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Development studies on thermochemical cycles for hydrogen production
International Journal of Hydrogen Energy, Vol. 2, pp. 107-121. Pergamon Press, 1977. Printed in Northern Ireland
DEVELOPMENT STUDIES ON THERMOCHEMICAL CYCLES FOR H Y D R O G E N PRODUCTION D. VAr~ VELZENAND H. LANGENKAMP C.C.R. Euratom Ispra, Italy &ImPart--The experimental work in the field of the development of cycles of the Fe/C1 family is described. It appeared that application of a homogeneous support material for the gas-solids reaction was not feasible due to unexpected side reactions. The hydrolysis of ferrous chloride has been found to give good results and the development and operation of a continuously operating bench-scale reactor for this reaction has been described. The chlorination of the resulting magnetite could not be performed according to the originally proposed scheme. A satisfactory alternative has been found, i.e. chlorination at 150-200°C with only hydrogen chloride and introduction of the reverse Deacon reaction as a fourth reaction in the cycle. INTRODUCTION THE NUMaeR of proposed cycles for the thermochemical decomposition of water is very considerable. The stage of development of a large majority of these cycles does not pass the thermodynamically calculated feasibility, often accompanied by rough estimates of expected thermal etticiencies. The number of cycles on which work on laboratory scale has been carded out is very limited and the number of cycles which are subject of a development directed study is most probably less than ten. The iron-chlorine family cycles belong to the last category and can even be considered the most important member of this group, as investigations are being carried out in Germany, Japan, the U.S. and Italy. The major attraction of cycles of this family is the fact that the chemicals involved are common and relatively cheap, so that minor losses would not directly jeopardize the economics of the process. In this paper the development study carded out at the J.R.C., Ispra, Italy, will be discussed. The main objective of this work has been to obtain the necessary data for the construction of a bench-scale continuous demonstration plant. This implies that the orientation of the study has been sometimes more towards the development of practical solutions for existing problems than towards a purely scientific approach. As an example, a considerable effort has been spent on the search for a suitable support material for the solid reactants to minimize transport problems. This part will be treated more extensively below. The cycle acting as the point of departure was the well-known Mark-9 cycle proposed by Hardy [2] in 1972. It consists of the following three reactions 6FeC12 + 8H20 ~ 2Fe304 + 12HC1 + 2H2 2Fe304 + 12HC1 + 3C12 --* 6FeC13 + 6H20 + 02
(600-700°C) (150-200°C)
6FeC13 ---*6FeC12 + 3C12 (300-400°C)
(1) (2) (3)
This paper deals with the study of a possible homogeneous support material, the hydrolysis of ferrous chloride and development work in the chlorination of magnetite. The last reaction, the disproportioning of ferric chloride, will not enter into the considerations, as practical work on this reaction has been carried out at the Teclmical University of Aachen in Western Germany, together with the fact that data for this equilibrium can be taken from the literature [2, 3, 4]. D E V E L O P M E N T STUDIES F O R A H O M O G E N E O U S SUPPORT M A T E R I A L The idea to perform chemical reactions involving solids and gases where the solid reactant or catalyst is homogeneously distributed on an inert support material is not new. This technique is most extensively applied in heterogeneously catalyzed gas reactions. The main advantages are highly increased reaction rates due to the large accessible reaction surface t07
108
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
and the increased ease of solids handling. It seems thus very attractive to apply this technique also to the gas-solid reaction for the Fe-Cl-cydes. The study has been carried out by a systematic analysis of retention capacity and internal surface for a n u m b e r of candidate materials. T h e r e are four criteria to be considered for the selection of suitable supports: (a) In the first place it must be noted that a drawback of the use of a support material is the increase of the (already very large) bulk weight solids transport. Therefore, it is important that the support retains a maximum amount of reactant. (b) A second condition is that a good support must be physically and chemically resistant to the various reaction conditions. (c) Thirdly, the support should not exert an inhibiting action on vital reactions, nor give rise to unwanted side reactions. (d) Finally, it must be noted that a good support should not be excessively expensive. Ten possible supports have been tested, and a survey of the results is given in Table 1. The retention capacity of each support has been determined by saturation with concentrated (60%) solution of FeC12-4H20 in water. The support, previously dried in vacuum at 150°C to constant weight, is stirred with an excess of this solfition for 20 min and then filtered. The retained weight of solution is determined and the retention capacity (expressed in g FeC12/100 g of dry support) is calculated. The internal surface of each support before and after loading with ferrous chloride has been determined by means of nitrogen absorption using an A r e a m e t e r of the Firm Strthlein. It follows that Attapulgus Clay, a mineral material mainly consisting of a hydrous aluminium silicate, shows the highest retention capacity a m o n g the materials tested. Its initial specific surface is 76 m2/g and after being loaded this figure decreases to 16 m2/g, indicating a large accessible reaction surface. The material, also known under the name "Fuller's earth", has frequently b e e n used in industry as an adsorbent or a cracking catalyst and is satisfactorily stable at the reaction conditions occurring in the Mark-9 cycle. It seems to be the most promising support material available. TABLE 1. Characteristics of various support materials
Support Attapulgus C l a y
Manufacturer
Active carbon
PhillipsM i n . and Chem. Degussa
Pumice
Merck
Silica Silica
Impianti Sist. Gel Carlo Erba
"v-Alumina
Doduco
Molecular Sieves ~/-Alumina
Grace
-Aluminamacropore Carborundum
Doduco Norton --
Form granules i mm irregular 2-5 mm irregular 4-12 mm irregular 5-7 mm irregular 6-10ram spheres 2--4mm pellets 2 x 6 mm spheres 5-8 mm spheres 2--4mm spheres 2-4 mm
Retention capacity (g FeC12" 100g ')
Specific surface (m2/g) original .loaded
84
76
16
45
n.d.
n.d.
45
0.01
0.01
32
440
27
32
440
4
32
247
27
29
8
9
25
226
57
12
0.14
0.04
12
0.14
0.14
D. Van VELZEN AND H. LANGENKAMP
109
Active carbon also has an acceptably high retention capacity, but it appears unsuitable as a support, due to the occurrence of unwanted side reactions. In the hydrolysis step, active carbon support reacts with excess water forming carbon monoxide and hydrogen, whereas also in the oxygen formation step a reaction between the support and a cycle reactant occurs, in this case with liberated oxygen. Here, carbon dioxide was the obtained product and the oxygen formation was completely suppressed. Pumice also offers good possibilities as a support material, showing a retention capacity of 45 g/100 g dry support. It is noteworthy that this material, although being very porous, has an extremely small specific surface. Its stability under reaction conditions is satisfactory. The retention capacity of silica seems to be rather low in spite of its very high specific surface, which makes it somewhat less attractive as a support material than the above mentioned ones. Its stability is fairly good. This is not the case with the 3,-A1203 supports. It appeared that upon reaction at high temperature, i.e. above 600°C, this material embrittles and partly degrades into a fine powder. The A1203 spheres still remaining entire had largely lost their crushing strength. The last three materials: molecular sieves, a-A1203 in macropore form and carborundum could be immediately rejected as possible candidate materials. The first one because it looses its physical properties and degraded severely when being loaded with ferrous chloride, and the other two due to their very small retention capacity. Thus, experimental work has been carried out with the following support materials: Attapulgus Clay, pumice, silicagel and 3,-alumina. The study of the hydrolysis has been performed in a batch-wise operated fixed bed reactor. The parameters studied were the reaction temperature and the type of support. The main point of interest in these studies are the hydrogen yield and the degree of water conversion; i.e. the ratio of water reacted/water introduced. It appeared that most support materials exert a catalytic effect on the hydrolysis reaction, resulting in high degrees of conversion. For instance, at 570°C with Attapulgus Clay as a support, 36% of the entered water is converted, whereas with pumice this figure is 25%. At higher temperatures still higher conversion figures have been found, e.g. 45% with Attapulgus Clay at 650°C. Here, it is noted that the degree of H20 conversion for the unsupported reaction is found to be 20% at 600°C and 28% at 650°C (see below). This catalytic action, however, is accompanied by an inhibiting effect on the hydrogen formation. In all cases, hydrogen yields are considerably lower than theoretically possible. The above is adequately illustrated in Table 2 where the complete results of some experiments with various supports are given. It follows clearly that, although the decomposition of Fecl2 is nearly complete, the hydrogen yields are extremely small (between 3 and 25%). For the silica-containing supports (Attapulgus Clay, pumice and silicagel) this may be due to the intermediate formation of ferrous orthosilicate (Fe2SiO4). This compound is very stable and known as a mineral under the name "Fayalite." It can be synthesized at high temperatures by reaction of Fecl2, SiO2 and water in a hydrogen atmosphere [5], i.e. under very similar conditions as in the hydrolysis reaction of the Mark-9 cycle. Indeed, wet chemical analysis of the solid products of the runs 51 and 54 showed that at least 75 to 80% of the total iron after reaction was still in the bivalent state. The reaction to ferrous orthosilicate is obviously a fast one under the present reaction conditions, considering the very low hydrogen yields obtained (less than 16%). With the "y-AI203 support a somewhat higher hydrogen yield has been found (25%), but it is still largely inferior to an acceptable value. Also here, the formation of an intermediate ferrous compound (e.g. ferrous aluminate) is a probable hypothesis. The conclusion from the above results is that supports containing silica are unacceptable and that, of the ten supports tested, none can be considered suitable. HYDROLYSIS OF FeCI2 W I T H O U T SUPPORT Reaction rates
The feasibility of the hydrolysis reaction without the use of homogeneously supporting agents has been tested. These tests have been carried out in a tubular fixed bed reactor with batches varying between 50 and 500 g at temperatures between 600 and 700°C.
