绝热反应量热试验安全相关设计参数的经验测定
这篇文章翻译自Jens Conzen的文章, 副主任, 出现在Wiley-VCH CITplus版7-8中, 2019. 它描述了如何为反应过程设计应急救援系统.
A good understanding of the thermal safety of all substances is required when storing or producing materials by chemical reaction on the factory scale. 如果不是这样的话, deviation from normal plant operation parameters could lead to dangerous consequences for plant workers and the environment. 因此，正确设计泄压装置(如.g. pressure relief valve and / or rupture disc) is of great importance to protect chemical reactors or storage tanks against overpressure. 在热失控反应的情况下，温度和压力会突然升高. 应特别注意反应性物质. 保护原理很简单:通过排气将容器压力限制在失效点以下.e. 气体或多相污水排放).
The Chemical Reactivity Worksheet published by the American Institute of Chemical Engineers (AIChE) or similar resources aid in identification and control of exothermic chemical reactions. The thermal stability of substances can be analyzed in the laboratory for both normal and abnormal operating conditions.
从过去吸取的教训
One can look at numerous examples in the past where inadequate understanding or misinterpretation of data have caused accidents. Several industry incidents have demonstrated the importance of proper maintenance and engineering design of safety systems. In 2017, the explosion of an intermediate bulk storage tank for crude methylene diphenyl isocyanate (MDI) demonstrated how devastating uncontrolled events can be. 事故后调查的结论是，一个阀门要么被损坏，要么没有完全关闭, 这会导致催化剂泄漏到罐中吗. 这引起了高温下的自聚合和二氧化碳气体的产生. 自聚合形成的污水堵塞了减压管道, 防止油箱减压. 最终, 二氧化碳气体产生的增加造成的压力上升导致了爆炸. 而每一个事件都提供了学习的机会, 世界各地仍有孤立事件发生.
如何确定安全相关的设计参数?
心烦意乱的情景可以安全地模拟在一个实验规模 绝热反应量热计. 这些数据可用于应急救援系统的设计。. Possible hazards and triggering events are typically identified in a process hazard analysis (PHA) and may include the following:
- 暴露于火
- 冷却损失
- 失去躁动(e.g. 造成失控反应的热层的形成)
- 反应物误充(如.g. 误加催化剂或水)
- 批处理污染
大规模技术反应的模拟
Various commercially available 绝热反应量热计s can simulate large-scale chemical reactions using relatively little sample material (10-100 ml). 延森休斯主要依靠PHI-TEC II在其内部过程安全实验室. 该仪器采用薄壁测试槽，热惯性低. 试验方法几乎是绝热的, 通过确保测试单元的热容远远小于测试样品的热容, 这是典型的全面工业应用. (高热惯性测试不具有代表性，也不能产生保守的测试结果.)另外, the internal sample heat generated during the test is continuously measured and heat is applied to the outside of the test cell through heating elements to further approach near-adiabatic test conditions. The pressure inside the test cell is also tracked and balanced from the outside with nitrogen gas in the secondary containment of the test device. The test apparatus can also be used to analyze mischarges or batch contaminations by utilizing various injection pumps than can feed these materials during the experiment and even at high pressures. 此外，流动型(e.g. 单相/两相)可以确定, 应急救援系统设计中的一个重要参数是什么. 最后, 获得的工艺参数可以在工厂规模上应用，而无需额外的操作或缩放.
有意义的测试策略
The success and safe operation of a series of calorimetric trials depends on a robust and meaningful test strategy. 人们必须区分三种反应类型:
- Vapor systems: These are steam-like systems in which the overpressure is analogous to the vapor pressure and boiling of solvent takes place. 反应温度可以通过排气来控制.
- 气体系统:这些系统形成不可冷凝气体，反应不能通过排气来控制
- Hybrid systems: A combination of the first two reaction types and where cooling by boiling of solvent takes place at the set pressure of the pressure relief device.
根据预期的反应类型，指定一个开放或封闭的测试单元. 蒸汽系统通常可以在封闭的测试室中进行分析. An open test cell may be beneficial to measure the temperature and pressure rise rates at the set pressure of the pressure relief device for gassy and hybrid systems. If no previous knowledge is available and it is not possible to make a good assumption on the expected reaction type, 通常最好先使用开放的测试单元进行测试，以尽可能有效和安全地进行测试. The obtained kinetic parameters can then be used to design the emergency relief system according to ISO-4126-10 (standard on safety devices for protection against excessive overpressure) or according to the DIERS method (Design Institute for Emergency Relief Systems, AIChE的一个分支).
管道反作用力
减压阀或破裂盘的打开可以在几毫秒内完成. The sudden opening of the pressure relief device during an overpressure event causes the chemical effluent to accelerate through the emergency relief system. 污水要么被输送到集水池(封闭系统)，要么被排放到环境中(开放系统)。, 根据毒性和可燃性的要求. The momentum change of the impulse generates temporary fluid force imbalances on the pipeline of the emergency relief system. These dynamic loads must be analyzed and combined with all static loads such as thermal and live loads in the structural design. 这对于封闭系统(关于这个主题的进一步讨论)尤其重要, 参考API出版物520和ASME B31.3).
此外，沿管道的压力分布也很重要. 如果其他船只连接到同一紧急救援系统集管, potential backpressures could fail rupture disks in reverse or prevent safety relief valves from opening at their set pressures. 因此, the transient pressure distribution in the entire emergency relief system should be simulated and compared against the maximum allowable working pressures (MAWP) of connected vessels as well as their relief devices. 可以使用各种软件程序来模拟管网的流体动力学. 简化的一维解决方法通常就足够了.
动应力分析
动应力分析应按照结构分析规范(e)进行.g. 有限元分析)，并考虑瞬态流体力. 要分析的各种负载情况是由当地要求决定的.g. ASME B31.(美国有3个). 过去，业界通常将动态负载系数设定为2. 这个因素通常用于简单的静态计算，以避免详细的动应力分析. 然而，这种方法不能充分解决所有可能的情况. 原因是在泄压装置突然开启的过程中, 在承重点和管跨处可能产生较大的反作用力和弯曲应力. 如果反作用力的动力激发管道的固有频率, 会产生较大的动应力. 在这种情况下，动应力可能比静应力高好几倍. 因此，对于快速减压瞬态场景，建议进行动态应力分析.
A complete emergency relief design also includes a dispersion calculation with validation of the catch tank stack height. The dispersion calculation will compute the size of the exhaust plume as well as the potential for toxicity and ignitability. The process data obtained from the calorimetric trials as well as fluid dynamic calculations can be used as input for the dispersion model.
Summary
在保护反应化学系统不受超压影响时，应考虑许多因素, 包括对正常和异常操作条件的基本理解, 救援系统的规模, 以及管网动力的评估. Only by comprehensively evaluating these factors can an owner reliably demonstrate a credible basis for safety that will protect employees, 业务连续性和环境.