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oxygen steam generators for sterilization processes and chemical engineering
~. J. Hydroyen Energy, Vol. 18, No. 11, pp. 945 951, 1993. inted in Great Britain.
0360-3199/93 $6.00 + 0.00 Pergamon Press Ltd. International Association for Hydrogen Energy.
H Y D R O G E N / O X Y G E N S T E A M G E N E R A T O R S FOR S T E R I L I Z A T I O N PROCESSES AND CHEMICAL ENGINEERING H. J. STERNFELD* and M. P A U L U S " *DLR, The German Aerospace Research Establishment, D-7101 Lampoldshausen, Germany tFhG, Fraunhofer Gesellschaft, D-8000 Munich, Germany
(Received for publication 17 February 1993) Abstract--The working principles of the HYDRONIC:~ steam generator for producing sterilization steam are described. The steam generator is based on the combustion of hydrogen and oxygen. Results of intensive bacteriological investigations which analyze the quality of sterilization of bioreactors by direct steam feed, as well as continuous steam sterilization of liquid or pasty media, are presented. It was found that the HYDRONIC generator provides absolutely pure bacteria- and pyrogen-free steam.
1. INTRODUCTION :liable sterile process guidance is an indispensable quirement of biotechnology for important production ocedures in the food industry as much as in the tarmaceutical industry. In order to avoid unwanted microbe contamination, it necessary to sterilize everything which is in contact th the organic material, the incoming medium and :uctural components. The use of damp heat that is ~rmally produced by appropriately designed steam nerators, has frequently stood out as the means of ~oice. In the pharmaceutical industry, it is certainly tportant that the manufacture of so-called parenterelia ltravenously applied medicines) is in the absence of ver-producing pyrogene (endotoxine). In these producm branches, therefore, by the purification and steriliza)n of utensils with pyrogen-free purification substances, ,rresponding pyrogen-free steam must be used. A novel steam generator with the trademark HYDDNIC (Fig. 1) was developed for the production of ghly purified, sterile steam at various temperatures. ae working principle of this steam generator is based on e combustion of hydrogen and oxygen and was derived ~m the H2/O2 steam generator technology reported in -~f. U], 2. WORK PRINCIPLES OF THE STEAM GENERATOR The controlling chemical reaction in the steam generar is: 2H 2 + 02 --~ 2H20 + heat. :~Trade mark of DODUCO company, Pforzheim, Germany.
In the flame zone, which has a temperature T > 3000°C, an additional quantity of water is injected. Through reception of the heat of reaction, this water goes instantaneously into the steam phase. The remaining energy leads to increase of the temperature of the steam to the desired value. Through the regulation of the mass flow rate of gaseous hydrogen and oxygen and of the water, the steam condition parameters, temperature and water content, and steam quantity are reproducibly adjusted. They can also be temporarily changed. Saturated as well as superheated steam up to 300°C can be realized through simple means [2]. The steam generator apparatus consists of three functional structural groups: the steam generator, the media control unit and the electronic control system and regulator. A schematic diagram is shown in Fig. 2. The steam generator is the core of the arrangement. Its layout is shown in Fig. 3. It consists of the ignition, the combustion and the evaporation chamber. In the ignition chamber, a combustible gas mixture at low oxidant/fuel mixture ratio is electrically ignited by means of a sparkplug. After conclusion of the ignition process, the flame shifts to the combustion chambers. The residual oxidant is supplied to the combustion process, to adjust the total mixture ratio exactly to the stoichiometric one. The water to be evaporated is injected into the rear section. Beforehand, the water passes through the double mantle of the combustion chamber and cools the wall. In the adjoining evaporation chamber, whic~ also serves to homogenize the steam, two temperatur~gauges are integrated to control the steam temperature and to monitor the apparatus. / The media control unit regulates th# gas supply (hydrogen and oxygen) and the water supply of the system. The gases can be taken from any arbitrary storage facility 945
946
H.J. S T E R N F E L D and M. P A U L U S
Fig. 1. H Y D R O N I C s t e a m g e n e r a t o r system.
KEYBOARD
1 POWER CIRCUITRY .
.
.
.
z
S T E A M GENERATOR
BIOREACTOR
GH2 .
O
:1 GO2 ~~
tl
GAS SUPPLY
I
W A T E R SUPPLY
Fig. 2. Circuitry of H Y D R O N I C s t e a m generator.
