155- In the flue gas denitration, desulfurization, and dust removal project for the catalytic units in refineries, the flue gas desulfurization scrubber utilizes a baffle mist eliminator with an upgraded packing design

Addressing the hazards associated with flue gases from catalytic units in refineries is a problem that most refining companies must face; many of these companies invest considerable effort and resources only to achieve unsatisfactory results in resolving these issues. Among them, the problems that are most frequently encountered include blockages in the flue gas desulfurization scrubbers, high operating pressure drops, unstable flue gas emissions, as well as issues such as flue gas \"water leakage\" and \"dust deposition\".

Catalytic flue gas desulfurization, washing, and dust removal consist of two parts: external Venturi washing for desulfurization and dust removal in the scrubber tower, and washing for desulfurization and dust removal inside the tower, as well as separation. The flue gas enters the ejector venturi horizontally, where it mixes intensely with the circulating liquid that is drawn in from above the venturi at its throat section. This intense mixing facilitates the desulfurization reaction as well as dust removal. In the diffusion section and at the elbow of the venturi, a stratified flow of the flue gas, circulating liquid, and solid particles is established.

The initially stratified flue gas and circulating liquid flow tangentially into the scrubber at its flue gas inlet. Under the effect of the swirl at the inlet, the circulating liquid flows in a plug flow and settles near the walls of the scrubber, falling into the liquid accumulation area at the bottom of the tower, while the gas stream, after being treated by the trays and liquid separation packing, is discharged into the atmosphere through the chimney at the top of the scrubber. Here, the flow diagram of the catalytic flue gas desulfurization scrubber tower from Exxon’s WGS process package is provided for everyone’s analysis and discussion.

For addressing the potential hazards associated with flue gas in catalytic devices, washing for desulfurization and dust removal is carried out at the venturi, the tangential inlet of the scrubber tower, the tray levels, and the packing. Issues such as blockages in the flue gas desulfurization scrubber tower, high operating pressure drops, unstable flue gas emissions, as well as problems related to water droplets in the flue gas and dust deposition, are primarily related to the final filtering mechanism before the flue gas enters the chimney – namely the so-called packing.

After unpacking the WGS process package at the design institute, the main requirement for the \"packing\" used in the scrubber tower is: ① The overall pressure drop across the packing must be ≤ 0.4 kPa. ②The removal efficiency of mist droplets with a particle size of 30 μm or larger in the flue gas emitted from the packing facility shall be no less than 99%, ensuring that there is no noticeable \"rainfall effect\" in the area surrounding the chimney under normal operating conditions. ③The selection of the filler provides good anti-clogging performance for the collected wet dust, enabling continuous operation over a long period of more than three years, with the pressure drop remaining within 100 Pa during this three-year operation period. ④The external dimensions of the packing must meet the requirements for installation within the tower. The packing is supplied in separate units, with each unit being large enough to allow access through the manhole; the specifications for the manhole are 600 mm. ⑤The packing facilities and their supporting components shall be made of S25073 or an equivalent material, and their surfaces must be subjected to acid washing and passivation treatment.

The last edit to this post was made by luoli519 on 2020-5-30 at 18:35. The design and selection of the \"packing\" used in the flue gas desulfurization scrubber of catalytic units are crucial for their operational performance and lifespan. Let’s first talk about the material issue. Since the intake catalyzed flue gas contains sulfur oxides, the material of the equipment at the front end of the system that is in direct contact with the process medium must be capable of withstanding acidic substances; therefore, it is reasonable to use S25073 or an equivalent material or lining for devices and pipelines that come into contact with the flue gas, such as venturi tubes and scrubbers. However, as the material for the \"packing\" located at the very end of the flue gas stream in the scrubber tower, should S25073 or an equivalent material still be chosen? Given that by the time the flue gas reaches the \"packing\" element, the sulfur oxides in the flue gas have already undergone multiple intense neutralization reactions with the caustic solution; the residual acidity, whether in gaseous or liquid form, is at such a low level that it is measured in ppm. Moreover, since the \"packing\" serves only as an internal component rather than a pressure-bearing element of the equipment casing, SS316L is sufficient to meet the requirements, and there is no need to specify that the material of the \"packing\" should be S25073. It has turned out that the previous packing used in wash towers had to be replaced every 1–2 years due to clogging, so there is even less need to require that the packing be made of S25073 or an equivalent material.

The last edit to this post was made by luoli519 on 2020-5-30 at 18:54. Regarding the type of packing used in the flue gas desulfurization scrubber of catalytic units, initially, in accordance with the requirements set out by the design institute after breaking down the process package, the EF-25 series of packing was chosen, as specified by the process package provider such as EXXON; this packing was supplied by an American company. As a result, after operation, the requirement of “① Overall pressure drop of the packing ≯ 0.4 kPa” is not met. ②The removal efficiency of mist droplets with a particle size of 30 μm or larger in the flue gas emitted from the packing facility shall be no less than 99%, ensuring that there is no noticeable \"rainfall effect\" in the area surrounding the chimney under normal operating conditions. ③The selection of the filler provides good anti-clogging performance for the collected wet dust, enabling continuous operation over a long period of more than three years, with the pressure drop during this three-year operation period remaining no more than 100 Pa. Due to blockage causing unstable separation, the actual operating life of the wash tower packing was less than 2 years. During the replacement, the EF-25A improved version provided by a well-known foreign filler supplier was chosen again; although the actual operating performance improved slightly, it still could not achieve a clogging resistance period of 2 years. During replacements, many owners also tried various types of packing promised by domestic universities and domestic packing suppliers to meet the requirements of the process operation; however, the actual performance was even worse than before.

The images of the EF-25 series packing as it was supplied new, as required in the original process package provided by the owner and the design institute, are shown below:

The next two images show the condition of filler blockage, taken when the plant was shut down for replacement less than 2 years after the filler was put into use:

This post was last edited by luoli519 on 2024-4-7 at 10:45. Since the problem of repeated clogging in the packing could not be resolved, some people took the opportunity to suggest to the owner and the design institute that a \"cyclone tube\" mist eliminator, which had been used in a certain power plant, be adopted. Originally, under normal pressure conditions in flue gas, the kinetic energy and momentum of the flue gas are low, so the \"cyclone tube\" could only be used for primary separation with modest requirements; it was barely sufficient to meet the lower emission standards for power plant exhaust gases in earlier years. The clogging problem has been solved, but the issues of excessive flue gas mist and dust removal have reappeared. It still needs to be rebuilt! The image below shows the bulk \"cyclone tubes\" on-site for the catalytic unit scrubber tower, provided by a domestic company to the client. The dynamically separated internal components in bulk at the site must undergo overall inspection and testing after assembly. However, the inspection and testing steps are often omitted due to the lack of comprehensive conditions for such tests on site, which leads to numerous defects in structural performance and potential hidden risks. Given that the performance of swirl tubes makes them suitable only for primary separation rather than fine separation, and considering the numerous structural defects and potential hazards remaining from their previous use in field applications, it became necessary to renovate them.