane
7.97
1.064
0
2,2-Dimethylpentane
9.25
2.140
0
3,3-Dimethylpentane
9.24
1.373
0
2,2,3-Trimethylbutane
8.00
0.959
0
(The pressure and temperature values are single-point data (rather than field data) at which hydrate of the guest gas molecule is stable.)
3. SUMMARY OF THE INVENTION
The present invention overcomes many limitations of the present practices for decontamination of various toxic gases by forming gas hydrate using the toxic gas as the hydrate-former and carrying out this process of gas hydrate formation under controlled conditions that allow the toxic gas to be captured safely and disposed of. Trapping toxic gas molecules in the form of crystalline gas hydrate renders the gases chemically non-reactive and essentially safe to handle.
Relatively small-diameter toxic gas molecules can form type sI or type sII hydrate. In order to capture larger-molecule toxic gases in hydrate, which larger-molecule gases are too large to fill the voids within type sI or type sII hydrate, type sH hydrate is caused to be formed. Because the larger-molecule toxic gases are also too large to fill the smaller voids in the sH type hydrates, but are able to fill the larger voids in the type sH hydrate, a “companion gas” or “companion agent” is provided. The companion gas or agent fills the smaller voids in the type sH hydrate, and the larger-molecule toxic agent fills the larger voids in the type sH hydrates. Presence of the companion agent renders the hydrate crystals stable, thereby facilitating hydrate-based decontamination of larger-molecule toxic agents.
Once captured in gas hydrate, the toxic gas molecules are chemically unreactive and generally safe to handle. The toxic gas remains safe so long as the hydrate is maintained under conditions of stability for the particular gas hydrate.
Specialized apparatus according to the invention is provided, which apparatus ingests air that has been polluted by toxic gas into a hydrate formation vessel that is pressurized and cooled (as required) so that when water is introduced into the hydrate formation vessel, hydrate of the toxic gas will form spontaneously. Alternatively, the hydrate formation vessel may be filled with water and the toxic gas introduced into the water-filled vessel to form hydrate. The toxic gas hydrate is then concentrated and collected by allowing it to settle within the hydrate formation vessel.
A pressurized hydrate formation vessel or vessels utilize(s) a water spray in a pressurized environment in which the pressure/temperature regime is suitable for the formation of gas hydrate using the toxic gas itself as the hydrate-forming agent. Water, which is obtained either from local sources or brought to the location, is brought into contact with the toxic hydrate-forming gas-and-air mixture that has been ingested into the apparatus so that solid gas hydrate will form and be concentrated. Thus, the hydrate-forming gas can be concentrated and removed from the decontamination site in a safe and expedient manner.
After the hydrate has been accumulated, thereby concentrating the toxic gas, the water and the toxic gas can be separated by altering the pressure and/or temperature conditions under which the hydrate is held to render the hydrate unstable, thus causing the toxic gas hydrate to dissociate under controlled conditions. The toxic gas is released from the hydrate and recovered in a relatively pure and highly concentrated form—either pressurized or as a liquid, depending on the pressure/temperature characteristics of the particular toxic hydrate-forming gases and on the desired nature of the product—thus minimizing the volume of toxic material to be handled. Water is recovered and reused for further formation of toxic gas hydrate, thus minimizing the amount of water to be treated for dissolved toxic gas and other pollutants following successful completion of the decontamination operation.
Toxic gas hydrate decontamination units according to the invention can be fixed or portable. I

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