147-Upgrade design scheme for the wire mesh liquid separation tank at the inlet of the synthesis main cycle compressor in coal-to-oil projects, utilizing vaned separation internals

For domestic coal-to-oil projects, the engineering design is often carried out by companies that possess their own process packages for coal-to-oil projects. Such coal-to-oil process packages and their designers undoubtedly have a comprehensive understanding of the technical implementation in various aspects of the process package. However, perhaps due to EPC considerations for the project, constraints on the owner’s investment budget, or tight project schedules, it has not been possible to update some of the traditional and inefficient technical equipment used in the original process packages. For example, the issue with the liquid separation tank at the inlet of the main cycle compressor in the oil synthesis unit of a coal-to-oil project discussed in this article warrants reflection and technical upgrades on the part of the owners, designers, and industry professionals involved in coal-to-oil projects.

To ensure that the technical analysis presented in this article is truly focused on practical technical aspects, it takes as an example the problems existing in the original wire mesh liquid separation tank D-6113 at the inlet of the main cycle compressor in the oil synthesis unit of a well-known domestic coal-to-oil project owner, who sought technical upgrade solutions from Novae Energy Technology Company. The design for upgrading this tank using vane separation technology is used as a case study to serve as a starting point for further discussion.

The original wire mesh liquid separation tank D-6113 was located at the inlet of the main cycle compressor in this oil synthesis plant; it was a wire mesh separation tank designed based on the original design scheme from a coal-to-oil process package utilized by a certain engineering company. The plant operator reported that in the main circulation compressor of the synthesis unit at this oil product facility, the original mesh liquid separation tank D-6113 led to gas-liquid entrainment during operation due to the high velocity of the circulating gas. As a result, the compressor operated with liquid present, causing frequent damage to its driving components and shortening its operational lifespan, which severely affected the safe and stable operation of the unit.

This post was last edited by luoli519 on 2020-4-9 at 11:14. The actual operating conditions of the wire mesh liquid separation tank D-6113 at the inlet of the main circulation compressor in this oil production facility are as follows: 1. Medium: circulating gas; the components of this gaseous medium are listed in the attached “Gas Data Analysis Table for the Liquid Separation Tank at the Inlet of the Main Circulation Compressor”. 2. Flow rate: 35*10^4 Nm^3/h. 3. Operating temperature: 35°C. 4. Operating pressure: 2.88 MPa (gauge pressure). 5. Condensate components separated: The condensate at the bottom of tank D-6113 consists mainly of water, with a density of 0.9962 g/cm3； The main component of the gas entering the compressor cylinder is oil, with a density of 0.7353 g/cm3, a viscosity of 1.167 mm^2/s at 20°C, and a viscosity of 0.9430 mm^2/s at 40°C.

The schematic diagram of the original wire mesh liquid separation tank D-6113 at the inlet of the main cycle compressor in this oil synthesis unit is as follows:

As everyone can see, this set of liquid separation tanks is equipped with a wire mesh demister. However, the internals of its wire mesh demister are based on a very simple traditional separation technology; even when standard YORK 431 wire mesh demister internals imported from the United States are used, the separation mechanism relies on the interconnection of fibers to form **small-sized “pores” that are distributed in a Gaussian pattern, which capture liquid droplets and foam. Smaller droplets can be trapped by these small pores, while larger droplets manage to pass through the larger pores and escape. Furthermore, when wire mesh demisters are designed and manufactured in full compliance with national or industry standards, their operating flexibility is usually between 60% and 110%. Some clients have reported that their wire mesh separation tanks experience \"fluid flooding\" or \"surge\" even at lower loads, resulting in a significant drop in separation efficiency. The wire mesh demister designed for the screen liquid separation tank D-6113 at the inlet of the main cycle compressor of this oil product synthesis unit has the following problems: 1. The flow area of the wire mesh demister is too small; it is arranged on a circular surface with a diameter of 1600 mm, occupying less than 1/3 of the cross-sectional area of the separation tank. 2. Not only is the flow area of the wire mesh demister very small, resulting in a high gas velocity, but the thickness of the wire mesh demister is also only ~150 mm. 3. The distance between the upper surface of the screen defoamer and the air outlet at the top of the liquid separation tank is only 862 mm, and the diameter of the air outlet pipe at the top of the liquid separation tank is 600 mm. Due to the excessive short distance between the upper surface of the wire mesh demister and the airflow outlet at the top of the liquid separation tank, the contraction effect of the flow pattern as the airflow reaches the outlet pipe causes a portion of the flow-through surface of the wire mesh demister to operate abnormally or even not at all. As a result, the actual flow-through area of the wire mesh demister, which already has a small designed flow-through area, becomes even smaller, leading to poor operational performance.

