WO2019117732A1

WO2019117732A1 - A dehumidification system and method

Info

Publication number: WO2019117732A1
Application number: PCT/NZ2018/050177
Authority: WO
Inventors: Richard Hamilton Archer; Qun Chen; James Robert JONES
Original assignee: Massey University
Priority date: 2017-12-14
Filing date: 2018-12-14
Publication date: 2019-06-20

Abstract

A dehumidification system has a first desiccant wheel process portion and associated first desiccant wheel regeneration portion. The system also has a second desiccant wheel process portion and associated second desiccant wheel regeneration portion. A closed circuit of gas is in fluid communication with the first desiccant wheel regeneration portion and the second desiccant wheel process portion to regenerate the first desiccant wheel regeneration portion.

Description

A DEHUMIDIFICATION SYSTEM AND METHOD

FIELD OF THE INVENTION

The present invention relates to a dehumidification system and method.

BACKGROUND OF THE INVENTION

Moist gas dehumidification is important for humidity control in various environments, such as industrial drying, water treatment plants, electronic equipment manufacturing, archive storage facilities, and food packaging and processing, especially where low dew- point dry air may be required.

In a typical dehumidification process, water vapour can be removed from a gas either by cooling the gas to condense it through refrigeration systems or by adsorption in desiccant based systems that need to be regenerated using extra heating. Both dehumidification approaches require large amounts of energy. With the growing concerns of energy security and environmental sustainability, more energy-efficient dehumidification technologies are vital.

Desiccant based dehumidification is often employed when the dehumidification requirement is greater than the cooling need or when the required dew point of gas is unusually low. For example, in supermarket refrigeration systems it is preferable to have dry store air with a dew point close to 0°C in order to reduce energy costs. Gas used in lithium-ion battery rooms and some processes of food and pharmaceutical industries usually require dew points as low as -40°C. A number of efforts have been made to develop and apply desiccant dehumidification systems in relation to these circumstances.

When desiccant based dehumidification is used to provide low-temperature, dry gas for a process, as is often the case in the beverage, food, pharmaceutical and chemical industrial, the excess sensible heat must be removed in an extra cooling unit, resulting in extra capital cost and low energy efficiency for the dehumidification process.

US2016/0250583 describes a desiccant dehumidifier regenerated by superheated steam. The superheated steam is reheated in a closed loop around a single desiccant wheel. This invention also discloses an embodiment consisting of two or more desiccant wheels that are individually regenerated by a source of superheated steam. These desiccant wheels are arranged in sequence to dry the supply gas successively.

l US5688305 describes a process for drying moist gas with a desiccant wheel. The process features a desiccant that is regenerated by hot regeneration air, the throughput of which is determined as a function of its temperature leaving the desiccant wheel.

US5667560 describes a process consisting of a desiccant wheel for both

dehumidification and removal of volatile organic compounds from air provided from the passenger compartment of a transportation vehicle. Mixtures of several rare earth zeolites are used as adsorbent in the wheel so that the regeneration temperature of the desiccant wheel can be less than 80°C.

US8328904 presents several methods for reducing energy usage of desiccant wheels by changing operation conditions such as process air flow, reactivation air flow,

temperature and rotation speed. One or more purge sectors are also incorporated between the regeneration and process sectors.

US6854279 describes a desiccant system for adjusting the humidity of air that is supplied to cooling coils on a gas turbine driven ship. In this system, the supplied air is dehumidified through a desiccant wheel and then transfers part of its sensible heat to an exhaust air through a rotatable thermal wheel. The desiccant wheel is regenerated by the mixture of the exhaust air and some of the dehumidified supply air.

In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents or such sources of information is not to be construed as an admission that such documents or such sources of information, in any jurisdiction, are prior art or form part of the common general knowledge in the art.

It is an object of at least preferred embodiments of the present invention to provide an energy efficient desiccant based dehumidification system and a method, and/or to at least provide the public with a useful alternative.

SUMMARY OF THE INVENTION

In a first embodiment, herein is described a dehumidification system, comprising a first desiccant wheel process portion and associated first desiccant wheel regeneration portion, a second desiccant wheel process portion and associated second desiccant wheel regeneration portion, and a closed circuit of gas in fluid communication with the first desiccant wheel regeneration portion and the second desiccant wheel process portion to regenerate the first desiccant wheel regeneration portion. Preferably, the first desiccant wheel process portion and/or the first desiccant wheel regeneration portion is composed predominantly of a desiccant material or material having adsorption properties.

Preferably, the first desiccant wheel process portion and/or the first desiccant wheel regeneration portion is composed predominantly of silica gel.

Preferably, the second desiccant wheel process portion and/or the second desiccant wheel regeneration portion is composed predominantly of a desiccant material or material having adsorption properties.

Preferably, the second desiccant wheel process portion and/or the second desiccant wheel regeneration portion is composed predominantly of a silica gel.

Preferably, the first desiccant wheel process portion is configured to dehumidify a gas passing through it.

Preferably, the second desiccant wheel process portion is configured to dehumidify a gas in the closed circuit and in fluid communication with the first desiccant wheel

regeneration portion, such that the dehumidified gas regenerates the first desiccant wheel regeneration portion.

Preferably, the second desiccant wheel regeneration portion is regenerated by heated gas from an external environment.

Preferably, the first desiccant wheel process portion and associated first desiccant wheel regeneration portion are portions of a first desiccant wheel and the second desiccant wheel portion and associated second desiccant wheel regeneration portion are portions of a second desiccant wheel.

Preferably, the first desiccant wheel and/or the second desiccant wheel has a diameter of between about 100mm and about 4m.

Preferably, the first desiccant wheel and/or the second desiccant wheel has a diameter of about 550mm.

Preferably, the first desiccant wheel has a greater diameter than the second desiccant wheel.

Preferably, the first desiccant wheel and/or the second desiccant wheel has a depth of between about 50mm and about 500mm. Preferably, the first desiccant wheel and/or the second desiccant wheel has a depth of about 100mm.

Preferably, the first desiccant wheel regeneration portion is about a quarter to half of the first desiccant wheel.

Preferably, the second desiccant wheel regeneration portion is about a quarter to half of the second desiccant wheel.