HYDROGEN/OXYGEN STEAM GENERATORS
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temperature sensors T2 IJ;~
spark
H
plug.~
~
~
,
_ _1 evaporation section
Icombustion chamber
inition chamber Fig. 3. Principle of H 2 / O 2 s t e a m generator. or example pressure bottles, gas tanks). A pressure tualizer k e e p s the supply pressure of the gases, hydro,~n and oxygen, equal. With the help of sonic nozzles mt are installed in the gas pipes, the stoichiometrie J x t u r e r a t i o is achieved. By changing the gas supply ressure, the gas mass flow r a t e s c h a n g e synchronously ad linearly, so that the mixture r a t i o remains stoichiotetric. The gas supply is adjustable by a pressure regutor valve with m o t o r drive. The w a t e r mass flow rate is rovided by a gear pump with r e g u l a t e d r o t a t i o n a l speed. The electronic r e g u l a t o r and c o n t r o l are realized on a .icroprocessor basis. They take on the following funcons [3]: valve control; r e g u l a t i o n of gas supply pressure, e.g. the s t e a m generator capacity; r e g u l a t i o n of w a t e r supply; ignition of the gas mixture; monitoring of the ignition process; r e g u l a t i o n of the s t e a m t e m p e r a t u r e ; and safety control. The o p e r a t i o n of the s t e a m g e n e r a t o r is very simple, nee after the selection of the desired c a p a c i t y and the eam t e m p e r a t u r e , the required mass flow r a t e s of the edia are automatically regulated. The same applies also , the c h a n g i n g of the o p e r a t i n g p a r a m e t e r s . The process eps are p r o g r a m m a b l e and can be s t o r e d so t h a t , for :ample, s t e a m temperature/time curves are realized ithout problem. In the employed example of Fig. 2, sensors are depicted .at measure the t e m p e r a t u r e , the p r e s s u r e and the t d r o g e n content at the vessel. The signals of these nsors can be used for the c o n t r o l of the s t e a m g e n e r a t o r Id safety c o n t r o l of the system. In Fig. 4, the s t e a m mass flow rate is represented as a nction of s t e a m g e n e r a t o r capacity for the realizable
2O TD
= 1 2 0 " E ~~/~
I-< 15
~'-TD=300"E
o,
10
I-
5
--//
5
10
15
STEAM GENERATOR CAPACITY, kW
Fig. 4. Steam mass flow rate as function of steam generator capacity. s t e a m t e m p e r a t u r e range from 120° to 300°C. From the figure, it is obvious that the s t e a m mass flow rate is mainly determined by the c a p a c i t y of the s t e a m generator. For a given c a p a c i t y the c h a n g e of s t e a m mass flow rate due to the m a x i m u m s t e a m t e m p e r a t u r e variation a m o u n t s only to a b o u t 15~o. At l o w e r s t e a m t e m p e r a t u r e , the unit p r o d u c e s l a r g e r s t e a m quantiqes per unit time, since a larger w a t e r mass flow is requirqd for c o o l i n g the hot b u r n i n g gases to the desired steam~emperature. The technical data of the H Y D R O N I C s t e a m g e n e r a t o r are listed in T a b l e 1.
H. J. S T E R N F E L D and M. P A U L U S
948
c h a n g e s in s t e a m c o n d i t i o n s are avoided, w h i c h can n o r m a l l y lead to c o n d e n s a t i o n . The H Y D R O N I C s t e a m g e n e r a t o r has been g i v e n the title " T e s t e d S a f e " t h r o u g h the G e r m a n A g e n c y for M a t e r i a l T e s t i n g ( B A M ) , Berlin. In 1987, it received the I • R 100 a w a r d as one of the most significant new technical p r o d u c t s of the year from
Table 1. Technical data of the HYDRONIC steam generator Thermal capacity Thermal efficiency Steam temperature Steam pressure Steam mass flow rate Start-up time Steam temperature gradient Hydrogen consumption Oxygen consumption Dimensions: length diameter Material
4-15 kW 0.95 120° 300~C 1-7 bar Up to 20.5 kg h ~* About 10 s About 20~C s 1 4.2 Nm3 h 1, 2.1 Nm3 h- 1, About 350 mm About 60 mm 1.457 I (AISI-316 Ti)
Research & Development Magazine. 4. B A C T E R I O L O G I C A L I N V E S T I G A T I O N S
*At 15 kW.
3. T E C H N O L O G I C A L D I F F E R E N C E S F R O M THE C O N V E N T I O N A L STEAM G E N E R A T O R TECHNOLOGY The new w o r k i n g principle offers a l a r g e n u m b e r of technological a d v a n t a g e s over the c u s t o m a r y s t e a m gene r a t o r t e c h n o l o g y : inside the m a x i m u m capacity and temperature boundaries, every arbitrary steam condition in the M o l l i e r d i a g r a m of w a t e r is adjustable. The s t e a m c o n d i t i o n s and the s t e a m q u a n t i t y can be a d j u s t e d r e p r o d u c i b l y and also c h a n g e d quickly. D u e to the electronics utilized, fully a u t o m a t i c runs are realizable. The s t a r t i n g time of the s t e a m g e n e r a t o r is e x t r e m e l y s h o r t . The s t e a m is practically at o n e ' s d i s p o s a l with the push of a b u t t o n . Therefore, the system has to be o p e r a t e d only w h e n e v e r s t e a m is actually needed. C o n t r a r y to c o n v e n t i o n a l syst e m s , e n e r g y c o n s u m p t i o n does not o c c u r d u r i n g the standstill time. Also, in c h a n g i n g the s t e a m c o n d i t i o n s d u r i n g a run, the system r e a c t s just as rapidly. A very i m p o r t a n t a d v a n t a g e of the H Y D R O N I C s t e a m g e n e r a tor is the p u r i t y of the s t e a m , in its chemical/physical as well as biological respects. The w a t e r v a p o r is fully condensible. All p a r t s in c o n t a c t with s t e a m are made from 1.4571 stainless steel. Siltation of the s t e a m g e n e r a tor systems is basically excluded. No e x h a u s t gases result from the s t e a m g e n e r a t o r and thus it is especially safe for the e n v i r o n m e n t . D u e to its s m a l l size, the s t e a m g e n e r a tor can i m m e d i a t e l y be p l a c e d w h e r e s t e a m is needed. Therefore, t r a n s p o r t losses in s t e a m p i p i n g and also
