Honeycomb matrix comprising macroporous desiccant, process and use thereof

Abstract

Macroporous desiccant based honeycomb matrix containing the macroporous desiccant synthesized “in-situ”, the desiccant having a differential water adsorption. Process for the “in-situ” preparation of the macroporous desiccant based honeycomb matrix including the steps of soaking honeycomb substrate impregnated with water glass, in aqueous metal salt(s) solution or acid solution, or combination thereof, until such time that the hydrogel honeycomb matrix is obtained and thermally activating the hydrogel honeycomb matrix to produce macroporous desiccant based honeycomb matrix.

Claims

1. A process for in-situ preparation of a macroporous desiccant based honeycomb matrix comprising the steps of: i. soaking a honeycomb substrate impregnated with water glass, in 4-25% by weight phosphoric acid, at a temperature of 10-80° C. until such time that a hydrogel honeycomb matrix is obtained; ii. washing the hydrogel honeycomb matrix; and iii. thermally activating the hydrogel honeycomb matrix at a temperature of 140° C. to produce the macroporous desiccant based honeycomb matrix.

2. The process as claimed in claim 1, wherein the hydrogel honeycomb matrix comprises at least one substrate selected from the group consisting of an organic substrate and an inorganic substrate.

3. The process as claimed in claim 2, wherein the at least one substrate comprises fiber or pulp.

4. The process as claimed in claim 1, wherein the water glass is neutral grade sodium silicate or potassium silicate.

5. The process as claimed in claim 1, wherein the honeycomb substrate is soaked for about 15 to 120 minutes to obtain the hydrogel honeycomb matrix.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1(a) depicts the working of a typical microporous, thermally activated, desiccant dehumidification wheel/rotor.

(2) FIG. 1(b) depicts a typical sectoral division of rotating desiccant wheel.

(3) FIG. 2(a) depicts the working of a typical macroporous “passive” desiccant dehumidification wheel/rotor.

(4) FIG. 2(b) depicts typical sectoral division of “passive” rotating desiccant wheel.

(5) FIG. 3(a) depicts of a typical macroporous, thermally activated, desiccant dehumidification wheel/rotor with a heat exchanger.

(6) FIGS. 3(b) and 3c depict the typical sectoral division of the desiccant wheel of FIG. 3a.

(7) FIG. 4(a-d) depicts the “in-situ” synthesized paper, single facer, winding of single facer, and honeycomb matrix.

(8) FIG. 5a depicts the typical Brunauer classification of isotherms.

(9) FIG. 5(b) depicts the isotherm for the adsorption and structural characteristics of some experimental silica gel.

(10) FIG. 5(c) depicts the typical type-I isotherm of microporous desiccant.

(11) FIG. 5(d) depicts the typical type-II and III isotherm of a macroporous desiccant.

(12) FIG. 6(a) depicts the isotherm for the honeycomb matrix comprising macroporous desiccant.

(13) FIG. 6(b) depicts the isotherm differentiating the prior art from that of the present invention.

(14) FIG. 7(a-c) depicts the chemical filter comprising the macroporous desiccant based honeycomb matrix.

DETAILED DESCRIPTION

(15) The present invention provides the synthesis and creation “in-situ” of a new desiccant of type II/III in the porosity of the substrate of the honeycomb. The desiccant wheel matrix created is such that maximum difference is achieved between the water holding capacity at 50% relative humidity and 100% RH. The isotherm for such commercially available type II, III desiccants is shown in FIG. 5(d). To create a sufficient driving force or vapor pressure differential, this difference in moisture holding capacity i.e. moisture to desiccant weight to ratio, should be at least 40%, but preferably 50% or more as illustrated in FIG. 6(a). In one of the embodiments of the present invention the percentage water is greater than 60% at a relative humidity of 100%.

(16) Desiccant wheels for dehumidifiers for the purpose of the present invention are intended to include “passive” or “active” desiccant wheels.

