200- Discussion on selection criteria for dust-containing gas washing towers in powder handling systems: high separation efficiency, anti-clogging properties, low pressure drop, and anti-siphoning functions as well as foam removal capabilities
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This post was last edited by luoli519 on 2024-4-8 at 16:33. The use of scrubbers to remove dust from dusty gas streams is a common device and technique in industries such as petroleum refining, coal chemical processing, the chemical industry, metallurgy, and environmental protection. Examples include wet desulfurization scrubbers for the flue gases generated during the regeneration process in catalytic cracking units in refining plants, wet desulfurization scrubbers for the flue gases from sulfur recovery units, wet desulfurization scrubbers for boiler exhaust gases in power stations, water-based dust removal scrubbers for the primary syngas in gasification units, wet desulfurization scrubbers for the exhaust gases from PVC production units, wet desulfurization scrubbers for the flue gases from the roasting of pyrite to produce sulfuric acid, scrubbers for dusty gas streams in VOCS treatment systems, and water-based dust removal scrubbers for dusty exhaust gases in powder processing facilities. For such flue gases from washing towers containing liquid-laden, humid dust, how can an appropriate exhaust gas separation and demisting device be selected from a technical perspective? This technical article takes the selection options for foam separators in water-based dust removal scrubbers used in powder processing workshops to remove liquid-containing, humid dust from exhaust gases as an example for discussion and analysis.
For the exhaust gas from the dust-containing waste gas water scrubbing towers in powder processing plants, this exhaust gas often contains a large amount of liquid droplets and foam, as well as wet dust slurry. Therefore, the selection of separators for removing these impurities from the exhaust gas must be based on principles such as high separation efficiency, resistance to clogging, low operating pressure drop, and the ability to operate at low cost over extended periods of time. Many property owners and even design firms lack experience in the operation of such devices, have insufficient understanding of them, or fail to pay adequate attention during the technical selection and design phase. As a result, environmental pollution issues such as excessive dust particles in the exhaust gases and water leakage from the exhaust gases occur during the actual operation of these scrubber units, and this can even lead to operational safety accidents. Below, an analysis and discussion are presented using the dust-containing waste gas water scrubbing tower in the powder processing workshop of a company in the southwest as an example.
This post was last edited by luoli519 on 2022-3-3 at 10:40. The following image shows a schematic diagram of the process for treating dusty exhaust gas from the powder handling equipment provided by the company. The production and packaging units in the powder processing workshop generate large amounts of dust-containing exhaust gas. First, exhaust fans are used to extract this dust-containing gas from the working units, and it is then sent through pipes to a cyclone separator for preliminary dust removal. The pre-dusted exhaust gas then enters a packed tower, where it is further cleaned and dedusted through water spray washing. After being washed, the gas rises to the upper part of the tower, where it undergoes gas-liquid (solid) separation using the final critical device, an demister, before being released into the atmosphere.
This post was last edited by luoli519 on 2022-3-3 11:21. From the image provided by the owner, only two dust-containing waste gas treatment systems can be seen. In fact, the company has a total of five such conventional treatment units. Each series of blowers has an air handling capacity of 25,000–30,000 cubic meters per hour, with a pressure head of 7,000 Pascals. The diameter of each washing tower in a series is 2,600 mm, and the height of the tower body is 8,900 mm. The water spraying rate for each series is 50–60 tons per hour. The demister located at the upper part of the scrubber uses a two-stage gable-type demister, as those employed in the desulfurization scrubbers of coal-fired boilers in traditional thermal power plants. The owner reported that during actual operation of the scrubber, there were issues with excessive dust concentration at the outlet as well as droplet carryover.
As the company is located in a remote area of this southwestern city, it remains a significant taxpayer there, so the local authorities and environmental protection agencies turn a blind eye. But the company is also located next to the road, with exhaust dust and liquid splashes falling to the ground; local farmers filed complaints with the authorities on multiple occasions, and it was only under pressure that the company was forced to make corrections. As can be seen from the figure below, the scrubber tower is not far from the office building, and the exhaust smoke is yellowish-white; it is obvious at a glance that the emissions are abnormal. Even the ground in front of its company’s office building was soaked with droplets and dust dropped from the emissions, and the equipment and supplies placed on the ground had to be covered to prevent damage.
