GB2120121A

GB2120121A - Filtration combined with ion- exchange to purify steam

Info

Publication number: GB2120121A
Application number: GB08213951A
Authority: GB
Inventors: Kenneth A Perrotta
Original assignee: Whatman Reeve Angel Ltd
Current assignee: Whatman Ltd
Priority date: 1982-05-13
Filing date: 1982-05-13
Publication date: 1983-11-30
Also published as: GB2120121B

Abstract

Pressurized, saturated steam is purified by being passed through a composite cartridge (38) in which particulate matter is removed by a filter tube (44, 46) of bonded-together glass fibres, and boiler-feed additives are removed by contacting the filtered steam with a bed (48) of a hydrogen-form, strong-acid, ion- exchange resin. Saturated, pressurized steam substantially free of particulate matter and volatile, alkaline, boiler-feed additives is recovered for use, for example, in food-preparation and sterilization. <IMAGE>

Description

SPECIFICATION Apparatus for and method of removing volatile boiler-fed additives from pressurized steam Pressurized, saturated steam, particularly centrally generated steam, can be used for many purposes, such as for air humidification, food preparation and for the sterilization of medical devices. The use of pressurized steam, particularly in hospital sterilizers, has given rise to problems, such as stained and spotted sterilized instruments, dirty sterilizers requiring frequent cleaning, malfunctioning of steam-controlled valves, and an excess of maintenance on sterilizer door seals and gaskets.A frequent cause of problems in hospital sterilizers in particular is poorquality steam; that is, a pressurized, saturated steam that contains particulate matter, such as dirt from the piping system, pipe-scale rust, particles of packings and grease and oil, as well as liquid or condensed water carrying a wide range of contaminants therewith, such as boiler-feed additives, soluble salts and other organic material present in the boiler or water supply, and in addition includes other impurities, such as volatile boiler-feedwater additives, typically volatile alkaline additives, such as amines.

The problems associated with the use of steam, particularly in sterilizers, that is, caused by dirty steam, are often controlled or eliminated by filtering out the solid particulate material and the condensed water or other liquids out of the saturated steam, before the steam enters the sterilizer or is otherwise put to use. Usually, a steam filter is employed, which is an extremely efficient solids filter having a rating of 1 micron or better. Filter tubes composed of a plurality of nonwoven, randomly disposed, borosilicate-glass fibers, having a bonding agent at the fibercrossover points to form a porous, self-supporting filter tube, have been employed within a filter housing, to provide for a filtered, saturated steam.

The filter tube also contains a bonding agent, such as a silicon resin or fluorocarbon resin, which may be used at the steam temperatures. The filter tube, for example, is placed within a housing within a cylindrical baffle. Unfiltered, saturated, pressurized steam enters the housing through an inlet port and is deflected around the centrifugal baffle, so that the change in direction causes much of the suspended water to drop to the bottom of the housing, from which it is removed automatically by a float-operated drain. The steam is essentially free of condensed water and passes through the walls of the filter tube from the outside inwardly, and filtered steam is removed from the inside of the filter tube through an outlet port and into the sterilizer for use.It has been found that an efficient filter tube can remove virtually all or a substantial portion of the particulate and nonvolatile, water-soluble impurities in saturated steam. However, it is desirable to provide an apparatus and a method for a very highly sterilized steam, particularly for use in hospital sterilizers.

The present invention relates to an apparatus for and a method of preparing highly purified steam essentially free of contaminants and volatile boiler-feed additives. In particular, the apparatus comprises a composite-cartridge filter and a method of removing volatile alkaline amines employed as boiler-feed additives from filtered, pressurized steam.

