208-Technical Proposal for a Special Fan Blade Separator for the Off-gas from the Raw Gas Washing Tower in Gasification Units-NOVEL TECH

A customer from the Northwest region contacted our company for technical advice. The raw gas produced by their gasification unit contained a significant amount of dust, so it was sent to a water scrubber to remove most of that dust. Due to the absence of precise gas-liquid separation components at the top of the wash tower, the gas exiting the tower contains a small amount of dust and liquid droplets, which enter the downstream pipeline equipment and significantly affect their proper operation. This impact is particularly severe on the compression units, causing them to stop working almost every month.

Generally speaking, when the water scrubber towers for raw gas in coal gasification plants are designed and manufactured according to their rated operating conditions, design agencies and owners, with an eye on controlling investment costs, reserve little headroom in terms of tower diameter, effective flow area, and the space at the top of the tower. In actual operation, due to fluctuations in operating conditions, especially when the system pressure is below the rated design value, the actual gas volume flow rate increases significantly and exceeds the rated value. This leads to obstacles in the downward flow of liquid in the wash tower, resulting in a phenomenon of gas-liquid mixture surging. As a result, the amount of slurry and dust particles carried by the gas and escaping from the wash tower is much higher than under normal, stable operating conditions.

Given that the actual design pressure of the customer’s gasification unit’s wash tower is 7.7 MPaG, classifying it as a medium-to-high pressure device, and considering the complicated approval processes required for carrying out welding work on such equipment, as well as the difficulties associated with conducting secondary inspections and obtaining certifications for the pressure vessels in the wash tower after welding, plus the limited space available inside the wash tower, the customer decided not to install the vane separation components inside the wash tower, but rather to place a separate, dedicated vane separator on the exhaust pipeline of the wash tower.

The customer’s gasification unit features an exhaust pipeline for the water washing tower, with a diameter of DN250 mm and made of 304 stainless steel. The operating temperature is 40°C, the operating pressure is 6.7 MPaG, and the standard-condition volumetric flow rate of the raw gas is 54,000–58,000 Nm^3/h. The airflow not only carries liquid droplets, mist, and dust, but also contains substances such as benzene, toluene, naphthalene, fluorene, anthracene, phenanthrene, and pyrene. The mol% composition of the raw gas is as follows: water 9.36, carbon monoxide 7.82, hydrogen 69.48, carbon dioxide 1.25, methane 10.35, hydrogen sulfide 0.03, ammonia 0.33, ethane 0.48, benzene 0.71, naphthalene 0.06, pyrene 0.001, anthracene 0.001, phenanthrene 0.001, fluorene 0.02, toluene 0.08, nitrogen 0.02.

Since the raw gas being processed contains liquid droplets, foam, dust, as well as viscous crystalline polycyclic aromatic compounds, separation technologies and equipment that are prone to clogging, such as screens, filter elements, packing, and filter media, cannot be used.

If a Chevron vane separator with a horizontal cross-section is used, it requires a larger diameter and cross-section for the separation tank in order to meet the requirements regarding the flow area. However, since its gas flow, as well as the separated liquid and particle streams, all flow in the same channels, there is no separate liquid flow channel or discharge system. Moreover, this technology can achieve a gas-liquid separation precision of only 15–25 microns or larger, with an operational flexibility ranging from 60% to 110%. This type of technology is not recommended.

Someone recommended to the client the cyclone separator commonly used in petrochemical companies. Since cyclone separators are only suitable for conditions of high stability, high gas velocity, and high pressure, and the flow directions of the gas stream as well as the separated liquid and particle streams are not independent of each other, their most significant drawback is that they are not suitable for conditions with fluctuations. Their separation efficiency is sensitive to such fluctuations, they offer limited operational flexibility, and the pressure drop is often in the range of several tens of kilopascals. The client confirmed that the operating conditions of their gasification unit fluctuate significantly; therefore, it is not recommended to choose a cyclone separator.