Preferably, the second desiccant wheel regeneration portion is about a quarter of the second desiccant wheel.

Preferably, the rotation speed the first desiccant wheel and/or the rotation speed of the second desiccant wheel is between about 1 revolutions per hour and about 50 revolutions per hour.

Preferably, the rotation speed the first desiccant wheel and/or the rotation speed of the second desiccant wheel is about 5 revolutions per hour.

Preferably, the first desiccant wheel and the second desiccant wheel rotate at substantially the same speed.

Preferably, the dehumidification system further comprises a third desiccant wheel process portion and associated third desiccant wheel regeneration portion, and a closed circuit of gas in fluid communication with the second desiccant wheel regeneration portion and the third desiccant wheel process portion to regenerate the second desiccant wheel regeneration portion.

Preferably, the third desiccant wheel regeneration portion is about a quarter of the third desiccant wheel.

Preferably, the first desiccant wheel has a greater radius than the second desiccant wheel.

Preferably, the first desiccant wheel process portion and associated first desiccant wheel regeneration portion and the second desiccant wheel portion and associated second desiccant wheel regeneration portion are contained within a single wheel. Preferably, the wheel is subdivided radially into a central region having the first desiccant wheel process portion and associated first desiccant wheel regeneration and an outer region having the second desiccant wheel portion and associated second desiccant wheel regeneration portion.

Preferably, the outer region of the wheel is regenerated by a closed circuit of gas passing from the central region of the wheel to the outer region of the wheel and back to the central region of the wheel.

Preferably, the dehumidification system further comprises a condenser.

Preferably, the condenser is located in the gas flow before the second desiccant wheel regeneration portion, such that the condenser heats gas to a suitably high temperature before it enters the second desiccant wheel regeneration portion.

Preferably, the dehumidification system further comprises a heat recuperator.

Preferably, the heat recuperator is located in the gas flow before and in the gas flow after the second desiccant wheel regeneration portion, such that the heat recuperator recovers heat from gas exiting the second desiccant wheel regeneration portion and transfers the heat to gas entering the second desiccant wheel regeneration portion.

Preferably, the dehumidification system further comprises a heater.

Preferably, the heater is located in the gas flow before the second desiccant wheel regeneration portion, such that the heater heats gas to a suitably high temperature before it enters the second desiccant wheel regeneration portion.

Preferably, the dehumidification system further comprises a refrigerant evaporator.

Preferably, the refrigerant evaporator is located in the gas flow after the first desiccant wheel process portion, such that the refrigerant evaporator cools gas exiting the first desiccant wheel process portion.

Preferably, the dehumidification system further comprises one or more fans positioned within the gas flow of the system to create a current of air through the gas flow of the system.

In a second embodiment, herein is described a method for dehumidifying a gas, comprising passing a first humid gas through a first desiccant wheel process portion; passing a gas through a second desiccant wheel process portion and delivering the gas exiting the second desiccant wheel process portion to a first desiccant wheel regeneration portion that is associated with the first desiccant wheel process portion to regenerate the first desiccant wheel regeneration portion; and returning the gas exiting the first desiccant wheel regeneration portion to the second desiccant wheel process portion in a closed circuit.

Preferably, the method of the second embodiment is carried out by the dehumidification system of the first embodiment.

Preferably, the first humid gas passing through the first desiccant wheel process portion is dehumidified by the first desiccant wheel process portion.

Preferably, heated gas is passed from an external environment through the second desiccant wheel regeneration portion to regenerate the second desiccant wheel regeneration portion.

The term 'comprising' as used in this specification and claims means 'consisting at least in part of'. When interpreting statements in this specification and claims which include the term 'comprising', other features besides the features prefaced by this term in each statement can also be present. Related terms such as 'comprise' and 'comprised' are to be interpreted in a similar manner.

It is intended that reference to a range of numbers disclosed herein (for example, 1 to 10) also incorporates reference to all rational numbers within that range (for example,

1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5, 7, 8, 9 and 10) and also any range of rational numbers within that range (for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7) and, therefore, all sub- ranges of all ranges expressly disclosed herein are hereby expressly disclosed. These are only examples of what is specifically intended and all possible combinations of numerical values between the lowest value and the highest value enumerated are to be considered to be expressly stated in this application in a similar manner.

To those skilled in the art to which the invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting. Where specific integers are mentioned herein which have known equivalents in the art to which this invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

As used herein the term '(s)' following a noun means the plural and/or singular form of that noun. As used herein the term 'and/or' means 'and' or 'or', or where the context allows both. The invention consists in the foregoing and also envisages constructions of which the following gives examples only.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the present invention are illustrated as an example and are not limited by the figures of the appended drawings, in which like references may indicate similar elements and in which:

Figure 1 is a schematic drawing of one embodiment of a desiccant based

dehumidification system having two cascading desiccant wheels with a closed loop of intermediate gas passing from one of the desiccant wheels to the other of the desiccant wheels.

Figure 2 is a schematic drawing of the desiccant based dehumidification system of Figure 1, but without the condenser coil at the regeneration air input to the second, higher temperature desiccant wheel.

Figure 3 is a schematic drawing of another embodiment of a desiccant based

dehumidification system, which is similar to the system of Figure 1 with an additional desiccant wheel and corresponding closed loop.

Figure 4 is a schematic drawing of an embodiment of a desiccant wheel having two radially subdivided desiccant elements.

Figure 5 is a schematic drawing of the radially subdivided desiccant wheel of Figure 4 used in a dehumidification system.

Figure 6 is a schematic drawing of a first embodiment of a sector subdivided desiccant wheel used in a dehumidification system.

Figure 7 is a schematic drawing of a second embodiment of a sector subdivided desiccant wheel used in a dehumidification system.

Figure 8 is a schematic drawing of the desiccant based dehumidification system of Figure 1, but with a closed loop providing the regeneration air of the second, higher desiccant wheel.

Figure 9 is a schematic drawing of one embodiment of a desiccant based

dehumidification system having two cascading desiccant wheels with a closed loop of intermediate gas passing from one of the desiccant wheels to the other of the desiccant wheels in accordance with an embodiment of the invention.

