166- Design of a feather-leaf separated drip catcher specifically designed for efficient anti-clogging separation in the wet desulfurization towers of coke oven and blast furnace gas treatment systems

In coke oven gas and blast furnace gas plants, gas desulfurization is one of the core units. Since coke oven gas and blast furnace gas are produced under oxygen-free or deficient oxygen conditions, the sulfur in them exists mainly in the form of hydrogen sulfide along with small amounts of carbonyl sulfide. For the removal of the vast majority of sulfur from gas, the classic wet desulfurization process is typically used, while the trace amounts of sulfur that remain are removed by dry methods in subsequent units as appropriate. Since the majority of the sulfur in gas is removed through wet desulfurization towers, it is an issue that requires attention from a technical perspective in industrial equipment design: how to ensure the long-term stable operation of downstream gas-handling devices by properly selecting and designing anti-clogging, high-efficiency droplet catchers for the exhaust gas exiting these towers.

Here, we will conduct a detailed analysis using as an example the design scheme for a specialized vane-type droplet separator designed to prevent clogging in the wet flue gas desulfurization tower of a gas processing plant with a capacity of 50,000 Nm3/h, for a project handled by our team. The following figure shows an excerpt from the document in which the project owner specified the use of feather-leaf type droplet traps:

First, the project overview is listed here: Project Name: Outsourcing project for the desulfurization system of a gas station with a capacity of 5*10^4 m3/h. ★Project Overview 1. The maximum gas processing capacity of this station is 50,000 Nm3/h. 2. Operating temperature: 40℃~45℃. 3. Operating pressure: Atmospheric pressure. 4. The sulfur content in coal is required to be ≤1%; when it enters the gas in the form of 70% hydrogen sulfide, the hydrogen sulfide content in the gas will be 2.1 g/m3 based on the inlet H2S concentration. 5. The hydrogen sulfide content in the gas after desulfurization is required to be ≤100 mg/m3. 6. The wet desulfurization method is adopted for the desulfurization process. ★Design Requirements 1. Improve the purity of the sulfur-containing gas while ensuring the required production capacity. 2. Meet the requirements of long-term operation without tower blockage and minimizing system resistance as much as possible.

The owner opted for the traditional tannin method for the wet flue gas desulfurization technology in this project. The desulfurization agent in the wet desulfurization tower is TM-S, a cobalt-titanium-cyanosulfonate compound. It is presumed that the owner chose this method mainly due to its advantages such as simple operation and low operating costs; however, this also brings technical challenges, including significant liquid and sulfur foam in the gas phase at the top of the desulfurization tower, as well as difficulties in implementing droplet capture techniques.

Tannin desulfurization is the process of removing hydrogen sulfide from gases using an alkaline tannin aqueous solution; it constitutes a binary redox process. Tannin is a polyhydroxy compound with a phenolic structure; it is an excellent oxygen carrier and can also chelate various heavy metal ions. Its desulfurization reaction mechanism is as follows: (1) The reaction in which an alkaline solution absorbs H2S: Na2CO3 + H2S → NaHS + NaHCO3. (2) NaHS reacts with sodium metavanadate to produce sodium pyrovanadate: 2NaHS + 4NaVO3 + H2O → Na2V4O9 + 4NaOH + 2S↓. (3) Na2V4O9 is oxidized to sodium metavanadate: Na2V4O9 + 2 oxidized tannin + 2NaOH + 2H2O → 4NaVO3 + 2 reduced tannin. (4) Oxidation of the reduced tannin: Reduced tannin + O2 → Oxidized tannin + H2O. In addition, a side reaction that produces sodium thiosulfate also occurs during production: 2NaHS + 2O2 → Na2S2O3 + H2O. The sulfur removed from gas using the tannin method floats in the working fluid of the desulfurization tower, especially accumulating at the liquid surface, and is easily carried away by the gas in the form of numerous liquid droplets and bubbles. A viscous sulfur film tends to accumulate on the inner walls of the tower top and on the surfaces of the internal components, even blocking the airflow channels.

Next, let’s take a look at the design conditions for the desulfurization tower that have already been entrusted to external parties by the owner: Desulfurization Tower Design Plan 1: The desulfurization system uses the wet desulfurization method. The desulfurization tower is a packed tower that reduces the hydrogen sulfide content in the gas from 2100 mg/m3 to below 500 mg/m3. 2. The desulfurization tower adopts a packed tower design, with a diameter of Φ5200 mm and a height of 28 m. The internal structure of the packed tower is as follows: (1) The packed tower is equipped with 2 sections of packing, each section having a height of 8 m; the type of packing used is regular packing. (2) The tower is equipped with two sets of liquid distributors. (3) Pressure differential gauge interfaces are provided at the gas inlet and outlet to connect to the control room. (4) Generally, the desulfurization operation temperature is controlled at 40°C–45°C.

Based on the parameters of its desulfurization tower, we have calculated the following process data: 1. The gas velocity in the empty tower is between 0.6 and 0.7 m/s. 2. For the desulfurization pumps, 2 units are to be used with 1 as a backup; each pump has a flow rate of 400 m3/h and a head of 45m. The sulfur capacity of the circulating solution meets the industry standard requirement of 0.15 g/l. 3. The liquid-to-gas ratio and spray density in the desulfurization tower are near the lower limit of the industry standard range of 10–20 L/m3; it is presumed that the desulfurization efficiency is at the upper end of this range, resulting in insufficient flexibility to cope with fluctuating operating conditions.

