Method for generating formaldehyde monomer vapor

A method of generating a desired gas is provided. The method includes introducing a matrix comprising media containing a parent compound and an inert media into an effusion tube comprising a first zone and a second zone. The first zone includes a micro-porous metal tube, and a closed end. The second zone includes a non-porous metal tube, and an open end. Heating the effusion tube, produces a desired gas.

BACKGROUND

Formaldehyde is a toxic chemical substance that is commonly present in indoor and outdoor air pollution. Indoors, materials like furniture, carpets and household chemicals emit formaldehyde; outdoors, formaldehyde is generated through incomplete combustion of coal and fuels, and is commonly found in automotive emissions and stationary sources (stacks) that burn carbon-based fuels. There is considerable interest in having the ability to make accurate measurements of formaldehyde to assess the fate and health consequences of formaldehyde emissions, as well as to promulgate new regulations to control these emissions.

The current manufacturing process for calibration standards for formaldehyde makes use of an analytical-scale permeation device. This device generates formaldehyde vapor by heating alpha-polyoxymethylene (a solid polymer of formaldehyde) in a sealed vessel and allowing the small amount of generated formaldehyde monomer vapor to diffuse through a length of Teflon® PTFE tubing into a flowing gas stream. Although it does successfully produce formaldehyde gas for mixtures, the extremely low rate of formaldehyde emission makes this process very lengthy and impractical for large quantities of cylinders.

Although it is a small molecule with a low molecular weight, formaldehyde does not persist as a gas phase at high concentration in the pure form. Formaldehyde undergoes self-reaction to form polymers of itself (such as paraformaldehyde) and a variety of larger organic molecules by condensation reactions. Formaldehyde can be stabilized as a monomer in solutions with organic solvents or water (formalin).

However, aqueous solutions are not suitable for component additions to gas cylinders, as moisture almost always adversely affects mixture stability. Therefore, pure, dry and uncontaminated formaldehyde must be generated in-situ as needed from materials that emit formaldehyde when heated, such as paraformaldehyde, trioxane and even gum rubber tubing.

Describe herein is a new technique and a new apparatus for controlled generation of formaldehyde monomer vapor. This new process generates larger quantities and higher concentrations of formaldehyde, thereby facilitating faster production of mixtures in gas cylinders. This new process also minimizes decomposition of formaldehyde via self-reaction as well as formation of undesired side products.

SUMMARY

A method of generating a desired gas is provided. The method includes introducing a matrix comprising media containing a parent compound and an inert media into an effusion tube comprising a first zone and a second zone. The first zone includes a micro-porous metal tube, and a closed end. The second zone includes a non-porous metal tube, and an open end. Heating the effusion tube, produces a desired gas.

DESCRIPTION OF PREFERRED EMBODIMENTS

Element Numbers

As illustrated inFIGS. 1-8, the general design of the apparatus comprises a closed-end, micro-porous metal tube100captured inside of an outer metal jacket200. The micro-porous metal tube100has a first zone101that consists of a micro-porous wall, and a second zone102that consists of a non-porous wall. The porosity of the first zone101may be 0.1 to 5.0 microns, such tubes are commercially available. The first zone101has a closed end103. The second zone102has an open end104. In the following example, formaldehyde gas is used as a non-limiting example. However, as discussed below, the instant apparatus may also be used to generate other desired gases.

A formaldehyde-generating precursor105is packed inside of the porous metal tube100. The formaldehyde-generating precursor105may be combined with an inert media106to form a matrix107. Matrix107is then introduced into the metal tube100, and the open end is capped gas tight, for example with steel cap201. The filled porous tube100, also referred to herein as the permeator, is then passed through the full length of a union tee, or run tee,202. The run tee202is attached to the outer metal jacket200and secured with at least one high pressure compression fitting, for example, a tubing reducer203.

The outer metal jacket200is equipped with compression tube fittings, such as204awhich is integral to run tee202, capable of high pressure (typically about 2000 psig). Another high pressure compression tube fitting is at the exit end of outer metal jacket200, such as reducing union204b, and the other on the leg portion204c, also integral to run tee202, through which carrier gas205flows.