This post was last edited by luoli519 on 2020-4-9 at 12:00. Please analyze again the design of the air inlet pipe for this liquid separation tank: 1. The air inlet pipe of this liquid separation tank uses a tangential swirl design. However, the diameter of the corresponding separation tank reaches 2800 mm; to enable the tangential swirl inlet pipe associated with such a large-diameter separation tank to function effectively, a high airflow velocity is required. Otherwise, the centrifugal acceleration generated is too low, and the desired effect cannot be achieved. 2. The distance between the top of the swirl tube and the bottom of the wire mesh demister is too small, being only 0.4375 times the diameter of the separation tank; as a result, the tangential swirl does not develop sufficiently, and the centrifugal separation effect is not significant. 3. The flow pattern formed by the swirl tube conflicts with the flow pattern required for the proper operation of the wire mesh demister. The airflow generated by the swirl tube is a rotating wall flow with a highly uneven spatial distribution; furthermore, its development is insufficient due to the excessive short distance between the top of the swirl tube and the bottom of the wire mesh demister ； The ideal flow pattern required for the proper operation of a mesh demister is a uniform plug flow. The flow pattern formed by the swirl tube conflicts with the flow pattern required for the proper operation of the wire mesh demister, further deteriorating its actual performance.

Let’s analyze together again the impact of the tangential swirl inlet pipe on the smoothness of liquid discharge: 1. Since the distance between the bottom of the swirl inlet pipe and the high liquid level is less than 2/5 of the diameter of the separation tank, the underdeveloped rotational wall flow inevitably creates a relatively strong negative pressure field above the liquid surface along the centerline of the separation tank, resulting in difficulties in liquid discharge. The negative pressure field above the liquid surface causes poor drainage, which is a common issue in other cyclone separators. 2. To reduce the strong negative pressure field generated by the underdeveloped rotating wall flow at the liquid level on the centerline of the separation tank, which leads to difficulties in draining the liquid, the designer installed an anti-rotation screen plate with a diameter of 2800 mm between the tangential inlet pipe for rotational flow and the liquid level in that separation tank. Such a porous sieve plate not only increases equipment investment, but its ability to prevent swirling and facilitate smooth liquid discharge actually depends on the opposing effects of the negative pressure field created by the airflow velocity and the liquid level in the liquid storage area.

I believe that through the above brief analysis and discussion of the design issues related to mesh liquid separation tanks, everyone should now realize that gas-liquid separation technologies and equipment, even the simplest ones such as mesh liquid separation tanks, fall under the category of dynamic separation technologies. Their design cannot be handled carelessly using arbitrary guesses or unprofessional approaches based on hunches alone. Attention must be paid to the many dynamic separation design details mentioned in the above analysis. Otherwise, the owner will definitely provide feedback on the actual performance of the liquid separation tank, and go to the extra effort and cost of carrying out further technical upgrades.

The owner has requested that Novai Energy Technology Company develop a technical upgrade and modification plan that meets the following requirements: 1. After separation, 100% of liquid droplets with a size of 8 microns or larger must be removed, and the total amount of liquid droplets carried in the gas after separation shall not exceed 13.4 liters per 100×104 Nm3； 2. All internal components of the liquid separation tank at the inlet of the main cycle compressor have been replaced with high-efficiency gas-liquid separation components ； 3. Redesign the high-efficiency gas-liquid separator while ensuring that the basic dimensions of the liquid separation tank at the inlet of the original main cycle compressor remain unchanged ； 4. The material required for the high-efficiency gas-liquid separation internals must be 304 (the material for the pre-welded components of these internals is also 304), or a stainless steel material that is at least as good as 304.