Note: Descriptions are shown in the official language in which they were submitted.
wo 93/15816 ~ ~ 3 Q PCT/~P93/00263
A proce~s for the improve~ removal of v~por~ in ~xy~g w~th
superheated ste~m
~. .
This invention relates generally to the drying of
water-containing preparations of useful materials and/or
ballast materials by treatment of the water-containing
material with drying gases based on superheated steam.
In the field of industrial drying processes, the
technology of superheated steam drying in its various forms
has recently been acquiring increasing significance~ The
circulation of the superheated steam used as drying gas in
a closed-loop system and the possibility of directly con-
densing the stream of vapors removed from the circuit
establish favorable conditions for the operation of drying
installations of the type in question with minimal environ-
ment-polluting emissions. In the specialist literature, it
is assumed that superheated steam dryers will attract
increasing attention, particularly when contact drying can
be combined with drying in a pure steam atmosphere. Corre-
sponding superheated steam dryers with integrated contact
heating surfaces are now being industrially used, for
example, in the drying of lignite and sewage sludge and in
the drying of sugar beet chips, biomasses and other organic
products for use in animal feedsl cf. for example D.
Gehrmann "Entwicklungstendenzen der Trocknungstechnik in
der chemischen Industrie (Development Trends of Drying
Technology in the Chemical Industry~", Chem.-Ing. Tech. 62
(1990) A 512 - 520, more particularly subchapters 2.2 and
3.1. Reference is also made there to the various possibil-
ities of designing superheated steam dryers of the type in
question. Thus, drying can be carried out, for example, in
variously designed steam fluidized beds. The drying zone
can also be in the form of a~ superheated steam jet dryer.
Corresponding processes have recently been used in practice
for the production of pulp and other dried natural materi-
2 ~ 3 û O O j !
W0 93/15816 2 PCT/EP93/00263
~ als, such as wood and coal.
However, the possibilities of applying the principle
of superheated steam drying are by no means con~ined to
such comparatively non-sensitive materials. In a number of
earlier patent applications, applicants describe the appli-
cation of this principle to useful materials and mixtures
of useful materials which are known per se for their
temperature sensitivity, but which are only accepted by the
user or consumer in a very high state of refinement, more
particularly in the form of pourable and/or free-flowing
powders or granules. Thus, in their earlier application
DE-A 40 30 688, applicants describe a process for the
production of fine-particle, solid, pourable or free-
flowing useful materials or mixtures of useful materials
for wetting agents, detergents and/or cleaning products
from aqueous preparations thereof, in which superheated
steam is used as the drying hot gas stream and drying of
the particulate material is terminated before it is en-
dangered by heat.
In a number of other earlier applications, applicants
describe particular embodiments and improvements of such
drying processes using superheated steam as the hot gas
stream and their application to useful materials and
mixtures of useful materials for wetting agents, detergents
and/or cleaning products, cf. in particular earlier German
patent applications P 42 04 035.3, P 42 04 090.6, P 42 06
050.8, P 42 06 521.6 and P 42 06 495.3.
The problem addressed by the present invention was
further to develop industrial-scale superheated steam
drying processes in the manner described in detail herein-
after and, at the same time, to optimize the operating
possibilities inherent in such superheated steam drying
systems. The teaching according to the invention seeks in
this regard to solve the following problem: drying with
superheated steam in the drying installation comprises
; 1 3 ~
Wo 93/15816 3 PCT/EP93/00263
circulation of the superheated steam through the drying
unit(s) and subsequent separation of the stream of vapors
corresponding to the amount of water evaporated in the
drying process. The stream of vapors removed from the
circuiating steam is laden with entrained useful materiais
and/or ballast materials, for example with powder-form use-
ful materials and/or ballast materials or with correspond-
ing materials more or less highly volatile in superheated
steam. Accordingly, these entrained useful materials
and/or ballast materials have to be removed as far as
possible from the circulating steam, normally before
removal of the stream of vapors. This may be done by
treatment with filters, although gravity separation tech-
niques, more particularly using cyclone separator~s, are
often applied. The entrained solids can never be complete-
ly removed. Those fractions which are volatile in the
stream of superheated steam under the working conditions
cannot be removed in any case. The stream of vapors
ultimately removed from the steam circuit is laden with
residues of entrained useful and/or ballast materials.
During the drying process, non-condensible gas constituents
optionally accumulating in small quantities are also
discharged from the system by the continuously removed
vapor stream.
