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·40· ； Kenan Hua, Issue 8, 1999 – A Brief Analysis of Crystallization on Melamine Surfaces, Zhao Xinhai. ’ _ - — — ～ ‘ Engineering reward pr6 withered pass master reply words some stems of the ink basket ； Polymer 1: An overview; 2.2 Adhesion to walls – During the rapid cooling process of melamine, a portion of it is… Currently, some fertilizer manufacturers use urea to produce trimelamine. In this process, urea is heated to a certain temperature (around 400°C) under normal pressure to bring it into a molten state. In a specially designed reactor, and with the action of a catalyst, urea is polymerized to form a gaseous mixture of melamine, carbon dioxide, and ammonia. The catalyst is then removed, and the mixture is rapidly cooled using quenching gas. This results in the formation of solid melamine powder, which is separated from carbon dioxide and ammonia to yield pure melamine. Carbon dioxide and ammonia are sent to recovery units for further processing. Since this process involves primarily gas and solid states after the reactor, and due to the properties of melamine, crystallization of melamine often occurs on the inner surfaces of equipment and pipelines during industrial production. Over time, an excessive amount of crystal accumulation can occur, leading to pipeline blockages and disrupting normal production processes. This forces shutdowns for cleaning purposes, which prevents the full utilization of the advantages of continuous production processes. As a result, raw materials are wasted, product costs increase, causing significant economic losses for enterprises. 2 Reasons for crystallization: Based on an analysis of this process, I believe the main reasons for crystallization are as follows: 2.1 Mechanical adhesion: Imperfections such as weld patterns on the surface of welds during equipment manufacturing, spatter and slag from the welding process, as well as defects like misalignment and undercutting within acceptable limits, and uncleaned burrs at the groove edges, contribute to this issue. The unevenness of the inner surfaces of equipment, along with these defects arising during pipeline welding, cause the rapidly cooled melamine crystals to adhere mechanically to these surfaces. Complete crystallization does not occur, especially in the areas around the equipment where cooling is poor; in such areas, small liquid droplets of melamine remain in a liquid state. Due to melamine’s own adhesive properties, these liquid droplets tend to stick to the equipment walls when there is a temperature difference. 2.3 Electrostatic deposition on walls: Since this organic compound is formed under high-temperature conditions, the catalyst must be removed in a gas-solid separator, and this can only be achieved through high-speed airflow. As a result, melamine in the mixed gas generates static electricity, and under the influence of this electric field, electrostatic deposition on the walls occurs. 3. Solutions: The principles behind the crystallization of melamine on the inner surfaces of equipment and pipes have been analyzed above. Due to different formation mechanisms and interacting conditions, regarding mechanical deposition and adhesion, increasing the flow rate of the mixed gas (or gas-solid mixture) can reduce or eliminate such phenomena. However, as the flow rate increases, collisions between melamine particles intensify, and the generation of charges among organic substances accelerates, leading to more severe electrostatic deposition on the walls. Finding a balance between these opposing effects requires experimental testing. Measurements were carried out to determine the relationship between static electricity generated by melamine particles and flow velocity. Taking into account heat transfer effects, the optimal point free from crystallization was identified among the first two factors and the static electricity-induced adhesion on the wall surface, so as to adjust the process parameters. Research departments are required to address this issue. Date of receipt: June 4, 1999. VIP Information: http://www.cqvip.com. As economic system reforms deepen, budgeting plays an increasingly important role in project management, contributing to reducing project costs, improving project quality, and enhancing efficiency. The project budget includes the main materials and auxiliary materials used in the project, machinery usage costs, labor costs, as well as all expenses incurred for organizing and managing the project. Project cost management is based on the construction drawing budget. Without construction drawings, project cost management is not possible. Therefore, the project cost must be kept within the budgeted amount; only when the actual cost is lower than the budgeted cost can the project be considered economically viable. How can project costs be reduced? We have gained some experience in the actual review of budgets, which is provided here for reference. 1. Enhance the use of computers in budgeting work: 1.1 Use computers to create various budgeting and statistical records. In 1998, we began using computers for budget registration and statistical compilation, thereby establishing a streamlined computerized management system that involves the construction unit, the planning department, the audit department, and the finance department. Further analysis shows that here, certain measures are taken under the existing process conditions in order to reduce the wall-adhesion phenomenon. 3.1 In equipment manufacturing and the installation of critical process pipelines, higher requirements are placed on the flatness of the inner surfaces. Proper protection must be provided during machining and welding processes to prevent mechanical damage to the inner surfaces as well as spatter from welding. The welds need to be smoothed out, and the surfaces must remain flat. In some critical devices, right angles or obtuse angles of less than 135° are not allowed; smooth transitions should be used (by employing spinning methods to ensure that no angular connections occur between the different parts) in order to reduce dead zones. After installation, acid washing and passivation treatments should be carried out to reduce or eliminate mechanical deposits on the surfaces. 3.2 Strict control is exercised over the insulation of the pipes associated with the equipment, both in terms of material selection and installation, in order to reduce the temperature difference between the equipment/pipe walls and the material flowing through them, thereby minimizing the temperature differences that occur along the pipe walls. 3.3 For equipment prone to crystallization, attention should be paid to the placement of the material inlets and outlets. The principle behind this arrangement is to increase the turbulence of the airflow without increasing the flow velocity; this is especially important in areas near the equipment walls, where the laminar boundary layer should be reduced while the turbulent boundary layer is increased. This approach improves the heat exchange efficiency or reduces the temperature differences within the equipment. It also helps to create a disordered state at low flow velocities, thereby reducing the energy of melamine particles and minimizing the generation of static electricity. This goal can be achieved by installing orifices in the pipes. 3.4 For the dead zones of the equipment and pipes in the aforementioned method, forced stirring and backflowing with quenching gas can be used to reduce the laminar boundary layer and lower the crystallization rate. 3.5 Pay attention to the quality of static electricity grounding; strict controls should be applied in this regard to minimize static electricity on equipment and pipelines and reduce the electric field strength. 3.6 For equipment for which the above methods yield unsatisfactory results, wall scrapers should be installed at the areas prone to crystallization in order to reduce the rate of crystallization and extend the time during which normal production can proceed. The above are some insights into the crystallization issues in the melamine production process; in practice, continuous efforts are needed to optimize and improve this process, so as to fully leverage the advantages of continuous manufacturing processes and further enhance both the yield and quality of melamine. References: 1 Gu Minzhen. Chemical Engineering. Beijing: Chemical Industry Press, 1986. 128 pages. 2 Niu Siming. Fundamentals of Chemical Engineering Equipment. Beijing: Ta Xue Industry Publishing House, 1985. 140 pages. 3 Tan Tiansi et al. Principles of Chemical Engineering. Beijing: Chemical Industry Press, 1984. 3R pages. Vip Information: http://www.cqvip.com