In one representative, but non-limiting, example, the closed-end microporous metal tube100may be nominally ⅜ inches in diameter, and outer metal jacket200may be nominally ¾ inches in diameter. Fittings such as run tee202, reducing union204b, and steel cap201in sizes such as these are commercially available.

The exterior of outer metal jacket200is then heated by heating device701to a controlled temperature, for example by variable input heating control704, sufficient to de-polymerize the formaldehyde-generating precursor105and liberate pure formaldehyde monomer vapor702. The formaldehyde vapor702escapes by passing through the interstices of the porous microporous metal tube101as a gas. While formaldehyde monomer vapor702is being formed, carrier gas205is passed through annular region208between the metal tube100and outer metal jacket200at a controlled rate to entrain the liberated vapor702. The carrier gas205is the same composition as the balance gas of the desired formaldehyde mixture703, typically nitrogen, helium or argon.

Close control of the temperature of the outer metal jacket200results in a predictable emission rate of formaldehyde vapor702. Higher temperatures increase the emission rate, but excessive heating results in undesirable side reactions and contamination. The composition controller710does not control the outer metal jacket temperature; the controllers for the heating devices do. The temperature may be monitored directly on the outer metal jacket200, for example, by temperature indicator705, or the temperature of the gas exiting the permeator may be monitored, for example, by temperature indicator706. The flow rate of carrier gas205may be monitored, for example, by flow indicator707, and/or the total flowrate of the formaldehyde mixture703may be monitored, for example, by flow indicator708. The final composition of the formaldehyde mixture703may be monitored, for example, by composition indicator709. Close control of the carrier gas flow rate205, in conjunction with close control of the permeator heating704, for example, by gas composition controller710, results in a flowing stream of formaldehyde gas (or vapor)702in the selected carrier gas205with a predictable, stable and controllable concentration.

In another embodiment of the current invention, the carrier gas205itself is externally heated prior to entry to outer metal jacket200, for example by external heat source712. In this embodiment, the heat of carrier gas205is used to cause the de-polymerization of the formaldehyde precursor107inside the permeator tube100. The temperature may be monitored, for example, by temperature indicator713, and provided as input to gas composition controller710, or carrier gas flow controller714.

In another embodiment of the current invention, as indicated inFIG. 5, the outer metal jacket200is wrapped with a tight coil of small diameter metal tubing701. In a non-limiting example, this metal tubing may have a nominal outside diameter of ⅛″. Carrier gas205ais passed through an external heater712as discussed above. The heated carrier gas205bis then passed through the coil401. The heated carrier gas205bheats the outer metal jacket200externally. Then the resulting warm carrier gas205c, enters run tee202as discussed above, and then heats the permeator internally, potentially resulting in improved temperature control.

It is known in the art that various materials emit formaldehyde when heated. Non-limiting examples include paraformaldehyde, trioxane and even gum rubber tubing. Commercially available analytical permeation systems typically use formaldehyde polymers as the source because of their convenient handling properties, such as being colorless granular solids. However, the skilled artisan would be aware of charring and melting of the paraformaldehyde when it is over-heated, thus resulting in reduced yield and purity.

In one early reference, U.S. Pat. No. 2,460,592, it is suggested to suspend finely ground paraformaldehyde powder in a stirred liquid medium, such as a non-reactive oil and fluid with a low vapor pressure. In this reference, very pure formaldehyde vapor is then generated when the liquid media is heated. This approach, however, will not work in a cylinder gas application, as such an application has stringent dryness standards, and cannot tolerate liquid media of any kind.

In one embodiment of the present invention, finely-divided paraformaldehyde powder (or other formaldehyde-emitting material)105is suspended in a non-reactive solid dispersing medium106, such as pure silica, alumina or other inorganic substrate. The dispersing media106is very pure, dry and completely free of organic materials that can react with formaldehyde; both silica and alumina are commercially available in adequate purities. The particle size distribution of the support media106is sufficiently small that it disperses the paraformaldehyde powder particles105from one another (to suppress self-reaction when heated), but sufficiently large to retain all of the support media106and paraformaldehyde particles105inside of the permeator porous housing100. The support media106also provides uniform heat distribution within the mixture107and may accelerate the rate at which formaldehyde vapor702is formed due to increased surface area. As the formaldehyde vapor702is formed, the dispersing media106stays behind in the permeator100, along with any other solids or non-volatile contaminants.

