Seven stages of chemical technology from creativity to industrialization (Issue 37/Total 100)--Material Balance

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This post was last edited by xiouxingzhe on 2026-6-10 23:49 The seven stages of chemical technology from creativity to industrialization (Issue 37/100 in total) - Technology finalization: Material balance calculation dear friends: Hello everyone! In the last issue, we talked about physical property data collection, and the basic data for all materials is available. This issue begins to enter the core "three balance" calculations in the preparation of process packages - material balance, heat balance, and momentum balance. Among them, material balance is the foundation and the starting point for all subsequent work. Simply put, the question to be answered by material balance is: How much material comes in, how much material comes out, what is the composition and state of each strand of material in between. There are no shortcuts to this process, just work it out layer by layer. 1. Sort out the data package and identify project amplification problems. After getting the pilot data package, don’t rush to open the simulation software. The first thing is to sit down, sort out the data package from beginning to end, and re-examine the technical solution of the entire process route. The pilot data package provides process flow and operating conditions validated on a few hundred liter plant. But at the scale of industrial installations, many issues that were simplified or ignored in the pilot stage must be faced again. For example, in the pilot stage, a certain material that is liquid at normal temperature and pressure reacts under high temperature and slightly positive pressure. The data package usually only provides the reaction temperature, pressure and product composition, but how to control the temperature and pressure of the feed and how to design the heat exchange of the reactor is often not considered in detail. These are exactly the problems that engineering scale-up must solve. During the grooming process, I usually ask myself a few questions. Is the phase transition process of key components clear? Where do logistics loops exist in the whole process, and how much is the loop volume? Which equipment involves discontinuous operation, and how to connect it in continuous industrial equipment? If there are technical providers, is there any critical engineering information missing from their data packages? After sorting it out, functionally decompose the process - dismantle the entire process from beginning to end according to the level of "device → process → unit → module → equipment" (this is my personal division habit). What are the functions of each level, what are the inputs and outputs, and what are the connection relationships. This decomposition process may seem like drawing a block diagram, but it is actually helping you to clarify your ideas. The clearer the decomposition is, the less likely it is to miss something during subsequent simulations. 2. Establish process simulation and open up the whole process. After the work is completed, establish the whole process model in Aspen Plus or PRO/II. The first step is to choose a thermodynamic equation. You cannot just use the software default options for this step. For polar systems, choose NRTL or Wilson, for non-polar systems, choose Peng-Robinson or SRK, and for systems containing electrolytes, choose electrolyte NRTL. If there are substances in the system with incomplete interaction parameters in the database, the UNIFAC method needs to be used to estimate group contributions. When you are not sure which equation to choose, you can first use the candidate equation to predict the azeotropic composition and temperature of the known azeotrope in the system, compare it with the literature data, and use whichever equation predicts the most accurately. When going through the process for the first time, the primary goal is to "get through", not "optimize". Let the model run first and observe where there are convergence problems and logistics conflicts. There will definitely be many problems found during the first pass process, some of which can be modified directly, and some of which need to be discussed and determined with the data package provider. After several rounds of this back and forth, a principled process was formed. The most common thing that gets stuck during simulation is convergence issues. For processes containing loops, the software needs to first guess an initial value and then iterate to approximate the true value. If the initial value is too far from the true value, the algorithm may diverge. The solution is: Design regulations are relaxed one by one, and the process is first opened up and then the regulations are tightened. The distillation column is the unit most prone to convergence problems in the simulation. The reflow ratio is set too small, the number of theoretical plates is not enough, and the discharging regulations exceed the physically feasible range, which may lead to non-convergence. Checking these settings one by one can usually find the problem. After the simulation results come out, a rationality test must be done. There are several methods of inspection. Select a few key components and do a material balance in the entire process. The total amount entering the device should be equal to the sum of the total amounts in all outlet streams. The temperature of the distillation column reboiler must be higher than the bubble point temperature of the bottom liquid at that pressure - if the simulation results show that the reboiler temperature is lower than the bubble point, there must be a problem with the results. The cooling water outlet temperature of the tower top condenser cannot be higher than the tower top temperature, and the outlet pressure of the pump cannot be lower than the inlet pressure. Check these physical rationalities one by one to avoid being misled by the "false convergence" of the software. 