CH630535A5 - Method of producing an adsorption/diffusion filter for removing a substance present as atomic or molecular particles from a flowing gas - Google Patents

Description

The invention relates to a method for producing an adsorption / diffusion filter for cleaning a flowing gas from a substance present as atomic or molecular particles according to the preamble of claim 1.

It is known to produce filters of the type mentioned at the outset, the filter material being arranged as a bed, for example made of granular material, in the gas jacket of the filter. For the design of the filter, for example the determination of the length 1 of the filter material through which flow occurs, empirical values based on the intended filter material are used. The disadvantage here is that very complex and lengthy experiments, in which different filter variants are subjected to the intended operating conditions, are required to determine a filter suitable for the given operating conditions.

It is an object of the invention to provide a method of the type mentioned at the outset, which makes it possible to use an adsorption / diffusion filter taking into account that for the

Interaction of the particles to be retained with the material properties of the filter material that determine the filter material, as well as its dimensions, which optimally fulfills the task set, without it being necessary to carry out complex test series 55 to adapt the filter to the requirements.

The object on which the invention is based is achieved in a method of the type described at the outset by the features of the characterizing part of the patent claim.

60 The mass transfer coefficient h is best calculated using the heat-mass transport analogy. If a filter is to be produced, which is to be used for cleaning a gas from radioactive particles, and Sil ',

"y x d> n then the condition

630535

4th

for all values of t and the relationship I in claim 1 is always valid.

The method according to the invention enables the production of a filter that is optimally adapted to the conditions and thus also to the special features of a system. In general, the above-mentioned variables are dimensioned such that the “von der Decken” number is as large as possible or at least reaches the value required to achieve the predetermined value for the passage coefficient. On the basis of the aforementioned mathematical relationships, it is advantageously possible to carry out tests on filters for large systems on a small model scale in a cost-saving manner. Two filters are equivalent in terms of deposit and thus in terms of their filter effectiveness if they have the same passage coefficient 8 and thus the same “von der Decken” number De. It is therefore also possible, starting from a simple filter variant, for example a straight pipe section through which the gas to be cleaned flows, to determine the parameter values required for producing a filter for a large-scale plant. The simple filter is subjected to different operating conditions and the parameter values are determined according to the relationship I in claim 1.

In the manufacturing method according to the invention, the diffusion of the particles into the filter material is also taken into account. It is therefore advantageously possible to produce filters which are also effective at temperatures above 400 to approximately 1000.degree. In contrast, only the desorption and adsorption of the particles on the surface of the filter material or chemical reactions of the particles with the surface of the filter material were taken into account in the production of the known filters. It was therefore also sought to use filter material with the largest possible surface. As a result, the known filters in the temperature range above 400 ° C. lacked the required effectiveness, which is why one was forced to keep the temperature in the filter low by cooling. In order to increase the effectiveness of the known filters, several filters were also connected in series, which however led to voluminous cleaning systems.

2 ^ t »1 and (X + 3 *)

In this variant of the filter according to the invention, the diffusion of the particles into the filter material is used.

In this case too, the dimensions of the filter are to be dimensioned such that De is as large as possible or has a value for which the transmission coefficient 8 reaches the predetermined value. Depending on the choice of filter material and the dimensions of the filter, such a filter is highly effective even at high temperatures up to 1000 ° C.

A variant of the method according to the invention is also very advantageous, which consists in that in order to achieve the longest possible operating time t with the same

An advantageous variant of the production method according to the invention consists in that at a temperature range below approx. 400 ° C. for a filter, during which the adsorption is carried out for the particles during the specified operating time t for the materials available as filter material. Desorption equilibrium for the adhesion of the particles to the surface of the filter material is not achieved if a material is provided as filter material for non-radioactive or long-lasting substances for which the relationships a) 2 \ jç t 1 and (X + 3 *) "t <: 1 or for radioactive substances the relationship b) X, 3 *

be valid. Under these conditions - for example, if a filter is to be used to purify a gas at low temperatures - the “from the ceiling” number simplifies the relationship

De = ^ - • St1 • -, (II)

eff a + h

If condition b) is fulfilled, then condition a) applies to all values of t and a filter produced according to relationship II is then effective for an unlimited period. According to relationship II, the dimensions of the filter are dimensioned using a material with a sufficient likelihood of adhesion for the particles to be retained in such a way that the intended transmission coefficient 8 is either as small as possible or corresponds to an intended value.

