210-CLAUS Claus sulfur recovery unit – Technical proposal for a specialized feather separator for removing liquid sulfur from off-gas – NOVEL TECH

In the CLAUS sulfur recovery unit, its exhaust gas is typically combined with the exhaust gas from the liquid sulfur storage tank for treatment. Especially when the liquid sulfur produced by sulfur recovery units contains a high amount of hydrogen sulfide, posing environmental and safety risks to subsequent storage, transportation, and packaging processes, gas injection degassing is often used to remove hydrogen sulfide from the exhaust gases generated by this process. These exhaust gases include those from the liquid sulfur storage tanks as well as those from the degassing units; the total flow rate of these exhaust gases is high, and they contain large amounts of sulfur in gas, liquid, and solid forms. It is therefore necessary to use efficient vane separators to effectively and stably remove the sulfur contained in these exhaust gases, in order to avoid problems such as sedimentation and blockages in downstream pipelines, difficulties in treating flue gases from combustion, unstable emissions, and high operational and maintenance costs – all of which represent challenges related to environmental protection and safe operation.

However, in the CLAUS sulfur recovery systems used by the vast majority of enterprises both domestically and internationally, owners and design firms still rely on traditional, simple mesh demisters that have been in use since the mid-to-late 20th century as the equipment for removing liquid sulfur from the exhaust gases generated by these systems. In practice, these systems suffer from various problems: frequent clogging to varying degrees, high maintenance costs and labor intensity, low separation efficiency leading to sulfur accumulation and blockages in downstream pipelines and equipment, high costs associated with exhaust gas treatment, unstable emission levels, and numerous risks related to operating conditions and safety. Therefore, both the owners and design institutes are actively seeking to upgrade the existing traditional and simple wire mesh demisters by adopting new technical equipment.

This post was last edited by luoli519 on 2022-10-14 at 11:41. The attached diagram shows the process flow diagram for the exhaust gas separation stage of the Claus sulfur recovery unit, designed by a certain design institute using a traditional and simple gas-liquid separation process for a particular enterprise:

The last edit to this post was made by luoli519 on 2023-3-23 at 13:59. As can be seen from the flowchart above, the design institute indeed uses a conventional, simple wire mesh demister as the main device for separating and removing the liquid sulfur droplets and mist from the exhaust gases coming from the liquid sulfur storage tanks. This wire mesh demister features a half pipe at the inlet where the exhaust gas enters the demister housing, while the main separation element is a conventional wire mesh. Please refer to the equipment structure diagram prepared by the design firm for the wire mesh demister, as shown below:

The plant operators reported that they and the design institute refer to this wire mesh demister as a “liquid sulfur catcher”. In actual operation, this wire-mesh type liquid sulfur trap experiences frequent clogging, resulting in poor exhaust gas flow. Almost every week, it is necessary to switch to a bypass line in order to unclog and clean the wire-mesh demister. Given the high temperature of the equipment itself and the high concentration of hydrogen sulfide in the medium, it is necessary to perform multiple rounds of purging and displacement before maintenance personnel can don bulky protective gear and enter the confined space to clean and replace the clogged screen blocks. This process involves heavy labor, incurs high maintenance costs, and poses significant environmental and safety risks, making it a daunting problem that everyone dreads dealing with.

This sulfur trap is suspended from the concrete floor of the workshop via brackets; due to the limited space, inspection and maintenance operations are rather inconvenient. The attached photo is a picture of the underside of the original trap:

The cleaning and maintenance operations for this catcher are primarily carried out from the top. On the upper side of the trap, both the equipment and pipelines are equipped with steam tracing and insulation layers. Therefore, every time it is necessary to clear a blockage, the steam must be shut off and the flange insulation removed; afterwards, everything must be restored to its original state. Every week or so, it’s necessary to clear the blockage; this has to be done repeatedly, which is extremely time-consuming and laborious, causing great hardship.

We have thoroughly understood the structure of the customer’s existing wire mesh sulfur capture unit as well as its operating procedures. The exhaust gas emitted from the liquid sulfur storage tank typically flows, under conditions of an operating temperature not exceeding 120°C and an operating gauge pressure of 0.03 MPaG, at a flow rate of 4,000–5,000 m³/h through pipes with a diameter of 300 mm, into the existing simple wire mesh capture and separation unit with dimensions of ID800 mm * TL/TL1950 mm. Due to the large fluctuations in parameters such as exhaust gas temperature, pressure, and flow rate during actual operation, the existing mesh-type collectors cannot meet the requirements of these operating conditions.

The reasons why the existing mesh-type liquid sulfur traps fail to meet the requirements under actual fluctuating operating conditions can be analyzed from the perspective of international specifications and practices for gas-liquid separators. It has been found that there are the following serious technical defects: 1. Design flaws in the shell: (1) The shell diameter is too small； (2) The straight edge length of the cylinder is too short ； (3) The steam jacket design is unreasonable ； (4) Defects such as the lack of necessary temperature control and liquid level control settings. II. Design defects of the screen internals: (5) The impact of physical and chemical properties such as the viscosity, apparent density, and surface tension of the gas flow and sulfur liquid phase under operating conditions on the efficiency of gas-liquid separation was not adequately considered in accordance with the requirements of professional dynamics separation techniques, resulting in poor actual operational performance ； (6) The mesh internals have a simple internal structure, consisting of only one theoretical separation unit; they are unable to perform the function of remedially capturing and recovering escaped liquid sulfur under fluctuating operating conditions ； (7) For the mesh internals, there are no separate flow channels for the gas and liquid phases respectively. Under fluctuating operating conditions, this easily causes the gas and liquid phases to shift from a “separated” state to a state of mutual “dispersion”. As a result, phenomena such as gas-liquid “backmixing”, liquid sulfur “entrainment”, and even “flooding and slugging” occur ； (8) Sulfur tends to accumulate in the screen under fluctuating operating conditions, blocking the air flow channels and causing pressure buildup, which results in poor exhaust emission.

Based on the actual operating conditions of the existing wire-mesh sulfur collectors in the sulfur recovery unit provided by the client, NOVEL utilized its precise kinetic separation technology diagnostic platform to determine that: under conditions where the operating temperature does not exceed 120°C, the operating gauge pressure is 0.03 MPaG, the flow rate of the exhaust gas ranges from 4,000 to 5,000 m³/h, the molar composition of the exhaust gas is as follows—61.06% carbon dioxide, 32.49% nitrogen, 4.7% hydrogen, 1.09% carbon monoxide, 0.01% sulfur dioxide, 0.01% hydrogen sulfide, and 0.5–0.8% oxygen—and the amount of entrained liquid sulfur is 1–2 g/m³; even when using the YORK 431 wire-mesh demister imported from the USA—a device widely recognized internationally for its superior separation efficiency—the average size of liquid sulfur particles remaining in the gas stream after treatment was found to be 36.42 micrometers. The pressure drop during normal operation was 18.41 mBar; however, in cases of clogging, this value increased several dozen times over. Consequently, the overall separation performance proved to be unsatisfactory.