Abstract:

Multi-effect distillation (MED) technology is one of the most important seawater desalination methods. As a typical slow-time-varying system, in the process of long-term operation, the heat transfer efficiency of the evaporator is often reduced due to fouling accumulation, resulting in production reduction or even shutdown. In order to avoid this kind of problem, designers usually adopt redundant designs to increase the heat transfer area, which leads to a significant increase in equipment investment. In order to ensure that the device can effectively operate in a full cycle and reduce the total heat transfer area as much as possible, a full-cycle optimization design method is proposed. This method takes the total heat transfer area as the objective function, and uses decision variables to perform segmental optimization throughout the full-cycle. At the same time, fouling accumulation, process changes and control requirements are considered when designing heat transfer area margins of each effect. It obtains the optimal operating conditions and the minimum heat transfer area in one step, and completes the optimal design of the slow-time-varying system. Finally, taking a MED desalination system with eight effects as an example, the system is designed by using equal area method, equal temperature difference method, steady-state optimization design method, and full-cycle optimization design method at the same time. The results show that the full-cycle optimization design method can minimize the heat transfer area and greatly reduce the equipment investment of the MED system. It is a multi-effect evaporative desalination system optimization design method with good application prospects.

Key words: seawater desalination, multi-effect distillation, margin design, full cycle, optimal design, slow-time-varying

摘要：

多效蒸发（MED）是最主要的海水淡化方法之一，作为典型的慢时变系统，该系统在长期运行的过程中，往往会由于结垢导致蒸发器传热效率降低，造成减产甚至停工。为避免出现这种问题，工艺设计者会采取冗余设计，增大传热面积，这会导致设备投资的显著增加。为保证MED系统能够全周期运行，且尽可能减少总传热面积，提出了一种全周期优化设计方法。该方法以总传热面积最小为目标，对决策变量在整个周期内进行分段优化，同时考虑结垢过程、工艺变化以及控制方面的需求，对各效传热面积进行裕量设计，通过一步优化求解得到最优操作条件与最小传热面积，实现对慢时变系统的优化设计。最后，以八效MED海水淡化装置为例，同时用等面积法、等温差法、稳态优化设计方法以及全周期优化设计方法对系统进行设计。结果表明，全周期优化设计方法能够最大限度减少传热面积，大大降低了系统的设备投资，是一种有着良好应用前景的多效蒸发海水淡化系统优化设计方法。

关键词: 海水淡化, 多效蒸发, 裕量设计, 全周期, 优化设计, 慢时变