Is it necessary to install a mist eliminator in the catalytic cracking flue gas desulfurization tower?

I just found out today that one set of mist eliminators for flue gas desulfurization in our factory was removed last year. The reason for demolition was internal corrosion in the desulfurization tower, which required plating work, and the mist eliminator got in the way. When work resumed, the demister was not reinstalled. The demister is made of plastic.

This post was last edited by luoli519 on 2020-5-21 at 15:16. A demister must definitely be installed in the flue gas desulfurization tower for catalytic cracking. Currently, the catalytic cracking flue gas desulfurization process packages used both domestically and internationally are basically those provided by two American companies, and the desulfurization washing and demisting equipment in their process packages is roughly the same. Domestically and internationally, the demisters used for flue gas desulfurization in catalytic cracking processes also generally adopt those provided by the companies recommended in the process packages, with all of them made of 316L material. However, the owners of petrochemical plants complain that the demisting effect is not satisfactory, and the catalytic cracking flue gas desulfurization towers experience severe water leakage; especially in winter, large areas of ground around the chimneys freeze over, making it difficult for workers to approach them for maintenance. The demister in the process package is a type of simple porous packing; its arrangement falls between that of random packing and structured packing, representing a transitional design. This structure works well for enhancing gas-liquid mass transfer in situations where gas and liquid phases need to be mixed, but it leads to backmixing and multiple entrainment events when used for efficient demisting and separation of the gas and liquid phases, thereby being detrimental to gas stream demisting. Professional dynamic separation technology companies, both domestic and international, rarely use fillers of this structure in high-efficiency gas-liquid demisting separators. Since the flue gas contains fine dust particles, alkali solutions will also crystallize within the demister; it is recommended to carry out a technical upgrade of such demisters by using vane-type mist eliminators. As for the material, 316L, as required by the process package, is the best choice. Plastic materials are preferably not used in catalytic cracking flue gas desulfurization units, especially those without temperature control systems; the plastic components inside such units are prone to thermal deformation or even collapse ； Furthermore, the plastic main components along with their flexible rubber sealing accessories are particularly prone to aging and damage, resulting in high costs and significant effort required for operation and maintenance.

Washing towers used for flue gas desulfurization generally contain packing material for separation; the issue can be addressed by improving the efficiency of this separation process, or by controlling the temperature of the flue gas. In most cases, water rises to the top of the chimney before it becomes saturated due to high temperatures, which prevents the separation packing from functioning effectively.

This post was last edited by luoli519 on 2020-5-21 at 15:18. Packing, used in gas-liquid mass transfer processes during distillation separation； Feather-leaf demister, used for the two-phase separation and mist removal of liquid droplets carried in air streams. In the former case, “separation” requires the generation of steam through external heat, which leads to intense mixing and mass transfer between the gas phase and the liquid flow; emphasis is placed on achieving “gas-liquid mixing” ； In the latter case, “separation” does not require the generation of steam through external heat; an attempt is made to avoid mixing of the gas and liquid phases, with emphasis on achieving foam removal through “gas-liquid phase separation”. The presence of liquid in the exhaust gases is more likely to be due to insufficient heating temperature of the gases; as a result, the liquid droplets in the exhaust gases do not vaporize in time and reach the top of the chimney.

For an example of the technical upgrade of the original mist remover in a catalytic cracking flue gas desulfurization tower using vane mist removers, please refer to https://bbs.hcbbs.com/thread-1944683-1-1.html. The 1.5 million tons per year heavy catalytic cracking unit at CNPC Wuhu Petrochemical uses vane mist removers to address the issues associated with the original flue gas desulfurization tower, and it has been operating efficiently for nearly 3 years now.

Practical use has shown that even when 316L is chosen as the material for the demister, it is difficult to ensure long-term operation. Hastelloy is the best choice for this area

Made of plastic, it has a lower cost, but it’s difficult to ensure consistency over a long period of time

Current mist eliminators in the traditional sense include venturi-type mist eliminators, packed-bed mist eliminators, and fan-blade types; in my opinion, none of these are effective for removing mist, and many refineries experience mist issues