Accordingly, the optimization of steam drying proces-
ses or other processes involving the use of a superheated
steam treatment stage, for example corresponding agglomera-
tion processes, presuppose a solution to the problem of
working up the steam removed in following process steps in
such a way that the materials entrained by the steam are
either returned to the drying process or at least can be
disposed of without harming the environment.
The problem addressed by the teaching according to the
present invention was to enable substantial improvements to
be obtained in this regard. More particularly, the teach-
WO 93/15816 4 213 Q ~ ~ ~ PcT/~p93/oo263
ing according to the invention seeks to enable the stream
of vapors removed to be worked up substantially free from
emissions, i.e. from waste gases and wastewater, at no
significant extra cost. Accordingly, the invention seeks
S to enable a vapor condensate of comparatively high purity
to be made available without substantial energy losses in
an aftertreatment process comprising one or more stages.
The technical solution to the problem addressed by the
present invention is based on the subsequent thermal puri-
fication of the vapor condensate initially accumulating,
the additional problem of optimizing the energy consumption
of this aftertreatment being solved by the circulation of
product streams or partial streams described hereinafter.
Subject of the inventio~
In a first embodiment, therefore, the present inven~
tion relates to a process for optimizing disposal of the
stream of vapors laden with entrained useful and/or ballast
materials which is removed as a steam product stream from
drying installations operated with superheated steam as the
drying gas and is then at least partly condensed by direct
and/or indirect heat exchange with a cooling medium.
In this embodiment, the process according to the
invention is characterized in that the vapor condensate
accumulating is subjected to an at least one-stage thermal
post-purification, vapor condensate obtained beforehand
being used as the cooling medium for condensation of the
laden vapor stream removed from the drying installation and
. then being subjected to thermal post-purification.together
with the heat of condensation taken up by the vapor stream
and the laden vapor condensate being delivered as aqueous
liquid phase to the thermal purification stage - herein-
after also referred to as the "distillation stage" - and
being partly evaporated again by the heat of condensation
transferred while a liquid stream with a high concentration
WO 93/15816 5 ~ -3 ~PCT/EP93/00263
of entrained useful and/~r ballast materials is removed
from the sump phase of the distillation stage.
In other embodiments, the invention relates to the
application of this process for the recovery of dried
useful mater als, mixtures of useful materials and/~r
ballast materials from water-containing preparations
thereof in the virtual absence of waste gases and waste-
water by treatment, more particularly drying, with super-
heated steam. The application of this process can be of
particular significance for the optimized emissionless
drying of useful materials and mixtures of useful rnaterials
which are suitable as and/or for use in wetting agents,
detergents and/or cleaning products. However, another
important embodiment of the teaching according to the
invention lies in its application to moist or water-con-
taining starting materials of which the treatment of drying
is known to involve considerable emission problems. One
example of this ~application of the teach ng according to
the invention is the drying of sewage sludge or animal
feces, such as liquid manure, which can lead to serious
odor emissions, as for example in the drying of anaerobic
sewage sludgesO
Another important application of the teaching accord-
ing to the invention is the treatment of toxic materials,
for example by drying and/or agglomeration using super-
heated steam. Reference is made purely by way of example
in this regard to highly toxic mixtures of useful materi-
als, such as pesticides or herbicides, of which the drying
and/or agglomeration can present considerable difficulties
from the point of view of unwanted emissions. If gas
phases are used as auxiliaries in cases such as these, for
example in the production of dry preparations (powders
and/or agglomerates), the entrainment of toxic useful
materials and their elimination from the gas phases used is
a serious technical problem. However, the elimination of
W0 93/15816 6 2.1~ PCT/~P93/00263
toxic or otherwise unwanted ballast materials from corre-
spondingly laden gas streams still presents considerable
problems in practice. The invention assists in solving
these various problems through its broad range of applica-
tions.
~articulars of the teaching according to the invention
The essence of the teaching according to the inventionlies in the following combination of elements:
The stream of vapors laden with entrained useful
andjor ballast materials which is removed from the super-
heated steam drying installation and which corresponds to
the water evaporated in the preceding drying installation
is now repurified thermally, more particularly by distilla-
tion, in an at least single-stage aftertreatment. This
additional purification step should have little or no
effect on the energy balance of the process as a whole.
