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Liquid level control in a recycle loop
Journal of Process Control ( IF 3.624 ) Pub Date : 2021-06-10 , DOI: 10.1016/j.jprocont.2021.05.014
Nitin Kaistha

A liquid tanks-in-series recycle process is analysed for the effect of the material balance (or inventory) control system (structure and tuning) on dynamics and stability. Two control structures, CS1 and CS2, differing only in the throughput manipulator (TPM) location, are systematically analysed. CS1 has a conventional TPM at the fresh feed flow while CS2 has the TPM at the total (fresh + recycle) flow. Analysis shows that CS2 dynamics can be an order-of-magnitude faster than CS1 for a high recycle fraction (R) and a large number of tanks (N). PI level control (LC) in CS1 may result in high frequency ‘ringing’ and potential instability even with conservative individual LC tuning. CS2, on the other hand, is always stable. Furthermore, all control structures with the TPM inside the recycle loop are dynamically agile and stable, similar to CS2. The work clearly illustrates that slow (or even unstable) dynamics in CS1 due to the positive feedback introduced by liquid recycle can be avoided by moving the TPM inside the recycle loop and bringing in the fresh feed as a make-up stream, as in CS2. This is further corroborated through dynamic results for a realistic reactor–separator–recycle process where the plantwide response with the TPM inside the recycle loop is about 5 times faster than a conventional TPM at the fresh feed.



中文翻译:

循环回路中的液位控制

分析了液体罐串联回收过程中物料平衡(或库存)控制系统(结构和调整)对动力学和稳定性的影响。系统地分析了两种控制结构,CS1 和 CS2,仅在吞吐量操纵器 (TPM) 位置不同。CS1 在新鲜进料流中具有常规 TPM,而 CS2 在总(新鲜 + 循环)流中具有 TPM。分析表明,对于高回收率,CS2 动力学可以比 CS1 快一个数量级(电阻)和大量坦克(N)。即使采用保守的单独 LC 调谐,CS1 中的 PI 电平控制 (LC) 也可能导致高频“振铃”和潜在的不稳定性。另一方面,CS2 始终稳定。此外,与 CS2 类似,所有在回收循环内带有 TPM 的控制结构都是动态敏捷和稳定的。这项工作清楚地表明,由于液体循环引入的正反馈,CS1 中的缓慢(甚至不稳定)动态可以通过将 TPM 移动到循环回路内并将新鲜进料作为补充流引入来避免,如在 CS2 中. 现实反应器-分离器-再循环过程的动态结果进一步证实了这一点,在该过程中,在循环回路内使用 TPM 的全厂响应比新鲜进料处的传统 TPM 快约 5 倍。

更新日期:2021-06-10
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