Figure 10A-10C are graphs showing the temperature, humidity and psychrometric chart of the process stream of a desiccant wheel in accordance with an embodiment of the invention.

Figure 11A-11C are graphs showing the temperature, humidity and psychrometric chart of the process stream of a desiccant wheel in accordance with an embodiment of the invention.

Figure 12A-12C are graphs showing the temperature, humidity and psychrometric chart of the process stream of a desiccant wheel in accordance with an embodiment of the invention.

Figure 13 is a schematic drawing of one embodiment of a desiccant based

DETAILED DESCRIPTION OF THE INVENTION

Introduction to desiccant based dehumidification systems and methods

Moist gas requiring dehumidification originates from external industrial processes and environment including, but not limited to, industrial dryers, water treatment plants, electronic equipment manufacturing, archive storage facilities, and food packaging and processing and other pharmaceutical or chemical industries. This moist gas is referred to herein as working gas and can be dehumidified by a desiccant based dehumidification system and method as described herein. The dehumidification system and method dehumidifies working gas to reach a near-zero or sub-zero dew point of between -70°C and 20°C, and preferably between -40°C and 0°C.

Each of the dehumidification systems described herein has one or more desiccant wheels for dehumidifying the moist gas from a drying chamber. Each wheel rotates about a central axis. Desiccant wheels are used for dehumidification without the use of refrigeration. Refrigeration removes humidity from a gas by lowering the temperature of the gas below the dew point, which causes the moisture to condense. Air

dehumidification by desiccants employs sorption, which uses differences in vapour pressure to attract water molecules out of the gas and onto the desiccant surface. The desiccant elements have reticulated or porous surface. A desiccant material, such as silica gel, zeolite, or similar, has a natural affinity for adsorbing moisture. The desiccant material is deposited on the perforated or corrugated surface of the desiccant wheel.

When a humid gas, for example a gas exiting a drying chamber of an industrial process, passes through a desiccant material, the gas first cools down the desiccant material.

The gas is then dehumidified by the desiccant and then recirculated back to the drying chamber. As the desiccant material adsorbs moisture from the gas, adsorption heat is released. The latent heat of the moisture becomes sensible heat. As a result, the temperature of the newly dehumidified gas increases to a certain degree due to the sensible heat converting from latent heat and being transferred from the hot

regeneration gas.

In operation, the humid gas is supplied to a part of the wheel (a process zone) and the desiccant in that part adsorbs the moisture. The desiccant loaded with moisture moves as the wheel rotates. Another part of the wheel (a regeneration zone) is supplied with a hot regeneration gas with lower relative humidity. The sensible heat of the

regeneration gas is transferred to the desiccant for heating the desiccant and

overcoming the desorption heat of the moisture. The moisture in the desiccant is desorbed to this regeneration gas. The desiccant is therefore regenerated or reactivated and is able to be used to dehumidify gas again. When the desiccant wheel rotates back to the process gas zone, it releases its sensible heat to the process gas and is able to absorb moisture from the moist gas again.

In some situations, it is desirable to maintain the temperature of the gas at a specified freeze-drying condition. In such situations, some of the sensible heat obtained from the regeneration side is removed. There are two ways to achieve this, firstly, a refrigeration evaporator can be used to chill the drying gas either before or after it passes through the low temperature desiccant wheel. When the drying gas is chilled before it enters the low temperature desiccant wheel, some of its moisture can be frosted in the evaporator and the relative humidity of the gas approaches 100%. This can improve the performance of the desiccant element. When the drying gas is chilled after it passes through the low temperature desiccant wheel, a little more sensible heat should preferably be removed from the gas compared to that chilled before the desiccant wheel. However, the refrigerant in the evaporator can work at a relatively higher temperature and thus a better coefficient of performance can be achieved for the compressor. On the other hand, the desiccant after regeneration can be cooled by a gas stream from the process, the temperature of which is lower than the regeneration gas. Preferred Embodiments

Figure 1 shows one embodiment of the dehumidification system. The dehumidification system features a first, lower temperature desiccant wheel 1 and a second, higher temperature desiccant wheel 2. The wheels are adapted to rotate about their central axis. The low temperature desiccant wheel 1 has a process sector 3 and a regeneration sector 5. The high temperature desiccant wheel also has a process sector 4 and regeneration sector 6.

The moist gas requiring dehumidification passes through the lower temperature desiccant wheel 1, as a process gas and is recirculated back to the industrial processes or environment after exiting the lower temperature desiccant wheel 1. The gas exiting the process sector 3 is preferably passed through a cooler or refrigerant evaporator 14 before being used for its intended use. This recovers some sensible heat from the working gas that can be carried over from the regeneration side of the desiccant wheel.

Closed circuit

The lower temperature desiccant wheel 1 is regenerated by a closed circuit 7 of intermediate gas with the higher temperature desiccant wheel 2. The dehumidified intermediate gas is in fluid communication in a closed circuit between the lower temperature desiccant wheel 1 and the higher temperature desiccant wheel 2. The desiccant material in the lower temperature desiccant wheel 1 is regenerated by a stream of low humidity intermediate gas in the closed circuit 7 of intermediate gas. In this closed circuit 7, a stream of air, or optionally other gas, is dehumidified as the process gas in the higher temperature or next higher temperature desiccant wheel 2.

The dehumidified intermediate gas exiting the higher temperature desiccant wheel 2 passes through the lower temperature or previous desiccant wheel 1 as a regeneration gas to reactivate the lower temperature or previous desiccant element. The moisture- laden intermediate gas exiting the lower temperature or previous desiccant wheel is then recirculated into the higher temperature or next desiccant wheel for

dehumidification as the process gas.

The temperature of the dehumidified intermediate gas exiting the process sector 4 of the higher temperature desiccant wheel 2 is higher than the process gas entering the lower temperature desiccant wheel 1. The temperature is preferably about 20°C to about 50°C, more preferably about 25°C to about 35°C higher. The temperature may be about 26°, about 27°, about 28°, about 29°, about 30°, about 31°, about 32°, about 33°, or about 34° for example. The relative humidity of the dehumidified regeneration air exiting the process sector of the higher temperature desiccant wheel 2 is lower than that of dehumidified working gas, typically between about 2% and about 8%. The ratio of volumetric flow rates of the process air to the regeneration air is between 1 : 1 and 8: 1 for the lower temperature desiccant wheel 1. The adsorbed moisture in the process sector 3 of the lower temperature desiccant wheel 1 is desorbed from the desiccant material in the regeneration sector 5 and carried out by the regeneration air stream.