In one preferred embodiment of the invention, this
problem is solved by the following process elements: the
vapor stream removed from the steam drying installation,
typically at temperatures above 100C, is first condensed
to form a continuous liquid phase. In a preferred embodi-
ment of the invention, virtually the entire vapor phase is
actually converted into the liquid vapor condensate. This
ensures that the entrained useful and/or ballast materials
- providing they are not gaseous at the process temperature
- are taken up by the lisuid phase of the vapor condensate
and are delivered in this form to the subsequent distilla-
tion-based purification stage.
Condensation of the vapor phase removed from the
drying circuit to form the liquid vapor condensate may be i~
carried out in a single stage or, in one important embodi-
ment, even in several stages. In the latter case, the
complete condensation of the vapor stream removed, as
ultimately required, is completed in successive stages, for
- - 2 1 ~
wo 93/1s8l6 7 PCT~E~93J002~3
example in 2 to 5 and preferably 2 to 3 condensation
stages. In the preferred embodiment of the invention, the
vapor condensates obtained in each stage are also subjected
to the purification step described in detail hereinafter.
Paxtial condensation of the vapor stream removed from the
drying circuit affords technical advantages, for example,
when the nature of the useful and/or ballast materials
entrained in the vapor stream enables the entrained mixture
to be separated up by the successive condensation stages so
that the various individual components recovered are easier
to re-use or to put to further uses. In the interests of
simplicity, however, the following description of the
invention is essentially based on the single-stage conden-
sation of the vapor stream removed and on the single-stage
or multi-stage purification of the vapor condensate ob-
tained. However, the following description of the inven-
tion applies equally to the individual stages involved in
the multi-stage partial condensation of the vapor stream to
be worked up.
Entrained components of the vapor stream removed from
the superheated steam drying installation which are gaseous
at the process temperature and which do not pass into the
liquid phase during condensation can be separated from the
vapor condensate and hence from its further treatment by
simple phase separation and disposed of in an ecologically
safe manner, as will be described in more detail herein-
after. The need to remove a gas phase in this first stage
of the condensation of the vapor stream generally arises in
special cases only, if at all. When it does arise, com-
paratively very limited quantities of a separated gas phase
are involved and can be safely disposed of as required
without significant technological difficulties.
The vapor condensate stream accumulating is trans-
ferred as a liquid phase to the concentration or distilla-
tion stage following the superheated steam drying stage.
:
21~10~
WO 93/15~16 ~ PCT/EP93/00263
In one important embodiment of the invention, however, the
following circuit is established or maintained with part of
this vapor condensate which is best removed from the sump
of the distillation stage:
The vapor condensate removed is delivered to an
indirect heat exchanger for condensation of the vapor
stream removed from the superheated steam drying stager In
this indirect heat exchanger, it takes up the evaporation
or condensation energy of the vapor stream delivered to the
heat exchanger through indirect heat exchange. This
results in the desired condensation of the vapor stream
removed from the drying installation. The condensate is
directly transferred to the heat treatment stage. After
leaving the indirect heat exchanger, the vapor condensate
lS used to condense the vapor stream and removed beforehand
from the sump of the distillation column is delivered as
carrier together with the condensation energy taken up to
the thermal treatment stage and, more particularly, is
returned to the distillation stage. In this way, the
energy released in the indirect heat exchanger is indirect-
ly or directly delivered to the distillation stage where
it can be used for evaporation of the vapor condensate
stream and hence for the distillation-based post-purifica-
tion stage of the process according to the invention.
In the particularly preferred embodiment of the
process according to the invention, therefore, the vapor
condensate is partly removed from the sump of the distilla-
tion stage, passed through the indirect heat exchanger to
take up the heat of condensation of the stream of vapors,
subsequently returned to the distillation stage and direct-
ly mixed therein with the liquid vapor condensate (sump).
However, the superheater or heat exchanger may al50 be
integrated in the evaporation/distillation stage as known
per se. Accordingly, by suitably controlling the process
parameters in known manner, the condensation or evaporation
WO 93/1581~ 9 213 0 ~ O ~i PCT/EP93/00263
energy can be directly transferred from the indirect heat
exchanger to the following distillation stage. It can be
useful in this regard, as known per se, to pass the vapor
condensate stream through the indirect heat exchanger under
elevated pressures, preferably under such elevated pres-
sures that the circulated vapor condensate stream is
present at least predominantly as a liquid phase, even
after leaving the exchanger.
