Note: Descriptions are shown in the official language in which they were submitted.
CA 022~6209 1998-12-1~
PROCESS AND DEVICE FOR RECOVERY OF THERMAL ENERGY
FROM CONTAMINATED VAPOR WHILE SIMULTANEOUSLY
SEPARATING VOLATILE ORGANIC SUBSTANCES
Field of the Invention
The present invention generally relates to recovery
of thermal energy from contaminated vapor while simulta-
neously separating volatile noxious organic substances
which would otherwise be present in the vapor condensate.
More specifically, the invention concerns recovery of
thermal energy from vapor generated in thermal treatment
of wood, especially fuel wood, while simultaneously
achieving such a quality of the discharged condensate so
as to allow it to be directly supplied to conventional
installations for water purification without any risk of
damaging these.
Background Art
In thermal treatment of wood, such as fuel wood,
volatile substances, e.g. terpenes, other extractive sub-
stances and, in case of increased temperature and time,
other decomposition products in gas phase escape. This
means that flue gases or exhaust air from drying of wood
contain these substances. If the flue gas or the air is
scrubbed with water for the purpose of cleaning and/or
recovering heat, the substances will be found in the
scrubber water. When drying in vapor atmosphere, the
volatile components of the wood will be found in the
excess vapor from the vapor drier. Especially terpenes
are well-functioning antiseptics, i.e. they aim at pro-
tecting the living wood from decay damage and bacteria.
These substances therefore interfere with the common bio-
logical wastewater purification, and the nitrification
step in modern wastewater treatment is particularly sen-
sitive.
CA 022~6209 1998-12-1~
The vapor emanating from vapor drying of e.g. fuel
wood contains a considerable amount of heat which is
recoverable.
Summary of the Invention
An object of the invention is to recover in one and
the same process heat from contaminated vapor, especially
vapor obtained in vapor drying of fuel wood, at the same
time achieving a sufficient quality of the condensate,
i.e. water, that is obtained in the process to make it
possible for this to be immediately subjected to conven-
tional purification including biological purification.
A further example is evaporation of water in, for
instance, black liquor evaporation in the pulp and paper-
making industry. The process can in this case replace or
supplement the stripping of terpenes whlch is made by the
condensates in a so-called condensate stripper.
The process can also be applied in the evaporation
of wastewater, e.g. consumed oil emulsions in which vola-
tile components such as ammonia, ethanol, methanol, ace-
tic acid and the like together with volatile solventstend to contaminate the condensates obtained in the
evaporation step. In many cases the heat exchanger is
arranged in the evaporator for recovery of heat by so-
called mechanical vapor compression. The vapor driven
off from the wastewater is then compressed in one or
more fans or compressors to a higher pressure. The higher
pressure results in a higher condensation temperature and
allows the vapor to condensate to heat the evaporator.
If the vapor is supplied to the lower part of the heat
exchanger, the volatile contaminants can be made to bleed
off in a concentrated state at the top while the purified
condensate flows out from the bottom.
A further object of the invention is to provide a
device for carrying out the process.
With a view to achieving these objects, the process
and the device have been given the characteristic fea-
tures that appear from the appended claims.
CA 022~6209 1998-12-1~
The heat exchanger used in the inventive process and
included in the inventive device is of a type having one
or more heat exchanger packages comprising, for instance,
tubes or lamellae of a conventional design, and being of
the encased type, i.e. the heat exchanger having an outer
casing or shell. In conventional fashion, the heat
exchanger has an inlet and an outlet for a medium, such
as distant heating water, feed water, which flows through
the heat exchanger to absorb heat. According to a prefer-
red embodiment, the height of the heat exchanger isgreater than its width, and the packages are arranged
concentrically about the vertical axis of the heat
exchanger. When the contaminated vapor is supplied to
the lower part of the heat exchanger, rectification
takes place, i.e. enrichment in the same way as in dis-
tillation. Water vapor condenses when moving upwards
while volatile components are evaporated from the con-
densate which flows downwards. The presence of the vapor
also causes some components to be steam-stripped, i.e.
their volatility increases in the presence of water
vapor. At the top of the heat exchanger there is a gas
phase which is enriched with volatile components, and in
the bottom there is a condensate which is poor in similar
components. From the top, the gas/ vapor mixture can be
condensed in a separate condenser or conducted to combus-
tion. With a view to reinforcing the procedure described
above, there are arranged, according to a preferred embo-
diment, elements which increase the effective contact
surface between gas and liquid, for instance Raschig
rings, in the respective spaces between the outer wall
and the outer boundary surface of the packages. This pro-
cedure contributes essentially to the provision of the
feature according to the invention, viz. the provision
of a condensate of such a purity to allow it to be imme-
diately supplied to a wastewater purification plant.
CA 022~6209 1998-12-1~
According to a preferred embodiment of the inven-
tion, the vapor has, when entering the heat exchanger,
a pressure of 1-5 bar and a temperature of 110-180~C.
Brief Description of the Drawings
The invention will now be described in more detail
with reference to a preferred embodiment while referring
to the accompanying drawing which is a schematic view
of a preferred embodiment of a device according to the
invention.