110
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION TABLE 2. Hydrolysis experiments with various supports Run No. Temperature [°C]
47/74 570
48/74 650
51/74 650
52/74 650
54/74 570
58/74 600
Attap. 51 76 0.382
Attap. 51 76 0.382
Pumice 64 50 0.252
Silica 100 90 0.453
Silica 64 56 0.282
~,-A1203 100 45 0.226
1.0 35.0 1.944
1.0 35.0 1.944
1.0 40.0 2.222
1.0 42.0 2.333
1.0 50.0 2.778
1.0 45.0 2.500
464 30 0.006 0.676
380 47 0.007 0.760
450 70 0.013 0.530
250 50 0.005 n.d.
380 70 0.011 0.562
730 85 0.026 n.d.
80 27.9 n.d. 6.6
75 27.5 n.d. 1.4
80 17.7 13.0 2.4
n.d. n.d. n.d. n.d.
84 18.7 14.8 0.1
IN Support (Type) (g) FeCI2.4HzO (g) (gmol) H20-Rate (g/rain) H20 Total (g) (gmol) OUT Gas (nil) H2 content (%vol) H2 (gmol) HCI (gmol) Solids (g) Fe~t(wt%) Fe2~ (wt%) Cl (wt%) Fe~o, (pat) Cltot (pat)
0.400 0.771
FeCl2-convers. (%) H2-yield (%)
88 5
0.370 0.790 96 6
0.254 0.584
n.d. n.d.
91 16
n.d. 3
0.282 0.564 100 12
112 10.7 5.5 0.6 0.215 n.d. 96 25
A sketch of the e q u i p m e n t is given in Fig. 1. The reactor consists of quartz tubes of 3 and 5 cm i.d. respectively, and a length of 60 cm. W a t e r is i n t r o d u c e d by m e a n s of a m e t e r i n g p u m p into an e v a p o r a t o r and s u p e r h e a t e r so that the s t e a m input rate is k n o w n a n d constant. T h e f o r m e d gaseous p r o d u c t s are first cooled d o w n to r o o m t e m p e r a t u r e , c o n d e n s e d and collected in the first p r o d u c t collector, t h e n the excess h y d r o g e n chloride is a b s o r b e d in a w a t e r s c r u b b e r and finally the evolved gas (hydrogen) is collected u n d e r water, m e a s u r e d and eventually analyzed. A f t e r each run, wet chemical analysis is applied to the c o n d e n s a t e and the w a t e r scrubber, so that the a m o u n t of u n r e a c t e d water, the a m o u n t of liberated h y d r o g e n chloride as well as the q u a n t i t y of evolved h y d r o g e n are quantitatively known. T h e weight and the chemical analysis of the solids r e m a i n i n g in the c o l u m n t h e n allows p r e p a r a t i o n of c o m p l e t e material balances for every e x p e r i m e n t .
Reoctor(I D.=Scrn,length-60 cm) S ~ E ~mrpp°rot°r ing
or
Firs?product collector
~_ _
Scrubber
H20 FIG. 1. Sketch of experimental apparatus.
o~co.ector
111
D. Van VELZEN AND H. LANGENKAMP
IO0--
6o 80--
~ ~ T . 6 5 0 ~
C
"o
40 --
.,~
FeCL2 in : 200 g ( 1.58 gmoL) H20 rote: 1.44gmin -~
£ 2o-
,
I
50
i
t
I
2
i
~
[
I00,
4
i
~
~
H20/FeCL 2in,
1,50
,Time, min
,
8
gmoL/gmoL
FIG. 2. Hydrogen evolution in hydrolysis of FeCl=, Run No. 14/?.5. During these experiments, it appeared that practically always very high hydrogen yields and FeC12 conversion were obtained (between 80 and 100%, depending on the total water throughput). A n example of the hydrogen evolution as a function of time for a typical hydrolysis run is shown in Fig. 2. The reaction time is proportional to and can be substituted by the normalized water throughput, i.e. the total amount of water fed to the reactor divided by the FeCI2 charge. The normalized water throughput is expressed as gmol H20/gat Fe. The hydrogen evolution is normalized to hydrogen yield, i.e. the amount of hydrogen obtained divided by the amount theoretically formed at 100% FeC12 conversion. The course of the curve is typical for the hydrolysis reaction. The slope is dependent on various parameters, which is shown in Figs. 3 and 4. Figure 3 shows the hydrogen yields as a function of normalized water throughput with reactor temperature as a parameter~ whereas Fig. 4 gives a plot of the same quantities with water feed rate as a parameter. It follows that the reaction is favoured by increased temperatures, but that the effect of temperature is less important than the effect of the water feed rate. When the H2-yield is plotted as a function of time with water feed rate as a parameter (Fig. 5), it can be concluded that an increase of water feed rate leads to considerable increases of the reaction rate. For instance, 50% hydrogen yields are reached at 20 min with a water feed rate of 10 g/rain and at 75 rain with 0.8 g/min. As the curves for 5 and 10 g/min nearly coincide, it can be anticipated that further increases of the water feed rate would not lead to substantially higher reaction rates. The gain in reaction rate, however, is more than balanced by a much higher excess of water (7.9 against 2.1 mol H 2 0 input/mol of FeCI2, where for 50% H2-yield the stoichiometric amount is 0.67 tool H20/mol FeC12). Obviously, optimum reaction conditions will have to be determined by an optimization procedure.
I00 --
650oC 6oo°C
700%
~
8O-"o 60
/ / /
FeCL2 in :200g(I.58gmoL)
40
20
I
2
5
4
5
Normalized water throughput,
6
7"
8
9
:C
g moL H20/~tool FeCL2
FIG. 3. Effect ef temperature on hydrogen yield.
112
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION IOC
-
6C
a~ >, I
1 7 ~ ~
2C
0
"1
I
I
2
I
3
reCL2:2 oo~
I
4
I
I
5
I
6
7
Normalized water t h r o u g h p u t ,
I
8
I
9
(l.ssgmoL)
I
I
I0
II
J
12
I
13
g m o L H~O/g real FeCL 2
FIG. 4. Effect of water feed rate on hydrogen yield.
Degree of water conversion Another point of importance for the hydrolysis reaction is the degree of water conversion, i.e. the ratio of reacted water to feed water. This value can be calculated for any period of the run lying between to and h by: 1
~y+x Z
where a = x= y= z=
water molal molal molal
conversion, amount of hydrogen formed between to and tl, amount of hydrogen chloride formed between to and h, amount of water introduced between to and h.
In Figs. 6 and 7, a is shown as a function of the hydrogen yield which can be assumed to be approximately equal to the FeCI2 conversion, respectively with temperature and water feed rate as parameters. It follows that generally ~ decreases with increasing FeC12 conversion, obviously becoming zero when all FeC12 has been converted. The maximum value obtained at 650°C is 0.32 for a H20 feed rate of 0.8 g/rain. The effect of increasing temperature on a is obviously positive whereas also a decrease of the water feed rate leads to a considerable increase in t~ (0.14 at 10 g/min to 0.27 at 0.8 g/min at 50% H2-yield).
Continuous bench scale unit A logical continuation of the type of work described in the preceding section is the design and construction of a continuously working bench scale unit, provided that sufficient data for the design of such a unit are available. In the present case, the data obtained on the laboratory scale are amply sufficient and, consequently, a bench scale plant has been constructed. The equipment is schematically drawn in IOC
-
8C
6c Fe CL2: 2 0 0 g ( I . 5 8 g reaL)
4c 2£
I
50
I
I00 Time,
I
150
I
200
rain
FIG. 5. Effect of water feed rate on reaction rate.
D. Van VELZEN AND H. LANGENKAMP
113
3C --
600%
2C o~
T
IC
H20 rote 1.44g min ~'
~
\& I
0
I
io
I
I
I
I
I
20 30 40 50 60 70 H2 yield, %
I
80
,~
90
,00
FIG. 6. Effect of temperature on H20 conversion. Fig. 8. It consists of a moving bed reactor where FeC12 travelling downward, is contacted with water vapour in countercurrent. The reactor consists of a 5 cm i.d. reactor tube of Hastelloy C with a heated length of 60 cm. Above the reactor there is a feed storage vessel. The reactor and part of the storage vessel are filled with the solid reactant. At the bottom of the reactor, there is an automatic discharge device. Here, a rod is moved at regular intervals through the pile of solids, at the reactor outflow, passing a certain quantity alternately through one of the two inlet ports of the product storage vessel. By application of a timer system, the flow of solids through the reactor can be varied. The water vapour flow rate is also here controlled by a metering pump, followed by an evaporator and a superheater. The treatment of the gaseous reaction products is essentially the same as in the batch fixed bed equipment. In a moving bed reactor, the solids have to be in the form of granules, pellets or spheres, as the presence ol fines gives rise to severe ditiiculties in the solids downflow. This statement has been confirmed by tests in the present reactor; if ferrous chloride in a finely powdered form is present in more than a few percents, the solids flow at the reactor inlet occasionally stops. For this reason, it has been decided to operate the reactor with ferrous chloride in a granulated form, mixed with ballotini of zirconium oxide. These ballotini have a diameter between 3 and 5 mm and the material is completely resistant to the present reaction conditions. There is another advantage for the use of an inert material, heterogeneously mixed with the reactant. The hydrolysis reaction is endothermic, about 81.5 kcal are required for the formation of I gmol of H2. It is obvious that severe difficulties are to be expected in large reactors to transfer this energy to the reactants. The addition of inert material could yield a suitable solution; e.g. if the reaction is carded out in a mixture of about 10 wt% FeC12 and 90 wt% ZrO2, a preheating of the solids to about 120-130°C above the solids outlet temperature could furnish practically all the necessary 30 --
~
1
T=650 °C FeCL2in200g( 1.58g tooL)
10
~
~
t
I
I
I
I
l
I
I
~0
~0
30
40
50
60
70
80
H2 yield,
\
I~11 90 ,00
%
FIG. 7. Effect of H20 feed rate on H20 conversion.
