(544e) Optimization-Based Design of Renewable Methanol Production Encompassing Waste Heat Utilization with the Fluxmax Approach

The transition from fossil to renewable sources of energy poses a great challenge for the chemical industry and introduces a need to couple energy generation processes with the chemical production. In this coupling, the energy efficiency of the chemical processes plays an important role as it directly affects the amount of installed capacity of the expensive energy generation processes, making the utilization of waste energy streams with a suitable utility system an important endeavor. Yet it is complicated by the fluctuations of the renewable energy sources and consequently by the need to consider a large spectrum of storage processes when designing an efficient production. To identify the optimal design under different renewable resource conditions, one should take advantage of all degrees of freedom of the complex subsystems of generation, utility, storage and chemical processes.

This can be achieved with optimization-based design approaches, which we apply in this contribution, where we design a renewable methanol production for different coastal locations in the US. Methanol plays an important role as a bulk chemical and shows potential as a synthetic fuel in the ongoing energy transition. To identify designs capable of functioning under strict environmental limits of the future chemical industry, only renewable sources of energy (solar, wind, ambient temperature heat source) and material (atmospheric CO₂, seawater) come into consideration.

We build upon the FluxMax approach for optimization-based design [1-3] and extend it to deal with the challenges of renewable chemical production. The foundations of the FluxMax approach are composed of three steps: 1) discretization of the thermodynamic state space, 2) modelling of the process functions to obtain a superstructure in the form of network flow problem 3) solving the network flow optimization problem, while simultaneously optimizing the heat integration.

Within this contribution, the extensions to the FluxMax approach include the introduction of storage nodes (mass, heat, work) and utility process functions (heat pump, heat engine, electric heating and heat exchange). A model for waste heat utilization is formed by a discretized temperature grid of the utility system and by defining the connectivity of the utility processes based on physical and technical limits a priori. Besides this, the time domain is discretized and yearly renewable resource data are exploited to capture the hourly, daily and seasonal fluctuations of the energy inputs. Moreover, we consider several generation process alternatives utilizing solar energy (PV, HCPV and thermal energy processes with various sun-tracking options) and wind energy (onshore and offshore wind turbines).

The resulting approach is capable of simultaneous design, scheduling and waste heat utilization using power-to-heat and heat-to-power processes while optimizing a techno-economic objective function (in this contribution, the levelized cost of methanol LCOMeOH). The advantage of this approach is that the resulting linear optimization problem allows identifying its global optimum. Furthermore, the overall set of generation, utility, storage and chemical processes optimized simultaneously is larger than previously reported for a renewable methanol production. This has led to novel process configurations and operating strategies, which also suggested co-production of other products.

Designs were identified with the LCOMeOH ranging from 1279 to 1785 $/t (with the current prices), depending on the location and operating strategy of the methanol synthesis and purification processes (Fig 1). One interesting process configuration featured consistently in designs with the stable operating strategy. In this configuration, the SOEC and PEM water electrolyzers operate together. The SOEC maintains a stable operation and the operation capacity of the PEM electrolyzer fluctuates with the energy input. The extra H₂, which is not immediately used by the methanol synthesis, is compressed and stored in tanks (daily storage) or in a liquid organic hydrogen carrier (seasonal storage). The heat needed for desorption in the direct air CO₂ capture process and for evaporation of water in the SOEC is supplied by the waste heat utilization system, leading to a more efficient operation and reduced installation capacity of the generation processes. This is facilitated by a synchronized operation of a heat pump (50 – 125 °C) and a steam turbine operating between 320 and 125 °C, which is supported by phase-change thermal energy storage (360 °C) supplied with heat from parabolic troughs.

[1] Liesche, G., Schack, D., & Sundmacher, K. (2019) The FluxMax approach for simultaneous process synthesis and heat integration: Production of hydrogen cyanide. AIChE Journal 65(7), e16554

[2] Schack, D., Liesche, G., & Sundmacher, K. (2020) The FluxMax approach: Simultaneous flux optimization and heat integration by discretization of the thermodynamic state space illustrated on methanol synthesis process. Chemical Engineering Science 215, 115382

[3] Schack D., Jastram A., Liesche G., Sundmacher K. (2020) Energy-Efficient Distillation Processes by Additional Heat Transfer Derived from the FluxMax Approach. Front. Energy Res. 8,134