66- Issues related to the accurate selection and design of feather-leaf demisters for foam removal from the gas at the top of water scrubbers and absorption towers

The last edit to this post was made by luoli519 on 2023-10-2 at 14:58. Regarding the selection of designs for demisters in washing towers, they can generally be divided into two categories: The first type of demister has a simple structure and does not feature separate secondary microchannels for liquid removal. Such demisters include screen-type, filter element-type, strainer-type, membrane-screen type, filter media-type ones, as well as baffle-type ones that do not have a separate liquid drainage system. The second type of demister are those equipped with separate secondary microchannels for liquid separation. Foam eliminators of this type are represented by vane separators.

Whether the demister has an independent secondary microchannel structure for liquid descent determines the principle by which gas-liquid separation is achieved, resulting in fundamental differences in separation efficiency and precision. Structure determines performance! Some defoamers, due to their structural design, simply do not allow for the installation of separate secondary microchannels for liquid removal, such as those of the screen type, filter element type, filter mesh type, membrane screen type, or filter media type.

This post was last edited by luoli519 on 2016-12-28 09:29. The importance of the independent liquid-drop descending two-stage microchannel structure for foam separators can be understood through the analysis of the following gas-liquid separation processes. The gas-liquid demisting separation process consists of the following two stages: The first stage is the growth of the liquid droplets carried by the gas flow within the internal components. The liquid droplets carried by the airflow enter the flow channels of the demisting internals, where they collide and coalesce with one another as well as with the surface of the internals material, resulting in the formation of larger liquid droplets. The second stage is the sedimentation and separation of foam and droplets, which is the key control stage. The enlarged droplets of foam break free from the surface tension of the wet surface of the internal component material and fall into the airflow to settle. Due to the upward pull exerted by the airflow on the liquid droplets, which creates an upward velocity component, only those droplets whose size is greater than a certain critical value have a downward gravitational settling velocity component that is strong enough to overcome this upward pull. Only such droplets can achieve complete and thorough gravitational separation before the airflow reaches the outlet of the device! Conversely, for droplets whose size is equal to or less than a certain critical value, the component of their downward settling velocity due to gravity is equal to or less than the upward pulling force exerted by the airflow on these droplets, which creates an upward velocity component. As a result, such droplets cannot achieve complete and thorough gravitational separation before the airflow reaches the outlet of the device.

This post was last edited by luoli519 on 2023-10-2 at 14:59. It features a demister with an independent liquid dropping secondary microchannel structure; the liquid droplets enter this structure directly from the short-distance structure through momentum transformation. In this independent falling liquid two-stage microchannel structure, the droplets and foam no longer experience \"secondary contact\" with the airflow before and after purification; the upward velocity component resulting from the drag exerted by the airflow on these droplets and foam can be completely ignored, as well as any resistance to their downward movement. As a result, the critical size of the droplets and foam that can be successfully separated is as small as just a few tens to a few hundred times that of demisters with simpler structures of the first type. Obviously, the second type of vane separator has a fundamental advantage in separation efficiency and precision over the first type, which lacks a separate secondary microchannel structure for liquid removal.

Furthermore, all first-type demisters belong to the traditional barrier-type demisting internals technology, and little technological advancement has taken place since their use began at the beginning of the last century. It primarily relies on the pores formed by the interlocking and \"bridging\" of the inner component materials to block and intercept liquid droplets and mists within a certain size range, thereby achieving separation. However, the pores formed by the interconnection of the materials are distributed in a Gaussian pattern on a small scale; while small droplets of liquid are trapped and separated by the smaller pore sizes, larger droplets are able to pass through the larger pores and escape. Therefore, traditional cellular barrier-type demisters find it difficult to achieve highly efficient separation of liquid droplets and mists of specified sizes. Furthermore, the flow channels of the first type of demister are prone to being clogged by solid particles and gels carried by air currents, which leads to a rapid decline in separation efficiency, high operating pressure drops, limited operational flexibility. It is necessary to regularly maintain and replace the internal components of the demister, resulting in high costs for operation and maintenance. However, due to past limitations in the understanding of demisting separation technologies, the first type of demisters is widely used across various industries. In situations involving high liquid load in gas streams, unstable operating conditions, or gas streams containing solid particles, gels, and liquid droplets, serious operational and maintenance issues arise.

This post was last edited by luoli519 on 2016-12-28 09:34. Through discussions with professional technicians from design institutes and engineering companies, it was learned that they used to make estimates based on the industry standards for domestic mesh-type demisters. For example, the HG/T21618 standard for wire mesh demisters. In terms of the practical application of this standard, there are the following significant drawbacks: 1. When estimating the actual flow velocity through the wire mesh demister, no account is taken of the large differences in viscosity and surface tension among different liquid phases such as water, oil, alcohols, and ammonia. As a result, the range of correction factors is very wide, and even professional engineers from design firms admit that they are not sure whether the estimated flow velocities and flow areas are reliable. 2. Is the maximum diameter of the internal components in the wire mesh demister 5200 mm? Did the drafters of the standard take into account the flow pattern contraction effect in the gas stream demisting process? The greater the difference between the diameter of the internal components of the mesh defoamer and the diameter of the gas outlet pipe, the more pronounced the gas contraction effect is. The airflow contraction effect causes the components in the edge areas far from the center of the internal component to operate inefficiently or even ineffectively, which in turn results in the flow velocity in the actual operating area exceeding the upper limit significantly, thereby worsening the demisting and separation process. 3. This standard indicates that it can be used for the removal of 3–5 micron foam droplets. As can be understood from the analysis of the aforementioned gas-liquid demisting and separation control process, the gravity sedimentation step is the demisting control mechanism in the wire mesh demister. There are only three laws governing gravitational settling: the Stokes law for laminar flow, the Allen law for transitional flow, and Newton’s law for turbulent flow. Regardless of which law formula is used to input the operating condition data, the critical separation size obtained is well above 3–5 micrometers. Severely misleading. Foreign counterparts have been shaking their heads and expressing disapproval at this standard.

This post was last edited by luoli519 on 2016-12-28 09:36. Early industry professionals in separation technology in China studied foreign data on separation curves and realized that the efficiency of gas-liquid demisting separation is related to the gas flow velocity; they thus established an empirical formula range for the gas flow velocity under gas-liquid separation conditions. However, there is always a significant difference between the actual operating performance of the designed gas-liquid demisting separator and the design requirements.

This post was last edited by luoli519 on 2016-12-28 09:39. Later, domestic engineers found through practical operation that the same demisting device performed fairly well in low-pressure conditions, but its demisting efficiency dropped significantly when the pressure increased substantially. They also proposed an empirical fitting curve for the gas-liquid separation efficiency and fluid momentum, indicating that gas-liquid separation is related not only to the fluid velocity but also to the fluid density.

This post was last edited by luoli519 on 2016-12-28 09:41. In recent years, in China, through the use of various foreign process packages and close cooperation with foreign specialist companies, as well as thorough research on dynamic separation technology models via international platforms, it has been found that gas-liquid separation is actually directly related to energy conversion, that is, it is directly related to the fluid density and the square of the fluid velocity. The unit fluid energy threshold becomes a key constraint for the design of quantitative and efficient separators.