The intermediate gas, after regenerating the lower temperature desiccant wheel 1, is recirculated back to the process sector 4 of the higher temperature desiccant wheel 2. The intermediate gas is optionally cooled first in a cooler or refrigerant evaporator to increase its relative humidity to about 0.6 to about 1.0, preferably about 0.7 to about 0.9 before the intermediate gas is recirculated back to the higher temperature desiccant wheel 2.

Higher temperature desiccant wheel

The regeneration sector 6 of the higher temperature desiccant wheel 2 is provided with warmed ambient air from an external environment. This is heated preferably by a heater 12 preceded by a heat recuperator 10 and preferably a refrigerant condenser 8, which heat the ambient air to a suitably high temperature. The regeneration sector 6 of the higher temperature desiccant wheel passes humidified air back to an external environment through the heat recuperator 10 which recovers heat from this air to be reused to heat the initial ambient air. The refrigerant evaporators and the refrigerant condensers are preferably integrated and work as heat pumps, for example a generic mechanical heat pump.

Cascading Wheels

With reference to the embodiment shown in Figure 1, the desiccant dehumidification system consists of two cascading desiccant wheels 1, 2. A stream of moist air with high moisture content, having a dew point preferably between -15 and 30°C, and being provided from an industrial process, such as an atmospheric freeze drying process, is passed through the process sector 3 of the lower temperature desiccant wheel 1 as a process gas, where the moisture is adsorbed in the desiccant material. This newly dehumidified process air exiting the lower temperature desiccant wheel 1 is then cooled by a cooler or refrigerant evaporator 14 to recover some sensible heat before being recirculated back to the external process. Heat transfer between the refrigerant condenser 8 and refrigerant evaporator 14 is shown by 18, and links the two

components together as a heat pump. In the embodiment of Figure 1, the desiccant material in the higher temperature desiccant wheel 2 is regenerated by hot ambient air from an external environment. In a system having two cascading desiccant wheels, high humidity ambient air, preferably at a temperature between about 10 °C and about 30°C and relative humidity around 65% is heated up by a refrigerant condenser 8, a heat recuperator 10 and a heater 12 to a moderate temperature of between 60°C and 100°C and works as regeneration air for the higher temperature desiccant wheel 2, and enters the regeneration sector 6 of this wheel 2. Some sensible heat from the regeneration air exiting the higher temperature desiccant wheel 2 is preferably recovered using a heat recuperator 10, preferably the same heat recuperator 10 used to heat the ambient air before it enters the heater 12, as it extracts heat from the gas exiting the regeneration sector 6 and passes this heat to the gas entering the heater 12.

In another embodiment, as shown in Figure 3, the desiccant based dehumidification system has three cascading desiccant wheels. The lower temperature desiccant wheel 1 is regenerated by a dehumidified gas stream in a closed circuit from the process sector 28 of the intermediate desiccant wheel 26. The intermediate desiccant wheel is regenerated in a regeneration sector 30 by a dehumidified gas stream in a closed circuit through the higher temperature desiccant wheel 2, as in the previous embodiment. Finally, the higher temperature desiccant wheel is regenerated by hot air, preferably generated by a heater and such, as in the previous embodiment.

Radially Subdivided Wheel

In a further embodiment, as shown in Figures 4 and 5, several desiccant elements are incorporated into a single wheel 35. The wheel is divided into several sub-divisioned (or sub-divided) desiccant elements: an inner desiccant portion 31 and an outer annular desiccant portion 33. These radially subdivided desiccant portion 31 and 33 are formed in a single wheel 35 as shown in Figure 4. Similar to the separate desiccant elements used in the previous embodiments, the central desiccant portion 31 serves a similar purpose as the higher temperature desiccant wheel 2, and the outer annular desiccant portion 33 serves a similar purpose as the lower temperature desiccant wheel 1, as shown in Figure 5.

In this embodiment, both the outer desiccant element 33 and the central desiccant element 31 of the wheel 35 are subdivided circumferentially, and have a corresponding process and a regeneration sector. In the outer element 33, the area of the

regeneration sector 38 is represented by the angle a, preferably, varying from between 40° to 180°. In the central element 31, the area of the process sector 32 is the same as that of the regeneration sector 34. The ratio of radius of the inner element, r to the radius of the outer element R preferably ranges from 1 :2.4 to 1 : 1.4.

Similarly to previous embodiments, the intermediate gas for regenerating the outer element 33 of the wheel 35 is first dehumidified as a process gas in the process sector 32 of the central element 31, which serves a similar purpose to the higher temperature desiccant element 2 in previous embodiments. The newly dehumidified gas is then preferably fed into the regeneration sector 38 of the outer element 33 as a regeneration gas. After exiting the outer element of the wheel, the moist intermediate gas stream is then recirculated back to the process region 32 of the central element 31 of the wheel for dehumidification.

The moist working gas from an external industrial process passes through the process sector 36 of the outer element for dehumidification. The dehumidified gas exiting the outer element is recirculated back to the external industrial process. The desiccant material in this outer element is regenerated by a stream of low-humidity intermediate gas in a closed circuit, as described in the previous embodiments. In this closed circuit, a stream of gas is first dehumidified as the process gas in the process sector 32 of the central element of the wheel. The dehumidified intermediate gas exiting the central element is then fed through the regeneration sector 38 of the outer element of the wheel to regenerate the desiccant material of this element.

In order to improve the dehumidification performance of the central element of the wheel, the moist intermediate gas exiting the outer element of the wheel is optionally cooled in a cooler 14 or preferably a refrigerant evaporator, to increase its relative humidity to preferably between 0.6 and 1.0, before being recirculated back to the central element of the wheel for dehumidification as a process gas.

