Membrane for the separation of carbon dioxide

A membrane for the separation of carbon dioxide is disclosed which includes a hydrogel film of a cross-linked, vinyl alcohol/acrylic acid salt copolymer impregnated with an aqueous carrier solution containing a carbon dioxide carrier dissolved therein. A composition containing a solvent, an alkali metal carbonate or bicarbonates, and a polydentate ligand capable of forming a complex with an alkali metal ion is suitably used as the carrier solution. This composition may also be used for the preparation of a liquid film of a carbon dioxide separation membrane.

BACKGROUND OF THE INVENTION 
This invention relates to a membrane useful for separating carbon dioxide 
from a carbon dioxide-containing gas mixture and to a method of preparing 
such a membrane. The present invention is also directed to a carbon 
dioxide carrier composition useful for the formation of such a membrane. 
One known method for the separation of carbon dioxide from a carbon 
dioxide-containing gas mixture uses a liquid film across which carbon 
dioxide is selectively transported. The liquid film contains dissolved 
therein a carrier substance, typically an alkali metal carbonate (Science, 
115, 44(1967); Science, 156, 1481(1967)). While the known liquid film has 
relatively good carbon dioxide selectivity and carbon dioxide 
permeability, the separation efficiency thereof is not fully satisfactory 
in actual, large scale utilization. In particular, the known liquid film 
poses a problem of leakage or drying up of the carrier liquid during use. 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a carbon 
dioxide separation membrane which is devoid of the drawbacks of the 
conventional membrane. 
Another object of the present invention is to provide a carbon dioxide 
separation membrane which can be used for a long period of time in a 
stable manner. 
It is a further object of the present invention to provide a membrane which 
has excellent carbon dioxide separation efficiency. 
In accomplishing the foregoing objects, there is provided in accordance 
with one aspect of the present invention a membrane for the separation of 
carbon dioxide, which comprises a hydrogel film of a cross-linked 
copolymer of vinyl alcohol with acrylic acid or a salt thereof, wherein 
the copolymer is impregnated with an aqueous solution containing a carbon 
dioxide carrier dissolved therein. 
In another aspect, the present invention provides a membrane for the 
separation of carbon dioxide, which comprises a liquid film of a solvent 
solution containing dissolved therein (a) at least one alkali metal salt 
selected from the group consisting of alkali metal carbonates and alkali 
metal bicarbonates and (b) a polydentate ligand capable of forming a 
complex with an alkali metal ion, the solvent being selected from the 
group consisting of water, polar organic solvents and mixtures thereof. 
The present invention also provides a carbon dioxide carrier composition 
which comprises (a) a solvent selected from the group consisting of water, 
polar organic solvents and mixtures thereof, (b) at least one alkali metal 
salt dissolved in the solvent and selected from the group consisting of 
alkali metal carbonates and alkali metal bicarbonates, and (c) a 
polydentate ligand dissolved in the solvent and capable of forming a 
complex with an alkali metal ion. 
In a further aspect, the present invention provides a method of preparing a 
membrane, which comprises the steps of: 
applying an aqueous solution of a cross-linkable copolymer of vinyl alcohol 
with acrylic acid or a salt thereof to a carbon dioxide permeable support; 
cross-linking the copolymer applied to the support to obtain a 
water-insoluble polymer layer; and 
impregnating the polymer layer with an aqueous solution containing 
dissolved therein a carbon dioxide carrier to form a hydrogel of the 
cross-linked copolymer. 
Further objects, features and advantages of the present invention will 
become apparent from the detailed description of the preferred embodiments 
to follow.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
The carbon dioxide separation membrane according to one embodiment of the 
present invention includes a hydrogel film of a cross-linked copolymer of 
vinyl alcohol with acrylic acid and/or a salt thereof. The copolymer is 
impregnated with an aqueous solution containing dissolved therein a carbon 
dioxide carrier. 
The cross-linked, vinyl alcohol/acrylic acid (or a salt thereof) copolymer 
has a high water absorbing power and the hydrogel thereof exhibits high 
mechanical strengths. The copolymer generally has an acrylic acid (or 
salt) content of 5-95 mole %, preferably 30-70 mole %. The acrylic acid 
salt may be, for example, an alkali metal salt, e.g. a sodium salt or a 
potassium salt, an ammonium salt or an organic ammonium salt. 