114
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
Storogefeed(FeCL2)
Gasoutlet. H2HCL H~) Reactor(T=6~
inlet (HzO)
5as
Autamal"ic
s°lids rernov°l-J TJ"-JIt~__ Storoge product (F'e304)
FIG. 8. Sketch of continuous moving bed reactor for the hydrolysis of FeCl2.
reaction heat and practically no additional heat would be required. This advantage could largely outweigh the drawback of a larger solids flow rate through the reactor. Some successful experiments in the bench scale plant for the hydrolysis of ferrous chloride have been carded out. Here, the 5 cm i.d. moving bed reactor has been fed with a heterogeneous mixture of zirconia ballotini and anhydrous ferrous chloride in a weight ratio of about 9 : 1 at 650°C. The solids feed rate was 78 g/rain zirconia and 8.8 g/min ferrous chloride. In countercurrent water vapour has been entered at a rate of 5.6 g/min, which resulted in a hydrogen development rate of 0:42 l/min. Moreover, the exit gases consist of 2.521/min HC1 and 5.75 l/rain unreacted water vapour; here all flow rates are expressed at S.T.P. This corresponds to a water conversion of 23% and a ferrous chloride conversion (and consequently, hydrogen yield) of about 80%. Thermodynamic calculations of the equilibrium at 650°C indicate a water conversion of 25.3%, so that the conclusion can be drawn that the present reactor operates in a satisfactory way. It must be noted that the above reactor conditions lead to a downward solids velocity of about 1.5 cm/min, whereas the actual calculated upward gas velocity (at actual reactor conditions) is approximately 20 cm/s superficially at 650°C.
Future development At this point it seems useful to make a statement about the significance of the above described equipment in the total process of plant development: The upward superficial gas velocity of 20 cm/s corresponds to a hydrogen production rate per unit reactor cross sectional area of about 14 m3/m2h at S.T.P. From this datum, the required reactor surface for a hydrogen production of 100,000 m3/h (which is a reasonable output for a full-size production unit) can easily be calculated. It follows that this reactor cross sectional area will be about 7 5 0 0 m 2, i.e. a cylindrical reactor of 100 m i.d. It will be clear that this is a completely unrealistic proposal. The extremely large reactor diameter required is mainly a consequence of the low H2 content of the exit gases, which is only about 5% by volume. The conclusion is obvious: for a future
D. Van VELZEN AND H. LANGENKAMP
115
development of large scale reactors, the moving bed is an unsuitable type of reactor, as here the gas velocities are limited to values of about 0.5 m/s. Reactor types are needed in which gas velocities of 10--20 m/s can be achieved, and entrained fluidized beds could offer good possibilities here. This type of reactor is already applied (e.g.) in the calcination of aluminium hydroxide, where in a reactor of 4 m i.d. approximately 30 t/h of A1203 is produced [6], with gas velocities of approximately 4 m/s, i.e. a solids throughput of 2.4 t/h m 2. In this respect it is interesting to note that for the production of 100,000 m3/h H2 about 1000 tha of Fe304 has to be processed. This would then call for a reactor cross sectional surface of 400 m 2, or one reactor of 22 m i.d., adopting the specific throughput of 2.5 t/h m 2 of Fe304. It must be noted that further optimization might be possible. It can thus be concluded that reactors of this type may offer reasonable possibilities for full scale application. The moving bed reactor has to be considered a valuable equipment in the framework of a small-scale demonstration plant, where basic data about the chemical side of the process can be obtained. Its potentials for a full-scale application, however, must be considered as extremely restricted. It is interesting to note that here the reactor dimensions are mainly dependent on the gas flow rate and that the solids throughput plays a minor role. This implies that the often heard objection against the thermochemical production of hydrogen, i.e. the large masses of solids to be transported per unit volume of hydrogen produced, may not be as important as has been supposed before. CHLORINATION OF MAGNETITE
Simplified reaction scheme It is useful to analyze the chlorination reaction of the Mark-9 cycle, before discussing the experimental results. In reality reaction (2) is not unique, but a composite of a number of competing reactions. In the first place there is the reaction 3Fe304 + 1½C12---, 4Fe203 + FeCI3.
(4)
This reaction is an exothermic one; it proceeds practically at all temperatures above 100°C, its equilibrium is strongly shifted to the right. It is highly probable that reaction (4) occurs wherever Fe304 comes into contact with chlorine and also that this reaction is very rapid in comparison to the other ones involved. Therefore, and for simplification purposes, it will be assumed that magnetite is first converted to ferric oxide by reaction (4) and that, subsequently, ferric oxide reacts, either according to reaction (5) or (6), or the two simultaneously. Fe203 + 6HCI---, 2FeCI3 + 3H20.
(5)
Fe203 + 3C12 --->2FeC13 + 1½02.
(6)
Reaction (5) is known to proceed well at low temperatures in the order of 100-200~C, whereas reaction (6) is best performed at 800-1000"C. Additionally, in the gas phase occurs the well-known Deacon reaction, either in one or in the other direction; H20 + C12 a=~2HCI + ½02.
(7)
The above reaction scheme is obviously a simplified one, there are still many other intermediate reactions possible. Nevertheless, it is believed that the above scheme is sufficient for a better understanding of the observed phenomena, which will be discussed below.
Low temperature The chlorination of magnetite has been carried out at atmospheric pressure in the fixed bed apparatus also used for the hydrolysis studies. In the first series of experiments the gas feed consisted of HC1/C12 mixture of various proportions, from 8 : 1 to 1 : 1, introduced at a rate between 20 and 40 l/h S.T.P. The amounts of magnetite charged into the reactor varied between 20 and 50 g. This iron oxide feed consisted generally of the reaction product from previous hydrolysis experiments, whereas Fe304 purchased from the British Drug House has also been used. In the course of the
116
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
study it appeared that the latter product is very impure and cannot be considered as pure Fe304. Its content of bivalent iron has been analytically determined as 19.4% (expressed as FeO), whereas the theoretical value should be 31.0%. It is interesting to note that the bivalent Fe content of the magnetite produced by the hydrolysis reaction has been found to be between 29.0 and 32.0% for various samples. The iron oxide feed for the chlorination experiments has been applied either in the form of granules or as a powder. The latter may be heterogeneously mixed with an inert material (e.g. Attapulgus Clay or zirconia ballotini), or distributed in a glass wool bed. The method of application of iron oxide has been varied because the results with the pure magnetite granules (Run 39/74) led to the formation of big lumps of a mixed solid product. These lumps apparently had a very low accessible reaction surface and were very cumbersome to handle. A feasible technique to overcome the difficulties in solids handling has been the premixing of magnetite with Attapulgus Clay or zirconia ballotini. When the magnetite and the inert material are brought into intimate contact, the magnetite adheres to the support's surface and a free flowing mixture results. The formed FeC13 remains finely divided on the support and the solid product after the reaction is still free flowing. Solids handling problems are thus apparently minimal. The results of the experiments can be summarized as follows: at a temperature of 150-180°C the yields of FeCl3 are satisfactory, between 66 and 85% obtained in 1 - 2 h . However, in none of the experiments, even with a HCI/C12 ratio of 1 : 1 , has a perceptible production of oxygen been observed. The absence of oxygen formation at low temperatures has been confirmed by the results of Japanese researchers [7], From the above considerations the conclusion can be drawn that the chlorination reaction originally proposed for the Mark-9 cycle, in which chlorination and oxygen formation occur at the same time and at low temperature, cannot be performed at 150-200°C.
Intermediate temperature The chlorination runs at intermediate temperatures, i.e. 300, 400 and 500°C, showed a strong decrease of the reaction rate, whereas the oxygen production still remained negligible. Obviously, in this region, reaction (5) as well as reaction (6) are proceeding extremely slowly. Increasing the temperature to approximately 700°C, the reaction rates become higher and appreciable amounts of FeC13 are formed. Above this temperature reaction rates continue to increase with temperature, however, staying considerably below the reaction rates observed at 150°C where reaction (5) is the rate-determining one.
High temperature The field between 730 and 850°C has systematically been covered by a series of chlorination tests. In these runs 23 g of magnetite has been inserted and chlorinated by a mixture of 101/h HC1 and 7.5 to 8.71/h C12 for 120 min. The runs all proceed in a characteristic way. In the first few minutes, a rapid formation of ferric chloride is observed. This occurs before any oxygen or chlorine has been monitored in the outlet gas. After approximately 10 min a very constant rate of development of oxygen and ferric chloride is stabilized. After the fixed period of 2 h, generally no sign of decrease of the oxygen formation rate is observed. The FeCI3 production obtained in 2 h varied between 16 and 40% of the theoretically possible amount. A suitable working hypothesis for these series of experiments is the following. (a) In the first few minutes a rapid reaction of magnetite with chlorine takes place according to reaction (4). For complete conversion of Fe304 into Fe203 and FeCI3 a total amount of 1.21. S.T.P. of C12 would be sufficient. This corresponds well with the quantity introduced during the time needed to arrive at steady state conditions. (b) (Subsequently, reaction (6) takes place accompanied by reaction (7) in the vapour phase (in reverse direction obviously). It is highly improbable that reaction (5) should proceed at a non-negligible rate, since it is known that the equilibrium of this reaction is negatively influenced by an increase of the temperature. This second part of the reaction scheme does not proceed to completion at the actual reaction conditions. Therefore, it is advisable to put more emphasis on what happens during this second phase for a proper judgement of the experimental results.
D. Van VELZEN AND H. LANGENKAMP
117
Oxygen production The oxygen production during chlorination occurs only in the last part of the reaction (reactions (6) and (7)). It may well be expressed as the ratio oxygen/ferric chloride formed by these reactions. This ratio is calculated from the experimental results by the relation
/3=
P q -0.111W
where /3 = p= q= W=
the the the the
oxygen production ratio (gat O/gmol FeC13 formed), amount of oxygen formed (gat), total amounts of FeCI3 (gmol), initial amount of Fe fed to the reactor (gat Fe).