The s t e a m from the H Y D R O N I C s t e a m g e n e r a t o r is e x t r e m e l y pure and sterile. In o r d e r to d e t e r m i n e the germicidal effect d u r i n g s t e a m g e n e r a t i o n , the feed w a t e r was i n o c u l a t e d each time in a series of tests with the spores of Bacillus subtilis and the t h e r m o p h i l i c m i c r o o r g a n i s m Bacillus stearothermophilus, respectively. The heat-resistant s p o r e s of t h e s e microorganisms are put in as i n d i c a t o r s to d e t e r m i n a t e the effectiveness of the sterilization process. T h r o u g h germ suspension, feed w a t e r was used at germ c o u n t s from 2 x 105 to 2 x 106 g e r m s m l - 1 In the c o n d e n s a t e of the s t e a m at different e x p e r i m e n t a l conditions, no s p o r e s of the Bacillus subtilis c o u l d be detected ( T a b l e 2). In a f u r t h e r series of t e s t s u n d e r identical c o n d i t i o n s with s p o r e s of B. stearothermophilus, the results are c o m p a r a b l e . Only at m a x i m u m germ exit c o n t e n t in the feed w a t e r (2 x 10 6 g e r m s ml 1) and m i n i m u m s t e a m t e m p e r a t u r e (120°C) c o u l d a c o m p l e t e a n n i l a t i o n not be achieved. A c c o r d i n g to the G e r m a n o r d i n a n c e for d r i n k ing w a t e r and w a t e r for food service from 22 M a y 1986, the m a x i m u m b a c t e r i a c o n t e n t a l l o w e d in waterlines is limited to 102 g e r m s m l - t . The high r e d u c t i o n f a c t o r ( > 105 to 106) c o n s e q u e n t l y m a k e s it o b v i o u s that also by u s a g e of nonsterilized w a t e r , the H Y D R O N I C s t e a m g e n e r a t o r delivers sterile s t e a m in e v e r y i n s t a n c e I-3]. In o r d e r to d e m o n s t r a t e the p y r o g e n - e l i m i n a t i o n effect of the H Y D R O N I C s t e a m g e n e r a t o r , the feed w a t e r was i n o c u l a t e d with a defined pure c u l t u r e of Citrobacter freundii. Subsequently, s t e a m was p r o d u c e d at different t e m p e r a t u r e levels and c o n d e n s e d out. In parallel, a c o m p a r a t i v e e x p e r i m e n t was c o n d u c t e d using a c o n v e n t i o n a l electric s t e a m g e n e r a t o r . The results of the electric s t e a m g e n e r a t o r show only a m i n o r r e d u c t i o n of the p y r o g e n c o n c e n t r a t i o n in the c o n d e n s a t e c o m p a r e d with the feedwater, w h i l e a significant p y r o g e n r e d u c t i o n c o u l d be achieved by the H Y D R O N I C s t e a m g e n e r a t o r ( T a b l e 3).
Table 2. Mortification effect of B. subtilis spores by operation of the HYDRONIC steam generator Steam temp. (°C)
Power (kW)
120 150 150 150 300
4 5 10 15 15
Germs ml- ~ in feed water 8 2 8 8 2
x x x x x
105 l0n 105 l0s 105
Germs ml- 1 in condensate
Reduction factor (exponent of 10)
0 0 0 0 0
>5 >6 >5 >5 >5
HYDROGEN/OXYGEN STEAM GENERATORS "able 3. Comparison of pyrogen reduction utilizing HYDRONIC or electric steam generator Pyrogen content (ng ml 1) HYDRONIC
Electricsteam generator
1.180 0.028 0.020 0.016
0.910 0.800
eed water rD: 120°C 200°C 300°C
5. STERILIZATION OF BIOREACTORS BY D I R E C T STEAM F E E D I N G The schematic assembly of larger bioreactors (volaes > 10 1) is given in Fig. 5. The metallic bottom part designed as a double mantle, while the upper part presents a cylindrical adapter which is provided with a ~tallic cover. Conventionally, such a bioreactor will be ;rilized by steam feeding into the double mantle (feedg position 2). By heat conduction, the aqueous medium the bioreactor will be partially vaporized. Sterilization en occurs due to condensation on the inner surfaces of e vessel.
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\ Y
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~7
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®
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g. 5. Sterilization of bioreactor by: (1) direct steam feeding; and (2) heating via double mantle. •
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In contrast with the above described method, the sterilization by direct steam feeding o¢curs when pure and sterile steam is fed at position 1 intoithe pipeline and then partially distributed to the aqueousl medium, as well as to the upper space of the vessel. The thermic sterilization process is subdivided into three sections: heating phase; sterilization phase; and cooling phase. By the direct injection of steam, the heating phase can be especially shortened. The length of the sterilization phase depends on the employed temperature, as well as the manner and the concentration of the microbe contamination. For safety reasons, reduction of the length of this phase is not possible. By direct steam injection, additional water is brought into the medium. The obtainable amount of condensate is simple to calculate, since it only depends on the run conditions of the steam generator and the operating time. Due to the reproducibility of the H Y D R O N I C steam generator, the quantity of additional water can be estimated in advance. In Fig. 6, the temperature history for heating via a double mantle and for direct steam injection are shown for a 15 1 bioreactor. The thermal capacity input amounts to 5 kW in both cases. The bioreactor used has a valve system which closes the air duct at 103°C. The temperature history in both cases is measured in the aqueous medium, as well as on the underside of the reactor cover, using thermocouples and Pt-100-sensors, respectively. For the direct steam injection, a rapid temperature increase is shown. In the beginning the temperature increase is almost identical both on the cover and in the medium. In the medium, the necessary sterilization temperature of 121°C has been reached already after approximately 12 min. The somewhat longer heating time of the cover is caused by the outward heat conduction. If the bioreactor is heated via the double mantle, a more distinct, slower temperature increase in the medium can be established. The sterilization temperature is not achieved before about 22 min. At that time, the temperature on the underside of the cover is still substantially lower. A more distinct temperature increase on the underside of the cover occurs only if the medium has reached 100°C and begins to evaporate noticeably. Through the condensation of the steam out of the aqueous medium, the temperature on the cover climbs relatively quickly to about 100°C. Nevertheless, the further temperature increase afterwards certainly takes place slowly. The cover temperature nears an asymptotic value of a b o u t 115°C. The sterilization temperature of 121°C is not achieved, even after 55 min I-4]. These measurements illustrate that for double mantle heating, the risk of an unattained sterilization operation exists. As a rule, the temperature on th~ inner surfaces in the head chamber of the bioreactor is nqt monitored. The start of the actual sterilization proces~and the holding time at sterilization temperature will be ~ndicated only by the temperature of the medium. Therefore, it can easily occur that microorganisms and germs s~rvive the sterilization process without detriment. This danger is eliminated through direct steam injection.