The hydrogel film may be obtained as follows. First, a film of a 
noncross-linked copolymer of vinyl alcohol with an acrylic acid (or a salt 
thereof) is prepared. Various noncross-linked vinyl alcohol/acrylic acid 
(or a salt thereof) copolymers are commercially available (for example, 
Sumikagel L-5H manufactured by Sumitomo Chemical Industry Inc.) and they 
may be suitably used for the purpose of the present invention. The vinyl 
alcohol/acrylic acid (or a salt thereof) copolymer is formed into a film 
and is then cross-linked to obtain a film of the cross-linked copolymer. 
The formation of the film may be performed by any known method such as 
coating, extrusion or spreading. The cross-linking may be carried out by 
any suitable known way such as by heating the film at a temperature of 
100.degree.-150.degree. C., by irradiating the film with an actinic light 
(e.g. a UV ray) or by reaction with a cross-linking agent. 
The resultant cross-linked film, which generally has a thickness of 100 
.mu.m or less, is then impregnated with an aqueous solution containing 
dissolved therein a carbon dioxide carrier, thereby to obtain the hydrogel 
film according to the present invention. Generally, the hydrogel film has 
a thickness of 0.1-500 .mu.m. 
Any conventionally known carbon dioxide carrier aqueous solution may be 
suitably used for the purpose of the present invention. The carbon dioxide 
carrier is an organic or inorganic substance soluble in water to form an 
aqueous alkaline solution and capable of capturing carbon dioxide. 
Illustrative of suitable carbon dioxide carriers are alkali metal 
carbonates, alkali metal bicarbonates, alkanolamines, alkali metal salts 
of organic acids and mixtures thereof. 
Examples of the alkali metal carbonates and alkali metal bicarbonates 
include lithium carbonate, sodium carbonate, potassium carbonate, rubidium 
carbonate, cesium carbonate, lithium bicarbonate, sodium bicarbonate, 
potassium bicarbonate, rubidium bicarbonate and cesium bicarbonate. The 
concentration of the alkali metal carbonate or bicarbonate in the aqueous 
solution is generally 0.1-5.0 mol/liter, preferably 1.0-4.0 mol/liter. 
Examples of alkanolamines include monoethanolamine, diethanolamine, 
triethanolamine, monopropanolamine, dipropanolamine and tripropanolamine. 
The concentration of the alkanolamine in the aqueous solution is generally 
at least 3% by weight. 
The hydrogel film of the above cross-linked copolymer is considered to be 
composed of polyvinyl alcohol-rich phases and polyacrylic acid salt-rich 
phases. The phases which are rich in a polyacrylic acid salt are swelled 
by absorption of a large amount of the aqueous carrier solution to form 
gel phases. The polyvinyl alcohol-rich phases, on the other hand, are only 
slightly swelled with the aqueous solution but are drawn and oriented upon 
the swelling of the polyacrylic acid salt-rich phases, so that the gel 
phases are supported by the oriented polyacrylic acid salt-rich phases. As 
a consequence of the above structure, even though the membrane is thin, 
the high water-content hydrogel film can retain its shape upon being 
subjected to a pressure and can function as the carbon dioxide separation 
membrane for a long service life while exhibiting good water-retentivity 
and weatherability. 
It is preferred that the hydrogel film be supported by a carbon dioxide 
permeable support. The support preferably has a carbon dioxide permeation 
rate of 10.sup.-5 cm.sup.3 (STP)/cm.sup.2.sec.cmHg. In the case of a 
porous support, the pore diameter thereof is preferably 10 .mu.m or less, 
more preferably 1 .mu.m or less. The use of a porous support having a high 
porosity is preferred for reasons of easy permeation of carbon dioxide. 
The support may be formed of a plastic material, a ceramic, a metal, a 
glass or any other suitable material and may be in the form of a film, a 
hollow fiber, a cylinder, a woven or non-woven fabric, a paper or any 
other desired shape. The support generally has a thickness of 5,000 .mu.m 
or less, preferably 10-500 .mu.m. 
