WO1997020079A1 - Modification of corn grits to provide superior water adsorption characteristics - Google Patents

Abstract

Described are biodegradable desiccant compositions with superior water adsorption capacity, a preferred composition being a corn grit modified by reaction with an alpha-amylase under conditions effective to increase the desiccant capacity of the corn grits. The inventive compositions are useful in a wide variety of applications where conventional desiccants are utilized including, for example, systems for drying gases, such as air dryers, and processes for drying organics, such as ethanol purification processes. Also described are methods for making and using the inventive compositions.

Description

MODIFICATION OF CORN GRITS TO PROVIDE SUPERIOR WATER ADSORPTION CHARACTERISTICS

REFERENCE TO RELATED APPLICATIONS

This application claims priority upon U.S. Patent

Application Serial No. 60/007,625 filed November 28, 1995 and U.S. Patent Application Serial No. 60/008,861 filed December 19, 1995, each of which is hereby incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to the removal of moisture from fluids to provide dried fluids. More specifically, the present invention relates in one aspect to modified corn grits useful in providing dried gases useful inter alia in the pressurization of power and communication cables, paint spraying and ozone generation. In other aspects of the invention the dried fluids are organic liquids, such as fuel-grade ethanol.

Discussion of Related Art

There are presently a wide variety of systems which utilize adsorbents to remove moisture from various fluids. For example, the presence of moisture in gases leads to difficulties in many industries and operations. For example, in processes for preparing a nitrogen-rich or oxygen-rich stream from air, if water and carbon dioxide are not first removed from the air, these components will freeze and block heat exchangers employed for cooling the gas prior to cryogenic distillation.

Additionally, in a wide variety of systems, a slight drop in temperature can cause condensation to occur in pipelines and reservoirs which can lead to corrosion, scales, freeze-ups, and dirt, which may damage instruments and controls, cause blockages in airlines, produce excessive pressure drops, increase down-time and reduce the life of tools. Similarly in chemical, food and metal working industries, the presence of moisture in air and other gases produces undesired oxidation. It has also been found in the robotics field that extremely dry air is required for the efficient operation of pneumatic systems.

To produce relatively dry air, e.g. air having a dew point of less than about 60°C, it is often necessary to use an adsorptive drying system, such as a pressure-swing adsorption system having two desiccant beds and two three- way valves, operating according to the "Skarstrom process". In this process, moist compressed air is passed over one desiccant bed and the moisture is adsorbed onto the adsorbent, or desiccant, providing a dry air stream. A portion of this dry stream is routed for use, and another portion is diverted at atmospheric pressure over the other, previously used, bed to regenerate the same. Since a large portion of the adsorbed water is contained at the bed entrance, during regeneration, the dry air is passed counter-current to the moist-air feed direction, so that the adsorbed water does not contaminate the rest of the bed. Removal of water occurs because the equilibrium favors transfer of moisture from the used desiccant to the dry air; and the bed is quickly depressurized, which stimulates a flash system. In a typical system, the valves are switched every 30 seconds or so, thus periodically using and regenerating each bed. The pressure swing adsorption system is thus a cyclic batch system where the desiccant beds are cycled from high pressure for loading to low pressure for regeneration. U.S. Patent No. 4,738,692 to Fresch at al. describes such a system and is hereby incorporated herein by reference in its entirety.

In a preferred pressure swing adsorption system, short cycle and low throughput per cycle are preferably used to conserve the heat of adsorption, thereby maintaining an isothermal operation. By using short cycles, a hot bed during adsorption and a cold bed during regeneration are avoided, which would, if present, hinder the separation. Additionally, the purge volume/feed volume ratio is preferably 1/1. the volume ratio being measured at their respective pressures. Thus the l/l ratio ensures complete displacement. The purge/feed ratio is a key parameter in determining the product purity and is usually between 1.1 and 2.0 in practice. Also, for a pure product, the absolute pressure ratio between the high and low pressures should be greater than the reciprocal of the mole fraction of the product contained in the feed (assuming purge/feed=l) ; and the sizing of the beds should be 15 to 30 actual volume/volume (abbreviated v/v) of feed per bed per cycle when the purge/feed ratio is l/l. These requirements are commonly referred to as "Skarstrom's Rules". One actual v/v of feed represents an amount of gas or vapor at the feed pressure in the empty bed. The feed throughput can be increased substantially if a high purge/feed ratio is employed. Pressure swing adsorption uses mechanical energy to provide regeneration rather than heat, used by temperature swing adsorption. Because of the low and high pressure requirements, pressure swing adsorption can only be used with gaseous systems. Pressure swing adsorption is particularly useful in air drying, because the adsorption composition removes water, which can be removed from the air stream without considering recovery. Pressure swing adsorption operates on the principle that a more strongly adsorbed species is to be removed from the weakly adsorbed feed stream. In air drying, water is the strongly adsorbed species and air is the weakly adsorbed species . The advantages of pressure swing adsorption are: (1) it works well when the weakly adsorbed species is the material needed in high purity; and (2) because of the rapid cycling, only a small volume of adsorbent is needed, thus limiting the size of the adsorption equipment, and therefore, also limiting the capital costs. Additionally, a pressure swing adsorbent has a longer useful life than an adsorbent used in temperature swing adsorbers, because temperature cycles to which the adsorbent is exposed are small, typically less than 5°C. Since heat in the form of thermal energy is not added, the Skarstrom cycle drier is sometimes referred to as a heatless drier. Applications of pressure swing adsorption include the heatless air drier, air separation processes, and hydrogen purification. In many present pressure swing adsorption operations, zeolites and activated carbon are the adsorption compositions used. The oxidation of the εurface of a normally hydrophobic activated carbon imparts polarity to the carbon surface inducing hydrophilicity and thereby improving its strength of water adsorption. In zeolites, the silica/alumina ratio can be adjusted to give higher affinity for water and other polar molecules.

However, the use of zeolites, activated carbon and other inorganic adsorbents is disadvantageous in that these substances are not biodegradable. Thus when their adsorption capacity becomes deficient or when a strongly adsorbed fouling agent becomes attached to them, their disposal becomes necessary and, thus, biodegradability and environmental compatibility of these desiccants is of utmost importance. Especially in light of the current regulatory climate, there is a great need for an adsorbent composition which is biodegradable and, thus, readily disposed of after use .

