158-Design scheme for a specialized vane-type separation and demisting device for scrubbers used in alkylation units, nitration units, and other facilities to handle high-temperature exhaust gases containing dilute strong acids

In petroleum refining plants, in units such as alkylation plants and polyamide plants where strong acids or mixed strong acids are involved in the reactions, the exhaust gases emitted from these reaction units often have temperatures ranging from 100–200°C or even higher. These exhaust gases contain water generated through dehydroxylation, highly corrosive sulfur oxides and nitrogen oxides, as well as some particulate matter resulting from the reactions. Currently, to treat such high-temperature and highly corrosive exhaust gases, dilute strong acids are used in high-intensity cyclic spray washing to absorb the highly corrosive gases, acidic droplets and aerosols, as well as reactive particulates present in the exhaust gases, followed by efficient mist removal to ensure that the exhaust meets regulatory standards before being released.

In petroleum refining plants, there are often sulfur recovery units and acid production units that oxidize the recovered sulfur to produce sulfuric acid. Both sulfur recovery units and acid production units generate medium-to-high temperature exhaust gases with strong corrosivity. These gases are treated by absorption through strong acid spray washing, a method similar to that used for treating the exhaust gases from alkylation units as well as nitration and oxime formation units.

Other units that require similar treatment for their exhaust gas conditions to those of sulfur recovery units, acid production units, alkylation units, and nitration-oximation units include, in the petrochemical industry, units for producing nitric acid, hydrofluoric acid, hydrochloric acid, and phosphoric acid, as well as units for producing acetic acid and its anhydrides. In the fine chemical processing industry, there are units for sulfonation and hydrogen chloride generation in PVC production. In the textile dyeing and finishing industry, there are direct-fired drying units for fabric setting, all of which have similar exhaust gas conditions and treatment methods.

This post was last edited by luoli519 on 2020-7-16 at 10:20. The problem of mist removal from the exhaust gases in these circulating washing and absorption towers, which are exposed to high temperatures as well as highly corrosive gases and acid mist, has always been a source of concern for plant operators and corporate technical managers. The main problems are as follows: 1. The exhaust emission indicators are unstable; either sulfur oxides and nitrogen oxides exceed the allowed levels, or the emitted gases cause acid rain to corrode the pipelines, equipment in the vicinity, as well as the surrounding area, turning it into a \"desolate rust-colored landscape\". 2. The demister has a short service life due to corrosion or becomes clogged, resulting in unstable operation of the foam and mist removal processes; this is the main cause of acidic rain and acid mist in the exhaust gases. 3. According to the feedback from the plant staff, when the spraying intensity in the dilute acid scrubber is high, it is possible to effectively control the levels of sulfur oxides and nitrogen oxides in the exhaust gases; however, the poor processing capacity of the demister results in precipitation appearing in the emitted exhaust gases ； By reducing the spraying density of the circulating acid solution in the scrubber tower, it is possible to prevent rain in the emitted exhaust gas; however, alarms are triggered due to excessive levels of sulfur oxides and nitrogen oxides in the exhaust gas. The operators are in a lot of trouble. 4. The enterprise also modified the demister, but after the modification, there was a high operating pressure drop, pressure buildup in the exhaust gases, poor emission flow, accumulation of acidic mist in the exhaust gases, and even backflow and reverse contamination.

For the demister in the absorption process unit of dilute acid scrubbers, used to handle medium-to-high temperature flue gases containing highly corrosive gas-liquid mists, the primary concern is the safety of the equipment with regard to corrosion caused by such medium-to-high temperature, highly acidic gas mists. In terms of material, the main types of materials used for mist eliminators at present are metallic materials and non-metallic materials. In highly acidic media at temperatures above 100°C, it is very difficult for almost all types of metal materials to withstand their corrosive effects over an extended period of time. Considering long-term safe operation, choosing demisters made of non-metallic materials will be a beneficial approach for the present and future.