Given the actual fluctuating operating conditions of the client’s gasification unit, as well as the fact that its raw syngas contains liquid droplets, dust, and viscous crystalline polycyclic aromatic hydrocarbons, we have analyzed the situation from a professional standpoint of kinetic separation technologies and recommend that the client adopt a vane separator. In the vane separator, the purified gas stream and the separated heavy-phase entrained substances flow in perpendicular directions with independent flow channels; moreover, a new type of \"anti-siphon\" liquid dropping system is employed. The precision of gas-liquid separation can reach 3 microns or larger, and the operating range is between 15% and 130%. In traditional structured separators, the liquid dropping and liquid sealing mechanisms rely on a \"traditional liquid dropping tube + liquid sealing cylinder\" design, which is unable to handle fluctuating operating conditions; in particular, it cannot withstand pressure drops even when there are fluctuations of 0.5% in the operating pressure (pressure drop = 6.7 MPa * 0.5% = 0.0335 MPa, or 3.35 meters of water column, which is far greater than the height of the liquid dropping tube). As a result, the liquid at the bottom of the separation tank is drawn into the airflow through the liquid dropping tube due to the \"siphon effect\" created by this pressure difference, resulting in a large amount of liquid being carried along with the airflow. Therefore, from a professional perspective in dynamic separation technology, we recommend that our clients adopt a solution consisting of vaned internals together with a new type of anti-siphon liquid dropping system.

The feather-leaf separator boasts the following significant performance advantages: 1. In terms of its separation mechanism, it makes efficient use of the kinetic energy and momentum carried by the airflow, as well as the surface free energy of the droplets; it combines various separation processes such as vector separation, coalescence separation, and surface free energy capture. It offers great operational flexibility, operates at low pressures, is less prone to clogging, and maintains stable separation efficiency. 2. In terms of the structure of the separation internals, the vaned separation internals feature independent conveying channels for the gas-phase flow and the heavy-phase flow; these channels are structurally separate from one another and have flow directions that are orthogonal to each other, which effectively prevents secondary entrainment and mixing. 3. In terms of the separation cascade, the vane-type separation internals feature 4–6 series-connected separation units along the gas flow path; any heavy-phase particles that escape from a previous separation unit are continuously captured and separated by the subsequent units, thereby ensuring high exhaust gas separation efficiency and excellent operational flexibility. For example, under certain fluctuating operating conditions, the separation efficiency of the first-stage separation unit may be only 90%; this means that the escape rate or residual rate of the heavy-phase components in the airflow released by this unit is 10% (i.e., 100% – 90%) ； The airflow is captured and separated by the second-stage separation unit in series, resulting in a total escape rate or residual rate of the heavy-phase contaminants in the airflow of 10%*10% ； The airflow is then captured and separated by the third series-connected separation unit, resulting in an overall escape rate or residual rate of the heavy-phase carriers in the airflow of 10%*10%*10% ； The airflow is then captured and separated by a fourth series-connected separation unit, resulting in an overall escape rate or residual rate of the heavy-phase carriers in the airflow of 10%*10%*10%*10% ； And so on. The greater the number of series-connected separation units arranged along the airflow path in a vane-separated internal component assembly, the lower the overall escape rate or residual rate of heavy-phase substances carried within the airflow finally released by the vane separator. 4. To ensure that the feather separator can adapt to operating conditions with an extremely wide fluctuation range of 15–130%, a proprietary “anti-siphon” new downcomer system has also been implemented.

In addition, feather separators have been widely used in similar operating conditions within the petrochemical, coal chemical, oil and gas extraction, processing and transportation, fine chemical, pharmaceutical, papermaking, metallurgy, salt production, environmental protection, and new energy industries. The feather separator also has the following comparative advantages: 1. The feather separator relies on NOVEL’s precise dynamic separation technology as its fundamental technical foundation to develop a reliable technical solution. It exhibits high efficiency in separating and removing liquid droplets, mist, and dust particles from airflow. Typically, it can achieve a separation efficiency of over 99.9%, precisely removing liquid droplets, mist, and dust particles with sizes of 3 microns or larger. The quantitative and precise separation efficiency of the vane separator under specific operating conditions is provided by the hydraulic calculation sheet for NOVEL’s precise dynamic separation technology. 2. The feather-type separator offers greater operational flexibility; its operating range is 15% to 130%. Even under conditions of significant fluctuations in operating parameters, it exhibits excellent resistance to “siphoning” and prevention of air flow “short-circuiting”, thus ensuring high separation efficiency. 3. Judging from the operating curve of the patented equipment for separating feather leaves, its operating pressure drop remains stable throughout its long service life, and its separation efficiency is consistent, making it an ideal choice that meets the requirements of quality management systems. The operating curve of traditional separators shows significant fluctuations, which runs counter to the requirements of the quality management system when it comes to technology selection.