According to the invention there is provided a method of preparing purified, saturated, pressurized steam from pressurized, saturated steam which contains particulate matter and a volatile, alkaline boiler-feed additive therein, which method comprises: passing the saturated, pressurized steam through the wall of a filter tube, which filter tube comprises a plurality of randomly disposed, nonwoven, inorganic glass fibers having interstices therebetween, to define the porosity of the filter tube, the fibers having a diameter of from about 0.01 to 10 microns, and bonded at the junction of the fiber-crossover points with a bonding agent to form a porous, self-supporting filter tube to filter the particulate matter from the pressurized saturated steam; passing the particulate-free, filtered, pressurized saturated steam downstream through a bed of strong acid hydrogen-form ion-exchange resin material, the resin material composed of a high temperature, steam-resistant polymer, the resin material having a sufficient bed-depth to provide for the reaction of the strong acid hydrogen of the ion-exchange resin with the alkaline, volatile, boiler-feed additive in the steam in an acid-base reaction, the saturated steam having a pressure of from about 15 to 90 psi (1 to 6 bar); and recovering a filtered, purified, saturated, pressurized steam essentially free of particulate matter and volatile, alkaline, boiler-feed additive therein.

Another aspect of the invention provides apparatus for preparing purified, saturated, pressurized steam from pressurized, saturated steam which contains particulate matter and a volatile, alkaline boiler-feed additive therein, said apparatus comprising a filter tube which comprises a plurality of randomly disposed, nonwoven, inorganic glass fibers having interstices therebetween, to define the porosity of the filter tube, the fibers having a diameter of from about 0.01 to 10 microns and being bonded at the junction of the fiber-crossover points with a bonding agent to form a porous, self-supporting filter tube, means for passing saturated, pressurized steam through the wall of the filter tube and through a bed of strong acid hydrogenform ion-exchange resin material composed of a high temperature, steam-resistant polymer, the resin material having a sufficient bed-depth to provide for the reaction of the strong acid hydrogen of the ion-exchange resin with the alkaline, volatile, boiler-feed additive in the steam in an acid-base reaction, and means for recovering a filtered, purified, saturated, pressurized steam essentially free of particulate matter and volatile, alkaline, boiler-feed additive from said bed.

A wide variety of boiler-water additives is employed for various purposes, and in particular volatile, alkaline, boiler-feed additives are employed in boiler feedwater to control the pH of the condensate to minimize corrosion in the steam-distribution system and boiler system. Typical volatile amine additives which may be employed include, but are not limited to: cyclohexyl amine; diethylamino ethanol; hydrazine; morpholine; octadecyl amine; or combinations thereof.

These volatile, boiler-feed additives have been found to be present in filtered steam. The solid, particulate impurities in the steam are and can be removed by the employment of a glass-fiber filter tube, while the nonvolatile additives used as boiler-feed additives are typically carried into the steam line in the entrained water and remain dissolved or suspended in the water.

Therefore, a filter which effectively separates the condensate from the steam, before the steam enters the sterilizer used, will remove essentially or substantially all of the nonvolatile, feedwater additives.

However and importantly, it has been found that the volatile, boilerfeedwater additives are not removed, either in the condensed water or through the employment of a giass-fiber filter tube, alone. It has been discovered that an efficient, glass-fiber steam filter can remove virtually all of the solid impurities and the nonvolatile, water-soluble impurities from the steam; however, the glassfiber filter tube has been found to remove only a small amount, if any, of the volatile amine additives present in the steam. The major portion of these volatile amines remains in the vapor phase in the steam and enters the sterilizer and contacts the contents with the filtered steam.

Thus, the volatile amines present in the filtered steam may be deposited on food during blanching or steam-cooking or on medical supplies, such as instruments or gauze, during sterilization. Steam containing such amines is also often used with a conditioned air supply, and the air is sometimes humidified by centrally generated steam, and, therefore, there is a possibility that such amine-contaminated steam may reach clean-rooms or critical areas of hospitals, including operating rooms, delivery rooms, recovery rooms and intensive-care units. A further concern is that some of the amines, such as morpholine, present in the steam may react with nitrites or other foodstuff additives, to form nitrosoamines or other compounds which may be or are potential animal carcinogens.

