127- Urgent follow-up: Discussion on upgrading the droplet separator for the EDV process using a specialized vane separator in flue gas wet desulfurization scrubbers

Many refining and chemical enterprises, especially those within the CNPC and Sinopec networks in the southeastern, eastern, northern, and even northeastern coastal regions, are urging their plants to adopt the EDV process package along with its associated droplet separators, in order to remove salt-containing droplets, mist, and dust particles from the flue gas inside the flue gas scrubbers. After it was put into operation, owners reported that the flue gas emitted from its chimneys contained high levels of water and salts, the plumes were long and extensive, and there were recurring issues such as \"rainfall\" of flue gas particles, \"salt precipitation,\" and ice formation on the ground around the equipment in winter. This posed significant environmental challenges as well as safety risks for the operators. The company has decided to upgrade it technically.

The water droplet separator, whose key internal components are the same as those of the curved-cone separator, is a simple single-stage swirl plate-type separator. It consists of a set of vanes arranged in a simple fan shape in parallel, that is, with their heads overlapping while their tails are dispersed at a certain rotational angle — this is the Chevron light panel. As the gas flow passes through the swirl plate internals, it undergoes slight rotation; the heavy-phase dispersed particles carried by the gas flow are displaced at a low centrifugal force, causing them to separate partially from the main gas stream. In such separators, the centrifugal force is less than 0.1g, which is only about one-tenth of the acceleration due to gravity, resulting in low separation efficiency. Such separators can only achieve preliminary separation of slug flows and large liquid droplets carried by the gas flow. This type of separator evolved from structural modifications of the simple Chevron sheet separator around the 1940s.

This post was last edited by luoli519 on 2018-11-19 at 18:59. The water droplet separator is essentially the same as the curved cone separator; it is simply a type of centrifugal plate separator. It’s just that the terms and names vary across different industries and application areas. In coal ore crushing plants, the pre-separator used in front of the bag filter is commonly referred to as a \"cyclone plate\". When used for the primary separation of flue gas in coal-fired thermal power plant boilers, it is called a “cyclone tube”. Used in the flue gas desulfurization systems of forced draft devices; some are called “curved cones” or “hyperbolic cones”” ； In the EDV process package, it is called a “droplet separator,” a name that is quite descriptive; it is used to separate large droplets. It cannot be used for precise mist removal, nor can it address issues such as smoke precipitation, salt deposition, white smoke, or freezing. To be precise, the moisture content of the flue gas after treatment by the EDV droplet separator should be higher, or even significantly higher, than that of the WGS packing demister. Most of the refining companies that use EDV droplet separators are located in regions such as East China, South China, Southeast China, and South China, where the average annual environmental temperature is relatively high and water resources are abundant. The phenomena of smoke from chimneys causing \"rain\", \"salt deposition\", or \"white smoke\" were once considered normal and not given much attention; it is only in the past two years, as environmental awareness has increased, that people have begun to pay attention to these issues. Refining and chemical enterprises located in northern regions, especially those in the northwest and northeast areas, that use EDV droplet separators experience winter temperatures well below zero degrees. At such temperatures, the salt-containing droplets and mist carried by the flue gas accumulate on the ground, rooftops, pipe racks, and instrumentation, forming ice layers that pose a significant risk to the safety of operators as well as to the safety of equipment and instrumentation. Therefore, refineries in the northern region that use EDV droplet separators have an even greater and more urgent need to upgrade the technology of their existing droplet separator systems.

The advantage of the water droplet separator is its good resistance to clogging. Although the gel-like substances or particulate matter carried by the airflow tend to accumulate and even block the areas near the fan shaft or cone tip due to the small space gaps there, the gaps in the more distant areas far from the fan shaft or cone tip are large enough to prevent such blockages.

It should also be noted that this type of swirl plate separator utilizes the swirl plate internals to force the fluid to rotate, thereby generating centrifugal force that enables the separation of the light and heavy phases through relative displacement. At the same radius of rotation, the greater the linear velocity of the airflow, the greater the centrifugal force, resulting in better separation ； However, the higher the linear velocity of the airflow, the greater the friction between the airflow and the swirl plates as well as between the airflow and the walls of the separator, along with the internal dissipation within the airflow. This leads to a greater increase in operating pressure drop, resulting in issues such as poor flue gas discharge and congestion. Therefore, the separation efficiency and operating pressure drop of such separators are contradictory factors that are difficult to reconcile under normal or low-pressure conditions; as a result, these separators are not suitable for treating flue gases from heavy catalytic units or coal-fired boilers in thermal power plants, where normal or low-pressure conditions prevail.

This post was last edited by luoli519 on 2018-11-19 at 18:10. The EDV process is used in the flue gas scrubbing system of the reformer unit; its advantage over another major process for flue gas treatment in such units, namely the WGS process, is that the EDV process provides a higher intensity of spray washing for the flue gas. For example, in the EDV process, as the flue gas enters the scrubber at its inlet, an initial stage of cooling and humidification is achieved through a spray system located at the inlet. Subsequently, the flue gas is further cooled, cleaned of dust, and desulfurized by a four-stage high-intensity spray system situated in the middle and lower parts of the scrubber. In the WGS process, a venturi spray washing tube is installed at the inlet; only when the flue gas enters the scrubber tower and rises to near the packing demister is another spray system installed, and the intensity of this spray system may not meet the requirements of the spray washing specifications.

This post was last edited by luoli519 on 2019-7-2 at 15:21. Please see the image of the flue gas inlet section of the EDV process scrubber tower:

The last edit to this post was made by luoli519 on 2019-7-2 at 15:21. At the cost of high-intensity spray washing and a large flow rate with high recirculation ratio for the spray washing solution, the EDV process includes a filtration module to carry out a sixth round of washing, dust removal, and desulfurization of the flue gas. Please see the figure below:

A set of nozzles at the upper part of the EDV filtration module sprays in a reverse direction along the upward flow of flue gas； The spray washing liquid comes into contact with the rising air flow in the area near the throat of the filter tube, forming counter-directional liquid bubbles with high surface areas, thereby carrying out the washing and desulfurization processes. It should also be added that in the middle of the filtration module, there are dedicated ports for fresh alkaline solution and water replenishment; the standard concentration of the alkaline solution at these replenishment ports is 30%. After such 6 rounds of high-intensity spraying, washing, dust removal, and desulfurization, there is no need to worry about flue gas dust and sulfur levels.

This post was last edited by luoli519 on 2018-11-19 at 18:35. After the aforementioned intensive, almost frantic desulfurization process, the dust and sulfur in the flue gas must have been reduced. Of course, the operating energy consumption and costs are also certainly high. However, with such 6 rounds of nearly frantic spray washing, coupled with the intense mixing of gas and liquid phases in the throat area of the filter tube, the amount of liquid droplets and foam carried by the flue gas, as well as the amount of salts dissolved in those droplets and foam, inevitably increases significantly. It poses severe challenges to the subsequent operation of the water droplet separator. Will a water droplet separator work?