Process and apparatus for removing contaminants from pulp digester vent gas

There is provided a process and apparatus for improving the yield of turpentine collectable from the vent gases emanating from a pulp disgester in a kraft paper pulp Process. The apparatus is characterized by the insertion in a conventional turpentine recovery system of a gas/liquid contact vessel for intimately contacting the hot turpentine containing vent gas from a pulp digester with liquid foul condensate recycle from a turpentine decanter and a condenser for condensing the condensible components of the recycle treated vent gas, whereby the foul condensate is greatly improved as to contaminant content and rendered suitable for reuse in the plant.

This invention relates, as indicated, to a process for removing 
contaminants, such as, noncondensible hydrocarbons and sulphur-containing 
organic compounds, from the vent gas issuing from a paper pulp digester 
and for enhancing the recovery of turpentine from paper pulp digesters. 
This process renders the resultant condensate suitable for process uses in 
the pulp mill, such as brown stock pulp fiber wash, dregs wash, or green 
liquor production. 
BACKGROUND OF THE INVENTION AND PRIOR ART 
The present state of the art process for the recovery of turpentine in 
Kraft pulp mill operation involves separation of the digester vent gas 
from black liquor carry over, then condensing the steam and turpentine in 
a condenser. The combined condensate (water and turpentine) is received in 
a decanter, and a noncondensible gas stream (NCG) is vented from the 
decanter, cooled in a gas cooler and discarded. The light sulfur compounds 
are thus dissolved in the hydrocarbon (turpentine) phase causing the 
sulfur content of the oil phase to achieve an unacceptable level for 
turpentine buyers. 
The turpentine is separated by gravity and allowed to drain off as 
produced. The foul condensate, or water phase, is hydraulically displaced 
to a sewer, and contains various organic compounds that raise the 
biological oxygen demand (BOD) level of the waste water disposal ponds. 
The present invention is an improvement on the prior art process. and 
contemplates a method whereby the foul condensate containing organic 
sulphides and hydrocarbons is stripped of most of these pollutants by 
recycling a liquid foul stream through a reconditioning drum, or 
gas/liquid contact vessel, e.g. a tower packed with Raschig rings or other 
conventional packing, that provides for good gas/liquid contact with the 
incoming hot vent vapor gas from the digester. The residual organic 
compounds in the foul stream are revaporized or azeotroped over into the 
recovery system of the present invention. As these compounds recycle in 
the foul condensate stream, they attain an equilibrium and are ultimately 
removed in the decanter with the main portion of the turpentine. The 
resultant level of contaminants in the off-gas stream is reduced from 
about 300-3500 ppm to a level below 40 ppm in the novel process hereof 
without adding prime steam to the system, thereby effecting a considerable 
saving in power requirements. Moreover, the present improved process does 
enhance the overall production of turpentine by a considerable amount 
e.g., as much as 3%-5%, from the foul condensate which was heretofore 
lost. The increase in turpentine production will allow most mills to 
recoup the cost of retrofitting existing equipment, as hereinafter 
disclosed, in a relatively short period, e.g., about 24 months. Systems 
currently available use stripping systems that require large volumes of 
costly prime steam to accomplish a similar result. 
BRIEF STATEMET OF THE INVENTION 
Briefly stated, the present invention is in a plant for stripping 
contaminants or pollutants from the gas issuing from a pulp digester, said 
plant including a pulp digester, a black liquor separator, a decanter to 
separate turpentine condensed from the digester gas, and a primary 
condenser, the improvement which comprises a gas/liquid contact vessel for 
intimately contacting the hot turpentine-containing pulp digester gas with 
liquid foul condensate recycle from the decanter to yield a gaseous 
effluent containing a condensible fraction useful in the plant (clean foul 
condensate), noncondensible gas, and turpentine, a condenser for 
condensing the gaseous effluent to yield a dirty foul condensate, and a 
gaseous fraction containing turpentine and noncondensible gases, and a 
stripper for removing the turpentine from the cooled, dry, noncondensible 
gases, the turpentine being collected or returned to the decanter. 