950
H.J. STERNFELD and M. PAULUS direct s t e a m injection
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!
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20
30
50
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IIIIl,,u~,,lll,.
BIOREACTOR
heat-up time/min
Fig. 6. Temperature history for direct and nondirect steam heating of a bioreactor (open symbols: cover; closed symbols: medium). 6. MIXING CHAMBER FOR D I R E C T STEAM I N J E C T I O N IN LIQUID OR PASTY M E D I A For continuous sterilization, steam is injected into a mixing chamber as counterflow to the sterilizing medium. The steam and medium mix. The steam delivers the entrained energy that exists mainly in the form of the heat of evaporation to the medium, and this raises its temperature [51.
In Fig. 7, the process of continuous sterilization by direct steam injection is schematically represented. The nonsterile medium is compressed by the pump (P) to a pressure of a maximum of 5 bar, and is supplied to the heat exchanger (W1). There, flowing counter to the already sterilized medium, the nonsterile medium is preheated. Afterwards it flows into the core of the device, the mixing chamber (M), where the sterilization tempera-
control unit
i I STEAM
7
non-sterile medium
To
[
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!
heat exchanger
--
~ - T
T~
I I I
u ' ~ ' l . r
Tz I
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sterile medium
1
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I container
xV
Fig. 7. F l o w d i a g r a m for c o n t i n u o u s s t e r i l i z a t i o n b y d i r e c t s t e a m i n j e c t i o n .
HYDROGEN/OXYGEN STEAM GENERATORS re is achieved by means of direct steam injection. The essure in the mixing chamber, which is adjusted by the rottle valve (3), must be higher than the steam pressure the medium at the sterilization temperature, in order avoid the formation of steam bubbles. The temperare in the mixing chamber is controlled by the microprossor of the H Y D R O N I C steam generator. At the start-up of the system, nonsterile medium is Lmped into the mixing chamber. At this time, the valve ) remains closed. As soon as the desired temperature is ~ched due to the addition of steam, the control unit ~ens the valve (2). This prevents the entrance of non;rile medium into the sterile part of the system. The medium expands through the throttle valve in the ntainer (B) to a pressure of approximately 1 bar. At the me time a part of the medium evaporates with simultaous cooling down of the liquid phase. The steam can ndense out in the condensor (W2) and be drawn away. this way, the quantity of water that is added through rect steam injection is removed again from the medium. le medium is extracted at a temperature of some 100°C '3) and leaves the system with a temperature of approxLately 40°C (T4), after it has gone through the heat changer (Wl). For the sterilization of the system through the three~y valve (1), steam is injected into the supply line rough valve (7), as well as into the container (B) rough valve (8) and into the condensor through valve ). The condensate can be drawn away through valves ), (6), (10) and (11). The integration of the H Y D R O N I C steam generator ithin the mixing chamber device for continuous steam ~rilization allows fully automatic and regulated operam. By additional combination with a computer, the :rilization of the whole system can be fully automated.
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7. SUMMARY The working principles of the H Y D R O N I C steam generator for producing sterilization steam are described. The steam generator, based on the coml~ustion of hydrogen and oxygen, has several advantages with respect to conventional steam generators. Within the last 2 years, intensive bacteriological investigations have been c~irried out to analyze the quality of sterilization of bioreactors by direct steam feed as well as continuous steam sterilization of liquid and pasty fluids in food and pharmaceutical production. It was found that the H Y D R O N I C generator provides absolutely pure bacteria- and pyrogen-free steam. This is one of the main differences from conventional steam generators. Another important advantage is the programmable and controlled production of steam by the H Y D R O N I C generator, allowing automatic process runs. In this paper, the unique features of the H Y D R O N I C steam generator and the bacteriological test results are described. REFERENCES 1. H.J. Sternfeld and P. Heinrich, A demonstration plant for the hydrogen/oxygen-spinning reserve. Int. J. Hydrogen Energy 14, 703 716 (1989). 2. M. Paulus, H. J. Sternfeld and W. Tr6sch, H2/Oz-Dampferzeuger ffir die Sterilisierung von biotechnologischen Anlagen. Presentation at ACHEMA 1985, Frankfurt/Main (1985). 3. H. Kissler, M. Paulus, T. Maier and K. Steffens, Dampf von hoher Reinheit. BIF-Biotech-Forum 6, 178 181 (1989). 4. H. Kissler, M. Paulus, T. Maier and K. Steffens, Sterilisation von Bioreaktoren durch Direktdampfeinleitung. Verfahrenstechnik 23, 22-26 (1989). 5. H. Kissler, M. Paulus, T. Maier and K. Steffens, Kontinuierliche Dampfsterilisation fl/issiger und past6ser Medien. I/erfahrenstechnik 23, 30-33 (1989).