(17) In “passive” wheels as depicted by FIG. 2(a), which do not employ any heated air or fluid for regeneration or reactivation of the honeycomb matrix, there is limited means to regenerate the desiccant wheels, unless a driving force can be created to drive the moisture out of the desiccant matrix. During regeneration, the “passive” desiccant wheels, therefore, have to rely on the way they are applied so as to make available or maximize this driving force. Typically, in the way the “passive” wheels are being currently applied, the process side of the wheels matrix is exposed to a saturated fluid stream i.e. at 100% relative humidity. To regenerate this wheel, fluid/air is used which is typically room return air, which typically, is at 50% relative humidity. Hence the driving force will be the vapor pressure difference that will be available and created by the relative humidity difference when desiccant is saturated, and for regeneration is surrounded by air having a relative humidity of 50%. As “passive” wheels do not see high or elevated temperature for regeneration, so far use has been made of commercially available type II or III desiccants by either coating these onto, or impregnating into the substrate of the wheel matrixes. However, to synthesize and create “in-situ” such a desiccant, in the porosity of the substrate of the wheel matrixes, is an unknown, and challenging, and yet very much needed requirement. Shown in FIG. 5(c-d) are isotherms of type I and type II/III material. As will be observed, type I desiccant has sufficient and considerable moisture holding capacity at mid 50s or lower relative humidity, at which such desiccants are used. On the other hand, type II/III desiccant have a very high moisture holding capacity at 90-100% saturation/relative humidity i.e. saturation to near saturation level.

(18) Another area of increasing and differentiating application for dehumidification wheel is where bulk of the water is first removed with the help of a refrigerant or chilled fluid fed heat exchanger following which air is saturated i.e. at 100% relative humidity, before entering the dehumidification wheel. While the application for such wheels often requires thermal activation in the regeneration air circuit, the desiccant synthesized in the matrix is required to perform with maximum moisture removal at such saturated conditions. All currently available desiccant dehumidification wheels which have type I desiccant synthesized “in-situ” exhibit limited performance i.e. moisture removal when exposed to saturated inlet conditions as these are mainly microporous desiccant matrixes. The honeycomb matrix with macroporous type II/III desiccant, synthesized “in-situ”, for such applications, with saturated feed air, exhibits about 10% more moisture removal compared to the microporous type I type desiccant wheels currently available on the market. Consequently this translates into a major 10% energy saving i.e. energy consumed, for the same amount of moisture removed, which is a significant advancement for a desiccant honeycomb matrix, which is thermally activated, and exposed to saturated “process in” air.

(19) Honeycomb type wheels, as currently available in the market, with “in-situ” silica gel/metal silicate synthesized in and around the substrate have a good moisture removal from an air stream having a relative humidity of 5 to 100%. In the development of such desiccant matrixes, the object has been to focus on microporous desiccants which have a high surface area and have a moisture equilibrium ranging from 7 to 9% at 10% RH to 35 to 40% at 90% RH. Such microporous desiccants are limited in pore volume, as the goal is to maximize surface area. Such microporous desiccants have an average pore size of about 18-25 {acute over (Å)}.

(20) While there are several definitions, in published literature, separating the pore size range of microporous, macroporous, mesopore materials, the increase in pore size, more specifically increase in pore volume, and the increased moisture holding capacity at 90% RH are all indicative of macroporous or mesoporous material, generally of the type II & III isotherm.

(21) The desiccant dehumidification wheels have hitherto, till a few years ago, been mainly thermally activated. Being thermally activated the object has been to maximize the performance of moisture removal over the widest range of inlet humidity condition. This is typically achieved with a micropore type desiccant which provides maximum/optimum surface area, and generally has pore distribution between 18 to 25 angstrom.

(22) In the prior art there has been an increasing system design application where maximum moisture is first removed by pre-cooling before feeding the “process air” through the desiccant dehumidifier wheel. In such cases, the desiccant wheel invariably sees saturated (100% RH) air entering the process part of the wheel. This throws up the need and opportunity to create a desiccant honeycomb matrix which will have a high moisture removal potential when the entering air is saturated or near saturated i.e. have >90% relative humidity. Highest moisture removal at >90% RH would typically require a macroporous matrix which should have high moisture holding capacity (>60%) at 100% RH, as opposed to micropore material which have been generally been available and applied to the whole spectrum including the high humidity applications as well.