In this way, from the total ferric chloride yield the amount presumably formed by reaction (4) is subtracted, the remainder being the FeCI3 produced by the combination of reactions (6) and (7). The dependence of/3 on the reaction temperature is given in Fig. 9. It follows that the oxygen production is strongly temperature dependent. The value of /3 required for reaction (2) is 0.375. From Fig. 9, it can be concluded that this value is obtained at 730-740°C at the present reaction conditions. Consequently, the Mark-9 cycle could be modified by introduction of this higher temperature for the chlorination step, working with a HCI/C12 feed ratio of 1.25 : 1. HCI and C12 conversion A point of paramount importance is the degree of conversion of the gaseous reactants, determining the excess of HCI and C12 which should be recycled in the process. Also in this case a correction has to be applied for the amount of chlorine reacted in reaction (4). A plot of the experimentally obtained HC1 and C12 conversion as a function of reaction temperature is shown in Fig. 10. It follows that the HCl-conversion decreases slightly with increasing temperature, from 6.5% at 730°C to about 4% at 850°C. The chlorine conversion is much more temperature dependent. It increases sharply with increasing temperature, i.e. from 1.5% at 730°C to approximately 18% at 850°C. In the preceeding section it was concluded that the Mark-9 chlorination reaction could be performed at 730-740°C. A t this temperature, HC1 and 0 2 conversions are very low, 6.5 and 1.5%, respectively. This leads to a recycle of HC1 of about 15 times the reacted quantity. For C12 this figure would be even higher, about 60 times! Therefore, the conclusion can be safely drawn that performing the chlorination step in the Mark-9 cycle according to reaction (2) at 730-740°C is not very attractive, and alternative solutions are to be investigated.
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,
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FIG. 9. Oxygen production as a function of chlorination temperature.
118
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION re304 feed : 23cj HCL feed IOL.h- S.T.R CL2 feed: 8L.h-~ S.'EE 2c -
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FIG. 10. HCI and Cz conversions in the chlorination of Fe203. A L T E R N A T I V E SOLUTIONS General In principle, there are three possibilities to close an Fe-C1 cycle. (i) Carrying out the chlorination so that the stoichiometric amount of oxygen is formed in one reaction only. (Mark-9, reaction 2)). (ii) Carrying out the chlorination so that an excess of oxygen is formed, followed by a Deacon-reaction for the off-gases. In this case chlorination at elevated temperatures (800-900°C) could offer a good possibility. (iii) Carrying out the chlorination producing no or a short amount of oxygen, followed by a reverse Deacon-reaction for the off-gases. In this case chlorination at 150-200°C would be appropriate. In all three cases the overall production of FeCla should be six times the overall molal amount of oxygen produced. The first possibility has been investigated in the preceeding section and rejected due to very low conversions of the gaseous feed components. The second possibility calls for very high temperatures of about 800-900°C. From the foregoing it can be concluded that here rather low chlorine conversions have to be expected (see Fig. 10). Moreover, when an excess of oxygen is produced in the chlorination reaction, the excess of oxygen has to be reconverted into chlorine in the reverse Deacon-reaction, to close the cycle. This will lead to a further increase of the amount of chlorine to be recycled. A n additional disadvantage of this solution is the extreme corrosivity of the mixture of ferric chloride, chlorine and oxygen at the reaction temperature, possibly still enhanced by the pressure of hydrogen chloride and water. For the above reasons, the second possibility is also not very attractive. The third possibility appears to be more promising. Here, the expected conversions are rather high, as may be illustrated by the following example (exp. 26/75). 23 g of magnetite mixed with 200 g of ZrO2 ballotini has been chlorinated in a stream of 211/h-lHC1 and 4.01/h-lC12. Fig. 11 shows at1 for this experiment as a function of time. It follows that during the first 30 rain a = 100% and then gradually drops to about 10%. It is noticed that after 95 rain the yield of F e C 1 3 has been 86% in this run. Besides the good yields and conversions, there is the advantage of avoiding the handling of a set of extremely corrosive materials at elevated temperatures. It has been stated before that the oxygen production at low temperatures is negligible. Consequently, chlorine reacts only to oxidize the bivalent iron of the magnetite. Elsewhere in the cycle, at the disproportioning of ferric chloride, all this bivalent iron has to be reconverted into the bivalent state. It would thus be advantageous to leave the ferrous part of the magnetite unoxidized during the chlorination. This can be done when the chlorination is carried out with hydrogen chloride only; in this case the reaction should be FeaO4 + 8HC1 ~ FeCI2 + 2FeC13 + 4H20. It is this reaction which has been the subject of further investigations.
D. Van VELZEN AND H. LANGENKAMP
119
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Chlorination with hydrogen chloride A series of experiments has been carried out with the reaction temperature as main parameter. The hydrogen chloride flow from the reactor (i.e. the unreacted chlorination agent) has been dissolved in water and titrated. Changing the contents of the wash bottles at regular intervals of time allows the determination of the iron oxide conversion as a function of time. Figure 12 gives an illustration of the experimental results. It appears that the overall reaction rate decreased largely with increasing temperature, e.g. in 60 minutes at 150°C a conversion of 80% is obtained, whereas at 350°C this figure is decreased to 52% and an increase of the temperature to 400°C yields only a conversion of 26% in the same time period. All runs represented in Fig. 12 have been carded out with magnetite produced by hydrolysis of ferrous chloride. Always 23 g of magnetite has been chlorinated in a tubular fixed bed reactor of 3 cm i.d. with addition of 150-300 g of ZrO2 ballotini. The HCl-flow rate has been varied between 40 and 80 l/h; this variable has no perceptible influence on the observed reaction rates. The HCl-conversion at'low temperature (<200°C) is very similar to the one given in Fig. 11. When the reaction temperature is increased, obviously the HCl-conversion drops: at 350°C the average HCl-conversion over the first 15 rain of the experiment has been about 20%. The addition of the zirconia ballotini has been carried out for the same reasons as mentioned in the part on hydrolysis of FeC12, i.e. as a heat sink and as a solids flow promoter. It must be noted that presumably the chlorination can be performed in the same continuous moving bed reactor as used for the hydrolysis of FeCI2 without major difficulties. Experiments of this kind are planned for the near future.
Effects of magnetite purity It has been mentioned before that commercially available magnetite (B.D.H.) is very impure. Not only that the bivalent iron content has been found 15.1 wt% against 24.1% theoretically,
80
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FIG. 13. Chlorination with hydrogen Chloride; commercial grade magnetite. also the presence of impurities as SiO2 (2.4%), carbon (1.7%) and sulphur (0.3%) has been ascertained. The behaviour of this impure form of magnetite during chlorination with hydrogen chloride is anomalous. In the first place, initial reaction rates at 380-400°C are superior to those of pure magnetite and, once having reached a conversion of about 45%, the reaction proceeds at a very slow rate only. In practice, this phenomenon limits the iron oxide conversion to about 55%. These features are illustrated in Fig. 13, where the iron oxide conversion is shown as a function of time. Another point of difference between the chlorination of commercial and pure magnetite is that pure magnetite yields ferrous and ferric chloride proportionally and that the amount of bivalent iron before and after the experiment, is virtually the same. The commercial grade magnetite shows a yield of ferrous chloride largely superior to the initial quantity of bivalent iron. For instance, in an experiment where 220 g of iron oxide (containing 17.2% moisture) has been chlorinated for 2 h with 82 !./h HC1 at 385°C, the initial amount of bivalent iron was 0.49 gat, whereas in the solid products after the reaction 1.03 gmol of ferrous chloride has been found. In another example, chlorination of 4 0 g of magnetite (containing 0.090gat Fe 2+) yielded 0.155gmol of FeC12 at 380°C. The mechanism of this oxidation/reduction reaction in the commercial grade product is not clear yet. Experimental work to elucidate this point is in progress. CONCLUSIONS The work described in the present paper can be summarized in a number of conclusions. The hydrolysis of ferrous chloride gives good results and can be carried out in a moving bed reactor for a small scale demonstration plant. This equipment is ready for direct use. For large scale application, the reaction has to be carried out in a reactor type allowing gas velocities of the order of 10 m/s. This could possibly be an entrained fluidized bed. A moving bed is unsuitable for this purpose. The chlorination step proposed for the original Mark-9 cycle cannot be carried out at 150 ° 200°C, as originally anticipated. Increasing the temperature to 730-740°C would enable the reaction to proceed, however with low HC1- and C12-conversions. The best alternative solution seems to be low temperature chlorination with hydrogen chloride and introduction of the reverse Deacon-reaction in the cycle. The low temperature chlorination proceeds with good yields and conversions. It is clear that the success of the Fe-CI cycle family thus depends neither on the hydrolysis nor on the chlorination step. The question that arises is whether the unfavourable equilibrium conditions in the disproportioning of ferric chloride and/or the expected high costs of investment could possibly jeopardize this success. The discussion of these points, however, is beyond the scope of the present paper and will be treated elsewhere. REFERENCES 1. HARDYC., Thernlal decomposition of water using cycles of the Fe--Cl-family, Report EUR 4958f, (1973). 2. SCH3,FERH., Z. Anorg. Allgem. Chemie 266, 268 (1951).
D. Van VELZEN AND H. LANGENKAMP
121
3. Wmsor~ L. E. and GREGORY N. W.; J. Phys. Chem., 62, 433 (1958). 4. HAMMER R. R. and GREGORY N. W., J. Phys. Chem., 66, 1705 (1962). 5. GMELIN, Handbuch der Anorganischen Chemie, Verlag Chemie, Weinheim, 8. Auflage, Band 59 Fe, Teil B, p. 763. 6. B6nM E. and SCHMIDT H. W., Chemische Prozesse in Expandierten Wirbelschichten, I. Chem. Eng. Syrup. Series No. 43, paper 4. 7. ONo M., Mitsubishi Research Laboratories, Private communication.