0360-3199/93 $6.00 + 0.00 Pergamon Press Ltd. International Association for Hydrogen Energy.
H Y D R O G E N / O X Y G E N S T E A M G E N E R A T O R S FOR S T E R I L I Z A T I O N PROCESSES AND CHEMICAL ENGINEERING H. J. STERNFELD* and M. P A U L U S " *DLR, The German Aerospace Research Establishment, D-7101 Lampoldshausen, Germany tFhG, Fraunhofer Gesellschaft, D-8000 Munich, Germany
(Received for publication 17 February 1993) Abstract--The working principles of the HYDRONIC:~ steam generator for producing sterilization steam are described. The steam generator is based on the combustion of hydrogen and oxygen. Results of intensive bacteriological investigations which analyze the quality of sterilization of bioreactors by direct steam feed, as well as continuous steam sterilization of liquid or pasty media, are presented. It was found that the HYDRONIC generator provides absolutely pure bacteria- and pyrogen-free steam.
1. INTRODUCTION :liable sterile process guidance is an indispensable quirement of biotechnology for important production ocedures in the food industry as much as in the tarmaceutical industry. In order to avoid unwanted microbe contamination, it necessary to sterilize everything which is in contact th the organic material, the incoming medium and :uctural components. The use of damp heat that is ~rmally produced by appropriately designed steam nerators, has frequently stood out as the means of ~oice. In the pharmaceutical industry, it is certainly tportant that the manufacture of so-called parenterelia ltravenously applied medicines) is in the absence of ver-producing pyrogene (endotoxine). In these producm branches, therefore, by the purification and steriliza)n of utensils with pyrogen-free purification substances, ,rresponding pyrogen-free steam must be used. A novel steam generator with the trademark HYDDNIC (Fig. 1) was developed for the production of ghly purified, sterile steam at various temperatures. ae working principle of this steam generator is based on e combustion of hydrogen and oxygen and was derived ~m the H2/O2 steam generator technology reported in -~f. U], 2. WORK PRINCIPLES OF THE STEAM GENERATOR The controlling chemical reaction in the steam generar is: 2H 2 + 02 --~ 2H20 + heat. :~Trade mark of DODUCO company, Pforzheim, Germany.
In the flame zone, which has a temperature T > 3000°C, an additional quantity of water is injected. Through reception of the heat of reaction, this water goes instantaneously into the steam phase. The remaining energy leads to increase of the temperature of the steam to the desired value. Through the regulation of the mass flow rate of gaseous hydrogen and oxygen and of the water, the steam condition parameters, temperature and water content, and steam quantity are reproducibly adjusted. They can also be temporarily changed. Saturated as well as superheated steam up to 300°C can be realized through simple means [2]. The steam generator apparatus consists of three functional structural groups: the steam generator, the media control unit and the electronic control system and regulator. A schematic diagram is shown in Fig. 2. The steam generator is the core of the arrangement. Its layout is shown in Fig. 3. It consists of the ignition, the combustion and the evaporation chamber. In the ignition chamber, a combustible gas mixture at low oxidant/fuel mixture ratio is electrically ignited by means of a sparkplug. After conclusion of the ignition process, the flame shifts to the combustion chambers. The residual oxidant is supplied to the combustion process, to adjust the total mixture ratio exactly to the stoichiometric one. The water to be evaporated is injected into the rear section. Beforehand, the water passes through the double mantle of the combustion chamber and cools the wall. In the adjoining evaporation chamber, whic~ also serves to homogenize the steam, two temperatur~gauges are integrated to control the steam temperature and to monitor the apparatus. / The media control unit regulates th# gas supply (hydrogen and oxygen) and the water supply of the system. The gases can be taken from any arbitrary storage facility 945
946
H.J. S T E R N F E L D and M. P A U L U S
Fig. 1. H Y D R O N I C s t e a m g e n e r a t o r system.
KEYBOARD
1 POWER CIRCUITRY .
.
.
.
z
S T E A M GENERATOR
BIOREACTOR
GH2 .
O
:1 GO2 ~~
tl
GAS SUPPLY
I
W A T E R SUPPLY
Fig. 2. Circuitry of H Y D R O N I C s t e a m generator.
