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
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.
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.
Condensate stripping systems
Transport and treatment of pulp mill effluents that
contain methanol, dissolved reduced sulfur gases (TRS) and other
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.
Foul condensates are collected, transported and
treated by stripping technology. Steam stripping is a desorption process
where steam is used to heat a liquid to a point where the dissolved
volatile contaminant will vaporize.
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.
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.