Particulate cereal grain derivatives are known to be useful for moisture adsorption. The desiccant capacity of such materials, for instance corn grits, is thought to be primarily related to their major component, starch. Starch is a polysaccharide composed of glucose monosaccharides linked together by glycosidic bonds, typically containing 25% amylose and 75% amylopectin. Amylose is a linear amorphous polymer consisting of D-glucose units bound together by 1,4 O-glycosidic bonds. Amylopectin is similar to amylose but also has 1,6 O-glycosidic branches every 25- 30 monomer units. The fiber fraction of a cereal grain may also contribute to moisture adsorption, but the protein component is thought to be less important.

It has been reported that the amylose found in corn has a degree of polymerization between 930 and 990, while the amylopectin found in corn has a degree of polymerization between 4800 and 10200. Starch is considered a branched molecule because of the 1,6 O-glycosidic branches in amylopectin. Corn starch is approximately 40% crystalline, as has been shown by x-ray diffraction Crystalline polymers have been shown to have extensive secondary intermolecular bonding. The crystallinity thus is believed to hinder the adsorption of water in the corn starch, because the hydroxyl groups on adjacent glucose units are complexed with each other and cannot adsorb water unless this secondary hydrogen bonding dissociates The branched structure of amylopectin has overlapping hydroxyl groups which correspond to more hydroxyl groups per unit area of the starch surface. Thus an adsorption composition high in amylopectin has a greater adsorption capacity. As such, selectivity for water in corn starch is improved by maximizing the ratio of amylopectin to amylose. Another way to improve adsorptivity is to provide smaller starch granules, thus increasing the surface area exposed to the target substance for drying. However, a reduction in particle size becomes prohibitive because as particle size is reduced, the particle became more tightly packed and it becomes correspondingly more difficult to move the target substance through the starch.

Corn grits can be advantageously used for water adsorption and are relatively inexpensive Corn grits are conventionally processed from whole corn seeds, and contain primarily the degerminated endosperm portion of the corn seed In turn, the endosperm consists of two parts, the horny endosperm, which is hard and translucent, and the floury endosperm, which is soft and relatively opaque. The horny endosperm contains more protein than the floury endosperm and the starch matrix is more compact. The ratio of the horny to floury endosperm is approximately 2:1 in dent corn; however, flour corn has little or no horny endosperm.

The surface of corn grits has been examined using a scanning electron microscope and, upon examination of the photomicrographs, it was noted that starch particles were abundant on the white regions, corresponding to the floury endosperm, but only sparsely distributed on the yellow portion, corresponding to the horny endosperm, of the corn grits.

The starch contained in the endosperm is generally found in starch granules which are oriented in a matrix within the individual cells. Large, smooth granules are found in the inner, loosely organized cells of the floury endosperm, and smaller, tightly packed, faceted starch granules are found in the horny endosperm cells. It is likely that the larger starch granules in the interior of the cells of the floury endosperm more strongly affects the adsorption properties of corn grits because as the corn is processed, they are on the surface of the corn grit where adsorption takes place.

As described above, corn grits are known to be useful as dehydrating or desiccating agents. Perhaps the greatest advantage of using corn grits as a desiccant, for example in a pressure swing adsorption system as described above, is that when the adsorption capacity becomes deficient, whether because of degradation of the corn grit or because a strongly adsorbed fouling agent has attached to the grit, the grit can be removed from the apparatus and easily disposed. The fact that grits can be disposed of by simply treating them as benign waste or as a feedstock for fermentation in the alcohol industry is an attractive feature relative to zeolites, activated carbon, or other inorganic adsorption compositions which are not biodegradable.

In the area of dehydrating materials which are liquids at room temperature, water is often separated from such materials using one or more of a wide variety of distillation techniques. For example, in the production of fuel-grade ethanol, the ethanol must be dried to 99.6% purity. Ethanol produced by fermentation has a very low ethanol concentration and, thus, downstream processing is required to remove water from the ethanol to obtain the 99.6%, fuel grade alcohol. In a conventional ethanol production process, 4% ethanol is produced by fermentation and run through a distillation column to yield an ethanol effluent having about 90% purity (i.e. containing about 10% by weight water) . At atmospheric pressure, the maximum ethanol concentration which can be achieved by distillation is about 95.6% due to the formation of a water/ethanol azeotrope.

One approach which has been used to remove much of the remaining water from the water/ethanol azeotrope involves the use of a molecular sieve. This type of purification, however, requires a substantial input of energy. Another approach involves azeotropic distillation, but this too requires a substantial input of energy. A more energy- efficient approach that has been used to reduce energy requirements for removing the last 5% to 10% of water from ethanol is a fixed bed adsorption system that utilizes a desiccant composition and a heated inert gas stream for desiccant regeneration. Adsorption uses about 4 times less energy for the same final ethanol composition, starting from a 90 weight % feed, than does azeotropic distillation. This is because, for adsorption, the water-ethanol azeotrope does not have to be broken and the operating temperature of the system is much lower.

Although the use of corn grits is an improvement in the dehydration of ethanol and other materials, there remain needs for improved, degradable desiccant compositions having increased desiccant capacity, and for improved processes involving the manufacture and use of such compositions . The present invention addresses these needs. SUMMARY OF THE INVENTION

Briefly describing one aspect of the present invention, there is provided a desiccating agent comprising corn grits modified by reaction with an alpha-amylase enzyme.

Preferred such corn grits are characterized by an increase in desiccant capacity as compared to their corresponding native corn grits. Other characteristics of preferred modified grits of the invention include increased porosity, and increased external surface area.

According to another aspect of the present invention, there is provided a method for making an improved desiccant composition, comprising: (1) contacting corn grits in an aqueous medium with an alpha-amylase enzyme so as to increase the desiccant capacity of the grits; and (2) recovering the grits from the aqueous medium.

In another aspect of the present invention, there is provided a method for drying a moisture-containing gas, comprising contacting the gas with a composition comprising corn grits modified by reaction with an alpha-amylase enzyme under conditions effective to increase the desiccant capacity of the corn grits, so as to reduce the moisture content of the gas.