HYDROGEN/OXYGEN STEAM GENERATORS
947
temperature sensors T2 IJ;~
spark
H
plug.~
~
~
,
_ _1 evaporation section
Icombustion chamber
inition chamber Fig. 3. Principle of H 2 / O 2 s t e a m generator. or example pressure bottles, gas tanks). A pressure tualizer k e e p s the supply pressure of the gases, hydro,~n and oxygen, equal. With the help of sonic nozzles mt are installed in the gas pipes, the stoichiometrie J x t u r e r a t i o is achieved. By changing the gas supply ressure, the gas mass flow r a t e s c h a n g e synchronously ad linearly, so that the mixture r a t i o remains stoichiotetric. The gas supply is adjustable by a pressure regutor valve with m o t o r drive. The w a t e r mass flow rate is rovided by a gear pump with r e g u l a t e d r o t a t i o n a l speed. The electronic r e g u l a t o r and c o n t r o l are realized on a .icroprocessor basis. They take on the following funcons [3]: valve control; r e g u l a t i o n of gas supply pressure, e.g. the s t e a m generator capacity; r e g u l a t i o n of w a t e r supply; ignition of the gas mixture; monitoring of the ignition process; r e g u l a t i o n of the s t e a m t e m p e r a t u r e ; and safety control. The o p e r a t i o n of the s t e a m g e n e r a t o r is very simple, nee after the selection of the desired c a p a c i t y and the eam t e m p e r a t u r e , the required mass flow r a t e s of the edia are automatically regulated. The same applies also , the c h a n g i n g of the o p e r a t i n g p a r a m e t e r s . The process eps are p r o g r a m m a b l e and can be s t o r e d so t h a t , for :ample, s t e a m temperature/time curves are realized ithout problem. In the employed example of Fig. 2, sensors are depicted .at measure the t e m p e r a t u r e , the p r e s s u r e and the t d r o g e n content at the vessel. The signals of these nsors can be used for the c o n t r o l of the s t e a m g e n e r a t o r Id safety c o n t r o l of the system. In Fig. 4, the s t e a m mass flow rate is represented as a nction of s t e a m g e n e r a t o r capacity for the realizable
2O TD
= 1 2 0 " E ~~/~
I-< 15
~'-TD=300"E
o,
10
I-
5
--//
5
10
15
STEAM GENERATOR CAPACITY, kW
Fig. 4. Steam mass flow rate as function of steam generator capacity. s t e a m t e m p e r a t u r e range from 120° to 300°C. From the figure, it is obvious that the s t e a m mass flow rate is mainly determined by the c a p a c i t y of the s t e a m generator. For a given c a p a c i t y the c h a n g e of s t e a m mass flow rate due to the m a x i m u m s t e a m t e m p e r a t u r e variation a m o u n t s only to a b o u t 15~o. At l o w e r s t e a m t e m p e r a t u r e , the unit p r o d u c e s l a r g e r s t e a m quantiqes per unit time, since a larger w a t e r mass flow is requirqd for c o o l i n g the hot b u r n i n g gases to the desired steam~emperature. The technical data of the H Y D R O N I C s t e a m g e n e r a t o r are listed in T a b l e 1.
H. J. S T E R N F E L D and M. P A U L U S
948
c h a n g e s in s t e a m c o n d i t i o n s are avoided, w h i c h can n o r m a l l y lead to c o n d e n s a t i o n . The H Y D R O N I C s t e a m g e n e r a t o r has been g i v e n the title " T e s t e d S a f e " t h r o u g h the G e r m a n A g e n c y for M a t e r i a l T e s t i n g ( B A M ) , Berlin. In 1987, it received the I • R 100 a w a r d as one of the most significant new technical p r o d u c t s of the year from
Table 1. Technical data of the HYDRONIC steam generator Thermal capacity Thermal efficiency Steam temperature Steam pressure Steam mass flow rate Start-up time Steam temperature gradient Hydrogen consumption Oxygen consumption Dimensions: length diameter Material
4-15 kW 0.95 120° 300~C 1-7 bar Up to 20.5 kg h ~* About 10 s About 20~C s 1 4.2 Nm3 h 1, 2.1 Nm3 h- 1, About 350 mm About 60 mm 1.457 I (AISI-316 Ti)
Research & Development Magazine. 4. B A C T E R I O L O G I C A L I N V E S T I G A T I O N S
*At 15 kW.
3. T E C H N O L O G I C A L D I F F E R E N C E S F R O M THE C O N V E N T I O N A L STEAM G E N E R A T O R TECHNOLOGY The new w o r k i n g principle offers a l a r g e n u m b e r of technological a d v a n t a g e s over the c u s t o m a r y s t e a m gene r a t o r t e c h n o l o g y : inside the m a x i m u m capacity and temperature boundaries, every arbitrary steam condition in the M o l l i e r d i a g r a m of w a t e r is adjustable. The s t e a m c o n d i t i o n s and the s t e a m q u a n t i t y can be a d j u s t e d r e p r o d u c i b l y and also c h a n g e d quickly. D u e to the electronics utilized, fully a u t o m a t i c runs are realizable. The s t a r t i n g time of the s t e a m g e n e r a t o r is e x t r e m e l y s h o r t . The s t e a m is practically at o n e ' s d i s p o s a l with the push of a b u t t o n . Therefore, the system has to be o p e r a t e d only w h e n e v e r s t e a m is actually needed. C o n t r a r y to c o n v e n t i o n a l syst e m s , e n e r g y c o n s u m p t i o n does not o c c u r d u r i n g the standstill time. Also, in c h a n g i n g the s t e a m c o n d i t i o n s d u r i n g a run, the system r e a c t s just as rapidly. A very i m p o r t a n t a d v a n t a g e of the H Y D R O N I C s t e a m g e n e r a tor is the p u r i t y of the s t e a m , in its chemical/physical as well as biological respects. The w a t e r v a p o r is fully condensible. All p a r t s in c o n t a c t with s t e a m are made from 1.4571 stainless steel. Siltation of the s t e a m g e n e r a tor systems is basically excluded. No e x h a u s t gases result from the s t e a m g e n e r a t o r and thus it is especially safe for the e n v i r o n m e n t . D u e to its s m a l l size, the s t e a m g e n e r a tor can i m m e d i a t e l y be p l a c e d w h e r e s t e a m is needed. Therefore, t r a n s p o r t losses in s t e a m p i p i n g and also