DEVELOPMENT STUDIES ON THERMOCHEMICAL CYCLES FOR H Y D R O G E N PRODUCTION D. VAr~ VELZENAND H. LANGENKAMP C.C.R. Euratom Ispra, Italy &ImPart--The experimental work in the field of the development of cycles of the Fe/C1 family is described. It appeared that application of a homogeneous support material for the gas-solids reaction was not feasible due to unexpected side reactions. The hydrolysis of ferrous chloride has been found to give good results and the development and operation of a continuously operating bench-scale reactor for this reaction has been described. The chlorination of the resulting magnetite could not be performed according to the originally proposed scheme. A satisfactory alternative has been found, i.e. chlorination at 150-200°C with only hydrogen chloride and introduction of the reverse Deacon reaction as a fourth reaction in the cycle. INTRODUCTION THE NUMaeR of proposed cycles for the thermochemical decomposition of water is very considerable. The stage of development of a large majority of these cycles does not pass the thermodynamically calculated feasibility, often accompanied by rough estimates of expected thermal etticiencies. The number of cycles on which work on laboratory scale has been carded out is very limited and the number of cycles which are subject of a development directed study is most probably less than ten. The iron-chlorine family cycles belong to the last category and can even be considered the most important member of this group, as investigations are being carried out in Germany, Japan, the U.S. and Italy. The major attraction of cycles of this family is the fact that the chemicals involved are common and relatively cheap, so that minor losses would not directly jeopardize the economics of the process. In this paper the development study carded out at the J.R.C., Ispra, Italy, will be discussed. The main objective of this work has been to obtain the necessary data for the construction of a bench-scale continuous demonstration plant. This implies that the orientation of the study has been sometimes more towards the development of practical solutions for existing problems than towards a purely scientific approach. As an example, a considerable effort has been spent on the search for a suitable support material for the solid reactants to minimize transport problems. This part will be treated more extensively below. The cycle acting as the point of departure was the well-known Mark-9 cycle proposed by Hardy [2] in 1972. It consists of the following three reactions 6FeC12 + 8H20 ~ 2Fe304 + 12HC1 + 2H2 2Fe304 + 12HC1 + 3C12 --* 6FeC13 + 6H20 + 02
(600-700°C) (150-200°C)
6FeC13 ---*6FeC12 + 3C12 (300-400°C)
(1) (2) (3)
This paper deals with the study of a possible homogeneous support material, the hydrolysis of ferrous chloride and development work in the chlorination of magnetite. The last reaction, the disproportioning of ferric chloride, will not enter into the considerations, as practical work on this reaction has been carried out at the Teclmical University of Aachen in Western Germany, together with the fact that data for this equilibrium can be taken from the literature [2, 3, 4]. D E V E L O P M E N T STUDIES F O R A H O M O G E N E O U S SUPPORT M A T E R I A L The idea to perform chemical reactions involving solids and gases where the solid reactant or catalyst is homogeneously distributed on an inert support material is not new. This technique is most extensively applied in heterogeneously catalyzed gas reactions. The main advantages are highly increased reaction rates due to the large accessible reaction surface t07
108
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
and the increased ease of solids handling. It seems thus very attractive to apply this technique also to the gas-solid reaction for the Fe-Cl-cydes. The study has been carried out by a systematic analysis of retention capacity and internal surface for a n u m b e r of candidate materials. T h e r e are four criteria to be considered for the selection of suitable supports: (a) In the first place it must be noted that a drawback of the use of a support material is the increase of the (already very large) bulk weight solids transport. Therefore, it is important that the support retains a maximum amount of reactant. (b) A second condition is that a good support must be physically and chemically resistant to the various reaction conditions. (c) Thirdly, the support should not exert an inhibiting action on vital reactions, nor give rise to unwanted side reactions. (d) Finally, it must be noted that a good support should not be excessively expensive. Ten possible supports have been tested, and a survey of the results is given in Table 1. The retention capacity of each support has been determined by saturation with concentrated (60%) solution of FeC12-4H20 in water. The support, previously dried in vacuum at 150°C to constant weight, is stirred with an excess of this solfition for 20 min and then filtered. The retained weight of solution is determined and the retention capacity (expressed in g FeC12/100 g of dry support) is calculated. The internal surface of each support before and after loading with ferrous chloride has been determined by means of nitrogen absorption using an A r e a m e t e r of the Firm Strthlein. It follows that Attapulgus Clay, a mineral material mainly consisting of a hydrous aluminium silicate, shows the highest retention capacity a m o n g the materials tested. Its initial specific surface is 76 m2/g and after being loaded this figure decreases to 16 m2/g, indicating a large accessible reaction surface. The material, also known under the name "Fuller's earth", has frequently b e e n used in industry as an adsorbent or a cracking catalyst and is satisfactorily stable at the reaction conditions occurring in the Mark-9 cycle. It seems to be the most promising support material available. TABLE 1. Characteristics of various support materials
Support Attapulgus C l a y
Manufacturer
Active carbon
PhillipsM i n . and Chem. Degussa
Pumice
Merck
Silica Silica
Impianti Sist. Gel Carlo Erba
"v-Alumina
Doduco
Molecular Sieves ~/-Alumina
Grace
-Aluminamacropore Carborundum
Doduco Norton --
Form granules i mm irregular 2-5 mm irregular 4-12 mm irregular 5-7 mm irregular 6-10ram spheres 2--4mm pellets 2 x 6 mm spheres 5-8 mm spheres 2--4mm spheres 2-4 mm
Retention capacity (g FeC12" 100g ')
Specific surface (m2/g) original .loaded
84
76
16
45
n.d.
n.d.
45
0.01
0.01
32
440
27
32
440
4
32
247
27
29
8
9
25
226
57
12
0.14
0.04
12
0.14
0.14
D. Van VELZEN AND H. LANGENKAMP
109
Active carbon also has an acceptably high retention capacity, but it appears unsuitable as a support, due to the occurrence of unwanted side reactions. In the hydrolysis step, active carbon support reacts with excess water forming carbon monoxide and hydrogen, whereas also in the oxygen formation step a reaction between the support and a cycle reactant occurs, in this case with liberated oxygen. Here, carbon dioxide was the obtained product and the oxygen formation was completely suppressed. Pumice also offers good possibilities as a support material, showing a retention capacity of 45 g/100 g dry support. It is noteworthy that this material, although being very porous, has an extremely small specific surface. Its stability under reaction conditions is satisfactory. The retention capacity of silica seems to be rather low in spite of its very high specific surface, which makes it somewhat less attractive as a support material than the above mentioned ones. Its stability is fairly good. This is not the case with the 3,-A1203 supports. It appeared that upon reaction at high temperature, i.e. above 600°C, this material embrittles and partly degrades into a fine powder. The A1203 spheres still remaining entire had largely lost their crushing strength. The last three materials: molecular sieves, a-A1203 in macropore form and carborundum could be immediately rejected as possible candidate materials. The first one because it looses its physical properties and degraded severely when being loaded with ferrous chloride, and the other two due to their very small retention capacity. Thus, experimental work has been carried out with the following support materials: Attapulgus Clay, pumice, silicagel and 3,-alumina. The study of the hydrolysis has been performed in a batch-wise operated fixed bed reactor. The parameters studied were the reaction temperature and the type of support. The main point of interest in these studies are the hydrogen yield and the degree of water conversion; i.e. the ratio of water reacted/water introduced. It appeared that most support materials exert a catalytic effect on the hydrolysis reaction, resulting in high degrees of conversion. For instance, at 570°C with Attapulgus Clay as a support, 36% of the entered water is converted, whereas with pumice this figure is 25%. At higher temperatures still higher conversion figures have been found, e.g. 45% with Attapulgus Clay at 650°C. Here, it is noted that the degree of H20 conversion for the unsupported reaction is found to be 20% at 600°C and 28% at 650°C (see below). This catalytic action, however, is accompanied by an inhibiting effect on the hydrogen formation. In all cases, hydrogen yields are considerably lower than theoretically possible. The above is adequately illustrated in Table 2 where the complete results of some experiments with various supports are given. It follows clearly that, although the decomposition of Fecl2 is nearly complete, the hydrogen yields are extremely small (between 3 and 25%). For the silica-containing supports (Attapulgus Clay, pumice and silicagel) this may be due to the intermediate formation of ferrous orthosilicate (Fe2SiO4). This compound is very stable and known as a mineral under the name "Fayalite." It can be synthesized at high temperatures by reaction of Fecl2, SiO2 and water in a hydrogen atmosphere [5], i.e. under very similar conditions as in the hydrolysis reaction of the Mark-9 cycle. Indeed, wet chemical analysis of the solid products of the runs 51 and 54 showed that at least 75 to 80% of the total iron after reaction was still in the bivalent state. The reaction to ferrous orthosilicate is obviously a fast one under the present reaction conditions, considering the very low hydrogen yields obtained (less than 16%). With the "y-AI203 support a somewhat higher hydrogen yield has been found (25%), but it is still largely inferior to an acceptable value. Also here, the formation of an intermediate ferrous compound (e.g. ferrous aluminate) is a probable hypothesis. The conclusion from the above results is that supports containing silica are unacceptable and that, of the ten supports tested, none can be considered suitable. HYDROLYSIS OF FeCI2 W I T H O U T SUPPORT Reaction rates
The feasibility of the hydrolysis reaction without the use of homogeneously supporting agents has been tested. These tests have been carried out in a tubular fixed bed reactor with batches varying between 50 and 500 g at temperatures between 600 and 700°C.
110
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION TABLE 2. Hydrolysis experiments with various supports Run No. Temperature [°C]
47/74 570
48/74 650
51/74 650
52/74 650
54/74 570
58/74 600
Attap. 51 76 0.382
Attap. 51 76 0.382
Pumice 64 50 0.252
Silica 100 90 0.453
Silica 64 56 0.282
~,-A1203 100 45 0.226
1.0 35.0 1.944
1.0 35.0 1.944
1.0 40.0 2.222
1.0 42.0 2.333
1.0 50.0 2.778
1.0 45.0 2.500
464 30 0.006 0.676
380 47 0.007 0.760
450 70 0.013 0.530
250 50 0.005 n.d.
