Abstract:
Ultraviolet photocatalytic oxidation (UV-PCO) air purification system includes controller that coordinates operation of photocatalytic reactor that removes volatile organic compounds from air and a regeneration mode that removes contaminants adsorbed in UV-PCO system. Controller coordinates operation of the regeneration mode and photocatalytic reactor so that when air purification system is turned on, the regeneration mode begins to operate before photocatalytic reactor is activated. The initial operation of the regeneration mode allows contaminants that have adsorbed in UV-PCO system to be removed before controller initiates a normal operation mode by activating photocatalytic reactor to cleanse the air.

Description:
BACKGROUND 
       [0001]    This invention relates generally to the use of ultraviolet photocatalytic oxidation (UV-PCO) technology for decontamination of air in air purification systems. More specifically, the present invention relates to a control system and method for coordinating operation of components of the air purifier system when the system is first turned on after a period of inactivity. 
         [0002]    Some buildings utilize air purification systems to remove airborne substances such as benzene, formaldehyde, and other contaminants from the air supply. Some of these purification systems include photocatalytic reactors that utilize a substrate or cartridge containing a photocatalyst. When placed under an appropriate light source, typically a UV light source, the photocatalyst interacts with oxygen and airborne water molecules to form active oxidation species such as hydroxyl radicals. The hydroxyl radicals then attack the contaminants and initiate an oxidation reaction that converts the contaminants into less harmful compounds, such as water and carbon dioxide. It is further believed that the combination of oxygen, water vapor, suitably energetic photons and a photocatalyst also generates an active oxygen agent like hydrogen peroxide. [W. Kubo and T. Tatsuma, Analytical Sciences, Vol. 20, 591-93 (2004)]. 
         [0003]    UV-PCO air purification systems are attractive because they convert volatile organic compounds (VOCs) to harmless compounds. The most common types of VOCs, which are pure hydrocarbons, are converted to water and carbon dioxide by the UV-PCO process. The typical operation of a UV-PCO involves periodic rather than continuous operation of the air purifier. The air purifier may be turned on by a timer, or by a control signal from an HVAC system. Typically, the photocatalytic reactor begins cleansing a flow of contaminated air when the UV-PCO air purifier is turned on. 
       SUMMARY 
       [0004]    The present invention is based upon the recognition that a UV-PCO air purifier may have been turned off for a considerable amount of time before it is turned on. During this off time period, considerable adsorption of volatile organic compounds may occur either on the photocatalyst surface of the reactor, or on an upstream filter. The concentration of volatile organic compounds can be excessively high. If this concentration of volatile organic compounds is present when the UV source of the photocatalytic reactor is turned on, the reaction occurring on the catalyst surface can lead to either incomplete oxidation or to generation of high molecular weight compounds that strongly adsorb on the photocatalyst surface and prevent other species from reaching the photocatalyst. As a result, the photocatalyst can suffer loses in activity, and the air purifier will suffer from reduced effectiveness. 
         [0005]    In one embodiment, operation of the photocatalytic reactor is coordinated with a fan that is used to move air through the air purification system and the UV lamps that irradiate the catalyst. When the air purification system receives a command to begin operation after a period of inactivity, a controller coordinates operation of the fan and the photocatalytic reactor so that the fan operates for a period of time before the photocatalytic reactor is activated. 
         [0006]    In another embodiment, the UV source is turned on under a minimum flow of clean air to quickly remove hydrocarbons such as toluene, xylene and ethylbenzene. In both embodiments, contaminants that adsorbed during the period of inactivity are removed from the system before the photocatalyst begins to convert volatile organic compounds in an airstream to harmless products. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The sole FIGURE is a block diagram schematically illustrating a photocatalytic air purification device. 
       
    
    