It has been found that volatile, alkaline materials, such as the volatile amines used as boilerfeed additives, may be removed effectively from filtered steam through the employment, directly downstream or adjacent to the filtering operation, of an ion-exchange resin in a strong-acid form. The ion-exchange resin, particularly in particulate form, should be composed of a hightemperature or steam-resistant polymer, such as styrene copolymers or the like, which is susceptible to use with steam, without degradation or substantial deterioration. The employment of the ion-exchange resin material removes the amine vapors present in the filtered steam, by the formation of a resin salt in a reaction analogous to an acid-base reaction.Thus, where a sulfonic-acid-type ion-exchange resin is employed, the volatile amines, such as morpholine and the like, react in an acid-base reaction, to form the resin salt of the polymer and water. Usually, pressurized steam is employed at a range of from about 15 to 90 psi (1 to 6 bar) in hospitals, and, therefore, the upper temperature limit of the ion-exchange resin to be employed in the invention is about 300 F (1 50'C) or higher, if possible, which would permit the steam to be purified, employing the ion-exchange resin at a pressure up to about 65 psi (4.3 bar).

The ion-exchange resin employed should be of the strong-acid type, particularly in the hydrogen form, and not the sodium or potassium form, because it is desired to effect an acidbase reaction with the volatile, alkaline material in the pressurized steam. The use of boilerfeedwater additives used in the preparation of steam that will contact food has been recognized by the FDA, and upper limits have been set for particular, common additives, set forth in the Federal Register, Section 1 73.31 0(d), entitled "Boiler Water Additives".

In the method and apparatus as later described, by way of example, in detail, the filtered steam is directed to and passes through a bed of ion-exchange resin of sufficient depth, to permit the reaction of all or essentially all of the volatile amines in an acid-base reaction with the strong acid on the ion-exchange resin, so that the pressurized steam recovered, after passing through the bed, is essentially free of a volatile, alkaline material or contains only very minor traces of such materials; for example, less than 3 ppm. The ion-exchange resin is typically in bead or other particulate form, and may be placed in a separate container and spaced apart from, and downstream of, the steam filter tube, but more desirably is directly adjacent the filter tube, so that filtration and neutralization of the steam occur within a single, composite cartridge, as more particularly will be described hereafter. Such cartridge is normally employed in a housing, to permit the change in direction of the steam and the removal of condensed water from the housing drain, with the pressurized steam then passing through the walls of the filter tube and, thereafter, through the bed of strong-acid, ion-exchange resin.

In one embodiment, a strong-acid, iron-exchange resin is placed in between two glass-fiber filter tubes or one upstream filter tube and a downstream, porous, tubular retainer or support, whereby the saturated steam is passed through one wall of the filter tube, is neutralized by passing through the ion-exchange bed and is further filtered or passed through the wall of the filter tube or retainer on the opposite side of the bed. For example, the filter tube may be arranged concentrically, with the inside of the inside filter tube supported by a perforated core, with the flow of the steam being from the outside to the inside, and the purified steam is removed from the inside of the concentric filter tube, which combination forms a composite cartridge.

In another embodiment, the ion-exchange resin may be placed directly adjacent a filter tube or a series of filter tubes in a solid-bed form; for example, directly downstream of the filtered steam, so that the saturated steam, containing the impurities, is filtered by passing it through the walls of a glassfiber filter tube, and, thereafter, passes through a solid particulate bed of ion-exchange resin directly downstream of the filter tube; for example, where the bed is axially aligned therewith, and the filtered, highly purified, neutralized steam is removed from the solid, ion-exchange bed.