(23) In recent years, system designs have been configured employing a “passive” dehumidification wheel which does not require external thermal/heat of reactivation. Such systems invariably see near saturated air entering the dehumidification wheel.

(24) The present invention provides honeycomb matrix comprising macroporous desiccant prepared “in-situ”, the said desiccant having differential adsorption more than 50% or at least 40% at relative humidities of 50% and 100%, a pore diameter of about 40 to about 200 Å and pore volume of about 0.40 to about 0.80 gm/cc.

(25) A “passive” desiccant wheel employing said macroporous desiccant has the highest water holding capacity at the high RH/near saturated conditions.

(26) The invention further provides process for the preparation of the honeycomb matrix comprising macroporous desiccant prepared “in-situ”.

(27) The process for the preparation of the honeycomb matrix comprising macroporous desiccant prepared “in-situ” comprises the steps of: soaking honeycomb substrate impregnated with water glass, in 4-25%, by weight, aqueous metal salt(s) solution or acid solution, or combination thereof, at a temperature of 10-80° C. till such time the hydrogel honeycomb matrix is obtained and; thermally activating said hydrogel honeycomb matrix to produce macroporous desiccant based honeycomb matrix

(28) The hydrogel honeycomb matrix may be optionally washed before the thermal activation step.

(29) A macroporous desiccant based honeycomb matrix comprises of a flat and corrugated sheet of active desiccant, having a wave height between 0.8 to 5 mm, preferably between 1.0-2.5 mm, synthesized into the pores of the substrate. The substrate can be organic or inorganic substrate such as fiber glass, kraft paper, ceramic paper etc.

(30) In one embodiment of the present invention the substrate used in the desiccant based honeycomb matrix is a fibre glass substrate which is a highly porous material having a fibre diameter of 6-18 micron, fibre length 6-15 mm and thickness of 0.1-0.5 mm and preferably 0.15-0.25 mm. The binder content of the fiberglass substrate used in the said matrix is between 6-20%, preferably 6-10%, said binder preferably being polyvinyl alcohol. When subjected to thermal heating/regeneration, the macroporous desiccant based honeycomb matrix using an inorganic fibre substrate with low binder content is also environment friendly.

(31) The basic weight of the substrate used in the process is 20-80 gsm, preferably 25 to 45 gsm. The other very important characteristics of the porous substrate used in the process are porosity which should be 350-500, preferably above 400 cfm/ft2@0.5 inch water. The flat and the sheet meant for corrugation are first passed through the desired concentration of water glass solution to get the right condition of the impregnated paper before corrugation. The matrix is treated with divalent or trivalent aqueous metal salts or any strong/weak acid, be it inorganic/organic or combination thereof. Examples of weak acids are phosphoric acid and acetic acid while strong acid can be selected from hydrofluoric acid, hydrochloric acid, sulphuric acid, nitric acid etc. In case of water glass, potassium silicate and sodium silicate are generally used. However, sodium silicate is preferred due to its low cost, high solubility of byproducts, better bonding strength and easy availability. It is observed that treating the matrix with salt makes it very water washable.

(32) The corrugated sheet is produced by methods well known in the art.

(33) The honeycomb matrix produced as above either in the block or cylindrical form is soaked in water soluble metal salt/salts in different proportions and other forms of solution to produce silicate hydrogel. The reaction between water glass silicates with metal salts to form insoluble metal silicate hydro gel is given in table below:

(34) TABLE-US-00001 Na.sub.2Si0.sub.3 + Al.sub.2(S0.sub.4).sub.3 .fwdarw. Al.sub.2(Si0.sub.3).sub.3 + Na.sub.2S0.sub.4 Na.sub.2Si0.sub.3 + MgS0.sub.4 .fwdarw. MgSi0.sub.3 + Na.sub.2S0.sub.4 Na.sub.2Si0.sub.3 + MgCI.sub.2 .fwdarw. MgSi0.sub.3 + NaCI Na.sub.2Si0.sub.3 + AICI.sub.3 .fwdarw. Al2(Si0.sub.3).sub.3 + NaCI Na.sub.2Si0.sub.3 + HCl .fwdarw. H.sub.2Si0.sub.3 + NaCl