     DETAILED DESCRIPTION 
       [0008]    The FIGURE is a schematic diagram of air purifier system  10 , which uses ultraviolet photocatalytic oxidation (UV-PCO) to remove contaminants from air. Air purifier system includes inlet  12 , outlet  14 , prefilter  16 , photocatalytic reactor  18  (which includes substrate  20 , photocatalytic coating  22 , and UV source  24 ), fan  26  and controller  28 . 
         [0009]    Airstream A passes through prefilter  16  and then through photocatalytic reactor  18  and fan  26  to outlet  14 . Prefilter  16  removes dust and particles from airstream A before they reach photocatalytic reactor  18 . Prefilter  16  may contain a carbon filter to remove VOCs such as volatile silicon-containing compounds (VSCCs) from airstream A. 
         [0010]    As airstream A passes through photocatalytic reactor  18 , it comes in contact with photocatalytic coating  22 . In the FIGURE, substrate  20  is illustrated schematically as a flat plate. In practice, substrate  20  can take a number of different forms, which may be configured to maximize surface area on which photocatalytic coating  22  is located (and thus maximize the surface area in which contact between photocatalytic coating  22  and airstream A can take place). One example is a honeycomb structure on which photocatalytic coating  22  is deposited and through which airstream A passes. 
         [0011]    Ultraviolet radiation from UV source  24  is directed for and is absorbed by photocatalyst coating  22 . The UV radiation causes photocatalyst coating  22  to interact with airborne water molecules to produce reactive species such as hydroxyl radicals, hydrogen peroxide, hydrogen peroxide radicals, and super oxide ions. These reactive species interact with VOCs in airstream A to transform VOCs into harmless byproducts such as carbon dioxide and water. Therefore, airstream A contains less contaminants as it exits system  10  through outlet  14  than it contained when it entered system  10  through inlet  12 . 
         [0012]    Controller  28  coordinates the operation of a regeneration mode and photocatalytic reactor  18 . System  10  typically operates intermittently or periodically, rather than on a continuous basis. Controller  28 , which may be, for example, a microprocessor based controller may receive commands from an HVAC system to initiate operation of air purifier system  10 . Alternatively, controller  28  may be programmed to initiate and terminate operation of system  10  based upon a stored operating schedule or upon sensed parameters. 
         [0013]    When system  10  is turned on, either by an external command received by controller  28  or as a result of a determination made by controller  28 , the regeneration mode is operated for a period of time before air is purified or cleaned. Different regeneration modes can be used according to the type of contaminant present. For example, compounds which contain only hydrogen, carbon, and oxygen atoms usually only cause reversible damage. Example compounds include hydrocarbons such as toluene, xylene and ethylbenzene. On the other hand, volatile or semi-volatile organic compounds containing heteroatoms such as silicon, nitrogen, phosphorus and/or sulfur can lead to irreversible deactivation. 
         [0014]    In one embodiment, the regeneration mode uses fan  26  to regenerate a surface in system  10 . Fan  26  causes airflow through system  10  from inlet  12  to outlet  14 . This airflow allows the concentration of VOCs that may have been adsorbed during the off period of system  10  to be moved through system  10  before UV source  24  is turned on and before photocatalyst coating  22  begins conversion of VOCs into harmless products. 
         [0015]    The concentration of VOCs that may have accumulated on pre-filter  16  or on photocatalyst coating  22  during a period of inactivity of system  10  could adversely affect the operation of system  10  if UV source  24  is turned on immediately when system  10  is turned on. A high concentration of VOCs could result in incomplete oxidation or generation of high molecular weight compound that cause photocatalyst  22  to have reduced photocatalytic activity. 
         [0016]    The period of time that fan  26  operates before UV source  24  is turned on can be a programmed time period within controller  28 . A typical amount of time may be, for example, between about 5 minutes and about 10 minutes. 
         [0017]    Controller  28  may include a real-time clock or other timer circuitry to determine how long system  10  was inactive before receiving a command to turn on. If the period of inactivity is relatively short, the time delay between operation of fan  26  and UV source  24  may be reduced, or may be eliminated in some circumstances. The amount of time required for fan  26  to move air through pre-filter  16  and photocatalytic reactor  18  may be controlled, therefore, as a function of the inactive period during which VOCs have been allowed to accumulate within system  10 . 
         [0018]    By not turning on UV source  24  when a high concentration of VOCs is adsorbed on photocatalyst coating  22 , the deactivation of photocatalyst coating  22  is significantly reduced. The time delay between operation of fan  26  and UV source  24  does not significantly affect the overall operation of system  10  in its ability to remove contaminants from ambient air. 
         [0019]    In another embodiment, the regeneration mode uses UV source  24  to regenerate photocatalyst coating  22  when compounds that cause reversible damage to photocatalyst coating  22  are present. For example, when photocatalyst coating  22  has been exposed to high levels of hydrocarbons (HCs), the hydrocarbons occupy some or all of the catalyst&#39;s active sites and are not efficiently and quickly removed from the surface just by purging it with clean air. UV light and a minimum flow of clean air removes these contaminates from photocatalyst coating  22 . 
         [0020]    Examples of high levels of hydrocarbons include when the total hydrocarbon concentration is 100 parts per billion or higher or when a specific hydrocarbon concentration (such as the toluene concentration) is 50 parts per billion or higher. Example hydrocarbons include hydrocarbons that are capable of chemisorbing onto a surface of system  10 , such as toluene, xylene and ethylbenzene. 
         [0021]    The minimum flow of clean air is the incidental or natural air flow through system  10  that is caused by the local heating of the air by UV source  24 , and the clean air may be from the same source as the air which flows through system  10  during the operation of system  10 . After flowing through system  10 , the clean air may be directed to the building air supply or alternatively may be exhausted outside through vents. 
         [0022]    The period of time that UV source  24  operates with a minimum flow of clean air before contaminated air is admitted into reactor  18  can be a programmed time period within controller  28 . A typical amount of time may be, for example, between about 2 to about 8 hours. 
         [0023]    Air purification system  10  may also include hydrocarbon (HC) measurement device  30 , which is located upstream of photocatalytic reactor  18 , such as at inlet  12 . HC measurement device  30  measures the hydrocarbon concentration of airstream A. HC measurement device  30  may measure the total hydrocarbon concentration of airstream A, a specific hydrocarbon concentration, such as the toluene concentration, of airstream A, or any combination thereof. HC measuring device  30  may use gravimetric, thermal, resistive, electronic, magnetic, photolytic, optical or related sensing strategies, or any combination thereof, as the means of measuring the hydrocarbon concentration. HC measurement device  30  may send signal S, which represents the measured hydrocarbon concentration, to controller  28  to determine the appropriate operation procedure for system  10  after a prolonged period of inactivity. For example, the controller may only initiate a regeneration mode when a total hydrocarbon concentration of 100 parts per billion or higher is measured by HC measurement device  30 , or when a specific hydrocarbon concentration, such as a toluene concentration, of 50 parts per billion or higher is measured by HC measurement device  30 . 
         [0024]    The regeneration mode used is based upon the environment in which the system is installed. Some environments may require using a plurality of regeneration modes. For example, in one system a regeneration mode performed by running a fan to remove VOCs is performed everyday while a regeneration mode performed by using a UV light source to remove hydrocarbons is performed on weekends. In another example, two different regeneration modes are performed in series in a system, such as regenerating a surface by removing VOCs followed by regenerating the same or different surface in the system by removing hydrocarbons. 
         [0025]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.