The carbon dioxide separation membrane including the support, and the 
hydrogel film supported thereby may be prepared by a method which includes 
the steps of: applying an aqueous solution of a cross-linkable vinyl 
alcohol/acrylic acid (or a salt thereof) copolymer to a carbon dioxide 
permeable support; cross-linking the copolymer applied to the support to 
obtain a water-insoluble polymer layer; and impregnating the polymer layer 
with an aqueous solution containing dissolved therein a carbon dioxide 
carrier to form a hydrogel film of the cross-linked copolymer supported by 
the support. 
The aqueous solution of the cross-linkable copolymer is generally up to 20% 
by weight, preferably 0.5-5% by weight. The cross-linking is preferably 
performed by heating the copolymer at a temperature of 
100.degree.-150.degree. C. for 0.5-2 hours. The thickness of the hydrogel 
film is generally 1-200 .mu.m. In the thus prepared composite membrane, at 
least part of the support is impregnated, in the thickness direction, with 
the hydrogel film. The degree of impregnation depends upon the 
hydrophilicity and porosity of the support. 
In the present invention, a novel carrier including (a) at least one alkali 
metal salt selected from alkali metal carbonates and alkali metal 
bicarbonates and (b) a polydentate ligand capable of forming a complex 
with an alkali metal ion is especially suitably used. 
Examples of the alkali metal carbonates and alkali metal bicarbonates 
include those described previously. Examples of the polydentate ligands 
include cyclic polyethers, cyclic polyetheramines, bicyclopolyetheramines, 
cyclic polyamines, non-cyclic polyethers, polyaminocarboxylic acids, 
polyaminophosphoric acids, oxycarboxylic acids, condensed phosphoric 
acids, non-cyclic polyamines, acetylacetone, oxine, natural products and 
salts or partial salts of these compounds. These polydentate ligands may 
be used by themselves or in combination of two of more. 
Illustrative of suitable cyclic polyether ligands are 12-crown-4, 
15-crown-5, 18-crown-6, benzo-12-crown-4, benzo-15-crown-5, 
benzo-18-crown-6, dibenzo-12-crown-4, dibenzo-15-crown-5, 
dibenzo-18-crown-6, dicyclohexyl-18-crown-4, dicyclohexyl-15-crown-5, 
dicyclohexyl-18-crown-6, n-octyl-12-crown-4, n-octyl-15-crown-5 and 
n-octyl-18-crown-6. 
Illustrative of suitable cyclic polyetheramine ligands are cryptand[2.1] 
and cryptand[2.2]. 
Illustrative of suitable bicyclopolyetheramines are cryptand[2.2.1] and 
cryptand[2.2.2]. 
Illustrative of suitable cyclic polyamines 
are1,4,7,10,13,16-hexaazacyclooctadecane and 8-azaadenine. 
Illustrative of suitable non-cyclic polyethers are polyethylene glycol, 
polyethylene glycol monoalkyl ethers and polypropylene glycol. 
Illustrative of suitable polyaminocarboxylic acids are 
ethylenediaminetetraacetic acid, iminodiacetic acid, nitrilotriacetic 
acid, hydroxyethyliminodiacetic acid, 
trans-1,2-diaminocyclohexane-N,N,N',N'-tetraacetic acid, 
ethylenediaminediacetic acid, triethylenetetraminehexaacetic acid, 
glycoletherdiaminetetraacetic acid, 
diethyltriamine-N,N,N',N',N"-pentaacetic acid and 
hydroxyethylethylenediaminetriacetic acid. 
Illustrative of suitable polyaminophosphoric acids are 
ethylenediaminetetrakis(methylenephosphonic acid) and 
nitrilotris(methylenephosphonic acid). 
Illustrative of suitable non-cyclic polyamines are ethylenediamine, 
diethylenetriamine and triethylenetetramine. 
Illustrative of a suitable oxycarboxylic acid is citric acid. 
Illustrative of suitable natural products are glycine, hemin, chlorophyl, 
valinomycin and nigericin. 
The novel carrier composed of at least one alkali metal salt selected from 
alkali metal carbonates and alkali metal bicarbonates and a polydentate 
ligand capable of forming a complex with an alkali metal ion may be 
suitably incorporated into the above-described hydrogel. Alternatively, 
the novel carrier may be used in the form of a conventional liquid film as 
a membrane for separating carbon dioxide. This embodiment will be 
described below. 