In continuous operation, the quantity of vapor conden-
sate (sump) present in liquid phase which is required forthe process according to the invention is comparatively
limited. On the one hand, it is determined by the circula-
tion of the vapor condensate from the distillation stage
through the indirect heat exchanger with subsequent conver-
sion of the condensation or evaporation energy back to thedistillation column.
Accordingly, in the partisularly preferred embodiment
of the process according to the invention, a stream of the
vapor condensate is run off from the bottom of the distil-
lation column, passed through the indirect heat exchangerto take up the heat of condensation of the vapor stream
delivered in vaporous form, subsequently returned to the
distillation column and directly mixed therein with the
liquid vapor condensate (the bottom fraction). However,
the superheater or heat exchanger may also be integrated
into the evaporation/distillation unit, as known per se.
In this way, the condensation or evaporation energy can be
directly transferred from the indirect heat exchanger to
the following distillation column by suitable control of
the process parameters, as known to the expert. To this
end, it can be useful, as known per se, to pass the vapor
condensate stream through the indirect heat exchanger under
elevated pressures. The pressures applied may advantage-
ously be elevated to such a degree that, even after leaving
the heat exchanger, the circulated vapor condensate stream
wo 93~1581C 10 213 ~ PCT~EP93/00263
is at least predominantly present as a liquid phase.
In continuous operation, the quantity of vapor conden-
sate (bottom fraction) present in liquid phase which is
required for the process according to the invention is
comparatively limited. It is determined on the one hand by
the circulation of the vapor condensate from the distilla-
tion column through the indirect heat exchanger with the
subsequent return of the condensation or evaporation energy
to the distillation column.
In addition, the object of the post-purification step
according to the invention has to be borne in mind. All
the non-gaseous useful and/or ballast materials entrained
accumulate in the liquid vapor condensate. In this form,
they may be removed as a partial stream from the subsequent
purification cycle according to the invention. According
to the invention, therefore, a partial stream in which the
materials entrained from the superheated steam drying
installation are present in an increased concentration is
removed in batches or preferably continuously from the
liquid sump of the distillation column. This partial
stream may be delivered directly or indirectly to the
superheated steam drying installation. The direct re-
~ycling of this concentrate does not require any further
explanation. One example of indirect recycling is de-
scribed in the following: if the useful materials dried inthe superheated steam drying installation are worked up
beforehand by selective conditioning with aqueous phases
into an optimal quality for use in the superheated steam
; drying installation, the concentrate from the sump of the
following distillation stage containing entrained useful
materials may first be used in the preparatory conditioning
of the useful material or mixture of useful materials to be
dried in the superheated steam drying installation.
It has been found that effective separation between
the vapors to be purified and the entrained useful or
WO 93/15816 11 213 D O ~ `1 PCT/EP93/00263
ballast materials can be achieved by a simple one-stage
thermal aftertreatment in accordance with the teaching of
the invention. So far as this possibility is concerned,
the following situation in particular also has to be taken
into account: the operating conditions in the superheated
steam dxying installation are determined primarily by the
desired result of the drying step. Accordingly, the vapor
stream removed can be under working conditions of pressure
and, more particularly, temperature which lead to a sub-
stantial entrainment of useful and/or ballast materialswith the steam removed. By contrast, the working condi-
tions in the subsequent distillation-based purification
stage of the process according to the invention no longer
have to be geared to the desired drying result of the
superheated steam drying installation, instead the selected
working condit~ons are determined by the desired optimal
separation between steam vapors and entrained useful and/or
ballast materials. It can immediately be seen that this
subsequent process step affords completely new possibil-
ities for improved system separation. The teaching accord-
ing to the invention not onl~ utilizes this possibility,
the described sequence and combination of process steps
also provides selective access to the modified working
conditions from utilization of the energy balance of the
system as a whole without significant losses of energy.
The vapor phase removed from the evaporation or dis-
tillation stage may be subsequently recondensed by prefer-
ably indirect heat exchange and - presupposing adequate
purification - may optionally be put to another use. The
indirect heat exchange in this second condensation stage
ensures that transfer of the condensation energy precludes
the unwanted entrainment of any useful and/or ballast
materials. Small quantities of a waste gas phase can also
be removed in this second condensation stage,-depending on
the quality of the laden vapor stream used. The quantities
WO 93/15816 12 1 ~ ~ Q ~ PCT/EP93/00263
in which such a waste gas phase occurs are so small that
they can be safely disposed of without any technological
difficulties.
