148-Discussion on the upgrade plan using a vaned separation internal component set for the spiral-flow washing dust removal device in the water washing tower of the polyvinyl chloride project’s drying process

In PVC projects, PVC polymer particles are often dried using air flow drying. The PVC particles carried by the air flow are separated and settled using a cyclone separator. The airflow passing through the cyclone still contains some unsettled PVC dust particles, which are sent by the exhaust fan to the water washing tower for washing and sedimentation before being released directly. In many PVC projects, the exhaust gases released from the washing towers result in excessive emissions of PVC dust particles; moreover, the emitted gases contain a significant amount of liquid, leading to a \"raining\" phenomenon, which has led to complaints and orders for corrective action. This is why many PVC manufacturers are carrying out technical upgrades to their washing towers.

The reasons for the excessive emission of PVC dust particles in the exhaust gas from the wash tower of PVC plants, as well as the phenomenon of \"rain-like\" emissions, can be attributed, from the perspective of PVC processing equipment, mainly to improper design and selection of the cyclone separation devices used for pneumatic drying of PVC, as well as inadequate design and selection of the internal components used for separation at the top of the wash tower. Here, an exhaust gas scrubber of a large PVC plant is taken as an example for analysis and discussion.

The example cited in this article is the technical upgrade of the water scrubber tower for drying off-gases in a 400,000 tons per year PVC plant in the northwest. The owner hopes that by upgrading the wash tower technology, it will be possible to ensure that exhaust gases are discharged in compliance with regulations and to prevent any adverse effects caused by these gases, while also capturing and recovering more PVC particles. The owner selected a company’s multi-tube rotary water washing dust removal technology and internal components to carry out a technical upgrade of the water washing tower.

The typical operating parameters for this wash tower provided by the owner are as follows: 1. Wind pressure: 9800 PaG； 2. Operating temperature: 25℃ ； 3. Air volume: 300,000 cubic meters per hour ； 4. Particle size distribution of PVC particles in exhaust gases (laser particle size analysis): 5 microns and below, 0.5% ； Below 10 microns, 1% ； Below 30 microns, 2% ； Below 50 microns, 5% ； The rest, 95%. The owner reported that the diameter of the wash tower is 4.5 meters.

This post was last edited by luoli519 on 2024-4-7 at 10:27. The internal components for the multi-tube rotary water washing dust removal technology provided by a certain company are 8-tube cyclone separators, which are installed inside the water washing tower. The top view of its internal component layout is as follows:

This post was last edited by luoli519 on 2024-4-7 at 10:28. Multi-tube cyclone separation equipment belongs to the category of dynamic separation technology devices, and it is widely used both domestically and internationally. However, the multi-tube cyclone separation devices used domestically and internationally have smaller duct diameters, and they offer good separation efficiency and operational flexibility; they are employed in applications such as high-pressure natural gas transmission systems and heavy oil catalytic units. The smaller the diameter of the air duct, the higher the separation efficiency and operational flexibility. The water washing tower uses cyclone equipment with eight air ducts; these devices are large in size. As can be seen from the attached drawings, the total height of these cyclone ducts exceeds 20 meters, which inevitably results in high costs.

Furthermore, the diameter of the cyclone tube reaches 1600 mm, which is on par with the size category of the standard cyclone separators used earlier. Due to the large size of the cyclone tube, its separation efficiency and operational flexibility are significantly reduced.

The owner reported that the efficiency of this cyclone dust removal system for particles with a diameter greater than 40 μm is 95–99%. Particle size < 5μm, separation efficiency < 5% ； Particle size: 5μm~15μm, separation efficiency: 50~80% ； The particle size ranges from 15μm to 40μm, and the separation efficiency is 80–95%.

This post was last edited by luoli519 on 2020-4-11 at 20:17. Please use the operating parameters of the wash tower provided by the owner, together with the information regarding the above-mentioned eight-tube cyclone separation equipment, to conduct a precise calculation; it will become clear that the separation efficiency of this equipment is indeed low. Once the operating conditions fluctuate, the situation becomes even worse. The separation mechanism of a cyclone separator relies on the fact that, as the fluid phases move in a rotational motion within the duct, the difference in kinetic energy and momentum per unit volume of these phases must fall within a certain threshold range in order to achieve effective separation. The fluid moves in a rotational motion within the ducts, resulting in significant energy loss and often high operating pressure differences. Therefore, effective cyclone separation technologies are often used in high-pressure and ultra-high-pressure applications; even though the fluid undergoes rotational motion within the ducts, resulting in significant energy loss and high operating pressure differences, these factors can be ignored in such high-pressure and ultra-high-pressure conditions. However, at conditions close to normal pressure, the driving force from the fluid pressure difference is not significant (the total pressure here is only 9.8 kPaG), while the proportion of the total pressure accounted for by fluid pressure losses is large. Moreover, with a duct diameter of 1600 mm, the separation efficiency and operational flexibility are not high to begin with. Furthermore, this cyclone separation device is a separate separation unit; once the particulate matter and liquid droplets carried by the airflow escape, it is unable to take any corrective action on its own.

To improve the exhaust gas separation efficiency of the water scrubber tower, the owner requested Nuowei Energy Technology Company to use its patented vane separation technology and internal components; these were installed in series after the rotary water scrubbing and dust removal components to address any particles that escaped detection by those components. The vaned separation technology and internal components utilize fluid dynamics separation techniques to guide fluids through specialized micro-channels within the vaned internal component assembly, thereby enabling efficient conversion of kinetic energy and momentum. This facilitates synergistic effects such as collision and coalescence of droplets, foam, and carried particles, vector separation, and capture of surface free energy, resulting in efficient separation. Moreover, it features a low operating pressure drop, high separation efficiency, and great operational flexibility. It is particularly suitable for separation under medium and low pressure, low pressure, atmospheric pressure, and vacuum conditions. It should be noted that NOVEL’s vane-separated internal component set consists of 4–6 stages of sequentially connected separation units; even if some particles escape from the previous stage, they can be effectively captured by the subsequent separation units, thereby ensuring high operational flexibility and excellent separation efficiency. For example, if 10% of the material escapes from the first separation unit, then the amount that escapes through the second separation unit is 10%*10%. The amount that escapes from the third separation unit is 10%*10%*10%, and the amount that escapes from the fourth separation unit is 10%*10%*10%*10%, and so on.