3. System optimization: Prioritize separation and consider the overall situation before entering the optimization stage. The direction of optimization usually starts with separation. Separation efficiency has the greatest impact on the economics of the entire device, while the room for improvement in reaction yield is often limited. When the reflux ratio of a distillation tower is reduced from 3 to 1.5, the steam consumption of the bottom reboiler is directly halved. This benefit is often more significant than a one or two percentage point increase in yield. During the optimization process, it may be discovered that some design variables and design requirements set in the pilot data package are no longer reasonable on an industrial scale. For example, in the case of azeotropic materials, excessive reflux ratio, etc., it is necessary to adjust the design requirements based on the actual project and find an economical and reasonable technical solution. One of my experiences in doing material balancing is: First do the balance of the entire device, then disassemble it into the balance of each process, then refine it to the balance of each unit, and finally the balance of each material. From macro to micro, approach step by step. There are two advantages to doing this: First, it is more efficient and you won’t get bogged down in details from the beginning. ； Second, the influence between each unit can be seen more clearly. The work of material balance cannot be rushed. Only when it is done well can subsequent work proceed normally. 4. After the export and optimization of the material balance sheet is completed, export the complete material balance sheet from the process simulation software. Each share of logistics must contain at least the following information:: Temperature, pressure, total flow, flow and fraction of each component, gas-liquid phase state, density, viscosity, volume flow, average molecular weight, specific heat. These data collections need to meet the requirements for all subsequent work. This material balance sheet is the cornerstone of all subsequent work. The operating conditions on the equipment data sheet come here, the heat loads for the heat balance calculations come here, and the flow and pressure for the momentum balance calculations come here. If one data on the material balance sheet is inaccurate, all subsequent work will be inaccurate. While exporting the material balance sheet, the preparation of PFD can be started simultaneously. Each logistics stream on the material balance sheet corresponds to a line on the PFD. The logistics data table is attached to the PFD drawing or behind the drawing for subsequent professional inquiry. The specific requirements for PFD preparation will be discussed later in a special issue. 5. Frequently Asked Questions and Precautions During the material balance calculation process, there are several error-prone areas worth paying attention to. One is to ignore minor components. The content of a certain component in the data packet is less than one percent, and some people ignore it because they think it has little impact on the overall balance. However, this trace component may accumulate gradually in industrial equipment, eventually affecting product quality or causing corrosion and blockage. For example, if a certain circulating stream contains a trace amount of non-condensable gas, which accumulates a little in each cycle, the system pressure may increase significantly after a period of operation. On the material balance sheet, all components whose content is within the detectable range should be included in the accounting. Another is the confusion between operating conditions and design conditions. The base operating condition of the material balance sheet must be clear - is it rated load, maximum load, or minimum load? The material balance under different loads is different. I generally require at least two sets of material balance data for rated operating conditions and maximum load conditions. If the operating flexibility requirements are high, minimum load conditions need to be supplemented. There is also the location of temperature and pressure. For the same logistics, the temperature and pressure at the equipment outlet and in the pipeline are not exactly the same. The location of the temperature and pressure data on the material balance sheet must be clearly marked on the sheet to facilitate subsequent reference of the heat balance and equipment data sheet. Next issue preview No. 38: Heat balance calculation - heat exchange network and utility engineering selection and material balance are completed, and the flow rate, composition, temperature and pressure of each stream of logistics are available. The next step is heat balance - figuring out where heating is needed in the entire process, where cooling is needed, how big the heat load is, and what working fluid is used to exchange heat. How to perform pinch analysis, how to optimize the heat exchange network, and how to prepare a utility consumption table. Expand next issue.