(35) The washing of the gel matrix is essential in order to remove the byproducts and excess reactants formed during the synthesis of the active materials. The higher acidity or lower pH of gel matrix due to presence of excess reactants deteriorates the construction material used in the system. The gelling pH of the matrix or the concentration of reactant/reactants, temperature, reaction time used in the process change the active material characteristics such as pore size, porosity, pore volume and surface area. Table 3 below gives the characteristics of active materials (of high humidity isotherms) produced at different concentration of salt/acid or salt and acid solution.

(36) The said element was further dried under specified conditions to convert the hydrogel into the aerogel. It has been found that type of silicates, types of salts its pH, concentration, temperature and time during which the gel is aged or otherwise treated greatly affects the gel characteristics such as pore diameter, pore volume, surface area, adsorption capacity etc. The other important factors which affect the gel characteristics are the salt contents and surface tension of the liquid medium as it is being evaporated from the pores of the gel.

(37) The comparison of the characteristics of the prior art honeycomb matrix with respect to the honeycomb matrix according to the present invention is given below.

(38) TABLE-US-00002 Characteristics Prior Art New Invention Pore 20-35  40-200 size (Å) Pore 0.20-0.30 0.40-0.80 volume (gm/cc) Surface area (m.sup.2) 400-500 300-400 % Water adsorption RH-I0 7-9 7.-8  RH-20 11-15  9-10 RH-30 12-18 11-13 RH-50 15-25 16-20 RH-70 20-35 35-45 RH-90 26-40 53-60 RH-I00 35-45 65-75 Difference @ 20 50-55 50% RH & 100% RH

(39) To produce a chemical filter the said produced aerogel is immersed/impregnated with oxidizing agents solution, alkaline solution and weak acid solution of different concentration at different temperature for different soak time. When the honeycomb is sufficiently impregnated with solution, the excess impregnated is drained of and kept in another vessel after readjusting the concentration.

(40) The time required for full impregnation is variable with the structure of the adsorbent, temperature and the other factors. The honeycomb material is then placed in an oven and heated until the free moisture and water are evaporated or driven out of the matrix to certain limits by leaving impregnating material within the pores of honeycomb silicate aerogel material.

(41) It is preferred that the temperature range of drying of the honeycomb material should be in the range of 60° C. to 140° C. The exposure time during heating is varied with quality and quantity of materials, heating efficiency and other factors. After preparation, the honeycomb matrix is impregnated with impregnants is preserved until ready for use.

(42) In the experiments, the impregnants used are potassium or sodium permanganate, sodium or potassium hydroxide and weak acid such as phosphoric acid. The loading of impregnants depends on various factors such as types of desiccants, concentration of impregnants, soak time, temperature, no of dips etc.

(43) The table shown below gives details of factors affecting percentage of loading of impregnant/impregnants (Potassium permanganate or sodium permanganate) and adsorption capacity of impregnated honeycomb desiccant based matrix.

(44) TABLE-US-00003 A. CONCENTERATION OF KMnO.sub.4 microporous macroporous Parameters C1 C2 C3 C4 C1 C2 C3 C4 1. % Loading 2.6 3.2 6 7 4.0 6.6 8.1 8.1 2.. % Adsorption 30 31 29 32 38 40 39 39 B. SOAK TIME OF KMn0.sub.4 microporous macroporous Parameters S1 S2 S3 S4 S1 S2 S3 S4 1. % Loading 3.0 5.0 6 11 6 9 17 21 2.. % Adsorption 33 31 25 26 37 36 35 32 C. TEMPERATURE OF KMn0.sub.4 microporous macroporous Parameters T1 T2 T3 T1 T2 T3 1. % Loading 1.5 5.0 6.3 5.8 8.3 8.3 2.. % Adsorption 35 30 32 40 37 36 D. SUCCESSIVE DIPPING IN KMnO4 microporous macroporous Parameters D1 D2 D3 D1 D2 D3 1. % Loading 6.8 8.5 8.4 7.5 10 12 2.. % Adsorption 30 31 30 38 38. 34