Description of the Preferred Embodiment of the Invention
Vapor from a fuel drier with a pressure of about
3-4 bar and a temperature of 140-150~C is supplied to the
lower part towards the casing side of the heat exchanger.
At the same time a heat-absorbing medium, in this case
water, is supplied through an inlet (not shown) and is
allowed to flow through the tubes or lamellae in the heat
exchanger packages and is withdrawn through an outlet
(not shown) for use as distant heating water or feed
water. The vapor introduced as described above condenses
on its way upwards on the inner surfaces of the casing
and also on the surfaces of the heat exchanger packages
while more volatile components are evaporated from the
condensate flowing downwards. This condensate accumulates
on the bottom of the heat exchanger and is withdrawn in
the lower part thereof. The condensate is cooled before
being supplied to a conventional municipal wastewater
purification plant. The gas phase enriched in the top of
the heat exchanger and containing volatile components can
either be condensed in a separate condenser as shown in
the drawing, or be conducted to combustion (not shown).
According to the invention, a combination of distil-
lation-condensation-heat exchange has now surprisingly
resulted in the essential advantage that in one and the
same process on the one hand a condensate is obtained
which has such a purity that it can immediately be sup-
plied to a water purification plant and, on the other
hand, heat is recovered from the contaminated vapor. In
CA 022~6209 1998-12-1~
other words, the condensate satisfies the specifications
required for precisely such as plant.
Below follow the results of nitrification-inhibiting
experiments involving nitrifying sludge and test water
carried out according to modified ISO 9509-1989 (E).
Samples
Marking of samples: MG 11331 (condensates withdrawn
at the bottom of the heat exchanger according to the
invention) and MG 11332 (samples withdrawn from "top con-
densates").
The samples were stored in chilled state up to thetime of the analysis.
Materials and Methods
Test Method
The experiment was carried out at room temperature.
The method is simplified since only two sample concen-
trations were examined. The test principle implies brief-
ly that different volumes of test water and a synthetic
medium are mixed in a number of cylinders. The mixtures
are diluted with distilled water to amount to 125 ml.
Then 125 ml of nitrifying sll-dge are added, and the
cylinders are incubated at room temperature for about 4 h
during airing. Samples for analysis of the amount of
nitrite and nitrate are withdrawn from each cylinder both
before and after incubation. The amount of nitrite and
nitrate formed in the cylinders which have contained sam-
ples, is compared with the amount formed in cylinders
which have not contained any sample.
Preparation of Samples
The samples were adjusted in respect of pH to about
7.6 before the experiment.
The following test concentrations (vol%) of the sam-
ple were examined: 20 and 40.
Inoculum
The nitrifying sludge was supplied by Klagshamns
Wastewater Treatment Plant and was aerated until it was
used.
CA 022~6209 1998-12-1
Analysis
After filtration (Whatman GF/A) the nitrite and
nitrate nitrogen content of the filtrate was analyzed
according to Swedish Standard SS 028133-2.
Results
Table 1
Amount of nitrite and nitrate nitrogen formed. Samples
compared with blind sample and inhibitor.
Sample marking Sample (vol%) Oxide N (mg/l) Inhibition (%)
Blind sample 0 29.9*
MG 11331 20 24.7 17
MG 11331 40 17.3** 42
MG 11332 20 0.0 100
MG 11332 40 0.45** 98
Inhib*** - 0 100
* Average of three blind samples
** Average of double samples
*** Reference inhibitor: Allyl thiourea 8 mg/l
The proportion of suspended substance in the experi-
ment was 1.52 g/l.
The specific nitrification rate of the sludge was
measured to be 7.9 mg/N/(g.h). Owing to the high nitrifi-
cation rate of the sludge, the time of the experiment was
shortened to 2.5 h.
The results show that samples marked with MG 11331
exhibited 17% inhibition at a concentration of 20% and
42% inhibition at a concentration of 40%. For municipal
purification a limit of 20% inhibition at a concentration
of 20% is stated, which means that the condensate obtain-
ed according to the invention can be finally purified in
municipal wastewater treatment plants.
On the other hand, samples marked with MG 11332
caused a considerable inhibition of the nitrification in
relation to the tested inoculum. The inhibition was total
(100%) when 20 vol% of sample was added.
CA 022~6209 1998-12-1~
An analysis of the above samples especially in
respect of COD (chemical oxygen demand) and BOD (bio-
chemical oxygen demand) was carried out:
Sample: MG 11331
Property of analysis Result Unit KRUT-code
pH 3.6 PH-25
Conductivity 10.3 mS/m COND-25
Biochemical oxygen 193 mg/l BOD7-NAE
demand, BOD7
Chemical oxygen demand, 400 mg/l CODCR-NH
chromate
Suspended substances 51 mg/l STR-STG
5 Sample: MG 11332
Property of analysis Result Unit KRUT-code
pH 3.9 PH-25
Conductivity 18.1 mS/m COND-25
Biochemical oxygen 3400 mg/l BOD7-NAE
demand, BOD7
Chemical oxygen demand, 13700 mg/l CODCR-NH
chromate
Suspended substances 50 mg/l STR-STG
Acceptable values of condensates for biological
purification are:
COD 1000-5000 mg/l
BOD 800-3000 mg/l
Conclusion:
The bottom condensate satisfies the above criteria.