The invention also contemplates in a process for treating contaminated vent 
vapors from a paper pulp digester including the step of separating black 
liquor and a vapor containing turpentine, steam, and noncondensible gases, 
the improvement which comprises contacting said vapor with recycle foul 
condensate from the decanter in a gas/liquid contact vessel to yield a 
turpentine - rich liquid which is separated out, and a vapor containing 
the foul condensate and noncondensible gases, condensing the foul 
condensate from said vapor, and stripping the remaining vapor into 
turpentine (for return to the decanter) and noncondensible gases. 
The present invention provides, then, a method and apparatus for removing 
all the steam condensed from the vent gas or relief gas stream and to 
recover it as "clean" foul condensate acceptable for use in the pulp 
process; to recycle the "dirty" foul condensate back into a stripper 
vessel to extract 80% to 90% of the residual organic compounds from the 
stream; and to control the amount of sulfur compounds allowed to remain 
with the turpentine thus reducing the level of solubilized sulfur 
compounds in the decanter turpentine (hydrocarbon) phase. 
The removal of contaminants in the foul condensate to a level to make it 
suitable to recycle back into the process.

DETAILED DESCRIPTION OF THE INVENTION 
As indicated above, the present invention is in a novel system or apparatus 
for carrying out a novel process. The essence of the invention is in the 
use of what I call "foul condensate" derived from the vent vapors from the 
digester, in a liquified state as a recycle for contacting with the vent 
vapor in a gas/liquid contact vessel. This has the effect of concentrating 
the "foul condensate" to an equilibrium state in the recycle stream, and 
enriching the turpentine concentration in the overhead stream. The "foul 
condensate" bottoms from the gas/liquid contact vessel are returned to the 
primary condenser for ultimate delivery to the decanter. The heavier 
aqueous liquid in the decanter is foul condensate returned to the system 
as recycle. The enriched vapor overhead then passes through a selective 
condenser where vaporized "clean" foul condensate liquid is condensed and 
removed from the from the selective condenser. The turpentine enriched 
vapor together with noncondensible gases (NCG) are passed to the primary 
condenser to separate the NCG, and the liquid turpentine and any foul 
condensate returned to the decanter for separation of these immiscible 
liquids. The turpentine, taken from the top of the decanter then goes to 
storage as a salable product. The foul condensate collected in the 
decanter is returned as recycle for contact with the vent vapor, as above 
stated. 
In a preferred embodiment of the invention, a stripper is provided for 
further refining the vent gas in the turpentine storage vessel. This is a 
relatively short tower which functions to remove water and turpentine from 
the NCG and to clean the water, turpentine and NCG by refluxing. The 
cleaned water and turpentine drain to the decanter. The vapors from the 
stripper go to disposal, such as a lime kiln. The drawing shows in 
diagrammatic form an eductor 54. A fan would work as well. 
Referring now more particularly to the drawing, there is here shown an 
apparatus in schematic form, for carrying out the novel process of the 
present invention. Valves existing in the typical old system that are not 
used in the present invention are in solid black indicating that they are 
normally closed in the improved system and process. 
There is thus provided at least one paper pulp digester 10 with a vent line 
12 leading to a liquor separator 14 from which is recovered residual 
digester carry-over black liquor and a small amount of turpentine and C-10 
alcohols. The vent gas exits the system at a relatively high temperature, 
e.g., above about 212 F. The turpentine content of the vapors discharged 
from the digester 10 will range broadly from 0.1% to about 15%, and as a 
median value, from about 0.3 to 11.5% by volume. The vapors discharged 
from the separator 4 through conduit 16 will also contain steam and NCG. 
