Discussion on the design of a specialized feather-leaf separator for upgrading traditional separation technologies in the ammonia absorption tower used in carbonization off-gas treatment systems for 150-caustic soda, double-alkali, and soda ash production facilities

Dozens of caustic soda, heavy caustic soda, and soda ash production plants are already in operation or under construction in the country, and the exhaust gases from their carbonization towers are usually discharged directly after passing through ammonia purification towers. However, a considerable number of ammonia purification towers discharge exhaust gases that contain significant amounts of liquid, with ammonia levels exceeding the allowed limits. This not only results in high ammonia consumption by the plants but also leads to air pollution from the emitted gases, as well as soil and water contamination due to the liquid droplets in those gases, prompting complaints from residents and intervention by environmental protection authorities.

For enterprises located in the northwest, northeast, and North China regions, the issue of liquid in the exhaust gases discharged from the ammonia purification towers poses a problem; especially in winter, the continuous droplets of liquid in these exhaust gases can cause freezing on the process pipelines and instrumentation lines, leading to an increasing thickness of the ice layer and ultimately causing the pipelines to become overloaded and break. The falling ice pieces also pose a significant risk of injury to pedestrians； The continuous droplets from the exhaust gases form an increasingly thick layer of ice on the ground around the device, posing a significant potential risk to the safety of the operators. Regarding the ice layer that forms on the ground, companies are forced to regularly send personnel to remove it, which is time-consuming and labor-intensive with little effect. However, it is powerless against the ice formed by droplets continuously falling from exhaust gases on the fictional process instrument pipelines.

During the discussion, the client mentioned that for a soda ash plant with a capacity of 200,000 tons per year, it is common to install 4–5 carbonation towers. And for each set of carbonization towers, a set of ammonia purification tower is required to wash the carbonization exhaust gas and the filtered exhaust gas with ammonia. The liquid used in the ammonia scrubbers of different units varies depending on local conditions. Generally, brine is often used for ammonia washing. Some companies use brine as the ammonia washing liquid; naturally, this leads to more scaling of the tower internals.

In most early caustic soda process packages, heavy caustic soda process packages, and soda ash process packages, the net ammonia tower did not usually have an exhaust gas-liquid separator. The reason for this is, firstly, that early projects had low requirements regarding environmental awareness and protection, and even contained some incorrect concepts ； Second, at the time, the designers paid far less attention to the gas-liquid separation of the off-gas from the ammonia scrubber compared to other process units ； Furthermore, the technicians at the time might not have had much knowledge of the gas-liquid separation technology itself.

These issues related to the separation of ammonia-containing gases from the tail exhaust of the ammonia purification towers, which were overlooked by the process package supplier, the design institute, and the project owner at the time, arose during actual operation: when the circulation rate of the washing liquid was low, significant amounts of ammonia escaped into the exhaust gases, making it difficult for the workers to breathe, and ammonia consumption was high； Even when the circulation volume of the washing liquid remains at a normal and stable level, liquid appears prominently in the exhaust gases; especially when operating conditions fluctuate, brine-like liquid falls from the exhaust gases like rain, leaving the installation site in a mess. These issues encountered in actual operation have forced the process package suppliers and design institutes to upgrade the technology used for exhaust gas separation in the ammonia purification tower, while the owner is seeking technical upgrades to this tower with a view to ensuring stable production operations, reducing consumption, and improving environmental performance.

The information we have gathered from the process package suppliers, design institutes, and the owner is that, for the gas-liquid separation internals in the exhaust gas of the ammonia purification tower, they usually simply install a layer of wire mesh demisters across the cross-section of the exhaust gas pipe or the cross-section of the ammonia purification tower. For example, some enterprises have installed an additional wire mesh demister on their existing ammonia purification towers with dimensions of ID3000mm*TL/TL11500mm; other enterprises have done the same on their existing ammonia purification towers with dimensions of ID3600mm*TL/TL33850mm. Some enterprises have several sets of ammonia purification towers, and the projects were undertaken through EPC general contracting by design institutes; the wall thicknesses of some of these towers as well as their foundations do not provide sufficient strength to support the additional internal components, so wire mesh demisters have to be installed in those towers.

For net ammonia towers operating at medium to low gas velocities, and especially at low velocities, the installation of a wire mesh demister certainly results in better separation of the exhaust gases compared to when no gas-liquid separation element is used. However, in a net ammonia tower that is not suitable for medium-speed or medium-to-high gas flow rates, the wire mesh demister has a stronger dispersing effect on liquids in the exhaust gas than a separating effect, resulting in poor performance. Some customers have reported that in ammonia purification towers that did not previously have gas-liquid separation components, the exhaust gases emitted under normal and stable operating conditions contained small droplets of liquid; when operating conditions changed, larger droplets would appear. After installing the wire mesh demister, there is still some liquid mist in the exhaust gases under normal and stable operating conditions; large droplets of liquid appear as well when the operating conditions fluctuate. Moreover, the wire mesh demister tends to scale up, causing the pressure drop to increase rapidly; this results in poor exhaust of pressurized gases, and the wire mesh components need to be replaced every few months.

If there is no significant improvement in the liquid content in the exhaust gases emitted from the pure ammonia tower after installing a mesh demister, the main reasons are as follows: First, the mesh demister selected is not appropriate, resulting in poor demisting efficiency. You can refer to the standards of the originally imported YORK 431 and above when making a choice. II. Improper installation location. The airflow flows from the net ammonia tower to the exhaust pipe, and the flow pattern of the fluid induces a constriction effect. If the placement position of the internal components is not confirmed as accurate through the calculations carried out by the precise dynamic separation technology design platform, then only a portion of the wire mesh demisters arranged across the cross-section of the washing tower will be operating properly, while the rest will be operating abnormally or not at all, resulting in poor separation efficiency. III. The original design for the diameter of the ammonia purification tower was too small; this diameter determines the flow area of the wire mesh demister ； The smaller the flow area of the screen, the higher the gas velocity through it ； The higher the flow velocity, the stronger the dispersing effect of the wire mesh demister on the liquid droplets in the exhaust gas, as compared to its separation effect. Therefore, in this case, no matter what type of wire mesh demister is installed on the cross-section of the net ammonia tower, the results will not be satisfactory. Needless to say, some wire mesh demisters also exhibit an effect when installed across the cross-section of the exhaust pipe.

Here, a specific operating condition of a certain enterprise is used as an example for further analysis: this enterprise has 4 ammonia purification towers for carbonization exhaust gases, and the operating conditions of each of these towers are roughly the same. The total circulation volume of brine is 1300 m^3/h; in other words, each pure ammonia tower has a brine circulation volume of 325 m^3/h. The off-gas treatment capacity of each set of ammonia scrubbers is 31,500 m^3/h. The temperature of the exhaust gas at the top of the net ammonia tower under operating conditions is 42°C, with an operating pressure of 70 kPaG. Each pure ammonia tower is equipped with a 4-m-high ring at its top; the tower’s diameter is 3.6 m, and wire mesh is installed on it. The diameter of the exhaust pipe is 1 m, and its height is 12 m. Under normal operating conditions, the gas-liquid entrainment in the exhaust pipe consists of small liquid droplets. During fluctuating operating conditions, a large amount of brine is carried away with the exhaust gases. The owner hopes that Novae Energy Technology Company will use a vane separator to carry out a technical upgrade for it.