Let’s consider a 2D area for a building with distant N rooms. We would like to monitor fluctuations in concentration of PM as carbon dioxide in each room. One solution is to sustain each room with a gas sensor to get the concentration. However if we want high accuracy we would need very expensive sensors. Our solution for this is to use mobile sensors to move across the building in effective time t such that the number of sensors is inversely proportional to t, meaning if we want to scan the whole building. Thus if the sensor detects any fluctuation on the levels of PM from the normal level, The purification system would be initiated using a PID control system.
It is very important in the current situation to maximize the indoor air quality with almost all the population staying and possibly working from home. It has been found out that the risk of exposure to indoor air pollutants can surpass that of the outdoor, for it contains up to 900 potentially hazardous chemicals and harmful biological materials. The decline of indoor air quality can be mainly attributed to the current drive towards air tightness and the increased occupancy period. This can gradually accumulate invisible pollutants that affect the occupant’s physical and mental health. A number of health conditions have been linked to the deterioration of indoor air quality ranging from cardiovascular diseases, asthma, allergy, and even lung cancer for those with preexisting unstable inclination. In 2012 poor air quality led to the death of nearly 100,000 Europeans as reported by WHO. This could only get gravely worse under such circumstances. The dominant types of indoor air pollution are volatile organic compounds, nitrogen oxides, CO, CO2, and molds. Prompted by these facts, it is essential to adopt new measures and methods in an attempt to improve the indoor air quality, which would positively reflect on the health and wellbeing of the worldwide population.
Our inspiration to tackle this environmental issue was primarily derived by an air monitoring project (ANITA) developed by the European Space Agency for the International Space Station. Air quality control is of crucial importance in space-crafts, especially for prolonged journeys, but it is also of equal importance to people on Earth to maintain a healthy level of air for proper cognitive functioning. ANITA is a monitoring device that mainly relies on the Fourier Transform Infrared interferometer to keep track of air contaminants supported by an advanced software calibrated on the ground before launch. Due to this technique’s considerable accuracy in detecting molecules of complex structure and its slow running time, we consider it to be a very prospective candidate for indoor air monitoring.
Although a typical human nose has 400 types of scent receptors enabling us to smell about 1 trillion different odors, many of us do not have the capacity to identify the type or concentration of gas present in our atmosphere. This is where Sensors’ role comes in, there are many types of Gas sensors that are used to detect the variation in the concentration of toxic gases in order to maintain the surroundings safe and avoid any unexpected threats. These gas sensors are essential to detect gases like oxygen, Carbon Dioxide, Nitrogen, methane. They also can be used to monitor the air quality in homes, factories, and offices.
There are many types of Gas sensors but the MQ type gas sensors are commonly used and widely popular, so we are proposing this type to be used in our Air Monitoring System. Gas sensors produce a corresponding potential difference based on the concentration of the gas. The potential difference can be measured and then mapped to real gas concentration value. Gas sensors are typically classified into various types based on the type of the sensing element it is built with.
ANITA was our inspiration to design an Air Monitoring System to detect any harmful gases and then the sensors’ data will be sent to the Purification System. We will use Gas sensors, Arduino board, buzzer, LCD monitor, and LEDs. Gas sensors that will be used in our solution are MQ-2 Sensor (to detect Methane, Butane, LPG, Smoke), MQ-3 Sensor (to detect Alcohol, Ethanol, Smoke), MQ-8 Sensor (to detect Hydrogen Gas), MQ-9 Sensor (to detect Carbon Monoxide, flammable gasses), MQ135 Sensor (to detect Air Quality), MQ138 Sensor (to detect Benzene, Toluene, Alcohol, Propane, Formaldehyde gas), and MQ216 Sensor (to detect Natural gas, Coal Gas). Buzzer, LCD, and LEDs will be used as an indication when a harmful gas is detected. Arduino board will be the controller for this Air Monitoring System.
2. Fourier Transform Infrared spectroscopy
The Fourier Transform Infrared (FTIR) spectroscopy utilizes infrared radiation to determine the structure of the molecules according to their unique absorption behavior of said radiation. The basic idea is that if you direct a beam of infrared radiation, that is invisible to the human eye, of a certain frequency range onto a sample of a molecular compound, organic or inorganic, some frequency bands are absorbed by the compound and some pass through to a detector placed at the other end, and according to the transmittance percentage of specific frequencies, useful information about the molecular bonds is obtained in the form of an infrared spectrum. An infrared spectrum can be analyzed to tell what air molecules are present in the sample. The chemical bonds in a molecule vibrate in a certain way when exposed to infrared radiation causing the bonds to stretch, bend, or contract. These vibrations, however complex, can be broken down to separate constituents, normal modes, very similar to the frequency mode of a stretched string. Different molecules, like guitar strings, vibrate at different frequencies based on their internal structure. A thorough analysis of the absorbed frequency can then be used to distinguish molecules by the infrared spectroscopy technique. Most targeted bonds are covalent bonds, which fortunately facilitates the process. In covalent bonds, atoms are not solidly linked together; they are both attracted to a pair of electrons, which causes both nuclei to vibrate back and forth around an equilibrium position. This vibration is unique for each bond, for it entails specific energy that is dependent on the bond length and atomic mass at both ends. Therefore, absorbing the right amount of energy from the radiation moves the bond to a higher vibration state, and this amount of energy is unique for each bond, which corresponds to the absorbed frequency of infrared radiation. By analyzing the produced infrared spectrum, abundant information is inferred about the molecule.