The filter tubes suitable for use include those filter tubes which are composed of a plurality of inorganic fibers, such as glass and typically borosilicate-glass fibers or alumina or zirconia fibers, randomly disposed to form a porous, self-supporting filter tube, with the diameter of the fibers ranging from about 0.01 to 10 microns; for example, 0.03 to 8 microns, and typically 0.1 to 4 microns, the fibers arranged so that the interstices between the fibers define the porosity of the filter tube, and with the crossover points of the fibers in the filter tube containing a bonding agent, such as a hardened material, and typically a hardened resin material, such as a silicone or fluorocarbon resin, which bonding material would be impervious or not degraded by the steam in use.A typical glass-fiber filter tube, employing a fluorocarbon bonding agent, suitable for use in the invention is described in U.S. Patent Specification No. 4,210,540.

The invention will be further described, by way of example, with reference to the accompanying drawings, in which:~ Figure 1 is an illustrative system for the furnishing of filtered, highly purified steam to a sterilizer using the method of the invention Figure 2 is an illustrative, cross-sectional view of a composite filter cartridge as used in the system of Fig. 1; and Figure 3 is an illustrative view of an alternative embodiment of composite cartridge; Fig. 1 illustrates a system for the supply of highly purified pressurized steam, which system 10 comprises a pressure vessel 12, such as a hospital sterilizer, a housing 36 for a composite cartridge filter 38, which housing includes a drain 26, such as an automatic drain.The housing is secured to a pressurized, saturated-steam inlet line 14 and an outlet line 16, with three-way valves 18 and 20 in the respective lines, and having a bypass line 22, with the outlet line including a pressure gauge 28. The pressure vessel 12 includes therein sterilized, filtered, pressurized steam 24, and the vessel includes a vessel outlet line 34 including a pressure gauge 30 which contains a trap 32 which passes an air-and-steam mixture and retains pure steam, which trap discharges air to the atmosphere.

In operation, a pressurized, saturated steam is introduced to inlet 14 and into the housing 36, and exterior of the outer wall of the composite cartridge 38, a cylindrical baffle (not shown) exterior of and spaced apart from the cartridge is optional, the steam passing through the cartridge 38 and out the outlet through valve 18 and line 16 into the pressurized vessel 12 as highly purified, pressurized steam 24. The automatic drain 26 discharges water, containing nonvolatile, boiler additives, from the saturated steam from the housing. Condensed water is passed through the trap 32.

Fig. 2 is a cross-sectional, illustrative embodiment of a composite cartridge useful in the system 10 of Fig. 1, which composite cartridge 38 includes fitted or sealed end caps 40 and 42, an outer filter tube 44 composed of borosiicate-glass fibers, with a silicone or fluorocarbon resin-bonding agent, and another internal, borosilicate-glass-fiber, inner filter tube which may be the same or different from the filter tube 44, and contained within the filter tubes 44 and 46 is a bed 48 of a sulfonic-acid, ion-exchange resin in particulate or bead form. A stainless-steel mesh or other porous support core 50 is employed, while the end cap 42 includes an outlet for steam, while end cap 40 closes off the bottom of the support core. The flow of the steam is schematically illustrated by the flow arrows.

In operation, saturated steam passes through the wall of the filter tube 44 through resin bed 48 and through the wall of filter tube 46 and into the interior passageway, and is discharged out outlet 52 as highly purified, filtered, pressurized steam ready for use in a hospital sterilizer.

Fig. 3 is an illustrative, sectional view of another form of composite cartridge, which includes end caps 54 and 56, with end cap 56 having a steam outlet 66, and with end cap 54 being imperforate. A glass-fiber filter tube 58 is employed in the lower portion of the cartridge. A bed of ion-exchange resin beads 60 is formed directly above the axial line as part of the composite cartridge. An open-mesh, porous, support grid 62, such as of stainless steel, separates and retains the ion-exchange resin from the interior of the filter tube 58. The ion-exchange resin is retained in a tubular, nonporous, plastics, sleeve element 64.