According to another aspect of the present invention, there is provided a method for drying a moisture-containing starting liquid, comprising: (1) vaporizing the starting liquid to provide a moisture-containing vapor; (2) contacting the vapor with a composition comprising corn grits modified by reaction with an alpha-amylase enzyme under conditions effective to increase the desiccant capacity of the grits, so as to remove moisture from the vapor; and (3) condensing the vapor to form a product liquid.

It is an object of the present invention to provide a modified corn grit which exhibits superior water adsorption characteristics, while substantially retaining its mechanical strength.

It is another object of the present invention to provide an inexpensive, non-toxic, biodegradable composition for use in drying operations. It is also an object of the present invention to provide superior methods for removing moisture from gases, for example, superior air drying methods.

Another object of the present invention is to provide superior methods for removing moisture from liquids, for example, superior ethanol purification methods.

Additional objects, advantages and features of the present invention will be apparent from the drawings and detailed description herein. BRIEF DESCRIPTION OF THE DRAWINGS

Although the characteristic features of this invention will be particularly pointed out in the claims, the invention itself, and the manner in which it may be made and used, may be better understood by referring to the following descriptions taken in connection with the accompanying FIGURES forming a part hereof.

FIG. 1 is a schematic drawing of the inner mechanism of a pressure swing drier which can be used in accordance with the present invention.

FIG. 2 is a schematic drawing of a pressure swing adsorption system which can be used in accordance with the present invention. FIG. 3 is a schematic drawing of an adsorption chamber for a pressure swing adsorption system which can be used in accordance with the present invention.

FIG. 4 is a schematic drawing of a fixed bed adsorption system which can be used in accordance with the present invention.

FIG. 5 is a schematic drawing of a fixed bed for a fixed bed adsorption system which can be used in accordance with the present invention.

FIG. 6 is a εcanning electron micrograph, 200X magnification, of an unmodified corn grit. FIG. 7 is a scanning electron micrograph, 320X magnification, of a corn grit modified by reaction with an alpha-amylase according to the present invention.

FIG. 8 is a scanning electron micrograph, 3000X magnification, of a corn grit modified by reaction with an alpha-amylase according to the present invention.

FIG. 9 is a plot of percent fines to test time which displays the results of attrition testing as described in Example 9 herein.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

For the purposes of promoting an understanding of the principles of the invention, reference will now be made to preferred embodiments thereof and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the invention, and such further applications of the principles of the invention as described therein being contemplated as would normally occur to one skilled in the art to which the invention relates.

The present invention provides modified particulate, starch-containing fractions of corn, such as corn grits, which have superior desiccant properties while also being biodegradable, and processes for preparing and using such compositions. Generally, such compositions can be obtained by modifying the corn grits or other similar starch- containing particulates by reaction with an alpha-amylase enzyme .

As for starting materials, corn grits useful in the practice of the invention can be obtained commercially or using known processes. For example, corn grits can be obtained from whole corn kernels in a process known as the "tempering-degerming" process. In this process, which is used by most large dry corn millers, cleaned corn is tempered to 20% moisture, and is then dehulled and degermed. The remaining part of the corn, often called the tail hominy fraction, contains the bulk of the corn endosperm, and is ground and screened to yield grits having various sizes. A wide variety of alpha-amylase enzymes are known and can be used in the present invention. Alpha-amylases are endo-acting enzymes that break alpha-1,4 glycosidic bonds randomly on the amylose and amylopectin chains of starch polymers. Alpha-amylase enzyme preparations have commonly been used to hydrolyze, liquify and/or convert starch containing materials into starch hydrolysates, such as glucose, maltose, maltotriose, maltotetraose, maltopentose, maltohexose and others.

Alpha-amylase enzymes are available from a wide variety of sources including both commercial sources and from preparative processes well-described in the literature. Most alpha-amylase enzymes are produced from bacterial sources such as Bacillus subtilis. Bacillus licheniformis , Bacillus stearothermophilus and other Bacillus micro organisms, which are cultivated in an appropriate culture medium. The cells produced therefrom are then destroyed and the enzyme preparation is thereafter separated from the broth and purified.

Many of the commercially available alpha-amylase enzymes produced are derived from Bacillus subtilis microorganisms . When these enzymes are used to convert starch to starch hydrolysates, they generally have an optimal use temperature ranging from about 80°C to about 85°C, and an optimal pH of about 6.0. Various heat stable alpha-amylase enzymes have also been developed. Examples of such heat-stable alpha- amylase enzymes include those produced from Bacillus stearothermophilis . More recently, alpha-amylase enzymes having good heat-stability in neutral or weakly alkaline solutions have been made available These alpha-amylase enzymes are produced by cultivating microorganisms of the species Bacillus licheniformis. Alpha-amylase enzymes have also been described which are heat- and acid-stable.

Although a wide variety of alpha-amylase enzymes may be used according to the teachings of the present invention, in a preferred embodiment the alpha-amylase enzyme utilized is MAXALIQ S alpha-amylase (Gist-Brocades International B.V., Charlotte, North Carolina) diluted 50 times by volume with phosphate buffer. This enzyme is derived from Bacillus licheniformis and, in its commercial preparation, contains sodium chloride, methyl and propyl parabens as preservatives. MAXALIQ S is most active between pH 6.0 and 7.0 and has a high thermal stability, with an optimum use temperature of 95°C.

To make a preferred modified corn grit according to the present invention, an unmodified corn grit is contacted with an alpha-amylase enzyme in an aqueous medium under conditions effective to increase the desiccant capacity of the grit. In this regard, increasing the desiccant capacity as referred to herein intends that the efficiency with which the corn grit adsorbs water vapor is increased. For example, such an increase in efficiency can be observed as the ability of the modified corn grits under similar conditions to remove a greater amount of moisture from a fluid than the corresponding native corn grits.