The s t e a m from the H Y D R O N I C s t e a m g e n e r a t o r is e x t r e m e l y pure and sterile. In o r d e r to d e t e r m i n e the germicidal effect d u r i n g s t e a m g e n e r a t i o n , the feed w a t e r was i n o c u l a t e d each time in a series of tests with the spores of Bacillus subtilis and the t h e r m o p h i l i c m i c r o o r g a n i s m Bacillus stearothermophilus, respectively. The heat-resistant s p o r e s of t h e s e microorganisms are put in as i n d i c a t o r s to d e t e r m i n a t e the effectiveness of the sterilization process. T h r o u g h germ suspension, feed w a t e r was used at germ c o u n t s from 2 x 105 to 2 x 106 g e r m s m l - 1 In the c o n d e n s a t e of the s t e a m at different e x p e r i m e n t a l conditions, no s p o r e s of the Bacillus subtilis c o u l d be detected ( T a b l e 2). In a f u r t h e r series of t e s t s u n d e r identical c o n d i t i o n s with s p o r e s of B. stearothermophilus, the results are c o m p a r a b l e . Only at m a x i m u m germ exit c o n t e n t in the feed w a t e r (2 x 10 6 g e r m s ml 1) and m i n i m u m s t e a m t e m p e r a t u r e (120°C) c o u l d a c o m p l e t e a n n i l a t i o n not be achieved. A c c o r d i n g to the G e r m a n o r d i n a n c e for d r i n k ing w a t e r and w a t e r for food service from 22 M a y 1986, the m a x i m u m b a c t e r i a c o n t e n t a l l o w e d in waterlines is limited to 102 g e r m s m l - t . The high r e d u c t i o n f a c t o r ( > 105 to 106) c o n s e q u e n t l y m a k e s it o b v i o u s that also by u s a g e of nonsterilized w a t e r , the H Y D R O N I C s t e a m g e n e r a t o r delivers sterile s t e a m in e v e r y i n s t a n c e I-3]. In o r d e r to d e m o n s t r a t e the p y r o g e n - e l i m i n a t i o n effect of the H Y D R O N I C s t e a m g e n e r a t o r , the feed w a t e r was i n o c u l a t e d with a defined pure c u l t u r e of Citrobacter freundii. Subsequently, s t e a m was p r o d u c e d at different t e m p e r a t u r e levels and c o n d e n s e d out. In parallel, a c o m p a r a t i v e e x p e r i m e n t was c o n d u c t e d using a c o n v e n t i o n a l electric s t e a m g e n e r a t o r . The results of the electric s t e a m g e n e r a t o r show only a m i n o r r e d u c t i o n of the p y r o g e n c o n c e n t r a t i o n in the c o n d e n s a t e c o m p a r e d with the feedwater, w h i l e a significant p y r o g e n r e d u c t i o n c o u l d be achieved by the H Y D R O N I C s t e a m g e n e r a t o r ( T a b l e 3).
Table 2. Mortification effect of B. subtilis spores by operation of the HYDRONIC steam generator Steam temp. (°C)
Power (kW)
120 150 150 150 300
4 5 10 15 15
Germs ml- ~ in feed water 8 2 8 8 2
x x x x x
105 l0n 105 l0s 105
Germs ml- 1 in condensate
Reduction factor (exponent of 10)
0 0 0 0 0
>5 >6 >5 >5 >5
HYDROGEN/OXYGEN STEAM GENERATORS "able 3. Comparison of pyrogen reduction utilizing HYDRONIC or electric steam generator Pyrogen content (ng ml 1) HYDRONIC
Electricsteam generator
1.180 0.028 0.020 0.016
0.910 0.800
eed water rD: 120°C 200°C 300°C
5. STERILIZATION OF BIOREACTORS BY D I R E C T STEAM F E E D I N G The schematic assembly of larger bioreactors (volaes > 10 1) is given in Fig. 5. The metallic bottom part designed as a double mantle, while the upper part presents a cylindrical adapter which is provided with a ~tallic cover. Conventionally, such a bioreactor will be ;rilized by steam feeding into the double mantle (feedg position 2). By heat conduction, the aqueous medium the bioreactor will be partially vaporized. Sterilization en occurs due to condensation on the inner surfaces of e vessel.
I®
\ Y
)
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---- E Z 3 = = C Z 2 E Z Z 2 = =:E222~
®
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g. 5. Sterilization of bioreactor by: (1) direct steam feeding; and (2) heating via double mantle. •
949
In contrast with the above described method, the sterilization by direct steam feeding o¢curs when pure and sterile steam is fed at position 1 intoithe pipeline and then partially distributed to the aqueousl medium, as well as to the upper space of the vessel. The thermic sterilization process is subdivided into three sections: heating phase; sterilization phase; and cooling phase. By the direct injection of steam, the heating phase can be especially shortened. The length of the sterilization phase depends on the employed temperature, as well as the manner and the concentration of the microbe contamination. For safety reasons, reduction of the length of this phase is not possible. By direct steam injection, additional water is brought into the medium. The obtainable amount of condensate is simple to calculate, since it only depends on the run conditions of the steam generator and the operating time. Due to the reproducibility of the H Y D R O N I C steam generator, the quantity of additional water can be estimated in advance. In Fig. 6, the temperature history for heating via a double mantle and for direct steam injection are shown for a 15 1 bioreactor. The thermal capacity input amounts to 5 kW in both cases. The bioreactor used has a valve system which closes the air duct at 103°C. The temperature history in both cases is measured in the aqueous medium, as well as on the underside of the reactor cover, using thermocouples and Pt-100-sensors, respectively. For the direct steam injection, a rapid temperature increase is shown. In the beginning the temperature increase is almost identical both on the cover and in the medium. In the medium, the necessary sterilization temperature of 121°C has been reached already after approximately 12 min. The somewhat longer heating time of the cover is caused by the outward heat conduction. If the bioreactor is heated via the double mantle, a more distinct, slower temperature increase in the medium can be established. The sterilization temperature is not achieved before about 22 min. At that time, the temperature on the underside of the cover is still substantially lower. A more distinct temperature increase on the underside of the cover occurs only if the medium has reached 100°C and begins to evaporate noticeably. Through the condensation of the steam out of the aqueous medium, the temperature on the cover climbs relatively quickly to about 100°C. Nevertheless, the further temperature increase afterwards certainly takes place slowly. The cover temperature nears an asymptotic value of a b o u t 115°C. The sterilization temperature of 121°C is not achieved, even after 55 min I-4]. These measurements illustrate that for double mantle heating, the risk of an unattained sterilization operation exists. As a rule, the temperature on th~ inner surfaces in the head chamber of the bioreactor is nqt monitored. The start of the actual sterilization proces~and the holding time at sterilization temperature will be ~ndicated only by the temperature of the medium. Therefore, it can easily occur that microorganisms and germs s~rvive the sterilization process without detriment. This danger is eliminated through direct steam injection.