In operation, pressurized, saturated steam passes through the walls of the filter 58 at the lower portion of the composite cartridge which is in the housing, and then passes upwardly through the center of the filter tube to the porous, support, open-mesh material 62 and through the tubular bed of particulate, acid, ion-exchange resin 60, and the highly sterilized, filtered, pressurized steam is then discharged from the top outlet 66.

Example 1.

195 grams of Amberlite (a registered trademark of Rohm s Haas Co.) 200 CH, strong-acid, ion-exchange resin were filled into the annular space between two hardened resin bonded glassfiber filter tubes of 0.75 ins (1.9 cm) diameter and 2.00 ins (5.1 cms) diameter. End caps were then potted on and the entire cartridge was installed in a steam-filter housing. The housing had been installed previously in a steam-distribution system, with a working pressure of 10 to 15 psig (0.7 to 1 bar) steam. Morpholine was then added to the boiler, feeding the system in an amount sufficient to generate 3 ppm by weight in the steam condensate. Samples of the steam condensate were taken upstream and downstream of the housing, by bleeding steam through a copper coil cooled with water.Glass collection bottles, which had been rinsed previously with distilled water, were used to collect the condensate. Using a Taylor Water Analyzer Kit No.

1300, the concentration of morpholine ppm by weight was determined. In the test used, morpholine reacts with carbon disulfide to form a thiocarbamate compound. This carbonate reacts with excess copper ion to form a brown-colored compound, with the intensity of the brown color being proportioned to the concentration of morpholine present.

The results were as follows: Concentration upstream filter 3.0 ppm Concentration downstream filter 0.2 ppm Example 2.

After 24 hours had elapsed, the procedure in Example 1 was repeated, and the results were as follows: Concentration upstream filter 2.0 ppm Concentration downstream filter 0.2 ppm Example 3.

Example 1 was repeated, but the ion-exchange filter cartridge was removed from the system, with no other changes. The results were as follows: Concentration upstream 1.5 ppm Concentration downstream 1.5 ppm Example 4.

Boiler-feedwater additives fall generally into the following categories: binding agents for calcium, magnesium and silicon, to prevent solids from plating out on the hot tube walls (examples of these materials are sodium tripolyphosphate, sodium alginate and tetrasodium EDTA (a chelating agent)); oxygen scavengers to retard tube corrosion, which can be accelerated by dissolved oxygen in the feedwater (examples are sodium sulfite and hydrazine); and alkaline buffers to keep the pH of the feedwater in the desired range; for example, 6.5 to 8.5.

In practice, steam condensate is encountered at pH9 or even higher. Sodium carbonate, sodium aluminate, morpholine and cyclohexyl amine are some materials used as alkaline buffers. The latter two products have the advantage of being volatile, and, therefore, they can protect the entire steam-distribution system and condensate return lines from acidic corrosion.

Further boiler-feedwater additives are volatile corrosion inhibitors which form a protective film on pipes or other metal surfaces. Octadecyl amine, a long-chain fatty amine, is typically added to boiler feedwater for this purpose.

Nonvolatile additives constitute the great bulk of the boiler-feedwater additives. The nonvolatile additives are carried into the steam line in the entrained water, and they remain dissolved or suspended in the water. Therefore, a filter which effectively separates condensate from steam, before the steam enters the sterilizer, will remove essentially all of the nonvolatile feedwater additives.

To obtain quantitative information on concentrations of volatile amines in the steam to hospital sterilizers, analytical determinations on the steam system of a hospital, which uses morpholine as one of its boiler-feedwater additives, were made. The testing was done at a sterilizer which has a filter housing containing a fluorocarbon resin bonded glass-fiber filter tube on the steam line. Tests were made on six days over about a one-month period. Each result is the average of two or three determinations. Steam was sampled both upstream and downstream of the filter tube, which is located immediately upstream of the sterilizer steam inlet. The concentration of morpholine inside the sterilizer was the same as the concentration downstream of the filter. The concentration of morpholine in the boiler feedwater, itself, was also determined.

