Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-04-20
2001-03-06
Padmanabhan, Sreeni (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S449000, C502S350000, C585S638000, C585S640000, C423S415100, C423S512100
Reexamination Certificate
active
06198005
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention broadly relates to a process for treating methanol-containing waste gas streams, such as encountered in a paper (pulp) mill. More particularly, this invention provides a method wherein a gas stream, such as from a paper pulp mill containing methanol, and other waste products, including methyl mercaptans, is passed in contact with a catalyst comprising a supported or unsupported bulk metal oxide in the presence of an oxidizing agent. In a preferred embodiment, the gas stream is contacted with the catalyst, in the presence of the oxidizing agent, for a time sufficient to convert at least a portion of the methanol to formaldehyde (CH
2
O).
2. Description of Related Art
Pulp mills that chemically digest wood sources, such as by using the Kraft process, generate a significant amount of by-product methanol, in addition to other potential pollutants such as reduced sulfur compounds, higher alcohols, terpenes, acetone, amines, acetaldehyde and methyl ethyl ketone. Significant methanol emissions are encountered during the overall pulping and paper-making operation, particularly from the following pulp mill sub-systems: pulp digesters, blow heat recovery units, and multiple-effect evaporators. In the past, this methanol was often discharged into the air or directly into wastewater. With growing environmental concerns over pollution caused by these past practices, however, more stringent emission requirements have evolved. As a consequence, processes must be developed to dispose of the methanol, and the other pulp mill by-products, in a more environmentally satisfactory manner. At the present time, two alternatives have been suggested as being suitable for complying with regulatory standards. Garner, Jerry,
Pulp
&
Paper
, (August 1996):59-62. In both approaches, the various waste steams generated in a pulp mill containing by-product methanol, and a variety of the other noted compounds, are first consolidated into a single condensate stream.
In a first alternative, this consolidated condensate stream is fed to a steam stripping column which is operated in a way to remove and concentrate, in the gas phase, a major portion of the pollutants from the liquid condensate. Over 80-90% of the methanol of the consolidated feed steam is removed in the stripping column, while the methanol concentration is increased from about 0.1-0.5% in the liquid feed to about 35-55% in the steam stripper gaseous overhead.
This methanol-containing stripper overhead is then treated by indiscriminate (noncatalytic) oxidation (incineration) to produce a waste gas that can be safely discharged into the environment. To safely complete the oxidation of the gas constituents, the incineration process must be operated at temperatures approaching 1000° C. Such operation generally requires the use of an auxiliary fuel source. In some facilities, the fuel value of the stripper overhead is recovered by using it, for example, to power a boiler or a lime kiln. In some cases, the methanol content of the stripper overhead is further concentrated by distillation to increase its value before use as a fuel.
The second alternative delivers the condensate stream to a wastewater treatment system where aerobic microorganisms use the methanol as a carbon source, converting it to carbon dioxide and water.
While highly dependent on the wood source and the basic operating conditions of a pulp mill, the consolidated waste methanol stream or condensate from a pulp mill can be expected to contain, among other materials, methanol and other higher alcohols, methyl ethyl ketone, acetaldehyde, acetone, terpenes, amines, ammonia and reduced sulfur compounds. The concentration of these materials in the aqueous condensate is enriched by steam stripping and results in a gas stream which typically contains about 40-55 wt. % methanol, 2-8 wt. % higher alcohols, such as ethanol and isopropanol, 2-8 wt. % ketones, including methyl ethyl ketone, methyl isobutyl ketone and acetone, 1-3 wt. % reduced sulfur compounds including hydrogen sulfide and methyl mercaptans, such as methanethiol (CH
3
SH), dimethyl sulfide (CH
3
SCH
3
) and dimethyl disulfide (CH
3
SSCH
3
), about 1-2 wt. % amines, including ammonia, 1-6 wt. % terpenes, such as &agr;-pinene and &agr;-terpineol, and the balance (generally 40-50 wt. %) water.
Simply incinerating or biologically degrading this methanol-containing waste stream constitutes a costly and inefficient use of the inherent resources present in the stream. While procedures used to recover the fuel value of the stream seek to reduce such inefficiencies, they create their own source of problems. In particular, the variable nature of the methanol-containing stream creates the risk of operational upsets in the operation of the lime kiln or a boiler. Further, if the lime kiln or boiler operation is interrupted, then the operation of the stripper must be discontinued since there is no way to dispose the concentrated stripper overhead. As a result of these potential problems, a pulp mill likely would need to operate with a dedicated incinerator using a power boiler as back-up.
In its preferred aspects, the present invention is directed to an improved process for using by-product methanol from methanol-containing waste streams, such as encountered in a paper (pulp) mill, to produce a valuable chemical commodity, formaldehyde.
BRIEF DESCRIPTION OF THE INVENTION
This invention provides a method wherein a methanol-containing waste gas stream, such as originates from a paper pulp mill, and containing other waste products, including methyl mercaptans, is contacted, under oxidizing conditions, with a catalyst comprising a bulk metal oxide. The bulk metal oxide catalyst can be either unsupported, or supported. The gas is preferably passed in contact with the catalyst in the presence of an oxidizing agent for a time sufficient to convert at least a portion of the methanol to formaldehyde (CH
2
O), and then recovering the formaldehyde as a product stream separate from the gas stream. In an alternative embodiment, the gas can be contacted with the catalyst for a time sufficient, and under oxidizing conditions sufficient, to oxidize the carbon-containing oxidizable components of the gas stream, including methanol, completely to carbon oxides (CO
x
) and the sulfur-containing components to sulfur oxides (SO
x
).
The oxidizing conditions can be established using an oxidizing agent such as oxygen or air. In the presence of the catalyst, other volatile organic compounds beside methanol are also oxidized, amines are generally reduced to nitrogen (though some portion may be oxidized to nitrogen oxides) and the sulfur is oxidized to SO
2
, and possibly a minor amount of SO
3
.
Accordingly, a preferred embodiment of the present invention is directed to a process for producing formaldehyde from methanol-containing waste gas streams, especially pulp mill condensates, which comprises (1) producing a methanol-containing gas from the condensate, such as by steam stripping the pulp mill condensates, (2) contacting said methanol-containing waste gas with a bulk metal oxide catalyst under oxidizing conditions for a time sufficient to convert at least a portion of the methanol to formaldehyde, and (3) recovering said formaldehyde from the gas stream. Following removal of the formaldehyde, the residual gas stream likely will be sent to an incinerator or after-burner for complete combustion of the residual impurities and products so as to produce a gas suitable for direct discharge into the atmosphere.
In carrying out the process of the present invention, known bulk metal oxide catalysts can be used. The catalyst can either be unsupported, or supported. Such catalysts generally constitute molybdates (Mo), chromates (Cr), vanadates (V), rhenates (Re), titinates (Ti), niobates (Nb), tungstates (W), antimonates (Sb), stannates (Sn), ceriates (Ce) and mixtures thereof. Such metal oxides also contain a wide variety of other metal species such as alkali metals (such as sodium (Na), lithium (Li), potas
Banner & Witcoff , Ltd.
Lehigh University
Padmanabhan Sreeni
LandOfFree
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