One reliable method of disposal is, for example,
combustion because the gas phases in question - for exampie
in the drying of useful materials or mixtures thereof in
the field of detergents - do of course gen~erally contain
non-condensible fractions of low~boiling constituents of
which the combustion is possible or even desirable. The
possibilities afforded by the process according to the
invention in this regard for optimizing the process as a
whole are clearly apparent from this example of disposal of
the non-condensible residual gas phase. In one preferred
embodiment, the non-condensible gaseous fractions ultimate-
ly removed are burnt in admixture with the fuel gaseswhich, in the preceding drying circuit, keep the circulated
stream of superheated steam at the required operating
temperature. The gas phase removed in the working up of
the vapor condensate may be premixed, for example, with the
air required for combustion and preheated with the waste
gases from the burnerf for example to 80 - 180~C, to reheat
the circulated stream of superheated steam. In this form,
the mixture of combustion air and non-condensible fractions
from the working up of the vapor condensate may be optimal-
ly utilized in the process as a whole.
Evaporation of the vapor condensate in the distilla-
tion-based purification stage can be carried out in a
single step by simple carryover. The simple design of the
distillation column which this involves can always be
useful when the laden vapor stream can be sufficiently
purified by the one-stage treatment with removal of the
entrained useful and/or ballast materials. In cases where
removal of the laden vapor condPnsate has to meet stringent
requirements, several possibilities are available for the
purification of the liquid stream in accordance with the
~ 1 3 ~
WO 93/15816 13 PCT/EP93/0026~
invention and may also be applied in combination with one
another.
In a first corresponding embodiment of the present
invention, purification by distillation is carried out by
using correspondin~ packed columns. ~ractionation columns
of the type in question can be designed, laid out and
operated on the basis of general specialist knowledge to
which reference is hereby specifically made. The optimal
rectification temperature is adapted to the particular
problem to be solved.
In another embodiment of the invention for improved
purification of the laden vapor condensate, which may also
be combined with the fractional distillation embodiment
just described, a plurality of distillation stages is used.
In a further embodiment, an evaporator comprising several
staqes, for example up to 5 stages and preferably 2 or 3
- stages, is used. In every case, any excess heat accumula-
ting may be recovered in the form of hot water.
In one preferred variant of the embodiment of the
invention where purification is carried out in a succession
of several separate evaporation or distillation units, the
plurality of purification steps is controlled almost
completely from the energy balance of the laden vapor
stream removed from the superheated steam drying installa-
tion~ To this end, the heat of condensation from thecondensation of the vapor phase of a preceding process step
is introduced in accordance with the invention into the
sump of the condensate to be evaporated in the following
distillation step by preferably indirect heat exchange. In
this case, too, a partial stream may again be used as
carrier for transferring this heat of condensation to the
following distillation stage. The procedure adopted in
this regard is the same as described at the beginning for
the single-stage distillation process. Accordingly, the
liguid stream containing the heat of condensation which is
2 1 3 Q ~
Wo 93/15816 14 PCT/EP93/00263
removed from the indirect heat exchanger is directly
introduced into the following distillation stage - best
under an appropriate excess pressure - and direc~ly mixed
therein with the condensate str am removed from the heat
ex~hanger. In this way, the condensate is re-evaporated in
the following process stage and may be accordingly divided
into a further purified vapor phase and a sump phase con-
taining useful and/or ballast materials in high concentra-
tions.
In cases where a plurality of successive-evaporation
or distillation stages are used, concentrated useful and/or
ballast material can be removed from the sump of each in-
dividual distillation stage or even from a selected rela-
tively small number of the distillation stage or even from
only a single distillation stage. In the last of these
three cases, the laden sump stream is generally removed
from the last distillation stage. The particular vapor
phases of the individual distillation stages may be com-
pletely transferred as condensate to the following distil-
lation stage. Equally, however, the condensed vapor phasefrom a preceding distillation stage may only be partly
transferred to the following distillation stage(s). The
particular procedure adopted is determined by the quality
of the laden vapor streams to be purified, by the quality
of the entrained useful and/or ballast materials and by the
particular disposal possibilities arising therefrom.