A further advantageous variant of the method according to the invention consists in that at a temperature range above approximately 600 ° C. for a filter, during which the intended operating time t for the materials available for selection as filter material for the particles, the adsorption -Desorption equilibrium for the adhesion of the particles to the surface of the filter material is achieved, such a material is provided as the filter material that has the largest possible penetration coefficient 45 and for which the relationships t> 2 yç t and r? * ■ *> \

(III)

small transmission coefficient 8 for the thickness s of the filter material for radioactive substances the relationship

-Vi and for non-radioactive substances the relationship

£ »y D t, where D is the diffusion coefficient for the particles in. That of the ceiling number De then simplifies to the relationship

De. 4JL_. st. ,

dgff h + (1-ß) • a

*

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Filter material is. In particular in this process variant - in contrast to the previously usual production processes - not only the adsorption-desorption behavior of the particles on the surface of the filter material, but also the diffusion of the particles into the filter material are used in the manufacture of the filter. While in the known filters the period of use of the filter depended exponentially on the reciprocal temperature, it is now possible to choose a filter material with a sufficient thickness e to produce a filter that, especially at high temperatures, utilizes the diffusion of the particles into the filter material and has a long operating time t has.

In the following, the possible variation of the design data for the manufacture of a filter from a predetermined material and for different operating conditions will be explained in an exemplary embodiment on the basis of diagrams shown in the drawing.

Show:

Fig. 1 is a graphical representation of the "von der Dek-ken" number De as a function of the mass flow rate and the Reynolds number and

2 shows a graphical representation of the transmission coefficient S as a function of the mass flow rate and the Reynolds number.

In two further exemplary embodiments, the results of experimental investigations on filters are compared with the values for the transmission coefficient 8 determined according to the relationship I in claim 1.

Furthermore, the design data required for producing a filter for different operating conditions are specified in further exemplary embodiments.

Show:

Fig. 3 shows a longitudinal section through a filter consisting of a bundle of parallel tubes

4 shows a cross section through the filter according to FIG. 3.

Embodiment 1

To determine the design data for the manufacture of a filter from tubes arranged in parallel to one another, the “von der Decken” number De is calculated from the relationship I and from this the passage coefficient 8 as a function of the mass flow m of the gas flowing through a tube.

The pipe on which the calculation is based has a length of 1 = 800 cm and a diameter of 1 cm. The range of mass flow rates considered is 10 "2 to 12 g / sec. The gas and wall temperature of the tube are assumed to be 950 ° C. and the gas pressure p = 40 bar. The particles to be filtered from the helium are assumed to be Cesium-137 atoms are taken into account, assuming two different wall materials with the penetration coefficients 1 - ß = 0.7% o and 1 - ß = 100% o, while the value of 0.7% o for Cs-137 for Characteristic of materials with face-centered cubic lattice, the penetration coefficient of 100% means that the material used is a perfect «diffuser».

For a clear representation of the characteristics of the filter, the mass flow rate is varied with a parameter K, which is determined by the relationship m = K-m0,

where rho is the mass flow rate that is used as a reference.

Since there is a linear relationship between the Reynolds number Re and the mass flow rate m, the following relationship also applies

Re = KRe0.

Re0 means the Reynolds number at mass flow rate m0.

As can be seen from the graphic representations in FIGS. 1 and 2, the filter characteristic has a jump at a value for K = 0.077. This value for K corresponds to a value for the Reynolds number of approximately Re = 2300. This discontinuity is due to the fact that the Sherwood number Sh and thus the mass transfer coefficient h or the Stanton number St 'during the transition from turbulent to laminar flow also has a discontinuity. The jump height depends on the geometry, i.e. the ratio 1 / d and the wall material used.

The flow range in which the passage coefficient reaches the smallest value, which is correspondingly large in terms of the number of ceilings, and from which the effectiveness of the filter also reaches its highest value can also be read from the graphic representations. As can be seen from the graphic representations, these minimum and maximum values lie in the region of the strongly laminar flow and in the transition region, for example, at Reynolds numbers between 2500 and 5000. When producing a filter, it is possible to set both regions simultaneously, for example by in the filter tube bundles are arranged, through which gas flows outside and inside.

