Thoughts on Reading “Chemical Process Design” – The Process Design Framework

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Thoughts on Reading “Chemical Process Design” – The Process Design Framework Liutaize. Generally speaking, producing a chemical product involves multiple steps, including reaction, separation, circulation, mixing, heating, cooling, and pressure changes. In the entire synthesis process, the system first needs to identify each individual unit, and then connect these units together to form a cohesive whole, thereby achieving the desired product outcome. Typically, a flowchart shows the relationships between all units and specific devices. Once the flow diagram is finalized, full-process simulation calculations can be carried out, and the process can be optimized by adjusting parameters such as temperature, pressure, flow rate, and material composition. To evaluate the quality of a process system, considerations are mainly drawn from the following aspects: First, economic efficiency: There are often multiple routes or processes available for producing a product; in other words, many ways can lead to the same goal. However, there are significant differences in terms of production costs, investment required, and space occupied. It is necessary to calculate and compare the economic efficiency of each possible process in order to select the option with the best economic benefits. Of course, economic benefits are a very important criterion for evaluation, but they are not the only one ； II. Sustainable development: Sustainability is a prerequisite; it requires minimizing waste, reusing resources, and ensuring that processes are harmless in the design of chemical processes, with the overall process maintaining balance between industry and ecosystems ； III. Adaptability: A good process system should have strong adaptability, meaning it can operate properly under various operating conditions. Additionally, processes such as starting up and shutting down, as well as emergency shutdowns in the event of accidents, should be easy to control ； IV. Safety: Safety is often difficult to quantify, but it must be given top priority in process design. Regarding various uncertainties, designers are required to possess strong judgment skills. Regarding the points mentioned above, the author has gained deep insights while designing certain technical processes; for a long time, I was engaged in calculating and comparing various possible processes. Case 1: Comparison between the cold feed gas phase in a certain absorption tower and the circulating liquid phase in the cold tower bottom. In the case of the cold feed gas phase, due to the low heat transfer coefficient of the gas, the overall heat transfer coefficient is low as well, resulting in poor heat exchange efficiency; therefore, a larger heat exchange area is required in the design of the heat exchanger. In the case of the liquid phase in the cold tower reactor, although the liquid heat transfer coefficient is high, the overall heat transfer coefficient is also high, resulting in a smaller required heat exchange area. However, due to backmixing, segmented cyclic cooling is necessary to achieve good separation effects, and the structure of the tower equipment becomes complex ； Case 2: A comparison is made between the process of heat recovery through heat exchange at the inlet and outlet of a reactor for an exothermic reaction, and the process without such heat exchange. For a single pass through the reactor itself, heat exchange is advantageous over the process without heat exchange. However, this is not necessarily the case for products that tend to crystallize, especially when there are subsequent product absorption towers and distillation towers involved; since the temperature of the material after heat exchange decreases, the temperature in the absorption towers also drops, resulting in a lower concentration of the product and thus higher energy consumption for distillation. When the product material without heat exchange enters the absorption tower, the high temperature results in a large amount of reflux at the top of the tower; meanwhile, the high temperature at the bottom of the tower leads to a high concentration. As a result, the energy consumption in the subsequent distillation tower is low. Overall, the process without heat exchange is superior to the one with heat exchange ； Case 3: A comparison was made between conventional distillation and double-effect distillation for the distillation of a mixture of methanol and water. Conventional distillation requires lower initial investment but has higher energy consumption, while double-effect distillation involves higher initial investment but lower energy consumption. Overall, double-effect distillation offers significantly better economic benefits than conventional distillation ； Case 4: In a certain reaction, side reactions result in the formation of acid. The general approach to addressing this issue is to adjust the reaction parameters and optimize the reaction equipment in order to reduce the production of these acidic by-products. Alternatively, the by-products can be recycled through reactions using the reactants present in the system. However, a 100% recovery rate is not possible; ultimately, alkali solutions must be used for neutralization, and the resulting salts must be treated as pollutants and disposed of, which represents end-of-pipe treatment. By optimizing the process flow, cycles are cleverly utilized to address pollution at its source ； Case 4: For a certain technical process, multiple reactors are used in parallel, in series, or in a combination of both configurations. In terms of the process itself, multiple reactors in parallel result in a larger gas flow rate and lower pressure drop, while multiple reactors in series or in a combined configuration lead to a smaller gas flow rate and higher pressure drop. Regarding the initial investment in pipelines, those used in parallel require larger diameters, whereas those used in series require smaller diameters. The energy consumption should be similar, but upon closer examination of the compressor, it can be seen that the outlet temperature of compressors in a parallel configuration is lower than that of compressors in a series configuration (in the case of multi-stage compression, inter-stage cooling requirements also come into play); in other words, part of the work is converted into heat, which increases the compressor’s power consumption. However, if the material exiting the compressor needs to be heated, then the amount of heat required in a series configuration decreases. Therefore, a