194-Discussion on the technical upgrade plan for using vane-type mist eliminators in the flue gas desulfurization scrubber of catalytic thermal cracking units

Here, I would like to use a practical example from my own work – the renovation of a flue gas desulfurization and washing tower at an existing catalytic thermal cracking plant operated by a petrochemical company – to analyze and discuss it with everyone. The owner’s unit was designed and constructed around 2005 by a design institute affiliated with Sinopec, using a fluidized catalytic process of its own development, for the catalytic thermal cracking of crude oil with high wax content. Due to the **environmental protection authorities’ raising of standards for air pollutants in recent years, owners have been forced to carry out modifications to the flue gas desulfurization and washing towers of their existing catalytic thermal cracking units in order to address potential flue gas-related issues. The owner’s catalytic thermal cracking unit is located in the north, where environmental temperatures are very low; therefore, the WGS process, developed by Exxon and adapted for use in flue gas desulfurization of catalytic units in refining enterprises in northern regions, is employed.

This post was last edited by luoli519 on 2021-9-10 11:31. The catalytic thermal cracking unit (CPPU) built around 2005 for processing high-wax crude oil generates flue gas (on a wet basis) at a flow rate of 88,000 standard cubic meters per hour. Its flue gas desulfurization scrubber operates at near-atmospheric pressure, with an inlet pressure of 4 kPaG; the inner diameter of the scrubber is 4000 mm, and the temperature of the flue gas inside the tower is 50.5°C. The owner hopes that through the renovation of the flue gas desulfurization scrubber tower, its operational flexibility can be increased to 60%–110%, with 8,400 hours of operation per year and continuous operation for 2 years. Based on the owner’s requirements for modifying the flue gas desulfurization scrubber, the required operational flexibility and continuous operation time are quite low, which indicates that the performance of the existing equipment is not satisfactory. Based on the operational performance of similar units in the petroleum refining industry in recent years, the operating flexibility of flue gas desulfurization scrubbers ranges from 30% to 120%, and their continuous operation duration is 3 to 5 years, which corresponds to the maintenance cycle for such units.

The operating conditions of the flue gas within the flue gas desulfurization scrubber provided by the design institute at the owner’s request are as follows:

The last edit to this post was made by luoli519 on 2023-3-23 at 13:26. The main hydraulic data related to the process flow within the flue gas desulfurization scrubber of this device are shown in the table below:

This post was last edited by luoli519 on 2021-9-10 at 11:50. According to the WGS process for flue gas desulfurization in catalytic units, the flue gas coming from these units mixes with the circulating slurry pumped by the slurry circulation pump of the desulfurization scrubber in a venturi mixer outside the tower; this results in thorough mixing of the two substances, forming a dispersed phase with particle sizes of less than 300 microns. Most of the sulfur dioxide and catalyst dust present in the flue gas are captured by the alkaline circulating slurry. The gas-liquid-solid mixed flow enters the flue gas desulfurization scrubber tangentially through the outlet pipe of the Venturi mixer; the large-sized heavy-phase dispersed particles fall into the bottom of the scrubber. The flue gas rises through the trays and then passes through the packing to undergo secondary gas-liquid separation, before being discharged into the atmosphere through the chimney at the top. Please refer to the following WGS process flow diagram:

From the WGS process flow for flue gas desulfurization in the aforementioned catalytic device, we can identify at least the following three aspects that differ from the operation of conventional flue gas desulfurization scrubbers: First, the WGS process uses caustic soda as a reactant for capturing sulfur dioxide; caustic soda not only reacts with sulfur dioxide in the flue gas to form soluble salts but also reacts with the catalyst dust present in the flue gas (silicon dioxide, aluminum oxide) to produce inorganic polymer binders such as metasilicate (a component of cement). Inorganic polymer adhesives, which are mixtures of metasilicate with crystalline salts and solid particles, easily form deposits on the internal components of flue gas desulfurization scrubbers; these deposits are difficult to remove and can easily clog the packing and other internal components of scrubbers designed in conventional ways.

Second, the main gas-liquid separation component in the flue gas desulfurization scrubber designed using the WGS process is the packing facility. The advantage of packing facilities is their large specific surface area, which increases the mixing path and residence time of gas, liquid, and solid. However, for conventional systems, this major advantage becomes a serious drawback in the flue gas desulfurization washing systems used in catalytic devices. That is, the mixture formed by inorganic polymer binders such as metasilicate, crystalline salts, and solid particles tends to accumulate more easily in the packing materials, making it harder to clean them and increasing the risk of clogging. Conventional spray washing is essentially ineffective; it fails to achieve proper cleaning at all. Based on the current operating conditions of the packing facilities in the WGS process, the blockage cycle of the new packing facilities is 1 to 2 years; in some cases, they become blocked even in less than a year, forcing shutdowns to replace the packing completely. This results not only in a low continuous operation rate of the production units and disrupted operations, but also in high maintenance costs.

Here is a photo showing the clogged packing material that was removed from inside the scrubber tower and taken to the ground during the shutdown of a certain catalytic unit’s WGS flue gas desulfurization scrubber tower for packing replacement. This photo is provided so that everyone can gain an idea of the severe operating conditions of the packing system in such catalytic unit scrubber towers, conditions that are far beyond those of ordinary flue gas desulfurization scrubbers.

Here is another image of a larger clogged packing:

The packing used in the aforementioned WGS process is a type specifically designed by EXXON Corporation based on the operating conditions of the flue gas desulfurization scrubbers in catalytic units; it is completely different from conventional structured packings. The packing used in this WGS process has been fundamentally optimized in terms of structure compared to conventional packing, in order to withstand the harsh operating conditions in the flue gas desulfurization scrubbers of catalytic units. However, the actual service life of the packing used in this WGS process still fails to meet the requirements associated with the overhaul cycles of catalytic units in refining companies. It is said that the CNPC group has recently suggested extending the overhaul period for the catalytic units at its various refineries to 5 years.