After reaction with the enzyme for a period of time and under conditions sufficient to achieve advantages modification of the corn grits, the grits are recovered from the aqueous medium. The grits are then preferably washed several times, for example using water, to remove the enzyme from the grit. It is preferable that substantially all of the enzyme be removed from the grit; however, it is contemplated that a small amount of enzyme, or fragments of enzymes, might remain on the grit as impurities in the practice of this invention. In a preferred embodiment, the grit is then dried. Preferably, the drying is accomplished at an elevated temperature, e.g. a temperature of from about 30°C to about 100°C, suitably from about 35°C to about 50°C. For purposes of the present invention, it is desired that hydrolysis of the corn grits by the alpha-amylase enzyme be allowed to proceed for sufficient duration to obtain the desired surface modifications and improvements in desiccant capacity, but terminated prior to substantial degradation of the physical strength of the grit particles. In this regard, although it is contemplated that some measurable hydrolysis of the corn grits will occur, it is preferred that the extent of hydrolysis be relatively small, for example less than about 20%, more preferably about 10% or less. As is described in greater detail in the Examples, the percent conversion can be found for the alpha-amylase by injecting a sample of the liquid supernate from the corn grit/enzyme solution into a liquid chromatography column that separates sugars. The maltose and the glucose concentrations are measured and percent conversion is calculated. Characteristics accompanying such desirable enzyme modifications include increased porosity and increased external surface area.

In preferred methods, a buffer is used for maintaining the pH in the aqueous enzyme/grit suspension within a range encompassing optimal pHs for the functionality of the particular alpha-amylase enzyme used. For example, a preferred pH range for the buffer is about 5 to about 8, more preferably about 6 to about 7. A preferred buffer for use in the invention is a phosphate buffer, for example provided by sodium phosphate. More preferably, the buffer comprises sodium phosphate monobasic and sodium chloride. Additionally, the system also desirably comprises an amount of sodium hydroxide necessary to bring the pH of the buffer within the preferred pH range. It will also be readily understood by one of ordinary skill in the art that additional additives may be present in the aqueous medium, for example, preservatives such as sodium azide or sodium benzoate

In a preferred embodiment, the modified corn grits of the invention have an average particle size of at least about 0.5 mm, typically falling in a range of about 0 5 mm to about 3 mm. As is known and commonly used m the relevant field, such sizes represent the average particle size of a particle distribution measured using sieve analysis. Additionally, the modified grits of the invention desirably have an external surface area, determined using the Brunauer, Emmett, and Teller (BET) method, of at least about 0.22 m 2/gram, typically in the range of aoout 0.22 Vgram to about 0.3 m /gram Additionally, preferred modified corn grits of the invention have a starch content of at least about 70% by weight, and typically in the range of about 70% to about 90% by weight

Although it is not intended that the present invention be limited by theory, it is believed that the improved adsorptivity may at least m part be due to increased surface area of the grit particles and/or increased porosity of the particles. For example, FIGS. 6, 7 and 8 show an unmodified grit at 200x magnification, an alpha-amylase modified grit at 320x magnification, and an alpha-amylase modified grit at 3000x magnification, respectively As can be seen, the modified grit has a more porous surface and, therefore, provides much more surface area than the relatively smooth-surfaced unmodified grit.

The increased capacities of the inventive grits may also be due to the action of the alpha-amylase enzyme on the amylose and amylopectin fractions of the starch in the grits. It has been suggested that water binds more strongly to amylopectin than amylose, perhaps due to the branched chain nature of amylopectin as opposed to the linear structure of amylose. The branched chain nature of amylopectin allows the overlapping of hydroxyl groups, which strongly attract the water molecules and trap them in a branched matrix. The action of alpha-amylase is to break the alpha-1,4 bonds in the starch. This may fragment the helical amylose into smaller chains and improve accessibility to the amylopectin by exposing more branches and hence more hydroxyl binding sites. Again, however, it will be understood that the present invention is not limited by any theory.

Corn grits of the present invention are useful in a wide variety of applications in which it is desired to remove water from a fluid. In one aspect of the invention, the inventive modified corn grits are utilized to adsorb water from moist air, thus providing dry air. For instance, using methods of the invention, air can be dried to a dew point of about -60°C to about -80°C , for example starting with air at a dewpoint of about 0°C to about -10°C. A preferred use of an inventive composition is in a pressure swing adsorption system. There presently exist a wide variety of processes which require as a first step the generation of a stream of dry air such as that produced by a pressure swing dryer. Examples of these applications include pressurization of power and communication cables, paint spraying and ozone generation. The adsorption capacity of inventive modified grits is comparable or superior to many conventional desiccants presently used in these types of systems. Advantageously, inventive modified grits are biodegradable and thus readily disposed of and the primary starting material, corn grits, is readily available and relatively inexpensive. Additionally, inventive modified grits are extremely robust, capable of withstanding hundreds of thousands of pressure cycles in a pressure swing adsorption system.

In another advantageous aspect of the present invention, the inventive modified corn grits are utilized as a water adsorption composition in the purification of an organic compound which is liquid at room temperature (e.g. about 25°C) . The organic compound may be, for example, an aliphatic or aromatic compound, including e.g. alcohols, ethers, ketones, alkanes, acids, and the like. Preferably, the organic compound will be relatively volatile, for instance having a boiling point of less than about 120°C. The organic compound may also be one which forms an azeotrope with water. In this case, processes of the invention can be used as a convenient alternative or supplement to splitting the azeotrope by specialized distillation or other processes. For instance, inventive compositions can be used in an ethanol purification system to remove the final 5-10% water from a wet ethanol stream resulting from the distillation of a crude fermentation broth. In a preferred mode, the wet ethanol or other organic liquid is vaporized to provide a moisture-containing vapor, which is then contacted with the inventive modified corn grits to remove water. Then, the vapor is condensed and the resulting product liquid has a lower moisture content than prior to contact with the grits . In such processes the inventive modified grits are preferably used in a fixed bed adsorption system and, more preferably, the liquid is dried from an initial water content of about 2% to about 30% by weight, to a product liquid which has a water content of less than about 1% by weight, more preferably less than about 0.5% by weight, by contact with the modified corn grits. In application to the dehydration of ethanol or other fermentively-produced organic compounds, the use of the biodegradable modified grits is particularly advantageous as when the grits become deficient due to fouling or other means, they can be used as a feedstock in the fermentation process. This not only avoids the need to dispose of an environmentally unfriendly desiccant, but also provides a starting material for the manufacturing process in which it is first used as a desiccant.