The central element of the wheel is regenerated preferably by ambient air after being heated in a heater 12 preceded by a heat recuperator 10 and preferably a refrigerant condenser 8 to a moderately high temperature of preferably between 60°C and 100°C. The sensible heat of this regeneration air exiting the central element of the wheel is recovered by the heat recuperator 10, as in the previous embodiments.

Different sizes

Another aspect of the present invention is that the individual desiccant elements used in the dehumidification process can be of different radii. For the lower temperature desiccant element, a large quantity of drying air needs to be dehumidified. Therefore, the ratio of the area of the process sector to the area of the regeneration sector is preferably high, the ratio preferably 2.5: 1 or above. The size of the lower temperature desiccant element is preferably bigger, with a larger radius to provide more

dehumidification capacity. The size of the higher temperature desiccant element is determined by the area of the regeneration sector of the lower temperature desiccant element. The regeneration air stream for the higher temperature desiccant element removes moisture from the system. It preferably needs some working area of the desiccant element. Therefore, the area ratio of the process to regeneration for the higher temperature desiccant element is preferably set at between 1 : 1 and 3: 1.

The rotation direction and speed of the desiccant elements such as the desiccant wheels used in the previous embodiments shown affects the circumferential profiles of humidity and temperature of the gas exiting the desiccant elements. If the initial conditions for the gas from the external process are the same, when the two closely coupled wheels rotate in an opposite direction to each other, the sensible heat gain in this process air stream is lower than that when the two wheels rotate in the same direction, although the humidity is higher. The preference in rotation direction depends on the conditions of the gas required by the external process.

Other Divided Wheel Concept

In another optional embodiment, the dehumidification system consists of one desiccant wheel subdivided circumferentially into multiple desiccant elements having multiple sectors A - F, as illustrated in Figure 6. Moist gas from an external process is dehumidified through Sector A. The moisture-laden desiccant material turns from Sector A to Sector B as the wheel rotates, where it is regenerated by dehumidified regeneration gas from Sector C. The regeneration gas exiting Sector B is then recirculated back to Sector C for dehumidification and a closed loop is formed. The desiccant material absorbs moisture in Sector C and is then regenerated in Sector D by dehumidified regeneration gas from Sector E. This regeneration gas stream exiting Sector D is then dehumidified in Sector E and forms another closed loop. The desiccant material from Sector E is then regenerated by a stream of hot gas. In such

embodiments, the moisture in the gas from an external process is first absorbed in the desiccant material in Sector A and then transferred to Sector F by two cascading closed loops operating through Sectors B and C to Sectors D and E. The moisture is finally removed by the hot regeneration gas from Sector F. Preferably, the ratio of area of two consecutive sectors through A to F is between 1 : 1 and 3: 1.

In another optional embodiment, shown in Figure 7, a purge sector G can be positioned between the regeneration sector F and the process sector A. Purge gas is used to reduce the carryover of the sensible heat from the regeneration sector to dehumidified gas stream. However, it is also feasible to use a relatively lower temperature regeneration air to reactivate the desiccant element. This can not only reduce the carryover of sensible heat, but also reduce the temperature excursions that an individual desiccant wheel has to endure and prolong the usable life of the desiccant wheel.

Second Closed Loop Example

In another embodiment, as shown in Figure 8, the desiccant material in the higher temperature desiccant wheel 2 is preferably regenerated in a closed loop 19 consisting of a high temperature heat pump, comprised of a refrigerant condenser 8 and refrigerant evaporator 14. In this closed loop regeneration cycle, air is heated by the refrigerant condenser 8 of the heat pump and then enters the regeneration sector 6 of the desiccant wheel 2, where the desiccant material of the desiccant wheel 2 undergoes regeneration. The humid regeneration air exiting the desiccant wheel 2 is then cooled by the refrigerant evaporator 16 of the heat pump to a dew point. The heat transfer between the refrigerant condenser 8 and refrigerant evaporator 14 is shown by 18, and links the two components of the heat pump.

This embodiment is a heat pump driven regeneration loop for use with multiple desiccant wheels in a dehumidification system. It differs from the embodiments described previously which require additional heat input for desiccant regeneration. This embodiment preferably requires only electric power input to power a heat pump, working entirely as a closed loop system. This allows the desiccant based

dehumidification system of this embodiment to operate completely in a non-air atmosphere, as no ambient air input is required.

Further Embodiments

Figure 9 shows a variation of the embodiment of the dehumidification system shown in and described in relation to Figure 1. The system shown in Figure 9 is composed of two cascading desiccant wheels 1, 2 similarly to the embodiment shown in Figure 1 but without a heat recuperator. The regeneration sector 6 of the higher temperature desiccant wheel 2 in this embodiment is provided with warmed ambient air from an external environment. This is heated by a heater 12 preceded a refrigerant condenser 8, which heats the ambient air to a suitably high temperature.

In some embodiments, and alternatively to freeze drying, the present invention is intended to be used in spray drying applications, such as, but not limited to the drying of: milk powder, coffee, and other food products, as well as pharmaceutical and industrial applications. In some embodiments, such as those where spray drying is the intended use for the system, the moist gas requiring dehumidification by the system, which is entering the dehumidification system has a temperature in the range of 50-120°C.

Figure 13 shows a further embodiment of the dehumidification system previously described, which is modified for use in spray drying applications. The system shown in Figure 13 has two cascading desiccant wheels 1, 2 similarly to the embodiment shown in Figure 1.

In this embodiment, the regeneration sector 6 of the higher temperature desiccant wheel 2 is regenerated in a closed circuit 44, however it will be appreciated that the higher temperature desiccant wheel can be regenerated using any of the systems or methods previously described. The air used to regenerate the high temperature desiccant wheel is in fluid communication with the air exiting the regeneration portion of the higher temperature desiccant wheel, in a closed circuit 44. In this embodiment, the air exiting the regeneration sector 6 of the higher temperature desiccant wheel is reheated to the regeneration temperature by a heater 8 located in the closed circuit 44.

In this and previous embodiments, the moist gas entering the system in this

embodiment passes through the process sector 3 of the lower temperature desiccant wheel 1 and undergoes dehumidification as previously described.