380 70 0.011 0.562
730 85 0.026 n.d.
80 27.9 n.d. 6.6
75 27.5 n.d. 1.4
80 17.7 13.0 2.4
n.d. n.d. n.d. n.d.
84 18.7 14.8 0.1
IN Support (Type) (g) FeCI2.4HzO (g) (gmol) H20-Rate (g/rain) H20 Total (g) (gmol) OUT Gas (nil) H2 content (%vol) H2 (gmol) HCI (gmol) Solids (g) Fe~t(wt%) Fe2~ (wt%) Cl (wt%) Fe~o, (pat) Cltot (pat)
0.400 0.771
FeCl2-convers. (%) H2-yield (%)
88 5
0.370 0.790 96 6
0.254 0.584
n.d. n.d.
91 16
n.d. 3
0.282 0.564 100 12
112 10.7 5.5 0.6 0.215 n.d. 96 25
A sketch of the e q u i p m e n t is given in Fig. 1. The reactor consists of quartz tubes of 3 and 5 cm i.d. respectively, and a length of 60 cm. W a t e r is i n t r o d u c e d by m e a n s of a m e t e r i n g p u m p into an e v a p o r a t o r and s u p e r h e a t e r so that the s t e a m input rate is k n o w n a n d constant. T h e f o r m e d gaseous p r o d u c t s are first cooled d o w n to r o o m t e m p e r a t u r e , c o n d e n s e d and collected in the first p r o d u c t collector, t h e n the excess h y d r o g e n chloride is a b s o r b e d in a w a t e r s c r u b b e r and finally the evolved gas (hydrogen) is collected u n d e r water, m e a s u r e d and eventually analyzed. A f t e r each run, wet chemical analysis is applied to the c o n d e n s a t e and the w a t e r scrubber, so that the a m o u n t of u n r e a c t e d water, the a m o u n t of liberated h y d r o g e n chloride as well as the q u a n t i t y of evolved h y d r o g e n are quantitatively known. T h e weight and the chemical analysis of the solids r e m a i n i n g in the c o l u m n t h e n allows p r e p a r a t i o n of c o m p l e t e material balances for every e x p e r i m e n t .
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D. Van VELZEN AND H. LANGENKAMP
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8
gmoL/gmoL
FIG. 2. Hydrogen evolution in hydrolysis of FeCl=, Run No. 14/?.5. During these experiments, it appeared that practically always very high hydrogen yields and FeC12 conversion were obtained (between 80 and 100%, depending on the total water throughput). A n example of the hydrogen evolution as a function of time for a typical hydrolysis run is shown in Fig. 2. The reaction time is proportional to and can be substituted by the normalized water throughput, i.e. the total amount of water fed to the reactor divided by the FeCI2 charge. The normalized water throughput is expressed as gmol H20/gat Fe. The hydrogen evolution is normalized to hydrogen yield, i.e. the amount of hydrogen obtained divided by the amount theoretically formed at 100% FeC12 conversion. The course of the curve is typical for the hydrolysis reaction. The slope is dependent on various parameters, which is shown in Figs. 3 and 4. Figure 3 shows the hydrogen yields as a function of normalized water throughput with reactor temperature as a parameter~ whereas Fig. 4 gives a plot of the same quantities with water feed rate as a parameter. It follows that the reaction is favoured by increased temperatures, but that the effect of temperature is less important than the effect of the water feed rate. When the H2-yield is plotted as a function of time with water feed rate as a parameter (Fig. 5), it can be concluded that an increase of water feed rate leads to considerable increases of the reaction rate. For instance, 50% hydrogen yields are reached at 20 min with a water feed rate of 10 g/rain and at 75 rain with 0.8 g/min. As the curves for 5 and 10 g/min nearly coincide, it can be anticipated that further increases of the water feed rate would not lead to substantially higher reaction rates. The gain in reaction rate, however, is more than balanced by a much higher excess of water (7.9 against 2.1 mol H 2 0 input/mol of FeCI2, where for 50% H2-yield the stoichiometric amount is 0.67 tool H20/mol FeC12). Obviously, optimum reaction conditions will have to be determined by an optimization procedure.
I00 --
650oC 6oo°C
700%
~
8O-"o 60
/ / /
FeCL2 in :200g(I.58gmoL)
40
20
I
2
5
4
5
Normalized water throughput,
6
7"
8
9
:C
g moL H20/~tool FeCL2
FIG. 3. Effect ef temperature on hydrogen yield.
112
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION IOC
-
6C
a~ >, I
1 7 ~ ~
2C
0
"1
I
I
2
I
3
reCL2:2 oo~
I
4
I
I
5
I
6
7
Normalized water t h r o u g h p u t ,
I
8
I
9
(l.ssgmoL)
I
I
I0
II
J
12
I
13
g m o L H~O/g real FeCL 2
FIG. 4. Effect of water feed rate on hydrogen yield.
Degree of water conversion Another point of importance for the hydrolysis reaction is the degree of water conversion, i.e. the ratio of reacted water to feed water. This value can be calculated for any period of the run lying between to and h by: 1
~y+x Z
where a = x= y= z=
water molal molal molal
conversion, amount of hydrogen formed between to and tl, amount of hydrogen chloride formed between to and h, amount of water introduced between to and h.
In Figs. 6 and 7, a is shown as a function of the hydrogen yield which can be assumed to be approximately equal to the FeCI2 conversion, respectively with temperature and water feed rate as parameters. It follows that generally ~ decreases with increasing FeC12 conversion, obviously becoming zero when all FeC12 has been converted. The maximum value obtained at 650°C is 0.32 for a H20 feed rate of 0.8 g/rain. The effect of increasing temperature on a is obviously positive whereas also a decrease of the water feed rate leads to a considerable increase in t~ (0.14 at 10 g/min to 0.27 at 0.8 g/min at 50% H2-yield).
Continuous bench scale unit A logical continuation of the type of work described in the preceding section is the design and construction of a continuously working bench scale unit, provided that sufficient data for the design of such a unit are available. In the present case, the data obtained on the laboratory scale are amply sufficient and, consequently, a bench scale plant has been constructed. The equipment is schematically drawn in IOC
-
8C
6c Fe CL2: 2 0 0 g ( I . 5 8 g reaL)
4c 2£
I
50
I
I00 Time,
I
150
I
200
rain
FIG. 5. Effect of water feed rate on reaction rate.
D. Van VELZEN AND H. LANGENKAMP
113
3C --
600%
2C o~
T
IC
H20 rote 1.44g min ~'
~
\& I
0
I
io
I
I
I
I
I
20 30 40 50 60 70 H2 yield, %
I
80
,~
90
,00
FIG. 6. Effect of temperature on H20 conversion. Fig. 8. It consists of a moving bed reactor where FeC12 travelling downward, is contacted with water vapour in countercurrent. The reactor consists of a 5 cm i.d. reactor tube of Hastelloy C with a heated length of 60 cm. Above the reactor there is a feed storage vessel. The reactor and part of the storage vessel are filled with the solid reactant. At the bottom of the reactor, there is an automatic discharge device. Here, a rod is moved at regular intervals through the pile of solids, at the reactor outflow, passing a certain quantity alternately through one of the two inlet ports of the product storage vessel. By application of a timer system, the flow of solids through the reactor can be varied. The water vapour flow rate is also here controlled by a metering pump, followed by an evaporator and a superheater. The treatment of the gaseous reaction products is essentially the same as in the batch fixed bed equipment. In a moving bed reactor, the solids have to be in the form of granules, pellets or spheres, as the presence ol fines gives rise to severe ditiiculties in the solids downflow. This statement has been confirmed by tests in the present reactor; if ferrous chloride in a finely powdered form is present in more than a few percents, the solids flow at the reactor inlet occasionally stops. For this reason, it has been decided to operate the reactor with ferrous chloride in a granulated form, mixed with ballotini of zirconium oxide. These ballotini have a diameter between 3 and 5 mm and the material is completely resistant to the present reaction conditions. There is another advantage for the use of an inert material, heterogeneously mixed with the reactant. The hydrolysis reaction is endothermic, about 81.5 kcal are required for the formation of I gmol of H2. It is obvious that severe difficulties are to be expected in large reactors to transfer this energy to the reactants. The addition of inert material could yield a suitable solution; e.g. if the reaction is carded out in a mixture of about 10 wt% FeC12 and 90 wt% ZrO2, a preheating of the solids to about 120-130°C above the solids outlet temperature could furnish practically all the necessary 30 --
~
1
T=650 °C FeCL2in200g( 1.58g tooL)
10
~
~
t
I
I
I
I
l
I
I
~0
~0
30
40
50
60
70
80
H2 yield,
\
I~11 90 ,00
%
FIG. 7. Effect of H20 feed rate on H20 conversion.
114
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
Storogefeed(FeCL2)
Gasoutlet. H2HCL H~) Reactor(T=6~
inlet (HzO)
5as
Autamal"ic
s°lids rernov°l-J TJ"-JIt~__ Storoge product (F'e304)
FIG. 8. Sketch of continuous moving bed reactor for the hydrolysis of FeCl2.
reaction heat and practically no additional heat would be required. This advantage could largely outweigh the drawback of a larger solids flow rate through the reactor. Some successful experiments in the bench scale plant for the hydrolysis of ferrous chloride have been carded out. Here, the 5 cm i.d. moving bed reactor has been fed with a heterogeneous mixture of zirconia ballotini and anhydrous ferrous chloride in a weight ratio of about 9 : 1 at 650°C. The solids feed rate was 78 g/rain zirconia and 8.8 g/min ferrous chloride. In countercurrent water vapour has been entered at a rate of 5.6 g/min, which resulted in a hydrogen development rate of 0:42 l/min. Moreover, the exit gases consist of 2.521/min HC1 and 5.75 l/rain unreacted water vapour; here all flow rates are expressed at S.T.P. This corresponds to a water conversion of 23% and a ferrous chloride conversion (and consequently, hydrogen yield) of about 80%. Thermodynamic calculations of the equilibrium at 650°C indicate a water conversion of 25.3%, so that the conclusion can be drawn that the present reactor operates in a satisfactory way. It must be noted that the above reactor conditions lead to a downward solids velocity of about 1.5 cm/min, whereas the actual calculated upward gas velocity (at actual reactor conditions) is approximately 20 cm/s superficially at 650°C.
