The 180-blade separated demister is used for the technical upgrading of the wave plate demister in the flue gas desulfurization tower of the sulfur recovery unit in the butyl octyl alcohol project

A petrochemical company in the Northeast launched a 200,000-ton butanone-octanol production project several years ago; the exhaust gas washing and desulfurization system for its sulfur recovery unit was constructed through EPC services provided by an environmental protection company based in Anhui. The actual performance of this sulfur exhaust gas desulfurization system is very poor; the white smoke emitted from the chimney sinks, and its pungent odor has a significant impact both inside and outside the enterprise. The owner learned that the long-term stable operation of the units after technical upgrades using vane-type demisters to address the issues in the flue gas scrubbing towers of similar facilities at CNPC’s Wushan Petrochemical and Dushanzi Petrochemical plants was very good, and thus contacted us to carry out technical upgrades using the patented technology of vane-type demisters to solve the problems they were facing.

The owners stated that their sulfur plant uses the ammonia-based desulfurization process provided by this company in Anhui for removing sulfur from exhaust gases. During the actual operation in these years, the following issues have occurred: 1. Flue gas emissions were severely above the permitted levels. The SO2, ammonia nitrogen, and particulate matter levels in the chimney exhaust were about 20 times above the limits indicated by the CEMS; in some cases, these values caused the CEMS to exceed its measurement limits. 2. Significant liquid carried in the exhaust smoke; high moisture content, white smoke that sinks downward, and a pungent ammonia odor. 3. After the adjustments made by the Anhui EPC contractor, the pH value of the circulating slurry was reduced to 4.5–5.5, which reduced the ammonia odor in the exhaust gases; however, the desulfurization efficiency decreased, and nothing could be done about it.

The last edit to this post was made by luoli519 on 2021-3-17 at 13:50. Through on-site inspections and discussions with the plant operators regarding the desulfurization equipment, it was also learned that in order to reduce liquid carryover and ammonia odor in the flue gas emitted from the chimney, the environmental protection company and the plant management instructed the operators to reduce the intensity of the slurry circulation spraying, thereby preventing liquid from escaping from the chimney and forming a layer of pale yellow ammonium salt powder on the outer wall of the chimney, the tower structures, and the surrounding ground – something that would cause visual problems. Looking down, the trouble on the ground seemed smaller, but looking up, I saw that new troubles had appeared in the sky. The smoke emitted from the chimney is yellow. The reason for this is that if the pH value of the circulating slurry is kept low, the ammonia content is insufficient, resulting in inadequate absorption and neutralization of acidic gases in the flue gas ； Furthermore, if the spraying intensity of the circulating slurry in the desulfurization tower is low, the efficiency of contact between the circulating slurry and the flue gas decreases even further, resulting in excessive levels of sulfur oxide acidic gases and dust particles in the flue gas. The following diagram shows the layout of the process pipelines in the foundation layer of the desulfurization unit, which we use as a reference for investigating the operating conditions on-site:

Sulfur-containing exhaust gases are generated in the incinerator; after some of the heat from these gases is recovered using a waste heat boiler, they are sent to the desulfurization system via a GGH heat exchanger, at a temperature of around 200°C. Cooling nozzles are installed in the inlet pipeline of the desulfurization tower to reduce temperature. The flue gas enters the desulfurization tower, where 4 layers of circulating slurry spray washing and absorption facilities are installed at intermediate stages. The following figure shows a schematic diagram of the inlet section and the circulating slurry spraying section of this desulfurization tower T-6501:

As can be seen from the diagram of the inlet to tower T-6501 and the circulating slurry spray section, its layout is basically the same as that of conventional desulfurization towers. The hardware of the devices is basically the same; the difference lies in the irrational way they are operated. As is well known, compared with the sodium-alkali desulfurization process, the ammonia desulfurization process lacks in both the speed and efficiency of absorbing and neutralizing acidic gases in flue gas under the same operating conditions. Moreover, lowering the pH value of the circulating slurry and reducing the intensity of spraying and washing with this slurry actually undermines its ability to wash, absorb, and neutralize acidic gases in the flue gas, resulting in excessive SO2 emissions from the chimney. Furthermore, it reduces the intensity of spray washing of flue gas by the circulating slurry, preventing the dust particles carried in the flue gas from being effectively removed, which is the reason for the excessive particle content in the flue gas emitted from the chimney. Process problems can only be solved with the help of equipment technology. Providing the reaction conditions for the desulfurization reaction by using the ammonia concentration and pH value of the circulating slurry as required by the process, along with the necessary spraying intensity of the circulating slurry, is a process-related issue. As for the circulating slurry and the ammonia it dissolves, the loss of these substances along with the flue gas through the chimney is a problem that requires solutions related to gas-liquid separation equipment. The level of tooling refers to the synergy between processes and equipment. Under no circumstances should one violate the principles and mechanisms of chemical reactions in an irrational manner due to issues with inefficient equipment.