Where several successive distillation stages are used,
it can be useful to apply different pressures in the
individual stages. In a preferred embodiment, the working
pressure is reduced in stages from a preliminary distilla-
tion stage to a following distillation stage. According to
the invention, it is possible in this regard for distilla-
tion at normal pressure to be followed by vacuum distilla-
tion of the condensate from the first purification stage
under comparatively low pressures. However, in cases where
- .9 ~ {~
WO 93/15~16 15 PCT/EP93/00263
the working pressure is reduced in stages in the successive
distillation stages, the reductions in pressure between the
individual stages are generally comparatively small. Thus,
where this procedure is adopted, the working pressure in a
following distillation stage is only reduced to such an
extent that the boiling temperature of the water is lowered
by at most 20 to 30C, preferably by no more than 20C and,
more preferably, by no more than 10C. Comparatively small
reductions in the boiling temperatures in the following
distillation stage, which may be up to about 5C below the
boiling temperature of water in the preceding distillation
stage, can be effectively used for the teaching accordi~g
to the invention.
The measure of reducing the working pressure to a
limited extent simplifies the overall technology of the
process, promotes the energy balance of the purification
process as a whole and thus enables the working result to
be optimized by achieving the desired purification of the
laden vapor stream under financially and economically
acceptable conditions.
The teaching according to the invention specifically
includes the direct introduction of the vapors into the
condensate in one or more vapor purification stages in
accordance with the state of the art using evaporators,
although indirect heat exchange may be preferable.
In general, the energy content of the vapor from the
last distillation stage may in turn be used for indirect
heat exchange, for example to heat process water, and hence
is not lost to the process as a whole. Depending on purity
and requirements, the purified condensate streams may be
used as industrial water - for example as rinsing water for
cleaning industrial installations - or~ if desired, may
even be simply disposed of as at least substantially
ecologically safe wastewater.
It can clearly be seen that the purification process
: 2l3000~`
WO 93/15B16 16 PCT/~P93/00263
according to the invention is suitable for laden vapor
streams of any origin. Accordingly, the teaching according
to the invention is not only suitable as an addition to
spray dryers or fluidized bed dryers operated with super-
heated steam, other types of process, for example granula-
tion, more particularly pelletizing, using superheated
steam, correspondingly operated thin-layer evaporators,
more particularly falling-film evaporators with or without
forced circulation of the material to be dried or evapora-
ted in a thin layer, are also suitable intermediate stagesfor the process according to the invention. In any com-
bination, the teaching according to the invention is a
useful addition to the preliminary superheated steam stage:
circulation of the superheated steam itself considerably
simplifies the problems posed by waste gases by comparison
with the hitherto predominant processes where the gas phase
is circulated only partly, if at all. The teaching accord-
ing to the invention of working up the laden vapor stream
removed now also provides effectively - and for the first
time - for substantially pollution-free disposal. Any
small quantities of waste gases occurring can be safely
handled in the described process stages and can be safely
disposed of without harming the environment, for example by
selective thermal treatment, more particularly selective
combustion, by treatment in biofilters and the like~
Problems associated with the disposal of wastewaters are
eliminated. The teaching according to the invention thus
extends to the entire field of concentration, drying,
pulverization and/or granulation of useful and/or ballast
materials of any origin. Accordingly, the drying and/or
granulation of useful materials and mixtures of useful
materials from the field of wetting agents, detergents
and/or cleaning products as mentioned at the beginning on
the one hand and the drying of ballast accumulating in
large quantities, such as sewage sludges from communal
,~" 213()31'
WO 93/15816 17 PCT/EP93/00263
and/or industrial wastewater treatment plants are merely
intended to serve as examples of the scope of application
of the teaching according to the invention. They are
nevertheless two ch~racteristic examples which illustrate
the practical and technological significance of the teach-
ing according to the invention.
Applicants' DE-A 40 30 688 and their earlier German
patent applications P 42 04 035.3, P 42 04 090.6, P 42 06
050.8, P 42 0~ 521.6 and P 42 06 495.3 describe important
particulars of the drying of useful materials for deter-
gents and cleaning products with superheated steam. The
disclosures of these documents are hereby spec~fically
included as part of the disclosure of the present inven-
tion. One of the problems which seriously affects the
drying of useful materials or mixtures of useful materials
on an industrial scale is so-called pluming, i.e. the
carryover of, in particular, nonionic surfactant components
through their volatility in steam. In the process accord-
ing to the invention, nonionic surfactant components
carried over primarily by pluming are reliably removed by
distillation-based post-purification in one or more stages
in acçordance with the teaching of the invention.
