A Computational Framework for Design and Optimization of Dynamic Membrane Processes

据估计，世界上一半以上的人口受到淡水短缺的影响，由于地球气候变化、社会经济发展不平衡和人口持续增长，这一数字将继续上升。能源供应的地理不平衡也是如此，这是由平行因素驱动的。海水淡化为利用海水和微咸地下水生产清洁水提供了一个可行的解决方案。在脱盐技术中，反渗透（RO）膜分离器由于其相对于基于蒸馏的那些的能量效率而被最广泛地使用。然而，即使是先进的多级RO工艺也存在效率低下和固有的操作挑战，包括膜污染和结垢、由于需要定期冲洗膜而导致的瞬态操作、RO网络基于模型的优化中提出的计算挑战以及内陆地区对高水回收水平的需要，其中成本与盐水废物的处置相关。该研究计划将推进对动态（分批，半分批和循环模式）膜分离过程的基本理解，并将在动态操作过程中优化其设计和性能。具体而言，该建议旨在（i）调查盐保留，污垢，结垢和浓差极化（膜表面的溶质浓度梯度的测量）和使用计算流体动力学（CFD）的内部分离器几何结构设计，（ii）探索系统动力学并应用最优控制理论来增强系统性能，（iii）开发新颖的网络化过程设计，和（iv）验证结果使用试点规模的数据，从当地水域。这些目标将最终导致动态膜工艺的变革性创新，以帮助解决水和能源供应方面的紧迫问题。该项目还将为不同的本科生和高中生群体提供研究机会。将开发先进反渗透系统设计和操作的教育模块，以使该领域的学生、研究人员和工业从业人员受益。该提案提出了开发动态反渗透（RO）工艺的设想，以克服无限数量的膜级和级间增压泵的实际限制，（理论上）稳定操作下的最佳脱盐系统性能，当前标称操作模式。要解决的挑战是同时优化设计的多级网络膜分离器系统，以及确定最佳的时间周期模式的操作整个系统。精确的RO模块模拟需要动态三维计算流体动力学（CFD）技术-然而，在RO网络设计和优化任务中使用这些类型的模拟在此时在计算上是难以处理的。因此，将开发一个降阶一维模型，其参数根据详细的CFD模拟进行拟合，并根据实际海水淡化厂数据进行验证。这个简化的模型可以有效地离散正交配置，随后将被用来定义一个严格的最优控制问题，寻求最小化能源使用和最大限度地提高清洁水回收。该研究计划将利用变压吸附（PSA）工艺设计的现有知识，变压吸附工艺是一种成熟的气体分离工业技术，用于指导网络化RO脱盐系统的初步设计。与计划中的反渗透系统一样，PSA工厂在瞬态条件下运行，并在分离单元内具有空间浓度梯度。由PSA系统启发的用于RO网络的动态建模、优化和设计工具将被扩展到绿色发电系统，其有效地作为RO反向操作：由水通过膜渗透到盐溶液中产生的渗透压增加了后者的压力和体积流量，随后收获用于功率。总的来说，这项研究计划将从根本上推进一套系统工程工具，可用于从详细的时空分布的传输模型中产生降阶模拟，这些模型是基于模型的设计和优化RO系统外的瞬态化学过程所需的。与此研究计划相结合，一系列的教育和推广计划，重点是本科生研究人员的支持和一个独特的学生交流计划与加州大学洛杉矶分校的计划。这个奖项反映了NSF的法定使命，并已被认为是值得的支持，通过评估使用基金会的智力价值和更广泛的影响审查标准。

It is estimated that over half of the world’s population is affected by fresh-water scarcity and that this figure will continue to rise because of the Earth’s changing climate, unbalanced socio-economic development, and continued population growth. The same can be said for the geographic imbalances of energy availability, which are driven by parallel factors. Desalination offers a viable solution to clean-water production from seawater and brackish groundwater. Among desalination techniques, reverse osmosis (RO) membrane separators are the most widely used due to their energy efficiency relative to those based on distillation. However, even advanced multistage RO processes suffer from inefficiencies and inherent operational challenges including membrane fouling and scaling, transient operation due to the need for periodic flushing of the membrane, the computational challenges posed in model-based optimization of RO networks, and the need for high water recovery levels in inland areas where costs are associated with disposal of brine waste. This research program will advance the fundamental understanding of dynamic (batch, semi-batch, and cyclic modes) membrane-based separation processes and will optimize their design and performance during dynamic operation. Specifically, this proposal aims to (i) investigate salt retention, fouling, scaling, and concentration polarization (a measure of solute concentration gradient at the membrane surface) under transient conditions and internal separator geometry design using computational fluid dynamics (CFD), (ii) explore system dynamics and apply optimal control theory to enhance system performance, (iii) develop novel networked process designs, and (iv) validate results using pilot-scale data from local water districts. These objectives will ultimately lead to transformative innovations in dynamic membrane processes to help address the pressing issues in water and energy availability. The project also will provide research opportunities to a diverse group of undergraduate and high school students. Educational modules on advanced RO system design and operation will be developed to benefit students, researchers, and industrial practitioners in the field.This proposal presents a vision for developing dynamic reverse osmosis (RO) processes to overcome the practical limitation of the infinite number of membrane stages and inter-stage booster pumps that would be required for (theoretically) optimal desalination system performance under steady operation, the current nominal mode of operation. The challenge to be addressed is the concurrent optimal design of the multistage networked membrane separator system together with determining the optimal time-periodic mode of operating the entire system. Dynamic 3-dimensional computational fluid dynamic (CFD) techniques are required for accurate RO module simulations – however, using these types of simulations in the task of RO network design and optimization is computationally intractable at this time. Therefore, a reduced-order 1-dimensional model with parameters fitted from the detailed CFD simulations and validated against actual desalination plant data will be developed. This reduced model can be efficiently discretized by orthogonal collocation and subsequently will be used to define a rigorous optimal control problem that seeks to minimize energy use and maximize clean water recovery. This research program will leverage current knowledge on the design of pressure-swing adsorption (PSA) processes, a mature industrial technology for gas separations, to guide initial designs for the networked RO desalination systems. Like the planned RO systems, PSA plants operate under transient conditions and feature spatial concentration gradients within the separation units. The dynamic modeling, optimization, and design tools for RO networks inspired by PSA systems will be extended to green power generation systems that effectively operate as RO in reverse: osmotic pressure generated by the permeation of water through a membrane to a saline solution increases the latter’s pressure and volumetric flowrate which is subsequently harvested for power. Overall, this research program will fundamentally advance the set of systems engineering tools available for producing reduced-order simulations from detailed, spatiotemporally distributed transport models needed for the model-based design and optimization of transient chemical processes outside of RO systems. Coupled with this research plan, a range of education and outreach plans focusing on undergraduate researcher support and a unique student exchange program with UCLA is planned.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.