The seven stages of chemical technology from concept to industrialization (Issue 36/100) -- Collection of physical property data

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This post was last edited by xiouxingzhe on 2026-6-10 23:50. The seven stages of chemical technology from concept to industrialization (Issue 36/100) —— Technology refinement: Collection of physical property data. Dear friends: Hello everyone! In the previous issue, we discussed project definition and scope, clarifying the boundaries of the process package. In this issue, we discuss the second of the sixteen core tasks: the collection of physical property data. I say that physical property data is the foundation of the process package, and this metaphor is by no means an exaggeration. If the foundation is not proper, anything built on it will be skewed. All subsequent calculations—material balance, heat balance, equipment selection, safety valve discharge capacity—all require the use of property data. If a key physical property value differs by more than 10%, the calculated heat exchange area can vary significantly, which in turn affects the investment required for the equipment. I. List of media: First, sort out all the materials. The first step in collecting property data is to list all the chemical substances involved in the process. It sounds simple, but it’s easy to miss something. Many people only list the main materials – raw materials, products, and major by-products. But a set of processes involves far more substances than just these. Acids or bases used for pH adjustment, solvents used for cleaning, nitrogen gas used for purging, corrosion and scale inhibitors added to circulating cooling water, activation gases used in catalyst regeneration, neutralizing agents used in wastewater treatment, and sealants used in vacuum systems – although these auxiliary materials do not participate directly in the reactions, their physical properties still influence equipment selection, pipe material choice, and operational safety. My habit is, when creating a material list, to go through it from start to end with the PID, and write down any material name I come across. After completing it, go over it again with the R&D team and the analysis team to check for any omissions. It’s better to include a few extra entries that won’t be used later, rather than miss one that will make it impossible to find the data when needed. After listing them, divide the media into several categories. Main process media – raw materials, products, by-products, intermediates. Utility media — circulating water, steam, compressed air, nitrogen, chilled brine. Auxiliary materials — catalysts, solvents, absorbents, neutralizers. The main components of the three wastes—waste gas, waste water, and waste solids. The purpose of categorization is not to organize things neatly, but to remind oneself that every category needs to be covered. II. Basic physical property data: Consult authoritative sources. Once the list of media is prepared, the next step is to look up the basic physical property data for each one. What needs to be checked? Molecular weight, boiling point, melting point, density, critical temperature, critical pressure, eccentricity factor, standard enthalpy of formation, standard Gibbs free energy of formation. These are the basic inputs for subsequent process simulation and thermodynamic calculations. In addition, there are safety-related data—flash point, auto-ignition point, explosion limit, toxicity level, and occupational exposure limit. These directly affect the safety interlock design and operational protection standards of the device. Where do the data come from? The order of inquiry should be: international authoritative databases first — NIST Chemistry WebBook, IUPAC, DETHERM, DECHEMA; these databases have been tested over a long period of time and are highly reliable. Next are the professional handbooks — Perry’s Handbook, CRC Handbook, and Handbook of Properties of Chemicals and Materials. Finally are the appendices of journal articles or online sources, which require cross-verification from multiple sources. For commonly used materials, the property databases built into process simulation software such as Aspen Plus and PRO/II are also convenient sources. The databases of these commercial software products have been developed over a long period of time and tested, resulting in high reliability for common material systems. But it should be noted that the data in the software also comes from various sources; for non-standard materials or complex mixtures, the parameters built into the software may be inaccurate, and these need to be corrected using actual measurement data. During the query process, the source of each piece of data must be indicated. It’s not just for form’s sake; it’s to let those who use this data in the future know its reliability. For database searches, it should be marked as “Database Name”; for manual searches, it should be marked as “Manual Name and Page Number”; for software searches, it should be marked as “Software Name and Version”; for estimates, it should be marked as “Estimation Method” – having a clear source allows for tracing back when there are issues with the data. III. Phase equilibrium data: the key to separation design. Whether a separation scheme is viable depends fundamentally on phase equilibrium. Whether distillation can separate them depends on the gas-liquid equilibrium. Whether extraction can be separated depends on the liquid-liquid equilibrium. Whether crystals can be separated depends on the solid-liquid equilibrium. Regarding phase equilibrium data, my advice is: use measured values if possible, rather than those from literature ； Use literature whenever possible; avoid estimates. The uncertainty in phase equilibrium data is one of the most common sources of deviations in subsequent distillation design. In many processes, the separation efficiency in the distillation column fails to meet the required standards after scale-up to pilot scale; at the root of this issue is often an insufficient understanding of phase equilibrium data during the lab-scale testing phase. There are several issues that need to be clarified thoroughly. Does this system have azeotropes? If so, what are the azeotrope composition and temperature? What is the relative volatility between the various components? Has a minimum or maximum azeotrope been formed? Can the thermodynamic model accurately describe the gas-liquid equilibrium behavior of this system? If the phase equilibrium data are obtained from literature, multi-source cross-validation must be performed. For the