The test results are: Morpholine Concentration, ppm Date Boiler Upstream Downstream Days Feedwater of Filter of Filter November 19 2.6 1.8 November 20 - 2.9 2.0 November 24 4.0 4.0 3.2 December 1 - 5.5 4.0 December 19 - 4.2 3.6 December 22 - 3.5 2.5 On the average, the filter tube reduced the morpholine concentration in the steam by about 25%, presumably by removing the liquid water in which some of the morpholine is dissolved.

However, the major portion of the morpholine remained in the vapor phase and entered the sterilizer with the filter steam.

Thus, while an efficient, steam, glass-fiber filter tube can remove virtually all solid impurities and nonvolatile water-soluble impurities from the steam, the filter will remove only about onefourth of the volatile amines in the steam, while, as demonstrated, the employment of a bed of strong-acid ion-exchange resin immediately downstream of the filter removes the volatile, alkaline additives from the filtered steam.

Claims

1. A method of preparing purified, saturated, pressurized steam from pressurized, saturated steam which contains particulate matter and a volatile, alkaline boiler-feed additive therein, which method comprises: passing the saturated, pressurized steam through the wall of a filter tube, which filter tube comprises a plurality of randomly disposed, nonwoven, inorganic glass fibers having interstices therebetween, to define the porosity of the filter tube, the fibers having a diameter of from about 0.01 to 10 microns, and bonded at the junction of the fiber-crossover points with a bonding agent to form a porous, self-supporting filter tube to filter the particulate matter from the pressurized saturated steam; passing the particulate-free, filtered, pressurized saturated steam downstream through a bed of strong acid hydrogen-form ion-exchange resin material, the resin material composed of a high temperature, steam-resistant polymer, the resin material having a sufficient bed-depth to provide for the reaction of the strong acid hydrogen of the ion-exchange resin with the alkaline, volatile, boiler-feed additive in the steam in an acidbase reaction, the saturated steam having a pressure of from about 15,to 90 psi (1 to 6 bar); and recovering a filtered, purified, saturated, pressurized steam essentially free of particulate matter and volatile, alkaline, boiler-feed additive therein.

2. A method according to claim 1, wherein the volatile, alkaline, boiler-feed additive comprises cyclohexyl amine, diethylamino ethanol, hydrazine, morpholine and/or octadecyl amine and combinations thereof.

3. A method according to claim 1 or 2, wherein the pressurized saturated steam is passed through the wall of a filter tube and thereafter directly axially upstream or downstream through a bed of a hydrogen-form sulfonic-acid ion-exchange resin material.

4. A method according to claim 1, 2 or 3, wherein the glass fibers of the filter tube comprise borosilicate-glass fibers having a diameter from about 0.03 to 8 microns.

5. A method according to any preceding claim, which comprises passing the filtered steam from the filter tube directly through the wall of another filter tube on the opposite side and adjacent to a bed of the ion-exchange resin, whereby the saturated steam passes first through the one wall of the filter tube, through the ion-exchange resin bed, and thereafter directly through another adjacent wall of another filter tube.

6. A method according to any preceding claim, wherein the strong acid hydrogen-form ionexchange resin comprises a sulfonic acid ion-exchange resin.

7. A method according to any preceding claim, wherein the bonding agent comprises a hardened fluorocarbon or silicon resin.

8. A method according to any preceding claim, wherein the pressurized steam is recovered having less than about 3 parts per million of volatile alkaline boiler-feed additive therein.

9. A method according to any preceding claim, wherein the bed of ion-exchange resin is spaced apart from and directly downstream of the filter tube.

10. A method according to any preceding claim, wherein the filtered, purified, saturated, pressurized steam is passed directly into a sterilizer vessel for the sterilization of medical instruments.