The present invention, i.e., dispersing materials onto a solid support and then forming a controlled gas stream using the porous metal permeator, may also be applicable to other materials that require moderate heating to force them into the gas phase, e.g., materials that emit a desired component when heated, or that have low native vapor pressure at room temperature.

The heating device701, whether directly attached to the outer metal jacket200, or integrated into the carrier gas flow system205(as discussed above), is equipped with variable input control capable704of making fine adjustments to the temperature. The outer metal jacket200of the formaldehyde generator is fitted with one or more temperature indicators, or thermocouples,705to measure the temperature, and provide feedback to the heating device to achieve closed loop control710of the temperature.

The generator system can also be equipped with real-time analysis710of the concentration708of the formaldehyde/carrier gas stream703as it is formed. To achieve this, a tee is installed in the carrier gas output line, downstream of the generator. A small bypass flow from the formaldehyde/carrier gas mixture stream is directed to a formaldehyde analyzer708. Analyzer708may be a chemical cell, FTIR or other formaldehyde sensor known in the art. Any changes in the formaldehyde emission rate or carrier gas flow can thus be measured directly, and adjustments made to temperature or carrier gas flow rate, for example by valve711. If desired or practical, the output from the real-time analyzer710can be fed back to the generator temperature controller704, or the carrier gas flow controller714, or both to achieve closed loop control.

In one embodiment of the present invention, a method of generating a desired gas is presented. The method includes matrix107which includes media containing parent compound105, i.e., a precursor to the desired gas, and inert media106. Matrix107is introduced into effusion tube100which includes first zone101and second zone102. The first zone101includes a micro-porous metal tube, and closed end103. The second zone102includes a non-porous metal tube, and open end104. Effusion tube100is heated, thus producing the desired gas702.

The two-zone effusion tube100may be fixtured inside of a larger cylindrical metal jacket200. The cylindrical metal jacket200may have gas entry port206and gas exit port207at opposite ends. Annular region208is formed between inner effusion tube100and outer cylindrical metal jacket200. Carrier gas205flows through this annular region208and over the exterior of effusion tube100.

Desired gas702may be formaldehyde, and parent compound105may be paraformaldehyde, alpha-polyoxymethylene (no space between poly and oxy), trioxane or other material that emits formaldehyde when heated. Desired gas702may be 1,4-dichlorobenzene; 1,3,5-trichlorobenzene; 1,2,4,5-tetrachlorobenzene; 1,2,4,5-tetramethylbenzene; camphor; hexachlorobenzene; hexamethylcyclotrisiloxane; indole; menthol; naphthalene; p-cresol; phenol; or any other low vapor pressure chemical. Parent compound105may be the solid, semi-solid or liquid form of the desired gas702.

Effusion tube100may be heated to a temperature no greater than 482 F. At least a portion of the heat to effusion tube100may be provided heating device701, which may be heating tape, rope heater, heating cable, or heating cord in contact with metal jacket200. At least a portion of the heat to the effusion tube may be provided by a tube furnace701that heats the outside of metal jacket200.

Desired gas702may be generated at a rate up to 50 grams per hour. The carrier gas205may be introduced into the annular region208, wherein the carrier gas205blends with desired gas702and exits the open end207of metal jacket200.

Blended gas702may have a first flow rate F1, (which will be approximately the same as the carrier gas second flow rate F2). The effusion tube may have a first temperature T1, and first flow rate F1may be controlled by adjusting the carrier gas flow rate F2. T1is not an adjustment for flow; it has very little effect on flow. Carrier gas205may provide at least a portion of the heat to effusion tube100. Carrier gas205may be nitrogen, helium, argon, air, or other gas that is non-reactive with the desired gas702. Blended gas703may be produced at a pressure up to about 2500 psig. Blended gas703may be produced at a pressure up to about 500 psi, preferably up to about 1000 psi, more preferably up to about 1200 psi, still more preferably up to about 1650 psi, and more preferably up to about 2200 psi.

The desired gas703, or the blended gas, may be introduced into gas cylinder715. Once the desired gas703is vaporized, the inert matrix106may retain some or all of the residue of the parent compound105.