The dehumidified gas is then preheated by a heater 40 before returning as dry air to the industrial environment for use. In this embodiment, the heater 40 is positioned within the system as shown. In other embodiments, the heater heating the dry gas is located in the industrial environment such as a part of the external spray drying process which the dry gas is fed into once dehumidified.

In these examples, the lower temperature desiccant wheel 1 is regenerated through the regeneration sector 5 by a stream of low-humidity intermediate gas in a closed circuit 7, as described in the previous embodiments. In this embodiment this intermediate gas in the closed circuit 7 is between about 80°C and about 155°C. The gas may have a temperature of about 85°C, 90°C, 95°C, 100°C, 105°C, 110°C, 115°C, 120°C, 125°C, 130°C, 135°C, 140°C, 145°C, or 150°C, for example. The stream of intermediate gas is dehumidified in the process sector 4 of the higher temperature desiccant wheel 2, as explained with respect to previous embodiments.

The higher temperature desiccant wheel 2 is. in some embodiments regenerated by a stream of superheated steam. In such embodiments, the stream of superheated steam has a temperature of between 130°C and 210°C. The gas may have a temperature of about 135°C, 140°C, 145°C, or 150°C, 155°C, 160°C, 165°C, 170°C, 175°C, 180°C, 185°C, 190°C, 195°C, 200°C, or 205°C, for example. In the embodiment shown in Figure 13, the higher temperature desiccant wheel 2 is regenerated by a stream of superheated steam in the closed circuit 44. In this closed circuit, the superheated steam first passes through the regeneration sector 6 of the higher temperature desiccant wheel 2. After exiting the regeneration sector 6 of the higher temperature desiccant wheel 2, a portion of the steam is exhausted or recirculated from this closed circuit through air path 42 to be used as a heat source for the heater 40 which heats the dry air exiting the process sector 3 of the lower temperature desiccant wheel 1.

The remainder of the superheated steam is reheated to the regeneration temperature by a heater 8 in the closed circuit 44 and then passed back into the regeneration sector 6 of the higher temperature desiccant wheel 2. In some embodiments, the amount of steam exhausted from the closed circuit 44 through air path 42 to the heater 40 is equivalent to the amount of moisture generated in the drying process.

Additional Features

In any of the embodiments described herein, one or more of the desiccant wheels in the system is composed of a silica gel rotor. Alternatively, one or more of the desiccant wheels is composed of a rotor that is predominantly silica gel, and may also comprise one or more other desiccant materials or materials which have adsorption properties.

In any of the previous embodiments, one or more of the desiccant wheels can have a diameter of between 100mm and 5m. In one embodiment the diameter of at least one of the desiccant wheels is 550mm. Other diameters may be 200mm, 300mm, 400mm, 500mm, 600mm, 700mm, 800mm, 900mm, lm, 1.1m, 1.2m, 1.3m 1.4m 1.5m 1.6m, 1.7m 1.8m, 1.9m, 2m, 2.1m, 2.2m, 2.3m, 2.4 m, 2.5m, 2.6m, 2.7m, 2.8m, 2.9m, 3m, 3.1m, 3.2m, 3.3m, 3.4m, 3.5m, 3.6m, 3.7m, 3.8m, 3.9m, 4m, 4.1m, 4.2m, 4.3m,

4.4m, 4.5m, 4.6m, 4.7m, 4.8m, or 4.9m, for example.

Furthermore, in any of the previous embodiments, one or more of the desiccant wheels has a depth of between 50mm and 500mm. In one embodiment, the depth of at least one of the desiccant wheels is 100mm. Other depths may be 150mm, 200mm, 250mm, 300mm, 350mm, 400mm, or 450mm, for example.

In any of the previous embodiments, the rotation speed of one or more of the desiccant wheels is between 1 and 50 revolutions per hour. In one embodiment, the rotation speed of one or more of the desiccant wheels is 5 revolutions per hour. In any of the previous embodiments, the rotation speed of one desiccant wheel in the system is the same as the rotation speed as the other desiccant wheels in the system. The speed may be 10 revolutions per hour, 15 revolutions per hour, 20 revolutions per hour, 25 revolutions per hour, 30 revolutions per hour, 35 revolutions per hour, 40 revolutions per hour, or 45 revolutions per hour, for example. The desiccant wheels may rotate at substantially the same speed.

In an optional embodiment, one or more centrifugal fans are provided in the system to provide airflow through the system. The centrifugal fans can be located before a desiccant wheel to drive air through the wheel. Alternatively, or in addition to having fans before a desiccant wheel, fans may be located after a desiccant wheel to drive air away from the wheel. In one embodiment, a first centrifugal fan is located in the drying chamber, forcing air towards the process area of the lower temperature desiccant wheel, a second centrifugal fan is located in the closed circuit as previously described between the process area of the higher temperature desiccant wheel and the

regeneration area of the lower temperature desiccant wheel, a third centrifugal fan is located before the condenser provided before the regeneration area of the higher temperature desiccant wheel to drive air towards it, and a fourth centrifugal fan is located after the process area of the lower temperature desiccant wheel and the evaporator or cooler to drive dehumidified air back to the drying chamber.

The dehumidification systems and methods described herein use a closed loop allowing fluid communication of gases between two or more desiccant wheels. This system produces dehumidified gas as a regeneration gas for regenerating the desiccant wheel employed to dehumidify the process gas of industrial processes that require a low dew point dry gas. In the dehumidification system, the humidity in the process gas of the lower temperature desiccant wheel is concentrated through the consecutive desiccant wheels into the regeneration gas of the last desiccant wheel. Furthermore, cascading desiccant wheels preferably gives hermetic separation of the process gas to be dehumidified and the regeneration gas of the last desiccant wheel. This facilitates the use of non-air gas in industrial processes or working enclosures.

Different gases may be used in the described dehumidifying system to fulfil different drying needs. Preferably, the gases that may be used as the drying gas in the lower temperature desiccant wheel include, but are not limited to, ambient air, nitrogen, and carbon dioxide. Preferably, the gas that may be used in the other circuits is, but is not limited to, ambient air.