The drying of sewage sludge is an example of another
problem area where an improved solution can be provided by
the teaching according to the invention. More particular-
ly~ the sewage sludges of anaerobic origin which now
accumulate in large quantities have hitherto led to almost
uncontrollable odor emissions during dryingO Only recently
, have any attempts been made to dry sewage sludge with
superheated steam and vapor circulation. Nevertheless,
problems are still involved in working up the laden vapor
stream removed which corresponds to the quantity of water
evaporated and which has to be disposed of without any
danger to the environment. The teaching according to the
invention provides in the described manner for the -
2 1 3 0 ~
WO 93/15816 18 PC~/EP93/00263
optionally repeated - removal of non-condensible and
particularly strong-smelling gas phases and their selective
destruction, for example by thermal treatment. At the same
time, the odor intensity of the vapor phase optionally
condensed in several stages can be reduced to such an
extent that no problems are involved in its conventional
disposal. As explained in detail, the purification process
in question can be carried out with hardly any additional
input of outside energy into the purification stage(s)
following the steam drying stage in accordance with the
invention.
Figures 1 and 2 illustrate exemplary flow charts of
embodiments of the vapor purification process according to
the invention. Figure 1 shows an experimental plant while
Fig. 2 shows a two-stage evaporation plant for use on an
industrial scale. In both cases, vapor pipes are represen-
ted by thick lin~s and water pipes by thin lines.
As shown in Fig. 1, the laden vapor stream from a
superheated steam drying installation is delivered by a
pump la through a pipe 1 to an indirect heat exchanger 2
where it is completely condensed to form a liquid phase.
Any small amounts of non-condensible gases and high-boiling
fractions accumulating may be removed, for example, through
the pipe 14 and suitably disposed of. More particularly,
this gas may be mixed with the feed air for the burner (not
shown) of the steam drying plant so that any impurities
present in the gas are also burnt. The liquid stream
removed from the heat exchanger 2 is pumped by the pump 3
~through the pipe 4 to the distillation stage 5. F~om the
bottom of the distillation stage 5, bottom condensate is
pumped by the pump 6 through the circulation pipe 7 into
the heat exchanger 2~ This condensate stream takes up the
condensation energy of the vapor stream delivered through
the pipe 1 and evaporates. The correspondingly heated
liquid stream is returned to the distillation unit 5 from
~ 213~0~
W0 93/~5816 19 PCT/EP93/00263
the bottom thereof through the pipe 8 and enters into
direct heat exchange with the vapor condensate delivered
through the pipe 4.
The bottom concentrate from 5 is partly removed
through the pipe 9 and may be returned directly or ,n~
directly to the steam drying installation.
The steam produced in the distillation unit 5 is
removed through the pipe 10, condensed in the indirect heat
exchanger 11 and used, for example, to produce hot water
which is returned through the pipe 13 to the indirect heat
exchanger 11 and again removed therefrom. The steam
condensate removed from the heat exchanger 11 through the
pipe 15 may be reused as industrial water or disposed of as
wastewater. Any non-condensible gaseous components from
the vapor stream leaving the distillation unit 5 may be
delivered through the pipe 12 and optionally via a pump 12a
to the pipe 14 and thence to a waste gas disposal facility.
Figure 2 illustrates by way of example the working up
of the laden stream in two stages by the process according
to the invention. The plant illustrated is particularly
suitable for use on an industrial scale.
The laden vapor stream is delivered to the indirect
heat exchanger 17 through the pipe 16 by the pump 16a and
leaves the heat exchangPr as condensate. The vapor conden-
sate is pumped by the pump 18 through the pipe 19 into thefirst distillation stage 20. From the bottom of this
distillation stage, the liquid stream is evaporated in the
heat exchanger 17, to which it is delivered by the pump 21
through the pipe 22, and returned to the distillation stage
20 through the pipe 20.
The vapor condensate introduced into the first distil-
lation stage 20 through the pipe 19 is partly re-evaporated
under the effect of the condensation or evaporation energy
delivered through the pipe 23 by direct mixing of the
product streams from 19 and 23. The vapor leaves the first
2 1 ~ O Q D s
WO 93/15816 20 PCT/~P93/002C3
distillation stage through the pipe 24 and the pump 24a and
is condensed to liquid in the indirect heat exchanger 25.
The condensate may be transported onwards in various ways
through the pipe 26. Depending on its purity, it may be
used as industrial water or may even be disposed c r as
wastewater. However, it is also possible - although not
specifically shown in the drawing - to deliver this conden-
sate component to the second distillation stage 28.