The last edit to this post was made by luoli519 on 2021-3-17 at 14:32. What is the root cause of the low efficiency of the gas-liquid separation system, which is responsible for addressing the issues of flue gas leakage and dissolved ammonia escape that occur during the operation of the T-6501 desulfurization tower, the core equipment of the existing desulfurization system? The plant’s technical personnel informed us that the gas-liquid separation internals installed in their desulfurization tower T-6501 are the simple corrugated plate demisters used in the separation units of many small and medium-sized chemical enterprises in China. Indeed, we have seen this type of separation element in separators for various chemical, papermaking, and environmental protection equipment. Please take a look at the schematic diagram of the gas-liquid separation section of desulfurization tower T-6501:

This post was last edited by luoli519 on 2021-3-17 at 14:53. As can be seen from the pictures of the demister installed in the desulfurization tower T-6501: 1. This type of Chevron corrugated plate demister with a simple structure is indeed one that is used by many companies in China. 2. This Chevron corrugated plate demister has a small number of separation units; the numerous escaping liquid droplets lack sufficient facilities and means for capture, and its separation accuracy is not high. It can only be used for preliminary pre-separation and is not suitable for flue gas treatment systems that require strict monitoring using CEMS. 3. In this Chevron corrugated plate demister, the liquid droplets that are pre-separated fall like a downpour into the upward airflow entering the demister, resulting in repeated transfer between gas and liquid. This increases the separation load on the demister, further worsening the already poor separation performance of the Chevron corrugated plate demister in actual operation. 4. The Chevron corrugated plate demister also belongs to the category of primary gas-liquid separators. To develop a complete and accurate technical solution, it is necessary to use a dynamic separation analysis along with a configuration design system platform to accurately calculate parameters such as temperature, pressure, gas phase composition, gas phase density, gas phase viscosity, gas phase eccentricity factor, droplet composition, droplet density, droplet viscosity, and droplet surface tension corresponding to the operating conditions. This approach is different from what the vast majority of environmental protection companies at home and abroad do, where they rely on experience and rough estimates to provide solutions and separation equipment. In wave plate demisters installed in a desulfurization tower such as T-6501, the air flow velocity is very high in the central 1/3 of the tower, while it is very low in the areas near the tower walls. As a result of these aerodynamic conditions, the demisters located near the tower walls cease to function, whereas those situated in the central 1/3 of the tower become overloaded. Therefore, installing demisters throughout the cross-section of the tower inevitably leads to significant differences in the operating conditions and efficiency of the demisters in different areas, resulting in poor overall separation efficiency.

The last edit to this post was made by luoli519 on 2021-3-17 at 15:00. People might ask: since this Chevron corrugated plate demister can only be used as a primary separation element and not in flue gas treatment systems that require strict monitoring via CEMS, what is its actual separation accuracy? In fact, domestic companies also adopt Chevron corrugated plate demisters by following foreign examples. However, foreign companies are unaware of the limitations under which these demisters are actually used, as well as their separation accuracy; as a result, they end up imitating them in a misguided manner. The figure below shows the research results from abroad on the separation accuracy of this type of Chevron corrugated plate demister, for your reference:

As can be seen from this diagram, when the airflow velocity reaches 5–6 m/s, the droplet separation accuracy is approximately 35 microns; the area corresponding to such a velocity range is the central 1/3 portion of the desulfurization tower； When the gas flow velocity is 1 m/s, the droplet separation accuracy is approximately 80 microns. The area corresponding to this flow velocity is the inner wall region of the desulfurization tower; the closer to the inner wall surface, the lower the gas flow velocity, and the droplet separation accuracy drops above 100 microns. With this type of demister, it is inevitable that liquid will be carried in the outlet gas and fall as rain.

As is often seen, the diameter of a scrubber tower is much larger than that of a chimney, and the flow field and flow pattern of the air inside the scrubber tower differ significantly from those in a chimney. When airflow moves from a scrubber tower with a large diameter into a chimney with a small diameter, a transition section is often required. When the flue gas is discharged from the chimney, the presence of liquid in the gas, dripping water, and ammonia-containing liquids increase the ammonia nitrogen content; moreover, its flow field and flow pattern are more similar to those in the chimney rather than those in the scrubber tower. The droplet entrainment resulting from the spray washing of the circulating slurry originates from the upper part of the scrubber tower. Therefore, the transition section required for the gas flow to pass from the large-diameter scrubber tower to the small-diameter chimney becomes a crucial point in terms of the modeling and design constraints for the gas-liquid separator. It is essential that a well-designed separator be installed at this location, so as to remove from the flue gas any ammonia, ammonium salts, or dust particles that are dissolved in it, before the gas reaches the bottom of the chimney, thereby preventing future problems. Of course, the efficient vane-type separation demister designed by professional dynamics separation technology companies must also be installed in specific areas of the tapered section, based on the center of the chimney and in accordance with the aerodynamic flow pattern. Note: Follow the aerodynamic flow pattern with the chimney center as a reference, in specific areas of the tapered cone – not across the entire cross-section as in traditional demisters!