From the foregoing, it can be seen that the absolute values given in the graphic representations are only valid for the case in question, but that the filter characteristics given thereby also enable a qualitative statement to be made about other operating states and other geometrical arrangements of the filter material. The desired absolute size of the effectiveness of the filter is easily achieved by arranging a corresponding number of tubes arranged in parallel.

Embodiment 2

In order to check the effectiveness of a simple filter consisting of a straight pipe section, a helium gas stream containing the fission products Cs-137, Cs-134 and Ag-110m was passed through a pipe section made of 99.5% titanium in two separate tests a total filter was connected downstream to record the quantities of fission products emerging from the filter. The content of fission products in the helium was different in the two investigations. The tubes had a length of 2370 mm, an outer diameter of 24.5 mm and a wall thickness of 1.65 mm. The temperature of the helium when entering the filter was 825 in the first case, 750 ° C in the second case, and 210 ° C in both cases when exiting the filter. The temperature of the wall of the pipes was stable during operation and therefore easy to measure.

During the operation of the filter, the flow of the helium was adjusted so that a mass throughput of 15 Nm3 / hour. was achieved. The operating time was 785 hours in the first examination and 1029 hours in the second examination.

The following values were used in relation I to calculate the transmission coefficient of the filter and thus the effectiveness of the filter:

For Cs-137: 1-ß

= 0.2% »;

Q =

38

Cs-134: 1-ß

= 0.1% »;

Q =

38

Ag-110m: 1-ß

= 0.04% o;

Q =

50

andtt> 0 = 1.308 1011

sec-1.

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630 535

Because of the temperature gradient in the pipe, the filter was divided into several sections for the calculation using the relationship I. The integral activity values in (iCi were determined for the fission products passing through the filter during the aforementioned operating time and compared with the values measured in the total filter. The calculated and the experimentally determined values are compared with one another below.

First investigation:

calculated measured

Cs-137 1.00 1.20

Cs-134 0.79 0.84

Ag-110m 11.4 11.7

Second investigation:

calculated measured

Cs-137 0.52 0.59

Cs-134 1.6 1.7

Ag-110m 5.1 5.6

Embodiment 3 According to the investigations described in embodiment 2, a filter consisting of a tube made of stainless steel X10 CrNiTi 189 (older designation 4541) was used with the values for diameter and wall thickness given in embodiment 1, but with a length of 140 cm.

The temperature of the gas when entering the filter was 625 ° C in both cases and when leaving the filter it was 210 ° C. The operating time was 818 and 790 hours.

The following values were used in relation I to calculate the transmission coefficient 8:

For Cs-137 1 - ß = 0.7% o; Q = 45 Kcal / mol Cs-134 1 - β = 0.33%>; Q = 45 Kcal / mol Ag-110m 1 - β = 0.2% o; Q = 28 Kcal / mol and © 0 = 1.308 1011 T • sec-1

The following results were obtained:

First investigation:

calculated measured

Cs-137 2.1 2.2

Cs-134 1.07 0.96

Ag-110m 6.2 6.5

Second investigation:

calculated measured

Cs-137 1.92 2.1

Cs-134 1.03 1.1

Ag-110m 3.26 3.51

Embodiment 4

The design data for a filter shown in FIGS. 3 and 4 and consisting of tubes arranged in parallel were determined for predetermined operating conditions.

As can be seen from FIG. 3, the filter consists of a multiplicity of tubes 1 arranged in parallel and at the same distance from one another, which are fitted within a casing 2 enclosing the cavity of the filter. The outside diameter of the individual tubes of the tube bundle is there, the inside diameter of the tubes is di, the length of the tubes is 1 and the inside diameter of the casing Dt.

As can also be seen from FIGS. 3 and 4, the tubes of the filter are flowed through both inside and outside by the gas to be cleaned.

The gas to be cleaned is helium, which contains the fission products Cs-137 and Ag-110m.

The intended operating conditions are:

The mass flow rate of helium: m = 111.25 kg / sec.