azeotrope data of the same system, different sources may show differences of several degrees. When this difference is taken into account in the design of the distillation tower, it may relate to whether a higher or lower reflux ratio should be used, as well as the resulting difference in energy consumption. If the literature data is missing or unreliable, experimental measurements must be conducted. This step requires money and time, but it is much cheaper than building a distillation tower only to find that the components cannot be separated. My practical criterion is: if this separation unit is on the critical path of the entire process—such as affecting the quality or yield of the main product—then phase equilibrium data is worth measuring and verifying. If it is a auxiliary unit, the accuracy of data estimation may be sufficient. IV. Safety and environmental protection data: dimensions that are easily overlooked. This aspect seems simple in theory, but it is easy to turn into a mere formality in practice. Many people, when checking safety data, only look at the flash point, the explosion limit, and the toxicity level, and then fill them in a table and that’s it. But when it comes to safety data, one value alone is not enough; a range needs to be considered. The explosion limit changes with temperature and pressure. Under standard conditions, the explosion limit is 5% at the lower end and 15% at the upper end, but your operating temperature is 180 degrees—at this temperature, the explosion limit widens, and the lower end may drop below 3%. If you conduct security design based on standard conditions, the risks will be higher. The flash point is also affected by the composition of the mixture. The flash point of a material under standard conditions is 40 degrees, and your operating temperature is 38 degrees; it seems that there’s a difference of two degrees, so it appears to be safe. However, if low-flash-point impurities are mixed into the material, or if there is localized overheating during operation, this safety margin may be exceeded. Therefore, regarding safety data, it is necessary not only to check the values under standard conditions but also to understand the trends in changes under operating conditions. If necessary, entrust a professional institution to conduct actual measurements—especially for systems involving high-risk processes or new substances. Environmental protection data is primarily used to prepare for the subsequent design of waste treatment. The odor threshold, biodegradability, aquatic toxicity, and environmental half-life of each substance – these data directly influence the selection of environmental protection facilities and the determination of emission standards. Well done; in the future, environmental impact assessments and pollution discharge permit applications will go much more smoothly ； If it’s not done properly, it’s only later that you realize the emission standards for a certain component are extremely strict and that your treatment method doesn’t meet those standards, which makes rework a hassle. V. What to do in case of missing data? In real-world chemical engineering projects, it is common to encounter property data that cannot be found in databases. Especially for new products and new intermediates, there is absolutely no information in the literature. At such times, it is necessary to assess which data can be supplemented using estimation methods and which must be measured directly. There are several commonly used estimation methods: the group contribution method for estimating critical parameters and boiling points, the corresponding state principle for estimating transport properties, and the UNIFAC method for estimating activity coefficients and phase equilibrium. But it is necessary to determine whether the estimate is accurate or not. My approach is: if there are known similar substances in the system, first use estimation methods to calculate the physical properties of that known substance, and then compare the deviation with the measured values. This deviation is roughly the error range when estimating an unknown substance. If the deviation is within the acceptable range, the estimated value can be used, but it must be clearly indicated that it is an estimate, what method was used for the estimation, and what the approximate error range is. If the deviation is too large, actual measurement should be considered. For the key physical property data that have a significant impact on the results of process simulation calculations, it is recommended to conduct experimental measurements. For example, the relative volatility on which distillation column design depends – a 10% difference in this value can result in a significant difference in the number of theoretical plates in the column, as well as substantial differences in investment costs and energy consumption. Such critical data is worth spending money on to measure. VI. Preparation of the material property sheet: Once all the physical property data have been collected, they are compiled into a formal material property sheet. Standardize the format to facilitate subsequent use by all professionals. Each row represents a substance, and each column represents a physical property. Molecular weight, boiling point, melting point, density, critical temperature, critical pressure, eccentricity factor, standard enthalpy of formation, standard Gibbs free energy of formation, flash point, autoignition temperature, explosion limit, toxicity level, occupational exposure limit. The source is indicated after each piece of data. This material property sheet is one of the basic documents for preparing the process package. Subsequent equipment data sheets, safety analyses, environmental impact assessments, and pipe material selection must all be based on this characteristic sheet. If the data is inaccurate, everything in the subsequent design will be at risk ； With complete data and reliable sources, subsequent work has a solid foundation. Preview for the next issue: Issue 37 – Material balance calculations: Full-process simulation and system optimization. The collection of property data is complete, and all the basic data for the materials are now available. The next step is to use these data to carry out full-process material balance calculations – by establishing a model in process simulation software, connecting the entire process, and determining the flow rate, composition, temperature, and pressure of each process stream. Material balance is the most fundamental and core element in the three-balance calculations of the process package. To be continued in the next issue.