This post was last edited by luoli519 on 2020-7-16 at 14:08. Based on the types of mist eliminators currently in use to handle gases resistant to high temperatures and strong corrosion, the main types include mesh separation type, fiber separation type, swirl separation type, and vane separation type. They can be classified into two main categories. The first category consists of traditional barrier-type mist eliminators, namely screen separation type and fiber separation type. The second major category consists of dynamically separated mist eliminators, namely swirl separation type and vane separation type. Within the first major category of traditional barrier-type mist eliminators, the single filaments in wire mesh-type mist eliminators tend to be slightly larger in diameter, whereas the individual fibers in fiber-type mist eliminators have a smaller diameter but a larger specific surface area. Whether it is a fiber-separated mist eliminator or a mesh-separated mist eliminator, the mechanism by which they separate and capture the heavy-phase particles in the airflow is essentially the same; both rely on the “bridging” of fiber strands to form grids of varying sizes, using interception to carry out separation – visually, it is similar to a sieve filtering the airflow. Heavy-phase carriers with sizes larger than the “grid” are blocked and remain on the surface and deep layers of the fiber filaments, whereas those with sizes smaller than the “grid” are carried by the airflow and penetrate through the inner layer. When the heavy-phase load carried by the airflow is in the form of particulates, their equivalent size remains essentially constant as they pass through the internals’ ‘grid’, making it easy for them to be trapped by the internals. However, when the entrained phase carried by the airflow is liquid droplets, their equivalent size elongates and deforms as they pass through the internal component’s ‘grid’ due to the flow velocity; after passing through the internal component, they return to their original larger size and shape under the effect of the fluid’s deceleration and the surface tension of the liquid droplets. Therefore, such separation internals that rely on a \"grid\"-based interception method exhibit excellent gas-solid separation performance, but poor gas-liquid separation performance and limited operational flexibility. In the case where the gas flow carries entrained substances such as liquid droplets and solid particles, these particles are effectively trapped and accumulate on the surface of the internal components as well as deep within them, blocking the gas flow channels. As operation time increases, more and more channels become blocked, resulting in greater resistance to the flow of gas through those components. For exhaust gases released into the atmosphere, the operating pressure is originally close to atmospheric pressure ； As the operating resistance of the internal components increases, exhaust gas emission is hindered, resulting in phenomena of pressure buildup and air stagnation. If the heavy phase carried in the aforementioned gas stream contains particulates, wire mesh and fiber separation internals should be avoided as much as possible when selecting demisters. If screen-type or fiber-type internals have been selected, it is necessary to be prepared for periodic maintenance and replacement; it is advisable to have one in use and one as a backup, so that timely switching and maintenance can be carried out in case the internals become clogged, without disrupting the continuous smooth operation of the production line.

The second major category consists of dynamic separation mist eliminators, whose separation mechanism relies primarily on the principles of fluid dynamics; they are fundamentally different from the first major category of traditional barrier-type mist eliminators. Since the second category of dynamic separation mist eliminators does not rely on \"grids\" for interception, the spacing between their components is larger; this effectively solves the problem of blockage by particulate matter in the internal components of the first category of mist eliminators during operation. As a result, they offer significant advantages when used to handle air streams containing particulate matter and gel-like heavy-phase substances. As mentioned earlier, the second major category of dynamic separation mist eliminators includes swirl separation mist eliminators and vane separation mist eliminators. The cyclone-type demister relies primarily on the centrifugal force generated by the high-speed rotation of the fluid to separate the gas stream from the substances carried in the heavier phase. The greater the rotational speed of the fluid, the larger the centrifugal difference between the airflow and the heavy phase, and the better the separation efficiency ； And the greater the rotational speed of the fluid, the greater the frictional pressure loss between the fluid elements as well as between the fluid and the internal components, resulting in a larger pressure difference. Therefore, for cyclone-type mist eliminators, within their stable operating range, the greater the operating pressure difference, the better the separation effect ； Conversely, the smaller the operating pressure difference, the worse the separation efficiency. However, as in the case of direct exhaust emission into the atmosphere mentioned earlier, the pressure under exhaust conditions is close to atmospheric pressure, resulting in a small allowable pressure difference for operation. Therefore, using a swirl-type demister under these conditions leads to unsatisfactory operational efficiency. Some units are equipped with mist eliminators; cyclone-type mist eliminators have been chosen for this purpose. To enable operation at high pressure drops, one or more axial flow fans need to be added to the pipeline to increase the pressure. It should also be noted that cyclone separators have very strict requirements regarding operational stability; their separation efficiency is sensitive to changes in operating conditions, which is why they need to be used in situations where the operating conditions remain stable.

