Paper making and fiber liberation – Processes of chemical liberation – recovery or purification... – With regeneration – reclamation – reuse – recycling or...
Reexamination Certificate
2000-04-28
2002-02-19
Silverman, Stanley S. (Department: 1731)
Paper making and fiber liberation
Processes of chemical liberation, recovery or purification...
With regeneration, reclamation, reuse, recycling or...
C162S030100, C162S029000, C162S036000, C162S051000, C423S232000, C423S233000, C423S242600, C423S245100, C423SDIG003, C210S760000, C210S631000
Reexamination Certificate
active
06348129
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to processes using ozone to treat streams containing sulfur compounds.
BACKGROUND OF THE INVENTION
Part of the air pollution and most of the odor problems that occur in kraft pulp mills are caused by “total reduced sulfur” (TRS) compounds, which are formed during pulping by the reaction of hydrosulfide/sulfide ions with wood components. TRS compounds include one or more of hydrogen sulfide, methyl mercaptan, dimethyl sulfide and dimethyldisulfide. These compounds have a strong unpleasant odor at very low concentrations and are toxic at somewhat higher concentrations. They are released from the digesters, where wood is converted to pulp, and from the evaporators of the spent pulping liquor together with vapors and may also be found in the spent liquor itself. The vapors are condensed and the resulting condensates are aqueous waste streams that contain variable amounts of TRS. These condensates are aqueous streams which are also referred to as “contaminated condensate” or “foul condensate” depending on the level of their odor, their TRS concentration and/or their content of spent pulping liquor.
Due to their relatively high temperature, condensates are frequently used in the pulp mill e.g. for pulp washing and/or washing of lime applied in the recovery process of the pulping chemicals. Normally the condensates are treated to reduce their TRS content prior to their use. Condensate that is not utilized in the mill is collected together with other liquid discharges and is then treated biologically before release into a river or lake. TRS can escape from the condensates during re-use or during aeration in the biological treatment and can thus cause an odor problem unless its concentration has been reduced to a very low level.
The so called “Cluster Rules” promulgated in the United States by the Environmental Protection Agency (EPA) in 1998 limit the emission of TRS from pulp mills. For some mills, especially those with older equipment, meeting the Cluster Rule limits may be very challenging.
The TRS emission problem can be reduced by stripping the condensates with air or steam. The stripped gas is then incinerated to convert the sulfur of the TRS compounds to sulfur dioxide, for example in the mill's lime kiln, chemical recovery boiler or a stand-alone incinerator. In the latter case, the sulfur dioxide containing gas may be released into the atmosphere or may be scrubbed with a chemical solution that would then have to be disposed of. However, the stripper off-gas is a potential concentrated source of odor, and any mishap in handling this stream can create a very severe odor incident. Furthermore, the stripping efficiency may vary considerably depending on steam availability and pressure and the stripped condensate still contains some residual TRS, if the operating conditions are not optimized. However, under optimized conditions, steam stripping can remove almost 100% of the TRS. In contrast, air stripping is generally less efficient. In older mills, retrofitting of stripping equipment can be very expensive.
Biological treatment of effluents, both anaerobic and aerobic, is being developed to remove TRS and other organic compounds contained in the condensate. Anaerobic treatment requires considerable equipment, oxygen must be completely excluded and stringent process control is usually required due to the sensitive nature of aerobic microorganisms. Aerobic treatment requires aeration or addition of oxygen to the condensate. This mode of treatment can strip unoxidized TRS compounds into the off-gas unless special precautions are taken. Some bio-sludge has to be removed from both types of biological treatment.
Other methods proposed for TRS removal from condensates include:
activated carbon adsorption;
chemical oxidation with chlorine or oxygen;
chemical precipitation.
These three methods have not found application in the kraft pulp industry. Activated carbon adsorption and chemical precipitation generate a solid waste that needs to be further treated or perhaps disposed of in a special land-fill, while oxidation with chlorine is no longer environmentally acceptable. Oxidation with oxygen alone is of doubtful value, as shown herein.
BRIEF SUMMARY OF THE INVENTION
One aspect of the present invention is a process for treating an aqueous stream containing one or more reduced sulfur compounds, comprising:
(a) reacting an aqueous feed stream containing TRS contaminant selected from the group consisting of hydrogen sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, and mixtures thereof, with ozone to produce a first liquid product stream and a first gaseous product stream, wherein the total amount of said one or more contaminants in said first liquid product stream is less than 1% of the amount thereof in said feed stream, and wherein said first gaseous product stream contains ozone and one or more of said contaminants; and
(b) treating said first gaseous product stream with an alkaline aqueous stream to remove essentially all of said one or more contaminants from said first gaseous product stream.
Preferably, a second gaseous stream produced in step (b) and containing ozone is reacted with an aqueous stream containing one or more of said contaminants, to consume all the ozone in said stream.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is applicable to all types of condensates produced in a pulp mill as long as they contain reduced forms of sulfur. These include (1) highly contaminated streams obtained from wood pulping and black liquor evaporation systems. These condensates come more specifically from the flash tanks of continuous digesters, blow tank relief valves of batch digesters, turpentine recovery systems, later stages of the evaporation system (known in the industry as effects 2 and 3) and the concentrator; (2) secondary condensates coming from the earlier stages of the evaporation system (known in the industry as effects 4 and 5); (3) condensates previously purified via air stripping; and (4) condensates previously purified via steam stripping. The concentration of TRS (total reduced sulfur) present in these various condensate streams may vary from 1 mg/L to 1500 mg/L and are treatable by the process of this invention. The stream being treated may also contain one or more of carbonyl sulfide, methanol, ethanol, acetone, furfural, and butanone.
The process is also applicable to contaminated condensates from other industrial operations as long they contain TRS concentrations within the range above specified.
The present invention achieves the destruction of TRS contained in condensates with ozone followed by scrubbing of the off-gas from the ozone treatment with an aqueous alkaline stream which can be a waste stream from the pulp mill. A large portion of the TRS can be oxidized with ozone, for example to sulfate, while a smaller amount of said contaminants is removed by being stripped into the oxygen rich off-gas. With a sufficiently high ozone dosage, TRS is removed so completely from the condensate that the amount of said contaminants remaining in the liquid stream produced in this step is less than 1%, preferably less than 0.5%, and more preferably less than 0.1% of the amount originally present in the stream fed to this step. At the same time, the color of condensates that are contaminated with small quantities of spent pulping liquor (so-called black liquor) decreases significantly; the pH of these contaminated condensates is reduced, from alkaline values typically about 9 or higher to 7.5 or less (preferably 7 or less) and their dissolved oxygen content rises significantly. Due to this quality improvement, the condensate may then be used in the mill without causing TRS emission problems.
The ozone used for condensate decontamination may be produced from an on-site generation system at concentrations ranging from 2-20% in the gas stream. However, the off-gas from an ozone bleaching system can also be directly applied in the decontamination of condensates. In addition, combinations of ozone
Charlet Philippe
Colodette Jorge Luiz
Black Donald T.
Halpern Mark
Silverman Stanley S.
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