comprehensive comparison of the entire system is necessary to draw a conclusion ； Case 5: Calculation for various operating conditions – In technical processes such as sulfur recovery and acid production, there are scenarios involving low-sulfur, high-sulfur, oxygen-enriched, pure oxygen, air, as well as conditions at the beginning, middle, and end stages of catalyst use, along with combinations of these conditions. To ensure that the process design meets production requirements, design calculations must be carried out for each of these conditions ； Case 6: For safety reasons, vacuum distillation is used in the distillation tower to reduce the temperature. Although normal-pressure distillation can also meet the separation requirements in terms of process design, safety hazards exist at that temperature; therefore, vacuum distillation must be employed to lower the temperature of the material and achieve intrinsic safety. The “onion” model for chemical process design has the reactor at its core; surrounding the reactor is the separation and recycling system, outside which lies the heat exchange network, and beyond that are the utility systems. In the vast majority of chemical processes, the reactor is the only equipment capable of converting raw materials into products. The stream exiting the reactor contains unreacted raw materials, products, and by-products. To obtain products that meet quality requirements, it is necessary to separate and purify the reaction products. In order to improve the utilization rate of raw materials and make the process more economical, the unreacted raw materials need to be recycled. The reaction determines the separation and recycling system; through the optimization of the processes in the reactor as well as in the separation and recycling system, along with heat recovery, the cold and hot loads are determined. Any deficiencies are covered by the utility systems, and thus the heat exchange network determines the selection and design of the outermost utility systems. Therefore, in the design of a general process structure, the reaction is determined first. Due to various constraints such as the inability of laboratories to recycle unreacted materials, feed conditions that are more favorable than those in industrial plants, smaller laboratory equipment, and significant scale-up effects, there is a large difference between most laboratory data and those from industrial production. Therefore, pilot tests or side-stream experiments are necessary, along with theoretical calculations and relevant engineering experience, in order to obtain accurate data; accuracy is crucial, as any deviation can lead to deviations in the entire system. Then, separation and recycling design are carried out, followed by heat exchange network design and utility system design. Two main methods for chemical process design: First, establish a process that cannot be simplified. Following the logical structure of the onion model, start from the inside out – first determine the reactor, and then add separation and recycling systems based on the reactions that take place. Heat recovery is integrated into the process, and after making use of both cold and hot streams, the cold and heat loads can be determined. Based on the results of the heat balance, the necessary utility systems are selected and designed. Throughout the entire process, the best choice must be made for each step, that is, local optimal decisions must be taken. Since the information is incomplete when designing each step individually, to ensure the best decision, it is necessary to evaluate the designs related to other decisions. Furthermore, the relevant designs need to be optimized, which requires completing and refining many designs. But even after completing and evaluating a large number of process designs, it remains difficult to ensure that the optimal process has been found, as there are very complex interrelationships between the devices, and at each decision point, simplifying the process means sacrificing the benefits that come from a more complex one. The advantage of this method is that the designer can control the basic design decisions at each step; all uncertainty factors are incorporated into the overall decision-making process, allowing the designer to interact with the design as it progresses. II. Establish and optimize a simplified process. This approach involves creating a redundant process structure, that is, considering all feasible and interrelated optimal design alternatives, which include some unnecessary components; this entails numerous choices. Then, the design problem is transformed into a mathematical problem using design equations and design variables, which is solved employing optimization algorithms. During the structural and parameter optimization process, the objective function is to maximize economic benefits or minimize costs. There are some difficulties in using this method: 1. If all the individual unit operations in the process are represented accurately, the resulting curve showing the economic benefits that need to be optimized becomes both large and irregular in shape. 2. By using this method, the role of the designer is eliminated during the decision-making process; many uncertain factors, such as safety and layout, cannot be properly taken into account because it is difficult to include them in mathematical models. On the other hand, this method also has some advantages: it allows many different design options to be considered simultaneously, and all the design steps can be programmed into a computer program, enabling rapid generation of design solutions. Both of the above methods have their advantages and disadvantages. The author tends to adopt a approach that combines local optimal decisions with an overall system consideration: first, the optimal choice is made for each individual unit operation; once the entire system is formed, its performance is evaluated, followed by adjustments and further evaluations at the local level. Factors such as safety, environmental protection, and layout are also taken into account to determine the process that yields the best economic benefits. This also requires designers to have a deep understanding of the process itself. In summary, the chemical process design framework is a comprehensive issue that requires designers to have a deep understanding of the process itself, to employ systematic approaches, and to take into account factors such as safety and layout. They must continuously optimize the process structure, carry out repeated simulations and evaluations, in order to ultimately determine the best process design.