The inventive modified corn grits may also be utilized in an evaporative air cooling system to replace conventional air conditioners that cool and dehumidify homes and cars. There is a growing interest in developing alternative systems to standard air conditioners, and many manufacturers are developing evaporative cooler systems using, for example, zeolites as desiccators. The present invention provides an adsorption composition which can be utilized in such a system as an advantageous alternative to zeolites .

Since air conditioning involves both reducing the humidity and lowering the temperature of the air, conventional air conditioning units must cool atmospheric air to its dew point before any moisture is removed. Thus, the air is cooled to a low temperature to reach the desired humidity and, to correct the uncomfortably cold but drier air, the air often has to be reheated to the desired final temperature. In an improved air conditioner which utilizes a desiccator such as that provided by the present invention (a "latent air conditioner") , removing moisture from the air to provide a lower humidity level is accomplished by passing the air through desiccant wheels which dry air using desiccant compositions. During the drying process, the air temperature is increased, so a final sensible cooling step over a thermal wheel and an evaporative cooler is used to cool the air to the desired temperature.

More specifically, in this system, desiccants are contained in a large rotating wheel. Atmospheric air enters and passes through one half of the wheel (top or bottom) , which contains dry desiccants, so moisture is removed from the product air. Water is sprayed into the product air, and because the air is dry, some of the water will be evaporated. By using an energy balance, it can be readily seen that as the water is evaporated by the air, the temperature of the air must decrease. The resulting air will be cool and at a comfortable relative humidity, because the amount of water sprayed into the dry air is readily controlled. On the other side of the wheel, air will be heated and sent through. This action will regenerate the desiccant for drying the ambient air by the time the wheel revolves another half turn.

A comparison of energy requirements shows that the latent mode of air conditioning consumes much less power, only about one fifth, of that consumed by conventional methods. The lower power requirement of this desiccant-type air conditioner results from the splitting of the sensible and the latent loads. Drier air being easier to cool than moist air, the capacity of the mechanical cooling system is decreased. Also, the need for expensive reheating is eliminated. In addition to being less expensive to install and run, these systems deliver air that contains no CFCs or HFCs, and carries fewer microorganisms. Microbial growth is inhibited in this system because the moisture content of the desiccant cycles from approximately 2% to 3-5% every time the desiccant wheel revolves, thus hindering organism growth. With their improved adsorption capacity, inventive modified corn grits provide a low-cost, renewable, natural substitute for desiccants presently utilized in these new cooling systems .

In another advantageous use of inventive modified corn grits, the grits are simply placed in locations for which dryness is desired. One example of such a location is within packaging for storing or shipping products such as, for example, computers, tissue papers, chemicals, granular salts, sugars and the like, wherein it is of utmost importance to keep the product free from moisture . Another example of such a location is between two glass panes in a double-pane window. In many uses such as these, the modified grit may be encapsulated in a gas-permeable membrane, thus forming a desiccant packet. Alternatively, the grits can simply be scattered among packaging material, depending upon the specific use desired.

The invention will be further described with reference to the following specific Examples. It will be understood that these Examples are illustrative and not restrictive in nature.

EXAMPLE ONE MODIFICATION OF CORN GRITS USING

ALPHA-AMYLASE

The corn grits used were 8/10 and 18 sieve sizes. They were obtained from the J.R. Short Milling Co., Kankakee, IL. The grits had a starch content of 88.1% by weight (dry basis) . The native 8/10 size grits had an average particle diameter of 2.16 millimeters and an external surface area which was calculated to be 12.5 cm²/cm³ compared to the 18 size grits which had an average particle diameter of 0.978 millimeters and an estimated external surface area of 47.5 cm^/cm³.

The modification procedure was carried out at pH 6.9, which is the optimum for the MAXALIQ S alpha-amylase enzyme.

The phosphate buffer was prepared by adding 2.4 gm of NaH2Pθ₄, sodium phosphate monobasic anhydrous (Signma

Chemical Co.) and 0.391 gm of sodium chloride crystals (Mallinckrodt Specialty Chemicals Co.,) to 1000 mL of deionized water. The pH was adjusted to 6.9 using 4M NaOH, while stirring and monitoring the pH with a corning pH meter. A volume of 2 mL of the enzyme was mixed with 98 mL of this buffer. Sodium azide, 20 mg, was also added and the mixture was stirred thoroughly with a magnetic stir bar over a stir plate. The procedure consisted of first soaking the corn grits in the amylase enzyme/buffer mixture. This was done by adding 1 gm of grits to 10 mL of the amylase enzyme-buf er contained in a test tube. After 5 seconds over the vortex mixer, the test tube was covered and placed undisturbed for 8 days at room temperature (23°C) . Another sample was maintained at a higher temperature of 50°C in the shaking water bath for the same length of time. In addition, an enzyme blank (consisting of the buffer/enzyme mixture only) and a substrate blank (consisting of a grits/buffer suspension only) was maintained for both the cases.

After soaking, the supernate was poured off and the grits were rinsed repeatedly in the test tube with water. After washing, the grits were allowed to dry by placing them in an oven at 35°C for 48 hours.

EXAMPLE TWO

BULK TREATMENT The bulk treatment, which utilized the same compositions described in Example 1, involved measuring out 300 to 400 mL of grits and adding it to a measured volume (600 to lOOOmL) of the chemical/enzyme contained in a 4 L flask. The mixture was then left undisturbed for the 1 to 7 days .

After the desired treatment time, the enzyme was drained off and the grits were repeatedly rinsed with tap water. A washing protocol was used to assure that all extraneous reagents were removed. This was done by filling the 4 L flask with water, closing its mouth, shaking vigorously and then draining off the water. This was repeated about 10 times and then again after 24 hours when the grits had been soaked in water, to ensure removal of the residual enzyme. Drying was done as described before, only this time because the volume of grits was greater they were spread on an 8" x 13" glass tray and then placed in the oven.