For the preparation of the liquid film, the carrier composed of the alkali 
metal salt and the polydentate ligand is dissolved in a suitable solvent 
to form a carrier solution. Any solvent may be used as long as both the 
alkali metal salt and the polydentate ligand are soluble therein. 
Generally, water, a polar organic solvent or a mixture thereof may be 
suitably used. The polar organic solvent is a solvent containing a hetero 
atom such as N, S or O and preferably has a boiling point of at least 
100.degree. C., more preferably at least 150.degree. C. Illustrative of 
suitable polar organic solvents are imidazole, N-substituted imidazoles 
(e.g. N-methylimidazole, N-propylimidazole, N-phenylimidazole and 
N-benzylimidazole), dialkylsulfoxides (e.g. dimethylsulfoxide and 
dioctylsulfoxide) and N,N-dialkylformamides (e.g. N,N-dimethylformamide 
and N,N-dioctylformamide). 
The concentration of the alkali metal salt in the carrier solution is 
generally 0.1-5 mol/liter, preferably 1-4 mol/liter. A concentration of 
the alkali metal salt below 0.1 mol/liter is insufficient to effectively 
absorb carbon dioxide. Too high a concentration of the alkali metal salt 
in excess of 5 mol/liter, on the other hand, is undesirable because the 
transportation speed of the absorbed carbon dioxide is slow. The 
concentration of the polydentate ligand is generally 0.001-1 mol/liter, 
preferably 0.01-0.1 mol/liter. 
The above carrier solution is then supported and immobilized on a permeable 
support to obtain a carbon dioxide separation membrane. The 
above-described support for supporting the hydrogel may be used in this 
embodiment. Because of excellent carbon dioxide separation efficiency, the 
carrier solution may also be utilized as an absorbent solution for the 
separation of carbon dioxide by an absorption method or by a flow liquid 
membrane method. 
The separation of carbon dioxide from a gas mixture containing carbon 
dioxide with the use of the carbon dioxide separation membrane will now be 
described. The membrane is mounted on a conventional permeable cell. A 
first side of the membrane is contacted with the gas mixture maintained at 
a pressure higher than that on the opposite, second side of the membrane. 
Generally, the space on the second side of the membrane is maintained in a 
reduced pressure. Thus, the first side of the membrane serves to function 
as a carbon dioxide absorbing surface while the second side thereof as a 
carbon dioxide discharging surface. 
For example, when the membrane includes a hollow thread support by which 
the above-described carrier-containing hydrogel film or liquid film is 
supported, the inside or outside of the hollow thread is used as a carbon 
dioxide absorbing surface. Thus, for example, when the hollow thread-type 
membrane is disposed in a stream of the mixed gas to be treated, the 
exterior surface thereof serves as a carbon dioxide absorbing surface. The 
carbon dioxide preferentially absorbed in the carrier liquid is 
transported by the carrier to the interior surface of the hollow thread 
where the carbon dioxide is liberated from the carrier and is discharged 
to the inside space of the hollow thread. Because of the above function of 
the carbon dioxide carrier, the amount of the carbon dioxide which 
permeates through the membrane is much greater in comparison with a case 
where no carrier is used so that respective components in the mixed gas 
permeate through the membrane merely due to the pressure difference 
between the inside and outside of the hollow thread. 
The following examples will further illustrate the present invention. 
PREATION OF MEMBRANE HAVING HYDROGEL FILM 
Example 1 
An aqueous vinyl alcohol/sodium acrylate random copolymer solution 
(SUMIKAGEL L-5H manufactured by Sumitomo Chemical Industry, Inc.; vinyl 
alcohol content: 60 mole %, copolymer content: 5% by weight) was applied 
on a surface of a poly(vinylidene fluoride) porous film (GVWP manufactured 
by Milipore Inc.; hydrophilic film; pore diameter: 0.22 .mu.m; thickness: 
110 .mu.m) by a spin coating method (1,500 rpm, 12 seconds) so that the 
film was impregnated with the copolymer solution. The resultant film was 
then heated at 120.degree. C. for 1 hour to cross-link the copolymer and, 
thereafter, immersed in an aqueous potassium carbonate solution (potassium 
carbonate concentration: 2 mols/liter) for 30 minutes, so that the 
cross-linked copolymer was swelled to form a hydrogel film supported by 
the poly(vinylidene fluoride) film. The gel membrane was laminated on a 
silicone rubber film (thickness: 70 .mu.m) to obtain a laminate membrane. 