The temperature of the helium when entering the filter: T = 950 ° C.

The pressure of the helium: p = 40 bar.

The intended operating time t = 30 years.

Heat-resistant steels which have a body-centered cubic structure or which, for example, Incoloy-802 and In-conel-625, have a face-centered cubic structure have been provided as materials for the tubes. For the aforementioned materials, the penetration coefficient for Cs-137 1 - ß is 0.7% o and for Ag-110m 1 - ß = 0.2% o. For the binary diffusion constant, which is used to calculate the substance transition coefficient h, applies to

T = 950 ° C and p = 40 bar.

Dcs-He = 0.146 cm2 / sec.

DAg-He = 0.272 cm2 / sec.

The values for h and St 'required to calculate the “von der Decken” number De resulted from the VDI Warmth Atlas and from Int. J. Heat Mass Transfer Vol. 14 pp 1235-1259, Pergamon Press. Since in the present case the conditions / "v

2yc t »i and (X +> 3 *) t> 2 ^ ç t and, 3 *» X

are fulfilled, the design data for the filter was calculated according to the relationship HI.

Provided that the volume of the filter does not exceed 40 m3, that the pressure drop is not greater than 0.1 bar and the passage coefficient 8 for silver between 6-10 ^ to 8.8-10 "3 and that for cesium between 1 , 2-10 "5 to 2-10" 3, the design data for the filter given in the table below have been calculated. The values for dt and da as well as for Dj and 1 are given in cm. N is the number of parallel arranged pipes in the filter In addition to the design data, the value for the pressure drop Äp in the filter in bar as well as the values for the respective passage coefficients are given. 14 pp 1235-1259, Pergamon Press calculated.

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630 535

N

d,

there

D,

1

5

8th

Ap (bar)

Cs-137

Ag-110m

95,000

0.55

0.75

300

500

1.53 IO "3

8.76 10 ~ 3

0.063

95,000

0.55

0.75

300

600

4.64 10-4

4.16 10 ~ 3

0.076

145,000

0.35

0.55

300

600

3.162 10 "4

5.26 IO "3

0.085

190,000

0.35

0.55

325

600

1.26 10 "5

6.29 10 "4

0.089

190,000

0.35

0.55

325

500

4,886 10 "4

2.129 IO "3

0.074

190,000

0.35

0.55

325

400

1.97 10 ~ 3

7.228 IO "3

0.059

Exemplary embodiment 5 As in exemplary embodiment 4, the design data for a filter consisting of tubes arranged in parallel to one another were determined for the same operating conditions, but for an inlet temperature of the gas of 300 ° C.

Ferritic steel of type 15Mo03 was provided as the material for the pipes. For this material, the penetration coefficient and the desorption energy Q have the following values:

For Cs-137

1 - ß = 1.2% o; DCs.He = 0.039 cm2 / sec Q = 65 kcal / mol and ü> o = 1.308 10 "T sec" 1

For Ag-110m

1 - ß = 0.3% o; DAg_He = 0.072 cm2 / sec Q = 52 kcal / mol and o) 0 = 1.308 10 "Tsecr1

Because in the present case the conditions

2 y? t 1 and (X + *) t <S 1

are met, the design data for the filter was calculated according to relationship II.

Provided that the volume of the filter does not exceed 17.2 m3, that the pressure drop Ap is not greater than 0.11 bar and the passage coefficient 5 for cesium 1.06-10 "4 and that for silver 5.61 • 10 "6, the following design data were determined:

N = 105

di = 0.3 cm da = 0.55

i5 Di = 230 cm

1 = 350 cm

The pressure drop is Ap = 0.105 bar.

20 embodiment 6

For the operating conditions specified in exemplary embodiment 5, the design data for a plurality of rods arranged in parallel and at the same distance from one another, which, like the pipes, are attached in a casing, were determined. The following values were obtained for the materials specified in Example 5:

For the transmission coefficients 5 = 1.62 x 10 "3 for Cs-137 and 8 = 4.4 x 10" 5 for Ag-110m and under the assumption that the volume of the filter is <10.5 m3 and the Pressure loss Ap <0.125 bar is:

N = 1.2-105

there = 0.5 cm

35 Di = 230 cm

1 = 250 cm

The pressure drop is Ap = 0.124 bar.

