Discussion on the most popular UV photolysis techn

Discussion on UV photodegradation technology for VOCs

in recent years, UV photodegradation technology has been widely used in the treatment of volatile organic waste gas (VOCs) and deodorization. However, the effect of UV photolysis on VOCs is mixed. Although I am not in this field, I would like to share with you my understanding and experience of UV photolysis technology for your discussion

I. Introduction to UV light

the white sunlight we usually see is actually composed of seven monochromatic lights: red, orange, yellow, green, blue, indigo and purple. The visible light is between nm (some say nm). Ultraviolet light is the general term of the electromagnetic spectrum in which the wavelength is from 10 to 380nm, which cannot be seen by the naked eye

ultraviolet light starts at the short wavelength limit of visible light and overlaps with the long wavelength of X-ray. Ultraviolet light can be divided into four bands: long wave (UVA, 315-380nm), medium wave (UVB, 280-315nm), short wave (UVC, 200-280nm) and vacuum ultraviolet (UVD, 10-200nm). The corresponding sources are called long wave, medium wave, short wave and vacuum ultraviolet light sources respectively

II. UV light source

UV light source is a non lighting light source that produces UV radiation. UV light source has fluorescence effect, biological effect, photochemical effect and photoelectric effect, which is suitable for industry, agriculture, national defense, medical and other fields

UV light sources mainly include UV high-pressure mercury lamp (-- 365nm), low-pressure mercury lamp (-- 254nm and 185nm), etc. Low pressure mercury lamp is excited by low mercury vapor pressure (PA) to emit ultraviolet light, but its luminous spectrum has two main lines: one is 254nm wavelength; The other is 185nm wavelength, both of which are invisible ultraviolet rays

because ultraviolet light cannot pass through ordinary glass, quartz glass must be used, and it is high-purity quartz glass, which requires very high impurity content. If titanium is contained in quartz glass, it can cut off ultraviolet rays below 200nm, but has little effect on ultraviolet transmission at 254nm. Using this principle, the amount of 185nm ultraviolet radiation can be effectively controlled by controlling the amount of titanium added. Because 185nm ultraviolet light can stimulate oxygen in the air to generate ozone (O3), by changing the performance of quartz glass, control the amount of ozone produced to produce three kinds of UV lamps, including low ozone (no ozone), ozone and high ozone

III. photon energy and chemical bond energy

the calculation formula of photon energy is: e (energy) =h (Planck constant) × H (frequency), the calculation formula can be simplified as e=1240/λ （ λ Is the light wavelength, NM), and the photon energy can be calculated according to this simple formula. The shorter the photon wavelength, the higher the energy. 100nm photon energy is 12.4ev; The photon energy of 200nm is 6.2ev; 300nm photon energy is 4.1ev; The photon energy of 400nm is 3.1ev

Table 2 shows the bond energy of common chemical bonds in volatile organic compounds, while increasing the bond energy of oxygen (O2). It can be seen from the table that the bond energy of C-N and C-S bonds is low and easy to break. The bond energy of o=o in oxygen (O2) is 5.16ev. When the photon energy of ultraviolet light is greater than the chemical bond energy, ultraviolet photons can destroy the chemical bond. Due to the low bond energy of C-N and C-S bonds, UV light of any wavelength can dissociate C-N and C-S bonds. The bond energy of o=o bond in O2 is 5.16ev, and the corresponding ultraviolet wavelength is 240nm. Theoretically, ultraviolet light with a wavelength less than 240nm can dissociate oxygen molecules to form oxygen atoms. Oxygen atoms combine with oxygen molecules to produce ozone (O3), which is why 185nm ultraviolet light can produce ozone, while 254nm ultraviolet light does not produce ozone

IV. UV elimination review the force, displacement deformation (that is, extended VOCs and odor

from the previous analysis, when ultraviolet light irradiates VOCs, if the wavelength of ultraviolet light is above 240nm, oxygen cannot be excited to generate ozone, but C-N and C-S bonds with lower bond energy can also be excited to dissociate. However, when the wavelength of ultraviolet light is less than 240nm, not only oxygen can be excited to generate ozone, but also chemical bonds with higher bond energy can be excited and dissociated. If chemical bonds are excited Then the organic molecules become more active, making it easy to further oxidize

at present, UV light is widely used in the field of VOCs and odor elimination. UV light related technologies include UV photolysis and oxidation technology, photocatalysis technology and ozone oxidation technology, as follows:

1 Photocatalysis technology

photocatalysts are substances that can promote chemical reactions without changing themselves under the irradiation of light. Photocatalysts use light energy to convert into the energy required for chemical reactions to produce catalysis, so that the surrounding oxygen and water molecules can be excited into highly oxidizing oh- and o2- free negative ions. It can decompose almost all organic substances harmful to human body and the environment. At present, titanium dioxide (TiO2) photocatalyst has a good effect. TiO2 photocatalyst can only be effective under ultraviolet light, and visible light is ineffective. The key point of photocatalysis technology is to have high-performance photocatalysts. As far as I know, the reaction efficiency (speed) of photocatalysis is relatively low. The self-cleaning of glass is based on the principle of photocatalysis

