169-Analysis of the gas-liquid separation and demisting device in the flue gas desulfurization scrubber of Zhuhai Baota Petrochemical’s catalytic cracking unit and sulfur recovery unit

Indeed, over a decade ago, enterprise owners who commissioned flue gas desulfurization systems for heavy oil catalytic cracking units and tail gas desulfurization systems for sulfur recovery units knew very little about the foreign process packages utilized in such systems. Many design firms also took this opportunity to secure EPC contracts for projects. This was supposed to be something convenient for all parties. Unfortunately, there are many designers who fail to approach the matter properly, leading homeowners to choose domestic companies that offer demisters at low costs but lack the necessary technical expertise in terms of gas-liquid separation systems. The designer reaped huge profits from the low-cost demister under this EPC contract, but the owner suffered as a result. The owner was forced to carry out technical renovations time and again, but the designer was no longer the original one, so a replacement was necessary; this was detrimental to both others and oneself, and it serves as a lesson to be learned.

The gas-liquid separation and defoaming unit used for the flue gas emitted from the chimney of the flue gas desulfurization system for the fluid catalytic cracking unit and the tail gas desulfurization scrubbing system for the sulfur recovery unit, as reported in this consultation feedback from Zhuhai Bota Petrochemical, is a newly installed equipment system as part of a technical renovation project carried out in 2018. It is primarily aimed at flue gas desulfurization, denitrification, and dust removal for the 800,000 tons/year catalytic cracking unit, as well as flue gas desulfurization for the 9,000 tons/year sulfur recovery unit; both units share an ammonia-based flue gas desulfurization washing and demisting system along with a flue gas exhaust system. For this technical renovation project of the flue gas desulfurization system, a Shanghai-based environmental protection company is responsible for the EPC. During the operation of the system after the technical renovation, the indicators of the flue gas emitted from the chimney remained unstable; on sunny days, liquid-laden flue gas would fall to the ground, resulting in the formation of white ammonium salt crystals. The gas-liquid separation and demisting equipment for the washing tower in the technical improvement project in 2018 was a demister purchased by that Shanghai environmental protection company from local suppliers in Shanghai that same year.

This post was last edited by luoli519 on 2020-12-19 at 13:15. Please first take a look at the process design data sheet for the desulfurization tower of this project, prepared by a certain environmental protection company in Shanghai that is responsible for this EPC project, in order to understand the origin of the problem:

According to the process design data sheet for the desulfurization tower prepared by the EPC project designer, this flue gas desulfurization process utilizes the ammonia-based desulfurization method. Compared with the liquid alkali method commonly used in catalytic cracking units both domestically and internationally, this approach has lower desulfurization efficiency and accuracy, and ammonia leakage represents a major risk. According to the technical requirements for flue gas entering the tower, the dust content in the flue gas entering the tower is required to be no more than 30 mg/Nm^3. This stringent requirement is often difficult for the flue gas from many catalytic cracking units to meet. It is evident that the designers took advantage of the owner’s lack of understanding and concern regarding these technical parameters to set up a technical trap, thereby embedding \"technical exemption clauses\" in advance to cover cases where the upgraded equipment fails to meet the required standards. Secondly, for a desulfurization tower with a diameter of 4300 mm, what is the average water replenishment rate for the tower, in tons per hour? This parameter seems to indicate a truly very low water consumption. Can it actually achieve the \"ultra-low emissions\" goal it claims in practice, let alone ammonia-based desulfurization? It is recommended that designers familiarize themselves with the international standard criteria for spray washing intensity and density, and should not fabricate an extremely low water consumption figure to deceive the client. Moreover, it’s ridiculous to think that even under such extremely low spray washing intensity, the gas coming out of the gas-liquid separation demister used would still contain liquid. Thirdly, among the requirements for flue gas emissions, the most important indicator for ammonia-based flue gas desulfurization is missing: the ammonia nitrogen level in the flue gas ? ?

The efficiency of the scrubber in removing dust from flue gas, as well as the ability of the gas-liquid separator to remove foam particles, depend to a large extent on the size distribution of the dust in the flue gas and the physicochemical properties of that dust. Let’s take a look at the data on the particle size distribution of dust entering the tower, as provided by the design team:

Based on the data regarding the particle size distribution of dust entering the tower provided by the designer, the particle sizes of flue gas dust are all below 40 micrometers. Furthermore, it is mainly distributed between 1 micron and 20 microns; dust particles with a size of less than 3 microns account for nearly 2/5 of the total. This figure suggests that designers need to use a higher intensity of liquid spraying for wetting and cleaning such small-sized dust particles, rather than reducing the intensity of the liquid spraying. Regardless of whether it may lead to liquid-containing flue gas escaping from the demister, it is necessary first to ensure sufficient wetting for dust suppression and the absorption of SO2 in the flue gas; only after that should consideration be given to selecting a high-efficiency demister specifically designed for flue gas desulfurization, one that offers good operational flexibility and stable separation efficiency. It would be putting the cart before the horse to reduce the water supply to the desulfurization tower and the intensity of the spray washing just because inefficient demisters have already been ordered.

This post was last edited by luoli519 on 2023-3-23 at 12:39. The owner also conducted measurements on the particle size distribution and physicochemical properties of the catalyst dust carried in the flue gas, providing the following data to the design team. Let’s take a look at the data on the particle size distribution and physicochemical properties of the catalyst dust, which is the main component carried by the flue gas, as provided by the owners:

The data on the particle size distribution and physicochemical properties of the catalyst dust primarily carried by the flue gas, provided by the owner, is a very valuable contribution. Many catalytic cracking units at home and abroad lack data on the particle size distribution and physicochemical properties of soot, which can serve as important references. If people compare the two sets of dust data, they will find that there are huge differences in their distribution patterns. Which table is more useful for reference? We believe that both sets of tables are of great reference value; it’s just that the sampling conditions for the two tables are different. The catalyst dust particle size distribution and physicochemical property data provided by the owner refer to catalyst dust that is mostly collected from samples of catalyst dust accumulated in flue deposits. The data on the particle size distribution of dust entering the tower provided by the design party are mostly obtained from samples of catalyst dust carried by the airflow at the inlet of the desulfurization tower.

This post was last edited by luoli519 on 2020-12-19 at 13:16. Let’s take another look at the demister solution proposals provided by the demister manufacturers:

Upon inquiry with the owner, it was learned that the aforementioned demister was purchased and supplied by the EPC party from another demister manufacturer in East China. However, based on the technical solutions provided by the demister manufacturers and the predictions regarding their acceptance of these solutions as well as future purchases by the EPC contractors for such projects, it appears that the EPC contractors do not have a sufficient understanding of the demister technology; they fail to identify and address the technical issues present in the manufacturers’ solutions in a timely manner.