950
H.J. STERNFELD and M. PAULUS direct s t e a m injection
120
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. A&,AAALx
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0
I
!
0
0
20
30
50
40
IIIIl,,u~,,lll,.
BIOREACTOR
heat-up time/min
Fig. 6. Temperature history for direct and nondirect steam heating of a bioreactor (open symbols: cover; closed symbols: medium). 6. MIXING CHAMBER FOR D I R E C T STEAM I N J E C T I O N IN LIQUID OR PASTY M E D I A For continuous sterilization, steam is injected into a mixing chamber as counterflow to the sterilizing medium. The steam and medium mix. The steam delivers the entrained energy that exists mainly in the form of the heat of evaporation to the medium, and this raises its temperature [51.
In Fig. 7, the process of continuous sterilization by direct steam injection is schematically represented. The nonsterile medium is compressed by the pump (P) to a pressure of a maximum of 5 bar, and is supplied to the heat exchanger (W1). There, flowing counter to the already sterilized medium, the nonsterile medium is preheated. Afterwards it flows into the core of the device, the mixing chamber (M), where the sterilization tempera-
control unit
i I STEAM
7
non-sterile medium
To
[
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!
heat exchanger
--
~ - T
T~
I I I
u ' ~ ' l . r
Tz I
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mixing chamber
sterile medium
1
conder
I container
xV
Fig. 7. F l o w d i a g r a m for c o n t i n u o u s s t e r i l i z a t i o n b y d i r e c t s t e a m i n j e c t i o n .
HYDROGEN/OXYGEN STEAM GENERATORS re is achieved by means of direct steam injection. The essure in the mixing chamber, which is adjusted by the rottle valve (3), must be higher than the steam pressure the medium at the sterilization temperature, in order avoid the formation of steam bubbles. The temperare in the mixing chamber is controlled by the microprossor of the H Y D R O N I C steam generator. At the start-up of the system, nonsterile medium is Lmped into the mixing chamber. At this time, the valve ) remains closed. As soon as the desired temperature is ~ched due to the addition of steam, the control unit ~ens the valve (2). This prevents the entrance of non;rile medium into the sterile part of the system. The medium expands through the throttle valve in the ntainer (B) to a pressure of approximately 1 bar. At the me time a part of the medium evaporates with simultaous cooling down of the liquid phase. The steam can ndense out in the condensor (W2) and be drawn away. this way, the quantity of water that is added through rect steam injection is removed again from the medium. le medium is extracted at a temperature of some 100°C '3) and leaves the system with a temperature of approxLately 40°C (T4), after it has gone through the heat changer (Wl). For the sterilization of the system through the three~y valve (1), steam is injected into the supply line rough valve (7), as well as into the container (B) rough valve (8) and into the condensor through valve ). The condensate can be drawn away through valves ), (6), (10) and (11). The integration of the H Y D R O N I C steam generator ithin the mixing chamber device for continuous steam ~rilization allows fully automatic and regulated operam. By additional combination with a computer, the :rilization of the whole system can be fully automated.
951
7. SUMMARY The working principles of the H Y D R O N I C steam generator for producing sterilization steam are described. The steam generator, based on the coml~ustion of hydrogen and oxygen, has several advantages with respect to conventional steam generators. Within the last 2 years, intensive bacteriological investigations have been c~irried out to analyze the quality of sterilization of bioreactors by direct steam feed as well as continuous steam sterilization of liquid and pasty fluids in food and pharmaceutical production. It was found that the H Y D R O N I C generator provides absolutely pure bacteria- and pyrogen-free steam. This is one of the main differences from conventional steam generators. Another important advantage is the programmable and controlled production of steam by the H Y D R O N I C generator, allowing automatic process runs. In this paper, the unique features of the H Y D R O N I C steam generator and the bacteriological test results are described. REFERENCES 1. H.J. Sternfeld and P. Heinrich, A demonstration plant for the hydrogen/oxygen-spinning reserve. Int. J. Hydrogen Energy 14, 703 716 (1989). 2. M. Paulus, H. J. Sternfeld and W. Tr6sch, H2/Oz-Dampferzeuger ffir die Sterilisierung von biotechnologischen Anlagen. Presentation at ACHEMA 1985, Frankfurt/Main (1985). 3. H. Kissler, M. Paulus, T. Maier and K. Steffens, Dampf von hoher Reinheit. BIF-Biotech-Forum 6, 178 181 (1989). 4. H. Kissler, M. Paulus, T. Maier and K. Steffens, Sterilisation von Bioreaktoren durch Direktdampfeinleitung. Verfahrenstechnik 23, 22-26 (1989). 5. H. Kissler, M. Paulus, T. Maier and K. Steffens, Kontinuierliche Dampfsterilisation fl/issiger und past6ser Medien. I/erfahrenstechnik 23, 30-33 (1989).