EXAMPLE THREE

ANALYZING PERCENT CONVERSION OF GRIT BY LIQUID CHROMATOGRAPHY

At the end of the corn grit modification procedures described in Examples 1 and 2, 1.5 cm³ samples were collected from each run, microfused and analyzed for the presence of glucose and maltose using liquid chromatography. A 40 μL sample was injected in a 300 mm x 7.8 mm liquid chromatograph column containing HPX-87C (Biorad Laboratories, Hercules, CA) as the packing. The eluent, water, was pumped at 0.55 mL/min. Calibration curves (concentration versus peak area) for both maltose and glucose were generated by injecting 10, 20, 30 and 40 mg/mL of each sugar into the column, and noting retention times and peak areas as recorded by the 3390 A Hewlett Packard integrator. Concentrations of glucose were double checked by also running the samples on the glucose analyzer. EXAMPLE FOUR

SEPARATION OF PARTICLES BASED UPON SIZE The particle size of each of the native and modified 8/10 and 18 size corn grits was measured by using a sieve shaker (Fritsch Analysette, Germany) . Sieves were chosen in the range of estimated size of the corn grits. The sieves were placed in descending order of size in a stack on the shaker. The total weight of corn grits was measured and the grits were placed on the top sieve. The shaker was turned on for 7 minutes and then the grits were collected separately from each sieve and weighed. The weight percents were found to be similar for both the native and modified corn grits of both sizes; however, because the modified grits were wet and then dried, some of the particles stuck together, forming larger, stable agglomerates.

EXAMPLE FIVE

TESTING MODIFIED CORN GRITS FOR SURFACE AREA

It has been theorized that surface area of corn grits has an effect on the moisture adsorbed. Therefore, the well-known BET nitrogen adsorption procedure was conducted on each of the larger and smaller sizes of native and modified corn grits. BET adsorption uses liquid nitrogen at 78 K to find the monolayer adsorption of nitrogen on the surface of the adsorbent. The BET equation is used to analyze the adsorption data and allows a determination of the monolayer capacity. The procedure is: the sample is prepared for the experiment by being heated in an oven and then placed in a port on the nitrogen adsorption apparatus where atmospheric gases are displaced by nitrogen flowing over the sample. The surface area is determined by immersing the sample in liquid nitrogen at 78 K while gaseous nitrogen is passing over it. As the partial pressure of nitrogen in the carrier gas is increased, nitrogen will adsorb onto the surface until a monolayer is formed. After this point, the nitrogen will continue to adsorb onto the surface, making a nitrogen layer that is several molecules thick, and it will also begin to condense and fill the pores of the material. The pores will be filled starting with the smallest and continuing to the largest when the applied pressure reaches the condensation pressure for a pore of that size.

To analyze corn grits, approximately 30 grams of the grits were dried overnight in an oven at 100°C. Approximately 1-2 grams of the specific type of dried corn grits were placed into the sample holder and weighed. The QUANTASORB used nitrogen as the adsorbed gas and helium as the carrier gas. The gas tanks were opened and then the gas flow controller and QUANTASORB were both turned on. The sample holder was attached to the outgassing port on the QUANTASORB and an empty sample holder was attached to the sorption port. The QUANTASORB was calibrated and the flow rates of the gases were set on the flow controller. The flow rates were chosen based on the P/P₀ of nitrogen desired for that trial and a total combined flow rate of nitrogen and helium at 20 cc/minute. The sample holder and the empty sample holder were switched so that the sample holder was in the sorption port, and after further calibrations, the sample holder was immersed in liquid nitrogen. The sample was allowed to adsorb nitrogen until the nitrogen concentration in the helium returned to normal, as shown by the display returning to zero. After this, the dewar of liquid nitrogen was removed and the sample holder was immersed in room temperature water. The amount of nitrogen desorbed during this time was measured and recorded by the QUANTASORB, and a peak showing nitrogen concentration in the helium was graphed on a strip recorder. A calibration of the QUANTASORB was conducted by injecting a measured sample of pure nitrogen withdrawn from the nitrogen OUT port into the nitrogen IN port. The calibration was repeated until the resulting peak was within 10% of the desorption peak, and the calibration area was recorded. The procedure was repeated for the different partial pressures of nitrogen desired.

Once the tests were completed, the weight sorbed and the BET parameter was calculated. Also calculated was the monolayer nitrogen covering on the corn grits. This data was used to determine the surface area of the corn grits It was found that the modified corn grits indeed had a greater surface area than unmodified corn grits.

EXAMPLE SIX

TESTING MODIFIED CORN GRITS QUALITATIVELY FOR MECHANICAL STABILITY

A qualitative estimate of the physical integrity of the treated and dried grits was obtained by placing them between the fingers and applying a crushing force. Modified grits according to the present invention did not break apart when rubbed between the thumb and index finger.

EXAMPLE SEVEN

REMOVING MOISTURE FROM A GAS USING ALPHA-AMYLASE-MODIFIED CORN GRITS

The operational sorption capacity of the corn grits was tested in a commercially available pressure-swing dryer. The dryer was a PUREGAS dryer (Model HF 2000A 106-A130, General Cable Co., Westminster, Co.) . The dryer had two 152 mm desiccant chambers. Each chamber held 135 ml of desiccant which corresponded to 100 g of corn grits. The bed height in both cases was 125 mm. The dryer was fitted with other external components to provide a system in which inlet and outlet air flow rate and inlet and outlet moisture contents could be controlled and/or monitored. A schematic of the inner mechanisms of the PUREGAS system is found in Figure 1 and a diagram of the entire adsorption system including traps and the saturator is found in Figure 2. A cross-sectional diagram of a column is found in Figure 3. The chambers were loaded by placing the air tubes that had an aeration metal disk and sponge filter on it into a hole at the bottom of the chamber. The grits were loaded on top of this in the chamber and then another sponge filter and disk were placed on top of the grits. A spring, another metal disk, and a retaining ring were placed on top of this. The retaining ring was compressed to fit into the chamber and then released into a groove on the inside of the desiccant chamber. This apparatus held the adsorbent bed stationary during pressure changes. The beds were screwed into the pressure swing adsorption system, which could then be run.