2. Ozone oxidation technology

because ultraviolet light below 240nm can produce ozone, it is necessary to explain ozone here. Ozone (O3) is a very strong oxidant, which can destroy the floating bacteria in the air in a short time, decompose toxic gases and VOCs, and remove the odor. Therefore, ozone can be used to purify air, drinking water, disinfect, treat industrial waste and as bleach. Ozone can also react with VOCs to oxidize VOCs into non-toxic and harmless CO2 and H2O

photolysis technology

UV photolysis technology, as a popular technology to eliminate VOCs and odor, has many applications, especially in the treatment of low concentration VOCs. On the Internet, we can see that many environmental protection products are advertised under the name of UV photolysis and oxidation. However, from the name "UV photolysis", people feel that ultraviolet light dissociates organic matter and directly destroys VOCs. In practical applications, simple 185 and 254nm UV tubes are used. According to the previous introduction, it is easy for us to think that as long as there is 185nm ultraviolet light, ozone will be generated. Ozone is very oxidizing. It can react with all organic substances and destroy organic molecules. If there is enough ozone, it can eventually oxidize organic substances to carbon dioxide and water. Of course, ultraviolet light can also destroy organic matter, but it is unknown whether these organic debris can react with oxygen. Destroying organics does not mean converting organics into harmless carbon dioxide and water. If only macromolecules are broken into small molecules, VOCs still exist. It is estimated that these organic fragments cannot react with oxygen. If they can react with oxygen, then photocatalysis technology is not required. Therefore, I believe that the so-called UV photolysis (oxidation) technology is actually ozone oxidation technology without the cooperation of photocatalysts

for the deodorization of UV photolysis technology, because odor substances generally contain organic substances of N and s, and the bond energy of C-N bond and C-S bond in organic substances is low, it is easy to be separated from ozone by UV photolysis. As long as the C-N bond and C-S bond in organic substances are destroyed, the odor will be greatly reduced or disappear. This may be the reason why we often hear that UV photolysis has a better effect on odor

if UV photolysis equipment is not equipped with photocatalyst, it is assumed that VOCs are oxidized only by ozone. Taking toluene as an example, assume that the toluene concentration is 10ppm (41mg/m3) and the air volume is 10000m3/h. Generally speaking, there is only one active oxygen [o] in ozone. If the stoichiometric reaction is as follows:

c7h8 (toluene) + 18o3=7co2+h2o+8o2

assuming that ozone is fully utilized, no escape occurs. Then 10ppm toluene requires 180ppm ozone (O3), that is, 385mg/m3

a boss who makes UV light tubes told me a few days ago that 185nm UV light tubes produce 0.1mg of ozone per watt per minute, that is, 6mg of ozone per watt per hour. That is, 1m3/h gas needs to be equipped with a 64W UV light tube (assuming that the ozone is linear). If 1000m3/h is processed, 64KW light pipe is required. Such energy is consumed in the upper part of the fixture base 9, and the lower splint 8 is already very large. If the corona discharge ozone generator is used, the data found in the literature is that 1000g ozone consumes 7500w, which is required for 1m3/h, then 2.9w, and only 2.9kW for 1000m3/h. In other words, if only UV light ozone is used, the efficiency is very low

if the reaction occurs according to the following stoichiometry:

c7h8 (toluene) +6o3=7co2+h2o

then the former needs 21.3kw to process 1000m3/h organic waste gas (even if this is impossible in practice, the latter needs 0.97kw.

another possibility is that if the bonds of organic matter (C-C, C-H bonds) can be interrupted by UV light, whether it can directly react with oxygen to produce CO2 and H2O. Then, if this method is established, the photocatalyst mentioned above is not needed. It can be seen that organic molecules broken by UV are not easy to react with oxygen to produce CO2 and H2O. Even if this process is true, breaking organic molecules also requires energy. Due to the difficulty of calculation, quantitative data cannot be given here

due to the lack of relevant scientific papers in this field, I think more research is needed to improve the application level in this field. In my opinion, to improve the effect of UV photolysis, it is very important to introduce oxygen into (participate in) the reaction, and photocatalyst is needed. If only ozone participates in the reaction, it is very uneconomical to generate ozone with 185nm light. The most economical corona discharge ozone generator should be used to provide ozone. In addition, if we cooperate with other catalysts to activate oxygen molecules, let the activated oxygen molecules react with dissociated organics, get rid of the dependence on ozone, and achieve the elimination of VOCs, it is a good method with high efficiency and low energy consumption