In terms of the equipment for foam and droplet removal in the gas phase at the top of wet desulfurization towers using the tannin process, traditionally, mesh-type foam and droplet removers, packing-based foam and droplet removers, and simple Chevron baffle plates – which are low-cost but have poor structural performance and efficiency – are often chosen. In this project, the liquid-to-gas ratio of the desulfurization tower, as designed by an external party, is near the lower limit, which results in poor adaptability to varying operating conditions. Whenever the operating conditions change and the liquid-to-gas ratio needs to be increased, it inevitably leads to an even greater amount of liquid phase being carried along with the gas flow at the top of the desulfurization tower ； Furthermore, the sulfur film floating in the working fluid can easily be carried by air currents into the gas-liquid separation element. In light of this situation, the owner felt that the negative impacts resulting from the savings in investment for the desulfurization tower were significant; relying on traditional drip catchers with simple structural features and low costs would cause operational problems. Therefore, it was decided to use the feather-shaped separation drip catcher, a patented technology from Novenergy Technologies, for the desulfurization tower in this project.

Compared to traditional demisting and droplet-catching devices with simple structures and performance, the equipment based on the feather-leaf separation technology boasts the following significant advantages: 1. Thanks to its patented feather-leaf separation technology, it is highly efficient at separating liquid droplets and mist particles with a wide range of sizes carried by air currents; it can achieve a high removal efficiency of 99.99% when it comes to removing droplets and mist particles with sizes of 5–8 microns or larger. Grid-type interception demisters such as screen types, packing demisters, simple Chevron baffle plates, high-voltage electrostatic demisters, and (hyperbolic) swirl tube demisters all suffer from fatal defects in their separation technologies and internal components, which makes it difficult for them to operate effectively for quantitative separation under complex and variable operating conditions; as a result, they are replaced by vane separators, which offer better technical and economic performance, by professional designers and project owners. 2. The patented technology equipment for separating feather leaves offers greater operational flexibility under complex and variable working conditions, with an operational flexibility range of 15% to 130%. In contrast, grid-type interception demisters such as screen types, packed demisters, simple Chevron baffle plates, high-voltage electrostatic demisters, and (hyperbolic) swirl tube demisters have an upper operational flexibility threshold of 110%; excessive loading due to significant fluctuations in operating conditions often leads to problems such as \"liquid flooding\" and \"liquid surging\", as well as the escape of large amounts of liquid droplets, foam, and dust particles, resulting in emissions that significantly exceed regulatory limits. 3. The patented technology equipment for separating vane leaves enables stable operation over an extremely long period with low pressure drop under normal operating conditions, the pressure drop during such operation not exceeding 500 Pa. This is due to the principle of inherent separation of the vanes, as well as their structural advantages, which result in excellent anti-clogging performance, operation at low pressure drops, self-cleaning capabilities through airflow scouring, and the ability to operate stably over extended periods. These advantages are particularly significant in industries such as petrochemicals, coal coking, and oil and gas extraction, where systems are subject to crystallization and blockage caused by dissolved salts and dust particles carried in the airflow. In addition, the vane separator comes standard with an online spray cleaning system, and it also offers an offline, removable vane module design as an optional third layer of protection. Owners can choose to remove the core vane separation module from the scrubber tower during major repairs for external cleaning, thereby ensuring that the vane separator operates for extended periods of time with high efficiency, continuous stability, and extremely low maintenance costs. This is a patented separation technology device that is beyond the reach of other forms of demisters. 4. The patented technology for vane separation enables a smaller size and less floor space for the separator under the same operating conditions and separation efficiency. Under the same operating conditions and separation efficiency requirements, the diameter-to-wall thickness ratio of the vane separator is 30-40% smaller than that of conventional separators. After upgrading old devices with the same diameter using reuse technologies, their processing capacity can be increased by 1/3 or even more, resulting in significant improvements in capacity and output. 5. The patented equipment for feather vane separation is entirely manufactured from SS316L stainless steel; it can operate normally under alternating loads of cold and hot air flows in the range of -180°C to +520°C, and boasts excellent resistance to deformation and damage under abnormal operating conditions. 6. Judging from the operating performance curve of the patented equipment for separating feather leaves, this separator maintains a stable pressure drop and separation efficiency over an extremely long service life; its operating curve is essentially linear, making it an ideal choice that meets the requirements of quality management systems ； In contrast, for other types of demisters, the operating pressure drop and separation efficiency are acceptable at the beginning of operation of new equipment; however, as operation time increases, clogging worsens, the operating pressure drop soars, and the separation efficiency drops significantly, until it becomes necessary to replace the internal components again. Their performance curve is a zigzagged line with many large fluctuations, representing a type of traditional technical equipment that quality management systems aim to avoid as much as possible.

Since our feather-leaf type droplet separator can be easily installed in the space at the top of the desulfurization tower, eliminating the need for an additional housing for such a separator and thus saving on investment, let’s now consider the gas-liquid separation conditions in that area of the desulfurization tower: 1. Operating temperature: 40–45℃； 2. Operating pressure: 0-5kPaG ； 3. Gas flow rate: 50,000 Nm^3/h ； 4. Viscosity in gas phase: 0.020 cp ； 5. Liquid phase density: 984.05 kg/m^3; 6. Liquid phase viscosity: 0.561 cp ； 7. Liquid surface tension: 44.34 dyne/cm.