For the transmission coefficients ô = 5.84 x 10 "3 for Cs-40 137 and S = 3.272 x IO" 4 for Ag-110m - with the otherwise identical values for N, since, Di-the length 1 = 200 cm and Ap = 0.11 bar.

s

2 sheets of drawings

Claims (4)

$ 00 in cm / sec Diffusion coefficient for the particles in the filter material Density of the particles in the gas in atoms / cm3 the number of particles that the intended filter material can hold per cm3, in atoms / cm3. 630 535 1 for t> -v o for t <& v n = T1. -1/2 m sec V D n = D: NG: <Doo: _ (l-g) a N, (1) = ^ 1.308 • ÎO ^^ T in sec Debye frequency Q: Desorption energy in Cal / Mol R: universal gas constant in Cal / (°) mol X: Decay constant for the radioactive substance in sec-1 I1 (x): modified Besseische function of the first kind ç = • St1 • h ^ ® 7 in sec 1 eff (a + h) 2 I-ß: penetration coefficient; the probability that the particles will be irreversibly bound t: intended service life of the filter 1: Length 1 of the filter material in cm V0: within the casing in the area of length 1 according to the arrangement of Remaining void volume in cm3 F:, the surface of the filter material wetted by the gas in cm2 St '= -: second Stanton number h: mass transfer coefficient in cm / sec v: flow velocity of the gas in cm / sec a * = a * 3/63 • lo3 in cm / sec a: probability of adhesion for the particles on the surface of the filter material (a approximately equal to 1 at partial pressures P <10 ~ 10 atm if there are no activation processes) A: mass number of particles T: Temperature of the surface of the filter material in ° K J9 * -J- h + (1 -ß) a, -1 - * - - * - in sec a + h ^ e "^^: Desorption constant in sec 1 e O 1. Method for producing an adsorption / dif- the intended operating time t of the filter of the passage fusion filter for cleaning a flowing gas of egg coefficient g ^ Sub- 'present as atomic or molecular particles punch, in which to flow through with the gas 5 which is equal to the quotient of the particle stream j (o, t) provided, with a gas-tight sheathing, a cavity is created upon interaction with the particles (1, t) as it exits the filter material, and a predetermined the retention of the particle-causing filter material corresponds to this value, where over a length 1 in the direction of flow of the gas and with ^, ». «M -De a hydraulic diameter deff for the flow of the 10 '' 1S Gases is arranged, characterized in that the and being provided that Length 1 and the hydraulic diameter deff measured in this way and that such a material is provided as the filter material * becomes that with the given ^ containing the substance Gas, with a given gas flow and with pre-15 tuned substance to be retained on the filter material for De the following relationship I applies t- * v De = 4 ^ - • St '• g * - in eff ° + h e (t-4) + v> _ p (e ~ (X + ^ * K | P v?) y? Ti1 (2yçT) dT | - (i) With De: ^ «von der Decken» number (newly introduced name) deff = ■: hydraulic diameter in cm 2 fT »1 and (À + J3 * be valid. 2. The method according to claim 1, characterized in that for a filter in which during the predetermined operating time t for the materials available as filter material for the particles, the adsorption-de-sorption equilibrium for the adhesion of the particles to the surface of the Filter material is not reached, as a filter material for non-radioactive or long-lasting substances, a material is provided for which the relationships or for radioactive substances the relationship 20th t 1 and (X t «i apply. ^ 2nd PATENT CLAIMS 3. The method according to claim 1, characterized in that for a filter in which during the intended operating time t for the materials available as filter material for the particles, the adsorption-desorption equilibrium for the adhesion of the particles to the surface of the Filter material is achieved, such a material is provided as the filter material, which has the greatest possible penetration coefficient 1-B and for which the relationships 3rd 630 535 f » et t-i) - 4. The method according to any one of claims 1 or 3, characterized in that to achieve the longest possible operating time t with a constant small passage coefficient 5 for the thickness s of the filter material for radioactive substances, the relationship 40 t> 2 ^ ç t and /? * »X and for non-radioactive substances the relationship e â>] applies, where D is the diffusion coefficient for the particles in the filter material.