The attached figure shows the two-stage ridge-type demister that was once used in this company’s scrubber towers – namely, the ridge-type demisters employed in the flue gas desulfurization scrubbers of coal-fired boilers in traditional thermal power plants. These demisters have a simple structure, and it is the structure itself that limits their efficiency in removing mist and dust from flue gas; as a result, they are not suitable for use under conditions of fluctuating operational conditions. Before the year 2000, when environmental emission standards were not stringent or environmental regulations were not strictly enforced, it was still possible to manage somehow; however, the flue gas processed at that time would definitely fail to meet the current strict environmental emission standards when tested manually by third parties.
The owners reported that their dust-containing waste gas scrubbers would eventually need to undergo technical upgrades as required by the environmental protection authorities; they recommended a bag-type dust and oil mist filter provided by a local company, and asked professionals for advice and guidance. When selecting a gas separation and demisting device for exhaust gas from dust-containing waste gas water dust removal scrubbers, technical considerations such as high separation efficiency, resistance to clogging, low operating pressure drop, and cost-effective operation over extended periods must be taken into account. Bag filters fail to meet the requirements in terms of \"resistance to clogging,\" \"low operating pressure drop,\" and \"cost-effective operation over extended periods.\" As for \"high separation efficiency,\" it is necessary to consult the data on the separation precision of their filter media.
The figure below is Technical Data 1 for the bag-type oil mist filter provided by the owner. As can be seen from the diagram, it indeed belongs to the traditional bag filter type. Due to the high operating resistance and pressure drop of the filters, when operating under normal pressure conditions close to atmospheric pressure, it is necessary to apply pressure boost when using these filters; otherwise, it is difficult for air flow to pass through the filtering medium. This problem becomes even more severe over time as moist dust accumulates on the filter bags, causing blockages. Since this bag filter is specified for use with oil mist, that is, in conditions of humid dust, it is equipped with a booster fan of its own, as can be inferred from the mention of \"multiple power configurations\" in its specifications. If considering the use of a forced-air fan, it will not only **increase operating electricity costs**, but the maintenance of the equipment as well as the costs for spare parts represent significant expenses as well.
The performance curve shown in the figure above only indicates the static head or associated pressure loss required to handle a certain air flow rate; however, the performance curve should also include the separation efficiency corresponding to the particle size of the dust, which is not visible here. The performance curve of this bag filter should not deliberately conceal this important information. Low separation accuracy results in low pressure loss ； Conversely, it has high separation precision, but high pressure loss and high power consumption. The problem is that bag filters with low separation precision cannot address the issue of excessive dust levels in the exhaust gases.
This post was last edited by luoli519 on 2022-3-3 12:04. The image below shows Technical Data No. 2 for this bag-type oil mist filter, provided by the owner. According to this data, the maximum processing capacity of such bag-type oil mist filters is only 11,900 standard cubic meters per hour; given that the owner’s requirement for air flow per series is 25,000–30,000 standard cubic meters per hour, at least 2–3 sets of such devices need to be connected in parallel per series. According to the data, Series B also requires that the power unit be shut down for at least 4 hours per day for drainage purposes; therefore, at least one additional unit is needed, resulting in a configuration of \"3 in operation and 1 as backup\". The owner said that the costs associated with setting up the site and operating it are indeed a hassle for them.
The figure below is Information 3 on the bag-type oil mist filter provided by the owner. Information 3 also indicates that the maximum processing capacity of such bag-type oil mist filters is only 10,200 cubic meters per hour. It seems that the textual data doesn’t quite match the curves. Whether the maximum processing capacity per unit is 10,200 cubic meters per hour or 11,900 cubic meters per hour, for an owner with a gas handling capacity of 25,000–30,000 cubic meters per hour, it is still necessary to have a setup of \"3 in operation and 1 as backup\" to ensure reliability. After all, the booster fans are operating equipment, and having a backup unit is essential to prevent disruptions to normal production.