In a conventional pulp mill, these vapors go directly through a line 18 and 
valve 20 to a condenser 22. The condensate from condenser 22 is then 
conducted to a decanter 24 through a pipe 26. As shown in the drawing, the 
valve 20 (shown in solid black) is closed off in the present apparatus and 
what was before designated the "primary condenser" (22) is now designated 
a "secondary condenser" 22. 
For the purposes of the present invention, the vent line 16 carries the 
vent gas including noncondensible gas (NCG) at a temperature of from about 
212 F. to about 230 F. to a gas/liquid contact vessel 28 or stripping 
vessel 28. Here the vent gas is brought into intimate gas/liquid contact 
with recycle "foul condensate" at about 180 F. in a counter-current 
manner. The "foul condensate" is at this point what I call "dirty foul 
condensate." The hot vent gas enters the gas/liquid contact vessel 28 
through the line 30 near the lower end thereof and flows upwardly and in 
intimate contact with the downwardly flowing "dirty foul condensate" 
liquid entering at the upper end of the tower through the line 32. The 
"dirty foul condensate" amounts to about 20-40 volume % of the original 
vapor volume. The "dirty foul condensate" is received from the bottom of 
the decanter 24. The flow of "dirty foul condensate" to the tower 28 need 
not be controlled. Under the influence of a pump 25, the "dirty foul 
condensate" is passed over a weir 27 into a receiver 29a from whence it is 
pumped by the pump 25 through the conduit 32 into the gas/liquid contact 
vessel 28 or stripping vessel 28. The device 31 is identified by a circle 
indicates a conventional liquid level controller. 
The vapor is slightly cooled albeit still above about 212 F. and is 
conducted through a conduit 34 to a primary or selective condenser 36 
where from 50% to 90% of the vent gas stream is condensed. The remaining 
portion (mainly steam) is used as a carrier gas to purge the system of 
turpentine. The condenser 36 is operated at a temperature sufficient to 
condense the "clean foul condensate" contained therein to a temperature of 
about 206 to 209 F. by means of cooling liquid from a cold water source 
(CWS) for delivery to a "clean foul condensate" storage vessel 38. "CWR" 
in the drawing indicates a cold water return. The so called "clean foul 
condensate" is available for use in the plant. The "clean foul condensate" 
is contaminated, but the contamination is a small fraction of what it is 
in the conventional foul condensate normally going to sewer. The "clean 
foul condensate" is suitable for pulp washing or green liquor production. 
This usually amounts to about 60% to 80% by volume of the materials 
entering the condenser 36. 
The condenser 36 is selective, condensing only a portion of the 
condensibles in the vent gas and yields an off-gas containing the 
turpentine and NCG amounting to about 40% of the volume entering the 
condenser 36. This off-gas has a temperature of about 212 F. The off-gas 
is then fed from the primary condenser 36 into the secondary condenser 22 
through the line 40 where it is fully condensed using a cooling medium, 
e.g., water. The temperature of the condensate is reduced to about 180 F. 
for entry into the decanter 24 which temperature reduction condenses the 
turpentine and water (foul condensate) and separates out the NCG. The NCG 
consists primarily of air and light sulfur compounds, such as, dimethyl 
sulfide, carbon disulfide and mercaptans. The NCG portion is then routed 
through line 48 to a reflux tower 46 and further cooled to a predetermined 
controlled temperature by the reflux condensate from reflux condenser 50. 
The turpentine flows from the decanter 24 through an overflow line 42 to a 
turpentine storage 44 as a salable product. Any NCG is educted from the 
system through an eductor 54 for further treatment to remove any sulfur 
compounds that may be present. As indicated above, sale or in plant use of 
this turpentine enables recovery in a relatively short time of the cost of 
retrofitting a conventional pulp mill to include the present apparatus and 
process. 