Future development At this point it seems useful to make a statement about the significance of the above described equipment in the total process of plant development: The upward superficial gas velocity of 20 cm/s corresponds to a hydrogen production rate per unit reactor cross sectional area of about 14 m3/m2h at S.T.P. From this datum, the required reactor surface for a hydrogen production of 100,000 m3/h (which is a reasonable output for a full-size production unit) can easily be calculated. It follows that this reactor cross sectional area will be about 7 5 0 0 m 2, i.e. a cylindrical reactor of 100 m i.d. It will be clear that this is a completely unrealistic proposal. The extremely large reactor diameter required is mainly a consequence of the low H2 content of the exit gases, which is only about 5% by volume. The conclusion is obvious: for a future
D. Van VELZEN AND H. LANGENKAMP
115
development of large scale reactors, the moving bed is an unsuitable type of reactor, as here the gas velocities are limited to values of about 0.5 m/s. Reactor types are needed in which gas velocities of 10--20 m/s can be achieved, and entrained fluidized beds could offer good possibilities here. This type of reactor is already applied (e.g.) in the calcination of aluminium hydroxide, where in a reactor of 4 m i.d. approximately 30 t/h of A1203 is produced [6], with gas velocities of approximately 4 m/s, i.e. a solids throughput of 2.4 t/h m 2. In this respect it is interesting to note that for the production of 100,000 m3/h H2 about 1000 tha of Fe304 has to be processed. This would then call for a reactor cross sectional surface of 400 m 2, or one reactor of 22 m i.d., adopting the specific throughput of 2.5 t/h m 2 of Fe304. It must be noted that further optimization might be possible. It can thus be concluded that reactors of this type may offer reasonable possibilities for full scale application. The moving bed reactor has to be considered a valuable equipment in the framework of a small-scale demonstration plant, where basic data about the chemical side of the process can be obtained. Its potentials for a full-scale application, however, must be considered as extremely restricted. It is interesting to note that here the reactor dimensions are mainly dependent on the gas flow rate and that the solids throughput plays a minor role. This implies that the often heard objection against the thermochemical production of hydrogen, i.e. the large masses of solids to be transported per unit volume of hydrogen produced, may not be as important as has been supposed before. CHLORINATION OF MAGNETITE
Simplified reaction scheme It is useful to analyze the chlorination reaction of the Mark-9 cycle, before discussing the experimental results. In reality reaction (2) is not unique, but a composite of a number of competing reactions. In the first place there is the reaction 3Fe304 + 1½C12---, 4Fe203 + FeCI3.
(4)
This reaction is an exothermic one; it proceeds practically at all temperatures above 100°C, its equilibrium is strongly shifted to the right. It is highly probable that reaction (4) occurs wherever Fe304 comes into contact with chlorine and also that this reaction is very rapid in comparison to the other ones involved. Therefore, and for simplification purposes, it will be assumed that magnetite is first converted to ferric oxide by reaction (4) and that, subsequently, ferric oxide reacts, either according to reaction (5) or (6), or the two simultaneously. Fe203 + 6HCI---, 2FeCI3 + 3H20.
(5)
Fe203 + 3C12 --->2FeC13 + 1½02.
(6)
Reaction (5) is known to proceed well at low temperatures in the order of 100-200~C, whereas reaction (6) is best performed at 800-1000"C. Additionally, in the gas phase occurs the well-known Deacon reaction, either in one or in the other direction; H20 + C12 a=~2HCI + ½02.
(7)
The above reaction scheme is obviously a simplified one, there are still many other intermediate reactions possible. Nevertheless, it is believed that the above scheme is sufficient for a better understanding of the observed phenomena, which will be discussed below.
Low temperature The chlorination of magnetite has been carried out at atmospheric pressure in the fixed bed apparatus also used for the hydrolysis studies. In the first series of experiments the gas feed consisted of HC1/C12 mixture of various proportions, from 8 : 1 to 1 : 1, introduced at a rate between 20 and 40 l/h S.T.P. The amounts of magnetite charged into the reactor varied between 20 and 50 g. This iron oxide feed consisted generally of the reaction product from previous hydrolysis experiments, whereas Fe304 purchased from the British Drug House has also been used. In the course of the
116
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION
study it appeared that the latter product is very impure and cannot be considered as pure Fe304. Its content of bivalent iron has been analytically determined as 19.4% (expressed as FeO), whereas the theoretical value should be 31.0%. It is interesting to note that the bivalent Fe content of the magnetite produced by the hydrolysis reaction has been found to be between 29.0 and 32.0% for various samples. The iron oxide feed for the chlorination experiments has been applied either in the form of granules or as a powder. The latter may be heterogeneously mixed with an inert material (e.g. Attapulgus Clay or zirconia ballotini), or distributed in a glass wool bed. The method of application of iron oxide has been varied because the results with the pure magnetite granules (Run 39/74) led to the formation of big lumps of a mixed solid product. These lumps apparently had a very low accessible reaction surface and were very cumbersome to handle. A feasible technique to overcome the difficulties in solids handling has been the premixing of magnetite with Attapulgus Clay or zirconia ballotini. When the magnetite and the inert material are brought into intimate contact, the magnetite adheres to the support's surface and a free flowing mixture results. The formed FeC13 remains finely divided on the support and the solid product after the reaction is still free flowing. Solids handling problems are thus apparently minimal. The results of the experiments can be summarized as follows: at a temperature of 150-180°C the yields of FeCl3 are satisfactory, between 66 and 85% obtained in 1 - 2 h . However, in none of the experiments, even with a HCI/C12 ratio of 1 : 1 , has a perceptible production of oxygen been observed. The absence of oxygen formation at low temperatures has been confirmed by the results of Japanese researchers [7], From the above considerations the conclusion can be drawn that the chlorination reaction originally proposed for the Mark-9 cycle, in which chlorination and oxygen formation occur at the same time and at low temperature, cannot be performed at 150-200°C.
Intermediate temperature The chlorination runs at intermediate temperatures, i.e. 300, 400 and 500°C, showed a strong decrease of the reaction rate, whereas the oxygen production still remained negligible. Obviously, in this region, reaction (5) as well as reaction (6) are proceeding extremely slowly. Increasing the temperature to approximately 700°C, the reaction rates become higher and appreciable amounts of FeC13 are formed. Above this temperature reaction rates continue to increase with temperature, however, staying considerably below the reaction rates observed at 150°C where reaction (5) is the rate-determining one.
High temperature The field between 730 and 850°C has systematically been covered by a series of chlorination tests. In these runs 23 g of magnetite has been inserted and chlorinated by a mixture of 101/h HC1 and 7.5 to 8.71/h C12 for 120 min. The runs all proceed in a characteristic way. In the first few minutes, a rapid formation of ferric chloride is observed. This occurs before any oxygen or chlorine has been monitored in the outlet gas. After approximately 10 min a very constant rate of development of oxygen and ferric chloride is stabilized. After the fixed period of 2 h, generally no sign of decrease of the oxygen formation rate is observed. The FeCI3 production obtained in 2 h varied between 16 and 40% of the theoretically possible amount. A suitable working hypothesis for these series of experiments is the following. (a) In the first few minutes a rapid reaction of magnetite with chlorine takes place according to reaction (4). For complete conversion of Fe304 into Fe203 and FeCI3 a total amount of 1.21. S.T.P. of C12 would be sufficient. This corresponds well with the quantity introduced during the time needed to arrive at steady state conditions. (b) (Subsequently, reaction (6) takes place accompanied by reaction (7) in the vapour phase (in reverse direction obviously). It is highly improbable that reaction (5) should proceed at a non-negligible rate, since it is known that the equilibrium of this reaction is negatively influenced by an increase of the temperature. This second part of the reaction scheme does not proceed to completion at the actual reaction conditions. Therefore, it is advisable to put more emphasis on what happens during this second phase for a proper judgement of the experimental results.
D. Van VELZEN AND H. LANGENKAMP
117
Oxygen production The oxygen production during chlorination occurs only in the last part of the reaction (reactions (6) and (7)). It may well be expressed as the ratio oxygen/ferric chloride formed by these reactions. This ratio is calculated from the experimental results by the relation
/3=
P q -0.111W
where /3 = p= q= W=
the the the the
oxygen production ratio (gat O/gmol FeC13 formed), amount of oxygen formed (gat), total amounts of FeCI3 (gmol), initial amount of Fe fed to the reactor (gat Fe).
In this way, from the total ferric chloride yield the amount presumably formed by reaction (4) is subtracted, the remainder being the FeCI3 produced by the combination of reactions (6) and (7). The dependence of/3 on the reaction temperature is given in Fig. 9. It follows that the oxygen production is strongly temperature dependent. The value of /3 required for reaction (2) is 0.375. From Fig. 9, it can be concluded that this value is obtained at 730-740°C at the present reaction conditions. Consequently, the Mark-9 cycle could be modified by introduction of this higher temperature for the chlorination step, working with a HCI/C12 feed ratio of 1.25 : 1. HCI and C12 conversion A point of paramount importance is the degree of conversion of the gaseous reactants, determining the excess of HCI and C12 which should be recycled in the process. Also in this case a correction has to be applied for the amount of chlorine reacted in reaction (4). A plot of the experimentally obtained HC1 and C12 conversion as a function of reaction temperature is shown in Fig. 10. It follows that the HCl-conversion decreases slightly with increasing temperature, from 6.5% at 730°C to about 4% at 850°C. The chlorine conversion is much more temperature dependent. It increases sharply with increasing temperature, i.e. from 1.5% at 730°C to approximately 18% at 850°C. In the preceeding section it was concluded that the Mark-9 chlorination reaction could be performed at 730-740°C. A t this temperature, HC1 and 0 2 conversions are very low, 6.5 and 1.5%, respectively. This leads to a recycle of HC1 of about 15 times the reacted quantity. For C12 this figure would be even higher, about 60 times! Therefore, the conclusion can be safely drawn that performing the chlorination step in the Mark-9 cycle according to reaction (2) at 730-740°C is not very attractive, and alternative solutions are to be investigated.
