(590b) Controllability Evaluation of Intensified Separation Zones of a Carbon-Hydrogen-Oxygen Symbiosis Network

Carbon-Hydrogen-Oxygen symbiotic networks (CHOSYN) are settled for multi-plant mass integration among hydrocarbon processing plants as a strategy for reaching more sustainable industrial processes through performance targets such as reduction of raw material usage and waste generation, but also economic targets such as cost reductions and profitability enhancement [1]. Since these aspects are the guidelines used for the design of the networks, it is important to consider that the separation zones represent a significant contribution to the total cost and consume a large percentage of the energy requirements of the process, especially distillation processes that are well known for their low thermal efficiencies [2]. To address this concern, it has been proposed to include intensified distillation processes in the design of CHOSYNs as a strategy to improve the sustainability indicators of the networks such as economic and environmental aspects [3]. However, intensified distillation sequences such as thermally coupled sequences generally involve more complex designs than their conventional counterparts; therefore, in this work, it is proposed a controllability analysis performed in these intensified zones of the CHOSYN to determine whether the intensification improves or worsens its controllability. In this way, it is possible to ensure a feasible network design that incorporates the benefits of intensification and presents good controllability. Also, according to the principles of green chemistry by ensuring that the net design has good control properties a safer process can be expected. As a case study, it is proposed a CHOSYN configuration with two distillation sequences as areas for intensification [3]:

1. a) A three-column distillation sequence for ethylene purification from an ethane cracking process. For this sequence were proposed three retrofitting options replacing the reboiler from the first column (R-I option), the condenser of the second one (R-II option), or both (R-III option) for liquid/vapor recycles.
2. b) A two-column distillation sequence for propylene purification from methanol to propylene process. For this sequence were proposed two options: Petlyuk column and a side stripper arrangement.

The analysis was implemented under different scenarios with different intensification options considered for both sequences. The methodology implemented consists of two main analyses:

1) SVD method. First, the singular value decomposition technique was employed to assess the theoretical controllability of the different scenarios. This method allows us to measure qualitatively the controllability of a processing system through the concept of condition number, which is an indicator of how “well-conditioned” or sensitive is a matrix, in this case, this matrix is the steady state gain matrix, which relates the control and manipulated variables of the system. A large condition number indicates a sensitive system where a small disturbance in the system results in large changes in the control variables, conversely a small condition number indicates a less sensitive system. The matrix coefficients were determined by introducing a small disturbance in the manipulated variables and measuring the variation in the control variables, for this it was used Aspen plus simulations. For this criterion, the preliminary results indicate that some scenarios, which include intensification options, are as controllable as the conventional network. The intensified options for propylene purification present almost invariant condition numbers compared to the conventional sequence. Meanwhile, for ethylene purification, the intensified options presented higher condition numbers over the conventional option, which means that control is negatively affected by these options.

2) A dynamic study. Conventional L/V control structures based on PI controllers for the different intensified distillation options were proposed. These control loops consist of a feedback control scheme where the purity of the dome and bottom products are controlled with the reflux ratio and the heat duty. Aspen Dynamics simulations were used to verify the dynamic response of the network for the different scenarios, a Set Point change of 1 % for the product purity was proposed and the minimization of the absolute error integral (IAE) was used to determine the values of the controller parameters Kc (proportional gain) and τi (integral time). The responses were compared with the non-intensified options. The results show that PI-based structures of control are adequate to control de proposed sequences, and the time of response for some intensified options does only slightly differ from the response of the conventional case.

The joint results of these two studies indicate that the complexities introduced by the intensification do not necessarily mean worse control of the network. Therefore it is possible to integrate intensified separation processes into the CHOSYN, thus improving the performance of the network in terms of sustainability, lower cost, and energy consumption, as well as guaranteeing a feasible design in terms of control. In this way, processing schemes such as CHOSYNs are emerging as tools for greening existing refinery and petrochemical processing systems.

[1] Noureldin, M. M. B., & El‐Halwagi, M. M. (2015). Synthesis of C‐H‐O Symbiosis Networks. AIChE Journal, 61(4), 1242–1262. https://doi.org/10.1002/aic.14714

[2] Etchells, J. C. (2005). Process Intensification: Safety Pros and Cons, Process Saf. Environ. 83 (2), 85–89. https://doi.org/https://doi.org/10.1205/psep.04241

[3] Juárez-García, M., Contreras-Zarazúa, G., Segovia-Hernández, J. G., Ponce-Ortega, J. M. (2022). Sustainable Carbon–Hydrogen–Oxygen symbiosis networks: Intensifying separation sections. Chem. Eng. Process. - Process Intensif., 179, 109092. https://doi.org/https://doi.org/10.1016/j.cep.2022.109092