Example 2 
Example 1 was repeated in the same manner as described except that a 
dipping method was substituted for the spin coating method. 
Example 3 
Example 1 was repeated in the same manner as described except that the 
poly(vinylidene fluoride) film was substituted with a 
polytetrafluoroethylene porous film (FP010 manufactured by Sumitomo 
Electric Industries, Ltd.; pore diameter: 0.1 .mu.m; thickness: 55 .mu.m) 
and that the spin coating method was substituted with a casting method. 
SEATION OF CARBON DIOXIDE 
Example 4 
Each of the laminate membranes obtained in Examples 1-3 was tested for the 
carbon dioxide separation performance. A test gas composed of 10% by 
volume of carbon dioxide and 90% by volume of nitrogen was fed to one side 
of the membrane (effective surface: 9.62 cm.sup.2) under a saturated water 
vapor pressure at a flow rate of 60 ml/minute and a total pressure of 1 
atm at 25.degree. C., while maintaining the other side of the membrane in 
a reduced pressure of 2.3 cmHg. The gas which permeated through the 
membrane was analyzed by gas chromatography for the calculation of the 
carbon dioxide permeation rate Rc (cm.sup.3 /cm.sup.2.sec.cmHg) and the 
separation factor S. The separation factor S is defined as follows: 
EQU S=Rc/Rn 
where Rc is a carbon dioxide permeation rate and Rn is a nitrogen 
permeation rate. 
The results are summarized in Table 1. 
TABLE 1 
______________________________________ 
Membrane Permeation Rate Rc 
Separation Factor S 
______________________________________ 
Example 1 4.2 .times. 10.sup.-6 
294 
Example 2 3.2 .times. 10.sup.-6 
261 
Example 3 2.0 .times. 10.sup.-6 
224 
______________________________________ 
Example 5 
The carbon dioxide separation was continued for 30 days using the membrane 
obtained in Example 1 in the same manner as that in Example 4. The carbon 
dioxide separation performance after 1, 10, 20 and 30 days from the 
commencement of the separation test are shown in Table 2. 
TABLE 2 
______________________________________ 
Test period 
Permeation Rate Rc 
Separation Factor S 
______________________________________ 
1 day 2.8 .times. 10.sup.-6 
212 
10 days 1.7 .times. 10.sup.-6 
161 
20 days 1.5 .times. 10.sup.-6 
125 
30 days 1.8 .times. 10.sup.-6 
143 
______________________________________ 
Comparative Example 
Example 1 was repeated in the same manner as described except that the 
aqueous potassium carbonate solution was directly applied to the 
poly(vinylidene fluoride) porous film without the cross-linked, vinyl 
alcohol/sodium acrylate random copolymer layer, thereby to obtain a 
membrane having a liquid film supported on the film. Using the thus 
obtained membrane, Example 5 was repeated in the same manner as described. 
After one day, however, the carbon dioxide separation was no longer able 
to be continued because of the drying up of the liquid film. 
PREATION OF MEMBRANE HAVING LIQUID FILM 
Example 6 
Various aqueous carrier solutions No. 1-13 having the compositions shown in 
Table 3 were prepared. A poly(vinylidene fluoride) porous film (GVWP 
manufactured by Milipore Inc.; hydrophilic film; pore diameter: 0.22 
.mu.m; porosity: 75%; thickness: 110 .mu.m) was immersed in each carrier 
solution for 30 minutes for impregnation, thereby to obtain a liquid film 
supported by the poly(vinylidene fluoride) film. The thus obtained 
membrane was laminated on a PTFE (polytetrafluoroethylene) film (pore 
diameter: 0.1 .mu.m) to obtain a laminated membrane. 