Infrared spectroscopy works on all kinds of matter, solids, liquids, gases. FTIR spectrometer is considered particularly prominent and useful for our application due to several advantages, among which are: reduced noise and higher signal components than previous generation spectroscopy, very high accuracy in wavenumber, shorter scan time, higher resolution and a very wide range that allows us to scan a variety of molecules.
The basic components of any dispersive infrared spectrometer consist of a radiation source, which includes inert solids that are electrically heated to radiate thermal emissions in the infrared region, a monochromator which is used to separate a radiation beam into separate frequency bands and a detector. Generally, two beams are generated, one goes through the sample and the other reference champers for analytical purposes. After the incident beam travels through the sample, the wavefront beam reaches the detector, which responds by converting the analog spectral and producing electrical signal impulses. This signal is to be finally processed to produce the desired infrared spectrum. What makes this traditional process relatively slow is that it has to process one frequency at a time. FTIR has superior features to perform this task, producing enhanced output in considerably less amount of time. The FTIR spectrometer consists of a source, interferometer, sample compartment, detector, amplifier, and Analog/Digital convertor. After the source emits radiation that goes through the sample, it passes by the interferometer to the detector. The amplifier amplifies the signal, which is then converted to its digital components and finally reaches a computer that applies the Fourier transform. A key difference between the two discussed methods is the use of Michelson interferometer in FTIR. The Michelson interferometer is composed of two perpendicular mirrors, one is fixed, and the other is moveable, and a beamsplitter. The beamsplitter’s function is that to reflect half the light rays, which goes to the moveable mirror and refract (transmit) the other half, which goes to the fixed one. Both beams can then combine again, forming one light beam. The characteristics of the new light beam depend on the distance between both mirrors and the splitter. If the reflected light that hits the moveable mirror moves an extra distance of wavelength multiples and then meets back with the other half, constructive interference occurs (wave crests meet each other), and a maximum intensity is detected by the detector. If, on the other hand, the extra distance is a multiple of half wavelength, destructive interference occurs (crests meet troughs), and minimum intensity is detected. If the distance is anything else between these two extreme situations, the interference is a combination of constructive and destructive. If the mirror is designed to move back and forth, a sinusoidal plot is generated by the detector, which is known as the interferogram, which is a function of time. Here comes the use of Fourier transform, which transforms the interferogram into the frequency domain where you can finally get the infrared spectrum.
An FTIR spectrometer can identify basic background gases CO, CO2, and methane in addition to 28 organic and inorganic pollutants. It can weigh down to 10 kgs, which accounts for its portability and convenient usage in indoor activities. Furthermore, it can be supplied power from a battery and functions well in a wide temperature range. It has an excellent maintenance record as it only begins to lose optical alignment after three years of operating.
3. Purification systems
The current commercial purification systems use either activated carbons, Hepa , Ionizer or UV as a membrane to purify air. Despite that both HEPA and the activated carbons filter have a good particulate matter removal capability, they have a poor biocidal effect. On the other hand, UV filters are bactericidal however, there was not any scientific evidence that UV radiation will not cause any side effects. As a result, we thought of using metal organic frameworks as a filter layer in our purification system.
MOF is characterized by having a nano-porous layer and high selective permeability. There are different types of MOF each one has its own selectivity to certain types of molecules. Selectivity of certain MOFs could be enhanced or modified by modifying the connecting organic ligand for different applications.
For air purification purposes we would need a MOF membrane that cannot disintegrate in presence of high humidity and hydration. Mostly there are 5 types of MOFs that have this property in its periodic lattice structure without the need of post synthesis modifications. As reported by Li 2019, ZIF-8 is considered as the best candidate for air purification. We would like to implement this MOF in forming a purification membrane to be used in our system. This ZIF-8 MOF was found to be suitable due to the following outstanding results reported by the researchers,
https://drive.google.com/file/d/1gcqQ09LPJBQnUjo4M-XMGI9OoXydNGjg/view?usp=sharing
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