The invention is intended to be used with raw materials to be freeze-dried including, but not limited to, fruits, vegetables, and other food products such as organs and tendons. Depending on the original size, the frozen material to be dried can be treated in whole pieces, or be sliced into portion pieces or dices in order to shorten drying time. A dryer chamber linked with the dehumidification system described above can operate either batch-wise or continuously. Preferred embodiments of the invention have been described by way of example only and modifications may be made thereto without departing from the scope of the invention.

Experimental Data

In one embodiment, a number of temperature and humidity readings were taken over a period of time at a number of different levels. Figure 10A shows a graph of temperature readings over a period time, Figure 10B shows a graph of humidity readings over a period of time, and Figure 10C shows a psychrometric chart comparing the temperature and humidity readings as shown in Figures 10A and 10B, according to this embodiment.

In this embodiment, and as shown in Figures 10A, 10B, and 10C, the air process with temperature of 18.4°C and humidity of 5.4 g/kg d.a. was dehumidified by the desiccant based dehumidification system as previously described. After dehumidification, the temperature of the process air was 22.7°C and the humidity was 4.3 g/kg d.a. The dew point of the process air decreased from 4.8°C to 1.8°C. The low temperature desiccant wheel was regenerated by the intermediate air with temperature of 28.8°C and humidity of 4.7 g/kg d.a. The temperature of the intermediate air dropped to 23.9°C with the humidity increased from 4.7 to 5.7 g/kg d.a. The intermediate air was dehumidified by the process sector of the high temperature desiccant wheel in the closed circuit. The high temperature desiccant wheel was regenerated by hot air of 43.6°C with humidity of 6.8 g/kg d.a. The exhaust air from the high temperature desiccant wheel was 35.5°C in temperature with humidity of 8.4 g/kg d.a. The flow rates of the process, intermediate and regeneration air were approximately 0.11, 0.10 and 0.07 m³/s (normalised at 20°C), respectively. In this exemplification, the heat consumption of dehumidification was about 3.5MJ/kg water removal.

In another embodiment, a similar number of temperature and humidity readings were taken over a period of time at a number of different levels. Figure 11A shows a graph of temperature readings over a period time, Figure 11B shows a graph of humidity readings over a period of time, and Figure 11C shows a psychrometric chart comparing the temperature and humidity readings as shown in Figures 11A and 11B, according to this embodiment.

In this embodiment, and as shown in Figures 11A, 11B, and 11C, the process air with humidity of 1.31g/kg d.a. was dehumidified to 1.07 g/kg d.a. by the low temperature desiccant wheel while the temperature increased from -3.7°C to -0.1°C (dew point from -12.2 to -14.5°C). In the closed circuit, the intermediate air to regenerate the low temperature desiccant wheel was 27.6°C with humidity of 3.7 g/kg d.a. Its humidity increased to 4.2 g/kg d.a. as the temperature dropped to 20.9°C. The temperature of the regeneration air for the high temperature desiccant wheel was 45.1 °C and the humidity was 7.1 g/kg d.a. After regeneration, its temperature was reduced to 39.2 °C with humidity of 7.6 g/kg d.a.

In this embodiment, the flow rates of the process, intermediate and regeneration air were approximately 0.15, 0.07 and 0.07 m³/s (normalised at 20°C), respectively. When the process air was cooled to -3.0°C by the evaporator coil of the heat pump, the electricity consumption of the compressor was about 5MJ/kg water removal. The heat consumption of dehumidification was about lOMJ/kg water removal.

In yet another embodiment, a similar number of temperature and humidity readings were taken over a period of time at a number of different levels. Figure 12A shows a graph of temperature readings over a period time, Figure 12B shows a graph of humidity readings over a period of time, and Figure 12C shows a psychrometric chart comparing the temperature and humidity readings as shown in Figures 12A and 12B, according to this embodiment.

In this embodiment, and as shown in Figures 12A, 12B, and 12C, the air from frozen wood chip drying process (operating under partial loading) with temperature of -9.2°C and humidity of 0.92 g/kg d.a. was dehumidified by the low temperature desiccant wheel of the prototype. The dehumidified process air was -5.5°C with humidity of 0.75 g/kg d.a. The low temperature desiccant wheel was regenerated by the intermediate air with temperature of 24.9°C and humidity 3.51 g/kg d.a. in the closed circuit. The intermediate air of 20.7°C in temperature and 3.69 g/kg d.a. of humidity left the low temperature desiccant wheel and was then dehumidified in the high temperature desiccant wheel. The high temperature desiccant wheel was regenerated by the preheated air of 44.8 °C with humidity of 9.0 g/kg d.a. The temperature of the exhausted regeneration air was 40.7 °C and the humidity was 9.16 g/kg d.a. The flow rate for each of the process, intermediate and regeneration air was approximately 0.15m³/s (normalised at 20°C).

In this embodiment, when the process air was cooled to -8.0°C by the evaporator coil of the heat pump, the electricity consumption of the compressor was about 6MJ/kg water removal. The heat consumption of dehumidification of the prototype was about

20MJ/kg water removal.

Claims

1. A dehumidification system, comprising :

a first desiccant wheel process portion and associated first desiccant wheel regeneration portion,

a second desiccant wheel process portion and associated second desiccant wheel regeneration portion, and

a closed circuit of gas in fluid communication with the first desiccant wheel regeneration portion and the second desiccant wheel process portion to regenerate the first desiccant wheel regeneration portion.

2. The dehumidification system as claimed in claim 1, wherein the first desiccant wheel process portion and/or the first desiccant wheel regeneration portion is composed predominantly of a desiccant material or material having adsorption properties.

3. The dehumidification system as claimed in claim 2, wherein the first desiccant wheel process portion and/or the first desiccant wheel regeneration portion is composed predominantly of silica gel.

4. The dehumidification system as claimed in any one of the preceding claims, wherein the second desiccant wheel process portion and/or the second desiccant wheel regeneration portion is composed predominantly of a desiccant material or material having adsorption properties.

5. The dehumidification system as claimed in claim 4, wherein the second desiccant wheel process portion and/or the second desiccant wheel regeneration portion is composed predominantly of a silica gel.

6. The dehumidification system as claimed in any one of the preceding claims, wherein the first desiccant wheel process portion is configured to dehumidify a gas passing through it.

