CA2130005A1 - A process for the improved removal of vapors in drying with superheated steam - Google Patents

Abstract

The partial current of exhaust vapours loaded with entrained valuable and/or inert substances is transferred as a steam-product current from drying plants operating with overheated steam as the drying gas, then is at least partially condensed by indirect heat exchange with a cooling medium. The exhaust vapour condensate is subsequently cleaned by at least one distillation step. By recirculating a partial current from the liquid phase of distillation plant (5), exhaust vapour condensate is used for absorbing the condensation energy of the gaseous current of exhaust vapours in an indirect heat exchanger (2). The exhaust vapour condensate thus produced is supplied to the distillation plant (5), where it is brought into a direct energy exchange with the recycled partial current of exhaust vapour condensate drawn from the liquid phase. A
partial liquid stream having a higher concentration of entrained valuable and/or inert substances can thus be drawn from the liquid phase of the distillation plant (5). The vapour phase from the distillator (5) is further treated in an appropriate manner.

Description

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 !

~ 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' 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.

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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.

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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.
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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.

Claims (15)

1. 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, 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 conden-sation taken up by the vapor stream and the laden vapor condensate being delivered as aqueous liquid phase to the thermal purification stage (distillation stage) and being partly evaporated again by the heat of condensation trans-ferred while a liquid stream with a high concentration of entrained useful and/or ballast materials is removed from the sump phase of the distillation stage.

2. A process as claimed in claim 1, characterized in that the laden vapor stream is condensed in several stages and the vapor condensate obtained in each stage is subjected to thermal post-purification.

3. A process as claimed in claims 1 and 2, characterized in that the bottom of the distillation stage is superheated by direct or indirect heat exchange with the heat of condensation of the vapor stream introduced.

4. A process as claimed in claims 1 to 3, characterized in that, to condense the laden vapor stream, an indirect heat exchanger is integrated in the bottom of the distilla-tion stage or, preferably, part of the vapor condensate is removed from the bottom of the distillation stage, passed through an indirect heat exchanger to take up the heat of condensation of the vapor stream introduced, subsequently returned to the distillation stage and directly mixed therein with the liquid vapor condensate.

5. A process as claimed in claims 1 to 4, characterized in that the heat of condensation is transferred to the distillation stage by the fluid carrier medium under ele-vated pressure, preferably under such elevated pressures that the circulated stream of vapor condensate (bottom fraction) is at least predominantly present as liquid phase even after leaving the heat exchanger.

6. A process as claimed in claims 1 to 5, characterized in that the vapor phase removed from the distillation stage is recondensed by preferably indirect heat exchange and, if desired, is subsequently put to another use.

7. A process as claimed in claims 1 to 6, characterized in that any non-condensible gas phases accumulating in the individual condensation stages are safely disposed of, more particularly by burning with the fuel gases for producing the superheated steam to operate the drying plant.

8. A process as claimed in claims 1 to 7, characterized in that the liquid stream enriched with useful and/or ballast materials and removed from the bottom of the distillation stage is returned directly or indirectly to the steam drying stage.

9. A process as claimed in claims 1 to 8, characterized in that several, preferably 2 or 3, distillation stages are used for the laden vapor condensate, the heat of condensa-tion from the condensation of the vapor phase in the preceding distillation stage being transferred to the following distillation stage by heat exchange.

10. A process as claimed in claims 1 to 9, characterized in that pressures reduced in stages are applied in the successive distillation stages.

11. A process as claimed in claims 1 to 10, characterized in that the first distillation stage is operated at normal pressure and a second distillation stage is operated at moderately reduced pressure, preferably with a reduction in the boiling temperature of the water of no more than 20°C
and preferably no more than 10°C.

12. A process as claimed in claims 1 to 11, characterized in that evaporation of the vapor condensate in the distil-lation-based purification is carried out in a single stage and/or as fractional multistage distillation.

13. A process as claimed in claims 1 to 12, characterized in that the vapor stream to be disposed of emanates from the drying with superheated steam of useful materials or mixtures of useful materials which are suitable for use as and/or in wetting agents, detergents and/or cleaning products.

14. A process as claimed in claims 1 to 13, characterized in that the vapor stream to be disposed of emanates from the drying with superheated steam of an environment-pollut-ing water-containing material, for example from the drying and/or agglomeration of toxic materials, such as pesti-cides, herbicides and other toxic materials and/or waste solid at room temperature or from the drying of strong-smelling materials, such as sewage sludges, more particu-larly anaerobic sewage sludges, or animal feces.

15. The use of the process claimed in claims 1 to 14 for the recovery of dried useful materials and mixtures of useful materials, which are suitable for use as and/or in wetting agents, detergents and/or cleaning products, from water-containing preparations thereof in the substantial absence of waste gases and polluted wastewaters by drying with superheated steam.