The feather-leaf separated demister belongs to the second major category of dynamic separation demisters. It integrates various separation methods such as momentum and kinetic energy conversion separation, coalescence separation, vector separation, and liquid surface free energy capture separation, allowing them to work together dynamically. It combines the advantages of other types of demisters while overcoming their shortcomings; it features the ability to handle variable operating conditions, high operational flexibility, efficient and stable separation, and a low operating pressure drop. The vane-separated demister has a core internal component set that consists of four components: the G50D type pre-coalescing distribution internal component set, the G50 type vane-separated precision internal component set, the G50LD type anti-siphon short-circuit liquid dropping system, and the G50SP type online spray washing and cleaning system. A brief introduction to the functions of each section is provided below.

This post was last edited by luoli519 on 2020-7-16 at 16:08. The G50D type pre-coalescing distribution internal component set for vane-type demisters features a modular and detachable design, making installation, maintenance, and repair simple and convenient. It has three functions: First, it intercepts particulate matter that may be carried by the airflow, thereby protecting the subsequent precision vane separation components. Secondly, the kinetic energy and momentum of the mixed fluid are further distributed, so that the mixed fluid meets the requirements for dynamic separation before entering the subsequent precision vane separation components, thereby ensuring that these components operate within a range that enables stable and efficient separation performance. Thirdly, before the mixed fluid enters the subsequent precision vane separation internals, the tiny heavy-phase particles carried by the mixed fluid are pre-coalesced and enlarged, and then they proceed to the subsequent precision vane separation internals for further removal, thereby ensuring a higher quality of the gas produced by the separator. The vast majority of suppliers, both domestic and international, do not have this structure, resulting in unstable gas quality produced by the separator.

This post was last edited by luoli519 on 2020-7-16 at 16:32. The G50 type precision vane separation element set for vane-separated demisters is also designed and manufactured with a modular, detachable structure. It is based on the system platform designed and configured using NOVEL’s precise dynamics separation technology as its technical foundation, enabling the development of reliable and accurate solutions for separation processes. Within the structure of the vane separation elements, separation units with 4–6 stages in series are employed, allowing for repeated separation of small amounts of fluid that escape from the system. This approach offers high efficiency in separating drops and mist carried by airflow under conditions of high gas flow rates and fluctuations; it generally enables efficient and precise removal of such droplets and mist. For example, in cases of significant fluctuations in operating conditions, the liquid separation efficiency of the first separation unit drops to 90%, resulting in a residual liquid content in the gas at 10% (i.e., 100% – 90%). After the gas passes through the second separation unit, the residual liquid content is 10% * 10%; after the third separation unit, it is 10% * 10% * 10%; and after the fourth separation unit, it is 10% * 10% * 10% * 10%. In this way, the remedial separation effect on the residual liquid that escapes is significant. In contrast, screen filters, cartridge filters, packing materials, and conventional centrifugal separators typically have only one stage of separation; as a result, most of the liquid droplets and mist carried by the airflow fail to enter the specialized microchannels designed for liquid collection. This makes it difficult to address the issue of fluid leakage under fluctuating operating conditions, and it is also hard to achieve efficient, precise separation of liquid droplets and mist of various sizes present in the mixed airflow.