Integrated solutions for collection, transportation and treatment of non condensable gases from Kraft pulping process

Non-condensable gases are mainly TRS (total reduced sulfur), such as hydrogen sulfide (H2S), methyl mercaptan (CH3SH), dimethyl sulfide (CH3SCH3) and dimethyl disulfide (CH3SSCH3). Other gases are also present including other organic compounds such as turpentine vapor and methanol.

Non-condensable gases are toxic, corrosive and, if mixed with air, potentially explosive. Therefore, specific care must be taken in designing and operating NCG systems.

Non-condensable gases are divided in two categories: concentrated non-condensable gases (CNCG) and diluted non-condensable gases (DNCG). Generally, CNCG are considered as low volume, high concentration gases while DNCG are high volume, low concentration TRS gases.

CNCG sources are digesters (batch or continuous), turpentine recovery system, multiple effect evaporators and foul condensate storage tanks while DNCG sources are brown stock washers, knotters, screens and deckers, liquor storage tanks and, washer seal tanks. Volume and concentration from similar sources can greatly differ from mill to mill and vary in time depending on continuous or batch process using either softwood or hard wood.

Solutions

Concentrated non-condensable gases (CNCG) have a TRS concentration above the Upper Explosion Limit (UEL) which is often high enough to produce an explosive mixture, if in contact with air. Collection and transportation of CNCG is more complex and requires particular attention. The system should be designed to prevent air form getting into the ducts or pipes. To make sure that no such situation occurs, CNCG are transported with steam ejectors. The presence of steam dilutes the mixture and prevents air from entering the system. The use of rupture discs and flame arrestors adds a supplementary protection.

Diluted non-condensable gases (DNCG) have a TRS concentration which is normally below the Lower Explosion Limit (LEL) of the most explosive component. Special measures have to be taken to make sure that the mixture is safely maintained below the LEL: some dilution air may be injected to further decrease the contaminant concentration and make the mixture even less explosive.

The destruction of TRS, turpentine and other compounds, is generally achieved in a thermal oxidizer. The necessary condition for an adequate destruction of these substances is to maintain a temperature of around 1 500 °F (800 °C) for at least 0,5 seconds in a 3% to 4% excess oxygen.

In kraft pulp mills, these conditions exist in three places: the lime kiln, the power boiler and the recovery boiler. However, better oxidizing approach and technology adds to the advantages of using a separate dedicated incinerator. Combustion of TRS mixture, may produce undesired sulfur dioxide (SO2) emissions. This contaminant still has to be removed before the gas stream is sent to the atmosphere. The best technical and economical solution to treat SO2 is to use a wet scrubber with caustic liquor. The sodium hydroxide (NaOH) will react with the SO2 to produce sodium sulfite (Na2SO3) that can be reused in the process. MESAR/ENVIRONAIR INC. proposes new technologies for SO2 removal using maximum reagent available in process liquid and minimizing the use of fresh reagent.

MESAR/ENVIRONAIR INC. brings safer and efficient techniques and technologies for noncondensible gas systems. Different solutions are available for treatment of CNCG and DNCG with high energy efficiency oxidation. A proper design of a NCG system depends upon treating these gases as fuel. Through our new approach, both CNCG and DNCG can be treated in the same incinerator in order to take advantages of the volume, the concentration and the properties of each gas. This can be achieved with the use of a regenerative thermal oxidizer (RTO). When applicable and correctly integrated in NCG reduction systems, RTO has demonstrated a very high destruction efficiency and minimum external energy requirements. This leads to optimizing the energy available, minimizing the operating costs and providing reliability for noncondensible gases systems.

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Condensate Stripping

Condensate stripping systems

Transport and treatment of pulp mill effluents that contain methanol, dissolved reduced sulfur gases (TRS) and other organic compounds

The foul condensates are effluents from the pulping process that contain compounds responsible for environmental problems related to odors, BOD loading and toxicity. The major pollutants found in foul condensates are methanol and TRS. Different concentrations of turpentine, acetone, ethanol and 2-butanone (MEK) are also found. Sources of foul condensates are mainly digesters and evaporators. The turpentine recovery system also contributes, to some extent, to the foul condensates load.

While sulfur compounds present in foul condensates generate odor problems, the methanol increases the BOD5 loading to the mill waste water system. Other compounds, such as turpentine, contribute to toxicity in effluents.

Solutions

In typical steam stripping towers, the liquid is injected at the top of the column and goes down through the column. As it goes down, it meets the steam which is injected in the sump at the bottom. A temperature profile is established which generates a concentration profile for the contaminant. The contaminant is stripped from the liquid and is carried by the steam until it leaves the tower at the top. The purified liquid leaves the column at the bottom and can be reused in the process.

A reflux at the top of the column will knock down the vapor of the less volatile contaminant that has been carried to the upper part of the tower. It is then possible to obtain a purer distillation product.

The condensate stripping system can be either a stand-alone column or can be integrated into the evaporator system. Both systems offer different options for the destruction of steam stripper overheads that contain approximately 50% organics. The vent gases can be a valuable alternative to fossil fuel when routed to incinerators, boilers, or lime kilns.

Another solution provided by a stripping system is the possibility to send the stripper vent gases to a second distillation tower where enhanced separation is performed. The overhead product is liquid methanol with some other organics, containing between 1% and 5% water. Even though it is undesired in process effluents because of its toxicity, methanol is a valuable substance that can be used in the process as a combustible. It then appears interesting to recuperate the methanol found in these effluents. To perform this task at the lowest possible costs, MESAR/ENVIRONAIR INC. has developed a steam-stripping technology that ensures efficient, safe and reliable energy recovery.

Also, integration of the condensate stripper with the multiple effect evaporator system may significantly reduce capital and energy costs.

Solutions provided have to be perfectly integrated with the pulping process and secondary treatments. Leading-edge techniques and technologies may offer reliable, versatile and energy-wise solutions to condensate problems.

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Chemical oxidation of diluted TRS

MESAR/ENVIRONAIR INC., an Engineering Firm involved in industrial process and air pollution control is proud to announce a success story resulting from the newly commercialize process of chemical oxidation of TRS using residual chlorine dioxide from the Kraft pulp bleaching plant.

As a matter of fact, this new process (issued from dedicated R&D team and application engineers) is actually operating in a Kraft Pulp mill. Several trials have demonstrated the viability of this new technology for its efficiency and its economics. MESAR/ENVIRONAIR INC. now offers a new option for diluted TRS treatment that must be considered by the Kraft Pulp mills.

Advantages compared to incineration

1) Reduced capital cost.
2) Reduced maintenance cost.
3) Drastic reduction of manpower and operating cost.
4) Easy installation and low start-up cost.
5) Respect of air emission's laws and regulations.
6) Improved perceptions (environmental-friendly) of the mill.
7) Efficient, proven and flexible technology.

For more detailed information, please read the attached technical paper about this technology success. TRS_ChmOxydation.pdf

MESAR/ENVIRONAIR INC., has established itself over the past eight (8) years as a leader provider of non-condensable gas treatment and air emission system provider. In fact, we supply the most cost-effective environmental compliance technologies regarding any type of airborne pollutant (gas, odors and particulate). We would be very pleased to assist your team toward an environmental friendly approach and to optimize your process.

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