(45) The concentrations, soak time, solution temperature, successive dipping, and surface properties of active material play an important role in achieving the twin objective-of loading and absorption. The said objective is preferably achieved if concentration of soaking solution is in the range of 5-15%, most preferably 10-12%, with soaking time in the range of 15-120 minutes, more preferably 15 minutes, at 10-80° C., more preferably at 50° C. with macroporous desiccant only.

(46) The method of impregnation of alkali preferably sodium or potassium hydroxide or of acid preferably phosphoric acid with hybrid desiccant macroporous desiccants (Insoluble metal silicate and activated carbon acid) is described herein.

(47) In order to prepare a chemical filter impregnated with acid or alkali, a slurry of activated carbon is prepared in water glass solution and honeycomb matrix is wounded in the form of block or cylindrical. The said honeycomb matrix supported with water glass and activated carbon is treated with acid or base as described.

(48) The desiccants (Hybrid) supported honeycomb matrix is impregnated with different concentration of impregnates such as potassium permanganate, potassium hydroxide, sodium hydroxide, phosphoric acid etc, preferably in the range of 4-15%, more preferably 10% in the case of KMNO.sub.4, 4% in the case of alkali and 6% in the case of phosphoric acid at different temperatures, more preferably at ambient temperature for different soak time from 10 to 60 minutes, more preferably 15 minutes to get the maximum loading with high CTC adsorption capacity.

(49) The desiccant of the instant invention is prepared “in-situ” in a “passive” dehumidification wheel. Such a dehumidification wheel has a higher rate of adsorption at RH>50% as compared to the prior art. The differential adsorption between relative humidity of 50 and 100% for the desiccant being at least 40% as shown in FIG. 6(a)

(50) The advantage and performance improvement of the macropore matrix in the “passive” dehumidification” mode, and the high humidity (HH) thermally activated mode, are clearly made out from the well documented test data. All testing has been done relative to and on a comparative basis with micropore desiccant matrixes.

(51) Tests were conducted to bring out performance comparison between the macropore of the instant invention (G3MA) and the micropore (G3MH) of the prior art.

(52) Performance Comparison Between G3MA Series (Sample G3 MA-E) and G3MH Series (200 mm Deep) at 515/600 SFPM

(53) Reference—Test No 307 TO 311

(54) TABLE-US-00004 TABLE 1 DELTA W (gr/lb) G3MA SERIES (Sample G3MH % PERFORMANCE RPH G3 MA-E) SERIES CHANGE 4 11.9 6.3 88.9 6 12 6.8 76.5 8 12.2 7.9 54.4 10 11 7.8 41.0 12 10.9 7.6 43.4 15 10.6 7.4 43.2 57.9 AVERAGE G3MA stands for Generation 3 macropore; G3MH stand for Generation 3 micropore. It was found that G3MA series performance is better by 57.9% than of equivalent G3MH rotor.

(55) Tables 2 and 3 show an adsorption differential of 20% and 40% for desiccants adsorbing at medium and high humidity respectively. It can be seen from table 4 that the adsorption differential for the dehumidifier of the instant invention, between 50 and 100% RH is more than 50%.