TABLE 3 
______________________________________ 
Solution 
Alkali metal salt 
Polydentate ligand 
No. Kind Amount*1 Kind Amount*l 
______________________________________ 
1 K.sub.2 CO.sub.3 
2 18-crown-6 0.04 
2 K.sub.2 CO.sub.3 
2 18-crown-6 0.05 
3 K.sub.2 CO.sub.3 
2 cryptand[2.1] 
0.05 
4 K.sub.2 CO.sub.3 
2 cryptand[2.1] 
0.07 
5 K.sub.2 CO.sub.3 
2 cryptand[2.1] 
0.1 
6 K.sub.2 CO.sub.3 
2 cryptand[2.2.2] 
0.02 
7 K.sub.2 CO.sub.3 
2 EDTA*2 0.05 
8 Na.sub.2 CO.sub.3 
2 EDTA.2Na*3 0.03 
9 K.sub.2 CO.sub.3 
2 NTAA*4 0.05 
10 K.sub.2 CO.sub.3 
2 TETHA*5 0.05 
11 K.sub.2 CO.sub.3 
2 triethylenetetramine 
0.5 
12*6 K.sub.2 CO.sub.3 
2 -- -- 
13*6 Na.sub.2 CO.sub.3 
2 -- -- 
______________________________________ 
*1: mol/liter 
*2: ethylenediaminetetraacetic acid 
*3: ethylenediaminetetraacetic acid disodium salt 
*4: nitrilotriacetic acid 
*5: triethylenetetraminetetraacetic acid 
*6: comparative solution 
18-Crown-6, cryptand[2.1] and cryptand[2.2.2] have the following chemical 
structures. 
##STR1## 
SEATION OF CARBON DIOXIDE 
Example 7 
Each of the laminate membranes obtained in Example 6 was tested for the 
carbon dioxide separation performance. A test gas composed of 10% by 
volume of carbon dioxide and 90% by volume of nitrogen was fed to one side 
of the membrane (effective surface: 2.54 cm.sup.2) under a saturated water 
vapor pressure at a flow rate of 100 ml/minute and a total pressure of 1 
atm, while maintaining the other side of the membrane in a reduced 
pressure. The gas which permeated through the membrane was analyzed by gas 
chromatography for the calculation of carbon dioxide permeation rate Rc 
and separation factor S in the same manner as that in Example 4. The 
results are summarized in Table 4. 
PREATION OF MEMBRANE HAVING HYDROGEL FILM 
Example 8 
An aqueous vinyl alcohol/sodium acrylate random copolymer solution as used 
in Example 1 was applied on a surface of a poly(vinylidene fluoride) 
porous film by a spin coating method and the resultant film was thereafter 
heated at 120.degree. C. for 1 hour to cross-link the copolymer. This was 
then immersed in each one of the aqueous carrier solutions No. 1-11 having 
the compositions shown in Table 3 for 30 minutes, so that the cross-linked 
copolymer was swelled to form a hydrogel film supported by the 
poly(vinylidene fluoride) film. The gel membrane was laminated on a PTFE 
film to obtain a laminate membrane. Each of the laminate membranes thus 
obtained was tested in the same manner as described in Example 7 to reveal 
that these membranes exhibited good permeation rate and separation factor. 
It was also confirmed that these membranes exhibited satisfactory carbon 
dioxide separation performance even after 1 week continuous operation. 
TABLE 4 
______________________________________ 
Membrane Permeation Rate Rc 
Separation Factor S 
______________________________________ 
Solution No. 1 
1.1 .times. 10.sup.-5 
545 
Solution No. 2 
7.4 .times. 10.sup.-6 
670 
Solution No. 3 
2.0 .times. 10.sup.-5 
1,385 
Solution No. 4 
2.5 .times. 10.sup.-5 
1,859 
Solution No. 5 
2.0 .times. 10.sup.-5 
1,203 
Solution No. 6 
1.2 .times. 10.sup.-5 
647 
Solution No. 7 
2.4 .times. 10.sup.-5 
1,417 
Solution No. 8 
6.0 .times. 10.sup.-6 
600 
Solution No. 9 
1.1 .times. 10.sup.-5 
580 
Solution No. 10 
1.2 .times. 10.sup.-5 
600 
Solution No. 11 
1.6 .times. 10.sup.-5 
504 
Solution No. 12* 
6.1 .times. 10.sup.-6 
317 
Solution No. 13* 
3.0 .times. 10.sup.-6 
153 
______________________________________ 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embraced therein.