11. A method of preparing purified, saturated, pressurized steam from pressurized, saturated steam which contains particulate matter and a volatile, alkaline boiler-fed additive therein, which method comprises passing the saturated, pressurized steam through the wall of a filter tube, which filter tube comprises a plurality of randomly disposed, nonwoven, inorganic glass fibers having interstices therebetween, to define the porosity of the filter tube, the fibers having a diameter of from about 0.01 to 10 microns, and bonded at the junction of the fiber-crossover points with a bonding agent selected from the group consisting of hardened silicone or fluorocarbon resins to form a porous, self-supporting filter tube to filter the particulate matter from the pressurized saturated steam; passing the particulate-free, filtered, pressurized saturated steam directly downstream through a bed of hydrogen-form, sulfonic acid, ion-exchange resin material, the resin material composed of a high temperature, steam-resistant polymer, the resin material having a sufficient bed-depth to provide for the reaction of the strong acid hydrogen of the sulfonic acid ion-exchange resin with the alkaline, volatile, boiler-feed additive in the steam in an acid-base reaction, the saturated steam having a pressure of from about 15 to 90 psi (1 to 6 bars); and, recovering a filtered, purified, saturated, pressurized steam essentially free of particulate matter and having less than about 3 parts per million of volatile, alkaline, boiler-feed additive therein.

12. Apparatus for preparing purified, saturated, pressurized steam from pressurized, saturated steam which contains particulate matter and a volatile, alkaline boiler-feed additive therein, said apparatus comprising a filter tube which comprises a plurality of randomly disposed, nonwoven, inorganic glass fibers having interstices therebetween, to define the porosity of the filter tube, the fibers having a diameter of from about 0.01 to 10 microns and being bonded at the junction of the fiber-crossover points with a bonding agent to form a porous, self-supp.orting filter tube, means for passing saturated, pressurized steam through the wall of the filter tube and through a bed of strong acid hydrogen-form ion-exchange resin material composed of a high temperature, steam-resistant polymer, the resin material having a sufficient bed-depth to provide for the reaction of the strong acid hydrogen of the ion-exchange resin with the alkaline, volatile, boiler-feed additive in the steam in an acid-base reaction and means for recovering a filtered, purified, saturated, pressurized steam essentially free of particulate matter and volatile, alkaline, boiler-feed additive from said bed.

13. Apparatus according to claim 12, wherein the volatile, alkaline, boiler-feed additive comprises cyclohexyl amine, diethylamine ethanol, hydrazine, morpholine and/or octadecyl amine.

14. Apparatus according to claim 12 or 13, wherein the glass fibers of the filter tube comprise borosilicate-glass fibers having a diameter from about 0.03 to 8 microns.

15. Apparatus according to claims 12, 13 or 14, which includes another filter tube and means for passing the filtered steam from the first filter tube directly through the wall of said other filter tube on the opposite side and adjacent to the bed of the ion-exchange resin, whereby in one of the apparatus the saturated steam passes first through the one wall of the first filter tube, through the ion-exchange resin bed, and thereafter directly through an adjacent wall of said other filter tube.

16. Apparatus according to any one of claims 12 to 15, wherein the strong acid hydrogen form ion-exchange resin comprises a sulfonic acid ion-exchange resin.

17. Apparatus according to any one of claims 1 2 to 16, wherein the bonding agent comprises a hardened fluorocarbon or silicone resin.

18. Apparatus according to any one of claims 12 to 17, wherein the bed of ion-exchange resin is spaced apart from and directly downstream of the filter tube.

19. Apparatus according to any one of claims 12 to 18, wherein means are provided for passing the filtered, purified, saturated pressurized steam directly into a sterilizer vessel for the sterilization of medical instruments.

20. Apparatus for preparing purified, saturated, pressurized steam substantially as herein described with reference to and as illustrated in the accompanying drawings.

21. A method of preparing purified, saturated, pressurized steam substantially as herein described with reference to the accompanying drawings.