L)
~_ ,0-E
,
05
700
~
: 23g H CL-' f e e d : I 0 L.h-'
- /
I
750
cl2 feed:SLh-' 8Lo
I
800 Temperature
,
°C
FIG. 9. Oxygen production as a function of chlorination temperature.
118
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION re304 feed : 23cj HCL feed IOL.h- S.T.R CL2 feed: 8L.h-~ S.'EE 2c -
2(] - -
g
e
0 10 ~
k)
:c
o
7"00
°
I
800
Temperature, °C
I
900
0 700
800
I
900
Temperature, °C
FIG. 10. HCI and Cz conversions in the chlorination of Fe203. A L T E R N A T I V E SOLUTIONS General In principle, there are three possibilities to close an Fe-C1 cycle. (i) Carrying out the chlorination so that the stoichiometric amount of oxygen is formed in one reaction only. (Mark-9, reaction 2)). (ii) Carrying out the chlorination so that an excess of oxygen is formed, followed by a Deacon-reaction for the off-gases. In this case chlorination at elevated temperatures (800-900°C) could offer a good possibility. (iii) Carrying out the chlorination producing no or a short amount of oxygen, followed by a reverse Deacon-reaction for the off-gases. In this case chlorination at 150-200°C would be appropriate. In all three cases the overall production of FeCla should be six times the overall molal amount of oxygen produced. The first possibility has been investigated in the preceeding section and rejected due to very low conversions of the gaseous feed components. The second possibility calls for very high temperatures of about 800-900°C. From the foregoing it can be concluded that here rather low chlorine conversions have to be expected (see Fig. 10). Moreover, when an excess of oxygen is produced in the chlorination reaction, the excess of oxygen has to be reconverted into chlorine in the reverse Deacon-reaction, to close the cycle. This will lead to a further increase of the amount of chlorine to be recycled. A n additional disadvantage of this solution is the extreme corrosivity of the mixture of ferric chloride, chlorine and oxygen at the reaction temperature, possibly still enhanced by the pressure of hydrogen chloride and water. For the above reasons, the second possibility is also not very attractive. The third possibility appears to be more promising. Here, the expected conversions are rather high, as may be illustrated by the following example (exp. 26/75). 23 g of magnetite mixed with 200 g of ZrO2 ballotini has been chlorinated in a stream of 211/h-lHC1 and 4.01/h-lC12. Fig. 11 shows at1 for this experiment as a function of time. It follows that during the first 30 rain a = 100% and then gradually drops to about 10%. It is noticed that after 95 rain the yield of F e C 1 3 has been 86% in this run. Besides the good yields and conversions, there is the advantage of avoiding the handling of a set of extremely corrosive materials at elevated temperatures. It has been stated before that the oxygen production at low temperatures is negligible. Consequently, chlorine reacts only to oxidize the bivalent iron of the magnetite. Elsewhere in the cycle, at the disproportioning of ferric chloride, all this bivalent iron has to be reconverted into the bivalent state. It would thus be advantageous to leave the ferrous part of the magnetite unoxidized during the chlorination. This can be done when the chlorination is carried out with hydrogen chloride only; in this case the reaction should be FeaO4 + 8HC1 ~ FeCI2 + 2FeC13 + 4H20. It is this reaction which has been the subject of further investigations.
D. Van VELZEN AND H. LANGENKAMP
119
IO0 --'C
.~
80--
g
4o-
tn
>
40--
c
20-T
I
1
I0
I
20
50
I
40 Time,
I
50
I
I
60
I
70
80
I
90
I00
mln
FIG. 11. HCI conversion during run No. 26•75.
Chlorination with hydrogen chloride A series of experiments has been carried out with the reaction temperature as main parameter. The hydrogen chloride flow from the reactor (i.e. the unreacted chlorination agent) has been dissolved in water and titrated. Changing the contents of the wash bottles at regular intervals of time allows the determination of the iron oxide conversion as a function of time. Figure 12 gives an illustration of the experimental results. It appears that the overall reaction rate decreased largely with increasing temperature, e.g. in 60 minutes at 150°C a conversion of 80% is obtained, whereas at 350°C this figure is decreased to 52% and an increase of the temperature to 400°C yields only a conversion of 26% in the same time period. All runs represented in Fig. 12 have been carded out with magnetite produced by hydrolysis of ferrous chloride. Always 23 g of magnetite has been chlorinated in a tubular fixed bed reactor of 3 cm i.d. with addition of 150-300 g of ZrO2 ballotini. The HCl-flow rate has been varied between 40 and 80 l/h; this variable has no perceptible influence on the observed reaction rates. The HCl-conversion at'low temperature (<200°C) is very similar to the one given in Fig. 11. When the reaction temperature is increased, obviously the HCl-conversion drops: at 350°C the average HCl-conversion over the first 15 rain of the experiment has been about 20%. The addition of the zirconia ballotini has been carried out for the same reasons as mentioned in the part on hydrolysis of FeC12, i.e. as a heat sink and as a solids flow promoter. It must be noted that presumably the chlorination can be performed in the same continuous moving bed reactor as used for the hydrolysis of FeCI2 without major difficulties. Experiments of this kind are planned for the near future.
Effects of magnetite purity It has been mentioned before that commercially available magnetite (B.D.H.) is very impure. Not only that the bivalent iron content has been found 15.1 wt% against 24.1% theoretically,
80
350° C
60 c o 40
20
0
FIG.
I
20
I
40
I
I
60
80
Time,
min
I
I00
I
120
12. Chlorination with hydrogenchloride; chemically pure magnetite.
120
THERMOCHEMICAL CYCLES FOR HYDROGEN PRODUCTION ?'0--
[
I
I
[
I
I
6C-5C--
m c> o u
I~
o =Pellets
30
20
380"C
=Pellets
400%
=Powder
400°C
--
IO 0
20
I
40
I
60 Tame,
I
80 rain
I
I00
I
120
FIG. 13. Chlorination with hydrogen Chloride; commercial grade magnetite. also the presence of impurities as SiO2 (2.4%), carbon (1.7%) and sulphur (0.3%) has been ascertained. The behaviour of this impure form of magnetite during chlorination with hydrogen chloride is anomalous. In the first place, initial reaction rates at 380-400°C are superior to those of pure magnetite and, once having reached a conversion of about 45%, the reaction proceeds at a very slow rate only. In practice, this phenomenon limits the iron oxide conversion to about 55%. These features are illustrated in Fig. 13, where the iron oxide conversion is shown as a function of time. Another point of difference between the chlorination of commercial and pure magnetite is that pure magnetite yields ferrous and ferric chloride proportionally and that the amount of bivalent iron before and after the experiment, is virtually the same. The commercial grade magnetite shows a yield of ferrous chloride largely superior to the initial quantity of bivalent iron. For instance, in an experiment where 220 g of iron oxide (containing 17.2% moisture) has been chlorinated for 2 h with 82 !./h HC1 at 385°C, the initial amount of bivalent iron was 0.49 gat, whereas in the solid products after the reaction 1.03 gmol of ferrous chloride has been found. In another example, chlorination of 4 0 g of magnetite (containing 0.090gat Fe 2+) yielded 0.155gmol of FeC12 at 380°C. The mechanism of this oxidation/reduction reaction in the commercial grade product is not clear yet. Experimental work to elucidate this point is in progress. CONCLUSIONS The work described in the present paper can be summarized in a number of conclusions. The hydrolysis of ferrous chloride gives good results and can be carried out in a moving bed reactor for a small scale demonstration plant. This equipment is ready for direct use. For large scale application, the reaction has to be carried out in a reactor type allowing gas velocities of the order of 10 m/s. This could possibly be an entrained fluidized bed. A moving bed is unsuitable for this purpose. The chlorination step proposed for the original Mark-9 cycle cannot be carried out at 150 ° 200°C, as originally anticipated. Increasing the temperature to 730-740°C would enable the reaction to proceed, however with low HC1- and C12-conversions. The best alternative solution seems to be low temperature chlorination with hydrogen chloride and introduction of the reverse Deacon-reaction in the cycle. The low temperature chlorination proceeds with good yields and conversions. It is clear that the success of the Fe-CI cycle family thus depends neither on the hydrolysis nor on the chlorination step. The question that arises is whether the unfavourable equilibrium conditions in the disproportioning of ferric chloride and/or the expected high costs of investment could possibly jeopardize this success. The discussion of these points, however, is beyond the scope of the present paper and will be treated elsewhere. REFERENCES 1. HARDYC., Thernlal decomposition of water using cycles of the Fe--Cl-family, Report EUR 4958f, (1973). 2. SCH3,FERH., Z. Anorg. Allgem. Chemie 266, 268 (1951).
D. Van VELZEN AND H. LANGENKAMP
121
3. Wmsor~ L. E. and GREGORY N. W.; J. Phys. Chem., 62, 433 (1958). 4. HAMMER R. R. and GREGORY N. W., J. Phys. Chem., 66, 1705 (1962). 5. GMELIN, Handbuch der Anorganischen Chemie, Verlag Chemie, Weinheim, 8. Auflage, Band 59 Fe, Teil B, p. 763. 6. B6nM E. and SCHMIDT H. W., Chemische Prozesse in Expandierten Wirbelschichten, I. Chem. Eng. Syrup. Series No. 43, paper 4. 7. ONo M., Mitsubishi Research Laboratories, Private communication.