Humidification of Inlet Air

Air at approximately 320kPa and 298K was humidified to - 5°C dew point by bubbling it through 1 L of distilled water. Trace amounts of oil were removed by oil traps along the air line. The flow of air at the outlet was maintained at 1.8 L/min (STP, 273K, 101.3kPa) by a flow controller (Matheson, Model 8240) . The dry air flow used for the regeneration cycle was 11.5 L/min (STP) , with each of the two sorbent beds cycling between sorption and regeneration every 30 seconds. The low outlet/purge ratio was used in order to promote rapid equilibration of the bed to steady state outlet moisture conditions.

Measurement of Dew Point The dew point of the dry air was measured by a System 580 Hygrometer (Panametrics, IN., Waltham, MA) which has a range of -80 to 20°C dew point. The moisture sensor was made of an anodized aluminum oxide element inside of a probe shell. A gold coating was vapor deposited on the aluminum oxide surface to form the two electrodes of a capacitor. Water vapor could diffuse through the gold layer to be adsorbed on the pore wall of the oxide layer. The amount of water adsorbed could be related, through the electrical impedance, to the vapor pressure of water in the atmosphere around the sensor. The detector was insensitive to temperature and flow rate at these system conditions. However, 1.5 hours was required to achieve the final steady- state dew point of -55°C when dry helium from a cylinder was passed through the sensor. The output was attached by electrical wires to a data acquisition board (model CIO-DAS08-PGA, Computer Boards, Mansfield, MA) that was connected to a PowerFlex personal computer (Advanced Logic Research, Inc., Irvine, CA) and the signal was calibrated to give the dew point . The program that was used for the data acquisition was lablog2 v. 3.52 (Quinn-Curtis for Computer Boards, Mansfield, MA) . Dryer System Start-up and Operation

The dryer system was set up prior to each run. The water reservoir was emptied of any remaining water by slowly pressurizing the system with the feed line open. The system was then vented and the reservoir was refilled with 600 ml of deionized water. Finally, the feed valve was closed, the system pressure turned on, and the vent closed (in that order) . Air flow rates for both the product air and the purge air were measured by recording the time required for a 2 L graduated cylinder to fill with product or purge air. The 2 L cylinder was completely filled with water and inverted into a bucket of water (the small amount of water lost during the inversion was estimated) . Output air was transported into the cylinder by a tube. The time to fill the cylinder was recorded three times for each run and the average value was taken as the flow rate. The ideal gas law was then used to calculate the volume displaced at standard conditions of 393 K and atmospheric pressure.

Dryer System Shut Down

After a run was completed, consistent shutdown procedures for both the computer and the dryer shutdown were followed. On the computer, the I/O disk option was used to close and save the data file, and the scan option to stop the data acquisition. The dryer system was first vented, then unplugged and the chambers removed. The restraining ring in the chambers was removed (with care, since the spring was still compressed) as were the disks and filters which retained the sorbent. The desiccant was then emptied from the chambers .

Results and Discussion

Four types of corn grits were tested in the pressure swing adsorption system, 0.965 mm (modified) , 0.978 mm

(native) , 2.12 mm (modified) , and 2.16 mm (native) . For the same column of each, the enzyme modified grits weighed less, although their average particle size, as measured by sieve analysis, was the same as the starting material. Most of the conditions used in this work are within the ranges put forward by Skarstrom as reviewed previously in the Background section. However, the purge ratio in the laboratory system is much greater than Skarstrom' s 1:1. The ratio had been changed in order to use the pressure swing adsorption system as a tool for the evaluation of the grits. Corn grits show an inverse correlation between the external surface area/mL of column and the moisture content of the outlet air in a pressure-swing dryer. It was found that 2.16 mm grits treated with alpha-amylases at 1/51 dilution for 7 days at a pH of 6.9 gave outlet air dew points as low as -54°C with an insignificant change in particle size.

EXAMPLE EIGHT

EVAPORATION/DRYING/CONDENSATION PROCESS

A fixed bed adsorption system was used to adsorb moisture from a vaporized ethanol/water stream onto corn grits. The laboratory scale system used in the present work included an Ace Glass, Inc. size 25 jacketed glass column packed with 186 grams of 0.965 mm modified corn grits. A diagram of the system is found in Figure 4. The column was packed by the fill and tap method. Glass wool and glass beads were placed at each end of the column to keep the corn grits in the column. A diagram of the column showing the various parts is found in Figure 5.

The initial volume of the corn grits was 274 mL. After several runs the volume decreased and glass beads were added to the column. A 26.5 inch thermowall with an outer diameter of 1.8 inch and a inner diameter of 1.16 inch was inserted 24.5 inches into the column through a hole drilled in the Teflon end cap. Thermocouples were placed 20, 12 1/2, and 10 3/16 inches into the column bed. The thermocouples were attached to an Omega (Omega Engineering, Inc. Stamford, CT) channel scanner and then the output was attached to an Omega Thermocouple reader. The channel scanner sent the signal to an Omega DC Millivolt Amplifier and the amplifier sent the signal to the data acquisition system. The data was collected in a file on the computer using Lablog2. The data was written to disk every 5.0 seconds, and collected so that each channel would be read once per cycle of the channel scanner and then recorded in the file.

The fixed bed adsorption system was operated by pumping (miniPump, Laboratory Data Control, Milton Roy Co.) a 92 percent ethanol by weight ethanol/water solution at a rate of 1.89 mL/min (liquid rate) into the system where it was vaporized by a heat exchanger at 85°C. The vapor was kept from condensing in the inlet tubing by a heat tape attached to a variable autotransformer powerstat (Super Electric Co., Bristol, CT) set to 19 (max 140) . The column was jacketed and the jacket had water at 85°C flowing through at approximately 1.3 L/min, which was circulated by a MGW Lauda RM20 (Konigshofen, Germany) water bath. The vapor passed through the column and the product vapor was then condensed by a heat exchanger cooled with water at 5°C, which was circulated by a Neslab RTE 111 water bath (Neslab

Instruments, Inc.; Newington, NH) . The condensed liquid was collected in a small test tube directly from the outlet of the system for immediate determination of water concentration. Regeneration took place for 1.5 hours. Compressed air at 308 kPa (44.7 psia) and approximately 220°C, was metered to the column at 122 kPa (17.7 psia) and a measured flow rate through the column of 13 L/mm or 16.8 grams/mm. The air was heated to 87°C by a power tape attached to a variable autotransformer powerstat (Superior Electric Co., Bristol, CT) set up 61. The pressure drop through the column is approximately 17.2 kPa (2.5 psia) . The heated air passed through the column and picked up adsorbed water and ethanol and carried them out of the column. A septum was placed in-line and 0.5 mL gas samples were taken and analyzed for water and ethanol concentration by the gas chromatogram.