A large portion of the steam in the gas stream is removed in the primary 
condenser 36 leaving the condenser 36 as hot clean condensate suitable for 
use in the pulp mill. On start-up of the system, a diverter valve 64 
directs on-specification-condensate to the storage vessel 38 at a 
temperature controlled by a temperature controller 63 in the condensate 
discharge line 66. Condensate drained from the stripping vessel 28 is 
combined with the condensate from the primary condenser 36 and routed to 
the storage vessel 38 through the line 66. Off-specification-condensate is 
routed through the diverter valve 64 and line 68 to the decanter 24 for a 
brief period during start-up while the system is coming up to operating 
temperature, or during periods of unexpected upset when the system may go 
out of control. Condensate from the secondary condenser 22 is routed 
through a stilling well 26 to the decanter 24. 
In preferred embodiments of the present invention, there is provided a 
stripping column and a stripping step. Thus, there is shown in the annexed 
drawing a stripping column 46. This device accepts vapor or gas 
accumulating in the condenser 22 and in the outage of the turpentine 
storage vessel 44, and strips out any turpentine by cooling with cold 
water and recycle condensate, from the NCG. the off-gas or vapor from the 
secondary condenser 22 enters the bottom of the stripper 46 at a 
temperature of about 190 F. through a conduit 48 along with the vapor 
emanating from the turpentine storage vessel 44 at a lower temperature. 
Cold water is introduced into a heat exchanger 50 to liquify any 
condensible components of the combined off-gases and vapor and reduce the 
temperature thereof to about 150 F. for return through the conduit 52 to 
the decanter. The condensate depleted gas consists mainly of 
noncondensible gases that have followed the material through the entire 
process. These are exhausted in a conventional manner through the gas 
eductor 54 to a conventional lime kiln (not shown). 
The water insoluble hydrocarbons, or turpentine, in the decanter 24 are 
decanted through a line 42 to a storage tank 44. The water phase is then 
routed through a weir box 27 from which the rate of condensate removal can 
be monitored or measured. From the weir box 27, the aqueous phase or "foul 
condensate" enters a foul condensate receiver 29a. The flow of foul 
condensate from the receiver 29a is controlled by a liquid level control 
31 and moved by pump 25 through the line 32 into the stripping vessel 28. 
The liquid phase leaves the stripping vessel 28 by a drain line 29 and is 
combined with the condensate from the primary condenser 36 at the line 66 
to leave the system through the diverter valve 64. The system thereby 
ultimately discharges all "dirty" foul condensates as "clean" foul 
condensate through the line 66. 
A pump 56 and a temperature control loop 58 are provided for the primary, 
or selective, condenser 36 to maintain the high temperature (212 F.) in 
the condenser to hold selective condensing temperature conditions therein. 
Likewise, a pump 60 and a temperature control loop 62 are provided for the 
primary condenser 22 to hold the temperature conditions therein. 
The unnumbered circles and the circle 63 on the drawing indicate 
conventional temperature indicating controllers. 
The apparatus of the present invention is conveniently operated at 
atmospheric pressure although superatmospheric and subatmospheric 
pressures may be used. 
The art is replete with examples of different kinds of gas/liquid 
contacting devices, condensers, heat exchangers, stripping means and the 
like. Any of such known devices may be used herein. 
There has thus been provided a system and apparatus wherein residual Chd 
2-C.sub.10 hydrocarbons in the condensate are reduced by 80% to 90% of the 
level normally found in conventional kraft paper pulp turpentine recovery 
systems. Turpentine production is found to be increased by as much as 5%. 
Hot water in the form of "clean" foul condensate is suitable for reuse in 
the process, whereas in the past, the "dirty" foul condensate could only 
be sewered. The toxicity level of the effluent stream is greatly reduced. 
Organic contaminants can be removed from streams normally sewered to 
render those streams clean to a level suitable for reuse in the processing 
plant. The hot "clean" foul condensate is cleaned to a level that allows 
that stream to be used as process hot water for brown stock washing, dregs 
wash or geen liquor make-up thereby reducing the demand of the plant for 
fresh water. Little prime steam is required in the new process hereof.