(56) TABLE-US-00005 TABLE 2 Medium humidity Isotherm Relative Humidity % Water Adsorption 10 8.8 20 12.93 30 17.17 50 26.6 70 34.29 90 44.65 100 48.95

(57) TABLE-US-00006 TABLE 3 High Humidity Isotherm Relative Humidity HH-1 HH-2 HH-3 HH-4 10 8.32 9.01 7.02 7.74 20 14.6 10.76 9.01 10.41 30 17.2 16.91 12.24 11.64 50 20 21.54 16.43 18.57 70 40.7 35.48 43.3 31.26 90 60.95 53.77 58.55 51.88 100  65 62.2 69.42 68.69 Difference 45% 41% 53% 50% of adsorption at 50% & 100% RH

(58) In HVAC and air treatment, chemical filters are commonly applied, in addition to particulate filters. Chemical filters can be prepared, using the honeycomb matrix, by any conventional technique. Said filters use the principal of chemisorption. In a chemical filter, gases are “filtered” and “cleaned out” when they are passed through a chemical filter. The “cleaning” of the gas occurs at the molecular level when (pollutant) gas molecules react with “chemical” in the Chemical filter. This reaction is referred to as chemisorptions. The chemical(s) in/on the chemical filter get consumed over time as it reacts with the gases which are cleaned out through this reaction. The object is, therefore, for the chemical filter to have the largest amount of chemical, for reaction, for a given physical size and geometry, and air carrying capacity, for a given pressure drop.

(59) The testing has shown that macroporous desiccant matrix shows outstanding characteristics by way of increased % “loading” content compared to microporous desiccant matrix of an identical type, by a factor of 2 to 3 times.

(60) Adsorbates other than water vapour can also be adsorbed in the macroporous desiccant of the present invention and include a variety of gases, odor elements, volatile organic compounds etc. These adsorbates/contaminants are adsorbed by the macroporous material of the instant invention mainly by capillary adsorption and are substantially and regularly desorbed on the regeneration side of the desiccant wheel matrix, thereby exhibiting strong ability to remove contaminants from the supply air stream.

(61) Thus the honeycomb matrix with macroporous desiccants is having the advantage of adsorbing gaseous contaminants which phenomena substantially limits the passing through of the gaseous contaminants through the honeycomb matrix into the supply stream.

(62) In one of the embodiments of the present invention, the macroporous desiccant based honeycomb matrix is made bacteriocidal, by treating the same with antibacterial agents such as titanium and silver or salts thereof.

(63) A further embodiment provides a “passive”/“active” wheel and “passive/“active” wheel dehumidifier comprising the honeycomb matrix with macroporous desiccant prepared “in-situ” wherein differential adsorption (capacity) of the desiccant being more than 50% or at least 40% at relative humidities 50 and 100%, a pore diameter of about 40 to about 200 Å and pore volume of about 0.40 to about 0.80 gm/cc.

(64) In still another embodiment of the present invention, the macroporous desiccant based honeycomb matrix is water washable.

(65) In yet another embodiment, the present invention provides a “passive” wheel dehumidifier in HVAC applications comprising the honeycomb matrix based macroporous desiccant as described above.

EXAMPLES

(66) 1. A fibre glass substrate with a polyvinyl alcohol binder content of 15% is immersed in 20% sodium silicate solution and is dried at a temperature of 60-80° C. prior to corrugation. The substrate is formed into a single facer while adhering two similar substrates, the said single facer is then converted into a round honeycomb matrix. The said matrix is subsequently treated in a 15% aluminum sulphate solution bath at a temperature of 70° C. for 40 minutes. After the hydrogel has been formed in the matrix, the said matrix is water washed and then dried for one hour at a temperature of 60° C. and then thermally activated at a temperature of 120° C. 2. A fibre glass substrate with a polyvinyl alcohol binder content of 12% is immersed in 25% sodium silicate solution and is dried at a temperature of 80-100° C. prior to corrugation. The substrate is formed into a single facer while adhering two similar substrates, the said single facer is then converted into a block honeycomb matrix. The said matrix is subsequently treated in a 6% phosphoric acid bath at a temperature of 70° C. for 25 minutes. After the hydrogel has been formed in the matrix, the said matrix is water washed and then dried for two hours at a temperature of 80° C. and then thermally activated at a temperature of 140° C.

(67) The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

(68) Although the disclosure of system and method has been described in connection with the embodiment of the present disclosure illustrated in the accompanying drawings and examples, it is not limited thereto. It will be apparent to those skilled in the art that various substitutions, modifications and changes may be made thereto without departing from the scope and spirit of the disclosure.