The non-evaporated part of the bottom fraction from
the distillation stage 20 which is not required for
circulation is delivered through the pipe 27 to the second
distillation stage 28 where it enters into direct heat
exchange with a heated second recycle stream introduced by
removal of part of the sump from 28 by means of the pump 29
lS and circulation through the pipe 30, the indirect heat
exchanger 35 and the return pipe 31 to the distil~ation
stage 28. The vapor phase formed in the second distilla-
tion stage leaves the installation through 33 and may be
used, for example, to produce hot water (inlet and outlet
pipe 36~ via the indirect heat exchanger 34. The conden-
sate produced leaves the indirect heat exchanger 34 through
38 and may be put to various uses as industrial water
and/or as wastewater. Liquid phase components from the
bottom of the second distillation stage 28 containing
useful and/or ballast materials in high concentrations are
removed from the circuit through the pipe 30 and are
directly or indirectly returned to the superheated steam
drying stage.
~ In addition, Fig. 2 illustrates the possibility of
reducing the working pressure of the second distillation
stage in relation to the working pressure of the first
distillation stage by means of the pump 37.
The following Examples describe specific operating
parameters and results of the teaching according to the
invention with reference to Figure 1.
-~ 2 1 3 i~ 1) 0 `~ I
WO 93/15816 21 PCT/EP93/00263
E x a ~ p 1 ~ 8
Ex~mple 1
Using a Niro-Atomizer pilot-plant-scale experimental
spray drying tower, a powder was produced from a detergent
slurry in superheated steam. The slurry, which had a water
content of 50%, was introduced into the tower in a quantity
of 20.8 kg/h, corresponding to a volumetric flow rate of 16
l/h. The steam used as the drying medium was circulated
1~ and only that quantity of water removed from the product
was taken out of the circuit and processed in the following
step.
To this end, condensate which has accumulated from the
vapor stream removed from the circuit is concentratbd in a
forced circulation evaporator with an internal heat exchan-
ger, the evaporator being heated by the steam removed from
the drying circuit. The condensate issuing from the heat
exchanger was delivered to the evaporator as feed. The
superheated steam from the drying circuit entered the heat
exchanger of the evaporator during the test at a tempera-
ture of 179C. In establishing the return flow of concen-
trate to the heat exchanger, it is important to ensure
that, on the one hand, the volume flowing back guarantees
the condensation of the steam entering the evaporator and,
on the other hand, superheating of the concentrate flowing
~ack remains guaranteed. The procedure adopted to this end
in the test was to adjust the return flow in such a way
that complete condensation of the inflowing steam was just
achieved on the heat exchanger.
Hot water at 81C (entry temperature: 15C) was
produced by condensation of the vapor stream removed from
the evaporator through indirect heat exchange, the flow of
hot water amounting to approx. 74 l/h. The condensate
leaving the heat exchanger had a temperature of 62C for an
output of approx. 9.6 kg/h.
o a ~ ~
WO 93/15816 22 PCT/~P93/00263
Behind the condenser, there was a small flow of gas
consisting of air taken in from outside and uncondensed
components of the steam. This gas stream was not quantita- -
tively determined.
The flow of concentrate produced in the evaporator and
removed therefrom amounted to approx. 0.2 kg/h.
~.
Example 2
In another test carried out in a Niro-Atomizer spray
dryer of the "minor production'a type, detergent slurry was
dried by spraying in superheated steam. During the test,
the flow of slurry (50% water content) was 32.5 kg/h,
corresponding to a volumetric flow rate of 25 l/h. The
steam used as drying medium was circulated, the ~excess
steam being removed from the circuit~
The steam removed from the circuit was used to feed
the heat exchanger of the following evaporator and the
condensate issuing from the heat exchanger was subsequently
delivered to the evaporator as feed. The temperature at
which the steam entered the heat exchanger during the test
was 168C.
The vapor stream collecting in the evaporator was used
in a condenser to produce hot water at 79C ~temperature of
the cold water: 15C). Approx. 109.8 l/h hot water were
produced during the test. The temperature of the conden-
sate, as measured at the exit, was 66C, the flow of
condensate amounting to approx. 13.9 kg/h. In this test,
too, a gas stream was detected behind the condenser, but
, was not determined;
During the test, concentrate was removed from the
evaporator at a rate of approx. 0.4 kg/h.