The air/water/ethanol mixture was cooled by passing through two condensers. The first was a finger condenser trap (Ace Glass, Vmeland, NJ) with ice on the inside at 0°C and the second was a vacuum trap packed with 8 mm Raschig rings and immersed a dry ice/acetone bath at -70°C. The air leaving the second trap was analyzed by Gas Chromatography and no water or ethanol was found to be present, showing that the traps were sufficient to remove all condensables . The gas chromatogram output of water and ethanol and temperature data were loaded into Excel spreadsheets, calculations were performed, and the results were plotted using KaleidaGraph. The results demonstrated an advantageous removal of water from the initial ethanol/water feed. EXAMPLE NINE

ATTRITION TESTING

The stability of modified and unmodified corn grits to attrition was tested using a Crescent Dental MFG. Co. Wig-L- Bug Amalgamator (Lyons, IL) . A 0.5 gram sample of the grits waε placed in the cylindrical sample holder, which also held a metal ball, and set to shake for 2.5, 5, or 10 seconds. The sample was removed from the holder and sieved using a 0.075 mm sieve size. The final weight was the weight collected after sieving. The initial weight minus the final weight divided by the initial weight gave the percent fines.

The attrition test was designed to determine how grits would degrade over long periods of use in the pressure swing dryer. Comparative testing was accomplished by additionally subjecting a molecular sieve composition and a silica gel composition to the treatment described above. The results of this test are shown in FIG. 9. The results show that grits have lower attrition than both the molecular sieve and the silica gel compositions.

While the invention is described in connection with preferred embodiments, it will be understood that it is not intended to limit the invention to such embodiments . On the contrary, the invention is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the claims .

Claims

What is claimed is:

1. A desiccant composition, comprising corn grits modified by reaction with an alpha-amylase enzyme under conditions effective to increase the desiccant capacity of the corn grits.

2. The composition according to claim 1, wherein said modified corn grits have increased porosity.

3. The composition according to claim 1, wherein said modified corn grits have increased surface area.

4. The composition according to claim 1, wherein said modified corn grits have an average particle size of at least about 0.5 mm.

5. The composition according to claim 1, wherein said modified corn grits have an average particle size of from about 0.5 mm to about 3 mm.

6. The composition according to claim 1, wherein said modified corn grits have an average external surface area of at least about 0.22 m /gram.

7. The composition according to claim 6, wherein said modified corn grits have an average external surface area of about 0.22 m²/gram to about 0.3 m /gram.

8. A method for making a desiccant composition, comprising: reacting corn grits in an aqueous medium with an alpha-amylase enzyme so as to increase the desiccant capacity of the grits; and recovering the grits from the aqueous medium.

9. The method according to claim 8, further comprising washing the grits after said recovering.

10. The method according to claim 8, further comprising drying the grits after said recovering.

11. The method according to claim 10, in which said drying step comprises drying at a temperature of from about 30°C to about 100°C.

12. The method according to claim 10, in which said drying step comprises drying at a temperature of from about 35°C to about 50°C.

13. The method according to claim 8, said aqueous medium comprising a phosphate buffer.

14. The method according to claim 8, said aqueous medium comprising sodium phosphate monobasic and sodium chloride.

15. The method according to claim 14, said aqueous buffer further comprising an amount of sodium hydroxide necessary to bring the pH of said aqueous buffer to within a desired range.

16. The method according to claim 8, said alpha-amylase enzyme comprising an alpha-amylase enzyme isolated from a Bacillus organism.

17. The method according to claim 16, the Bacillus organism being Bacillus licheniformis.

18. The method according to claim 8, wherein in the contacting step, the grits undergo less than about 5% hydrolysis.

19. A product produced by the method of claim 8

20. A method for drying a moisture-containing gas, comprising contacting the gas with a desiccant composition comprising corn grits modified by reaction with an alpha- amylase enzyme under conditions effective to increase the desiccant capacity of the corn grits.

21. The method according to claim 20, wherein said gas comprises air.

22. The method according to claim 21, wherein the air is dried to a dew point of about -60°C to about -80°C.

23. The method according to claim 20, wherein said contacting step comprises contacting in a pressure-swing adsorption system.

24. A method for drying a moisture-containing starting liquid, comprising vaporizing the starting liquid to provide a moisture-containing vapor; contacting the vapor with a composition comprising corn grits modified by reaction with an alpha-amylase enzyme under conditions effective to increase the desiccant capacity of the corn grits, so as to remove moisture from the vapor; and condensing the vapor to provide a product liquid.

25. The method according to claim 24, wherein said starting liquid comprises an organic compound having a boiling point of less than about 120°C.

26. The method according to claim 24, wherein said starting liquid comprises an organic compound which forms an azeotrope with water.

27. The method of claim 25, wherein the organic compound is an alcohol .

28. The method of claim 27, wherein the organic compound is ethanol.

29. The method according to claim 28, wherein the product liquid contains less than about 1% by weight water.

30. The method according to claim 24, wherein said contacting step comprises contacting in a fixed bed adsorption system.

31. A process for preparing a modified, biodegradable desiccant composition, comprising: providing a starch-containing, particulate corn fraction; reacting the particulate corn fraction with an alpha- amylase enzyme in an aqueous medium to form a modified particulate corn fraction; and recovering the modified particulate corn fraction.

32. The process of claim 31 wherein the particulate corn fraction comprises corn grits.

33. A desiccant composition obtainable by a process according to claim 31.

34. A modified, biodegradable desiccant composition, comprising: a starch-containing particulate corn fraction modified by reaction with an alpha-amylase enzyme.

35. The composition of claim 34 wherein the corn fraction comprises corn grits.

36. The composition of claim 35 wherein said reaction with an alpha amylase enzyme increases the porosity and external surface area of the corn grits.