7. The dehumidification system as claimed in any one of the preceding claims, wherein the second desiccant wheel process portion is configured to dehumidify a gas in the closed circuit and in fluid communication with the first desiccant wheel regeneration portion, such that the dehumidified gas regenerates the first desiccant wheel regeneration portion.

8. The dehumidification system as claimed in any one of the preceding claims, wherein the second desiccant wheel regeneration portion is regenerated by heated gas from an external environment.

9. The dehumidification system as claimed in any one of the preceding claims, wherein the first desiccant wheel process portion and associated first desiccant wheel regeneration portion are portions of a first desiccant wheel and the second desiccant wheel portion and associated second desiccant wheel regeneration portion are portions of a second desiccant wheel.

10. The dehumidification system as claimed in claim 9, wherein the first desiccant wheel and/or the second desiccant wheel has a diameter of between about 100mm and about 4m.

11. The dehumidification system as claimed in claim 10, wherein the first desiccant wheel and/or the second desiccant wheel has a diameter of about 550mm.

12. The dehumidification system as claimed in any one of claims 9 to 11, wherein the first desiccant wheel has a greater diameter than the second desiccant wheel.

13. The dehumidification system as claimed in any one of claims 1 to 12, wherein the first desiccant wheel and/or the second desiccant wheel has a depth of between about 50mm and about 500mm.

14. The dehumidification system as claimed in claim 13, wherein the first desiccant wheel and/or the second desiccant wheel has a depth of about 100mm.

15. The dehumidification system as claimed in any one of claims 9 to 14, wherein the first desiccant wheel regeneration portion is about a quarter to half of the first desiccant wheel.

16. The dehumidification system as claimed in any one of claims 9 to 15, wherein the second desiccant wheel regeneration portion is about a quarter to half of the second desiccant wheel.

17. The dehumidification system as claimed in any one of claims 9 to 16, wherein the second desiccant wheel regeneration portion is about a quarter of the second desiccant wheel.

18. The dehumidification system as claimed in any one of claims 9 to 17, wherein the rotation speed the first desiccant wheel and/or the rotation speed of the second desiccant wheel is between about 1 revolutions per hour and about 50 revolutions per hour.

19. The dehumidification system as claimed in claim 18, wherein the rotation speed the first desiccant wheel and/or the rotation speed of the second desiccant wheel is about 5 revolutions per hour.

20. The dehumidification system as claimed in claim 18 or claim 19, wherein the first desiccant wheel and the second desiccant wheel rotate at substantially the same speed.

21. The dehumidification system as claimed in any one of claims 1 to 8, wherein the first desiccant wheel process portion and associated first desiccant wheel regeneration portion and the second desiccant wheel portion and associated second desiccant wheel regeneration portion are contained within a single wheel.

22. The dehumidification system as claimed in claim 21 wherein the single wheel is subdivided radially into a central region having the first desiccant wheel process portion and associated first desiccant wheel regeneration, and an outer region having the second desiccant wheel portion and associated second desiccant wheel regeneration portion.

23. The dehumidification system as claimed in claim 22 wherein the outer region of the wheel is regenerated by a closed circuit of gas passing from the central region of the wheel to the outer region of the wheel and back to the central region of the wheel.

24. The dehumidification system as claimed in any one of the preceding claims, further comprising a condenser.

25. The dehumidification system as claimed in claim 24 wherein the condenser is located in the gas flow before the second desiccant wheel regeneration portion, such that the condenser heats gas to a suitably high temperature before it enters the second desiccant wheel regeneration portion.

26. The dehumidification system as claimed in any one of the preceding claims, further comprising a heat recuperator.

27. The dehumidification system as claimed in claim 26 wherein the heat recuperator is located in the gas flow before and in the gas flow after the second desiccant wheel regeneration portion, such that the heat recuperator recovers heat from gas exiting the second desiccant wheel regeneration portion and transfers the heat to gas entering the second desiccant wheel regeneration portion.

28. The dehumidification system as claimed in any one of the preceding claims, further comprising a heater.

29. The dehumidification system as claimed in claim 28, wherein the heater is located in the gas flow before the second desiccant wheel regeneration portion, such that the heater heats gas to a suitably high temperature before it enters the second desiccant wheel regeneration portion.

30. The dehumidification system as claimed in any one of the preceding claims, further comprising a refrigerant evaporator.

31. The dehumidification system as claimed in claim 30, wherein the refrigerant evaporator is located in the gas flow after the first desiccant wheel process portion, such that the refrigerant evaporator cools gas exiting the first desiccant wheel process portion.

32. The dehumidification system as claimed in any one of the preceding claims, further comprising one or more fans positioned within the gas flow of the system to create a current of air through the gas flow of the system.

33. The dehumidification system as claimed in any one of the preceding claims, further comprising :

a third desiccant wheel process portion and associated third desiccant wheel

regeneration portion, and

a closed circuit of gas in fluid communication with the second desiccant wheel regeneration portion and the third desiccant wheel process portion to regenerate the second desiccant wheel regeneration portion. The dehumidification system as claimed in claim 33, wherein the third desiccant wheel regeneration portion is about a quarter of the third desiccant wheel.

34. A method for dehumidifying a gas, comprising :

passing a first humid gas through a first desiccant wheel process portion;

passing a gas through a second desiccant wheel process portion and delivering the gas exiting the second desiccant wheel process portion to a first desiccant wheel regeneration portion that is associated with the first desiccant wheel process portion to regenerate the first desiccant wheel regeneration portion;

and returning the gas exiting the first desiccant wheel regeneration portion to the second desiccant wheel process portion in a closed circuit.

35. The method for dehumidifying a gas as claimed in claim 35, wherein the method is carried out by the dehumidification system as claimed in any one of claims 1 to 34.

36. The method for dehumidifying a gas as claimed in claim 35 or claim 36, wherein the first humid gas passing through the first desiccant wheel process portion is dehumidified by the first desiccant wheel process portion.

37. The method for dehumidifying a gas as claimed in any one of claims 35 to 37, wherein heated gas is passed from an external environment through the second desiccant wheel regeneration portion to regenerate the second desiccant wheel regeneration portion.