The seven stages of chemical engineering technology from concept to industrialization (Issue 40/100) -- Preparation of PFDs

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This post was last edited by xiouxingzhe on 2026-6-11 at 11:14. The seven stages of chemical technology from concept to industrialization (Issue 40/100) —— Technology finalization: Preparation of PFDs. Dear friends: Hello everyone! The previous issue covered the calculation of momentum balance. All three types of balance – mass, heat, and momentum – have been achieved; the temperature, pressure, flow rate, and composition of each stream are known, and the operating conditions, heat load, and utility requirements for each piece of equipment are also clear. These data are currently scattered across simulation files, calculation sheets, and notes. What needs to be done in this phase is to consolidate them all on one diagram—the PFD, the Process Flow Diagram. This is the first and most important final document in the process package preparation process. All the previous steps, such as collecting property data, establishing process simulations, repeatedly optimizing yield and energy consumption, and calculating the heat load for each heat exchanger and the head for each pump, ultimately come together in this diagram. The subsequent equipment data sheets, instrument data sheets, pipeline data sheets, and even the boundary condition sheets – all of their data must be obtained from this diagram, and can only be obtained from it. I. A PFD is not an ordinary flowchart; many people tend to underestimate its importance when they first enter this field. It was thought that by following the results of the simulation process, by drawing the main equipment and listing a few key pipelines along with the logistics data, the task would be completed. Actually, it’s still a long way off. A truly qualified PFD that can serve as a project benchmark should contain at least the following information. All major equipment and their tag numbers. Reactors, towers, heat exchangers, vessels, pumps, compressors – any equipment involving three-equilibrium calculations must be shown on the PFD. Key parameters of the equipment. The diameter and height of towers should be indicated, the heat load of heat exchangers should be specified, the flow rate and head of pumps should be noted, and the volume of containers should be indicated. The detailed nature of logistics. On each major logistics line, flow rate, pressure, and temperature should at least be indicated. More detailed composition data is provided in tabular form as a logistics data sheet attached to or following the diagram. The operating pressure and operating temperature of the equipment. This data serves as the basis for subsequent equipment classification, pressure vessel design, and equipment material selection. The thermal load of the equipment. For equipment involved in heat exchange, such as heat exchangers and reactors, it is necessary to specify the design heat load, which serves as a basis for selecting the subsequent utility systems. Specifications and quantities of utilities. For steam, circulating water, heat transfer oil, etc., used for heating and cooling, it is necessary to indicate not only the location where they are connected to the equipment but also their specifications and estimated consumption levels. Important control scheme. Although the detailed control design is completed during the PID stage, the PFD needs to reflect the most fundamental control objectives—such as temperature control of the sensitive plates in distillation columns, cascade control of reactor temperatures, and flow ratio control for key fluids. Together, these pieces of information turn the PFD into a “data map”. Anyone who obtains this diagram can clearly understand the technological logic and key parameters of this system. II. Before drawing the PFD: Perform functional decomposition First, before opening the drawing software, I have a habit that I’ve maintained for many years: to carry out functional decomposition first. Take a blank sheet of paper and, following the five levels of “device → process → unit → module → equipment”, break down the entire process from top to bottom, from general to detailed. The very top layer is the facility – the boundary of the entire process package design scope. The second layer is the process steps – large sections within the plant that are divided according to their functional roles, such as raw material preprocessing, reaction, and separation and purification processes. The third layer is the unit – the unit operations divided according to functional tasks within each process step. The fourth layer is the modules – groups of devices within each unit that work together to carry out a specific sub-function; for example, a typical distillation module includes a distillation tower, a reboiler at the bottom of the tower, a condenser at the top of the tower, a reflux tank, a pump for transferring fluid from the bottom of the tower, and a pump for the reflux flow. The fifth layer is the devices – the individual units that make up each module. This decomposition process is not just meant to make the diagram look better; it has real engineering significance. When you break down a device from the top level to the bottom level, you naturally ask yourself: what is the function of each level, what are its inputs and outputs, and what are the relationships between them? This process of asking follow-up questions brings to light those functional requirements that are easy to overlook. For example, when breaking it down to the reaction process, you would ask: Does the feed need to be preheated? How to remove the reaction heat? How to separate catalysts? Each follow-up question corresponds to one or more modules or devices. If one skips the decomposition step and starts drawing the equipment right away, it’s easy to overlook some crucial functions; later on, one may realize that a heat exchanger or a buffer system is missing. III. Diagram Planning: Turning the drawing into a functional map. Once the functional breakdown is completed, the layout of the PFD has a logical basis. Devices from the same module are grouped together, using visual aggregation to reflect functional aggregation. The diagram becomes a map that shows the functional logic. Regarding layout planning, there are several issues that are easily overlooked in practice. First is the balance of information density. More information on the PFD is not always better. The main pipeline is highlighted with thick lines or a conspicuous color, and the data labels must be as complete as possible. Utilities and auxiliary materials are represented by thin lines; detailed data does not necessarily have to be included on the main diagram – it can be placed in a logistics data table. With an appropriate balance of detail, those who view the diagram can quickly grasp the core logic of the process. Second, to reserve space for additional equipment in the future. During the heat balance and momentum balance phases, it might be necessary to add more heat exchangers or pumps; if the diagrams are overfilled from the start, making subsequent modifications becomes very difficult. Third, standardize legends and representation methods. The linearity, line width, color, and out-of-bound area symbols must all be standardized to facilitate reading by various professionals. A practical approach is to first draw a draft to determine the locations of the equipment and the flow of logistics, then have the team review it to check whether the logic makes sense and if there are any omissions. Confirm that there are no issues before proceeding with the actual drawing. It’s easy to make changes during the sketch stage, but it becomes more troublesome once the final design is ready. IV. A core principle: The PFD is the only data source. Every piece of data on the PFD is not assigned arbitrarily. Temperature, pressure, flow rate – these data come from process simulation and are verified through three-equilibrium calculations. Equipment heat load and utility consumption – derived from heat balance calculations. The pump’s flow rate and head are derived from momentum balance calculations. I set a strict rule for myself: data that is not available on the PFD must not appear in any subsequent documents. If I see that the heat load of a heat exchanger listed in the equipment data sheet does not match the value on the PFD, I don’t ask \"Which one is correct\"; instead, I ask them to double-check it – because there can only be one source of truth. The purpose of this rule is not to lack trust in the judgment of downstream professionals, but to establish a traceable data chain. When the project reaches the detailed design stage, someone on site asks, \"Where does this temperature data come from?\" The answer must be only one: \"It comes from page X of the PFD.\" If it’s not on the PFD, then this data is worthless and cannot be used. Once the PFD is reviewed and finalized, all subsequent equipment data sheets, instrument data sheets, PID values, and boundary condition tables will use this finalized PFD as the sole data reference. Any subsequent modification to the data must first be made in the PFD, after which all downstream departments must be notified to update it accordingly. This is the core discipline of process package data management. V. Several details to note There are several details that are easily overlooked during the preparation of a PFD. First is the extraction of logistics data. For the same stream of fluid, the temperature and pressure at the equipment outlet are not exactly the same as those in the pipeline. It must be clearly indicated where the data shown on the PFD was taken. The usual practice is to take it at the device outlet – this way, the data from the material balance sheet, the device data sheet, and the PFD are all on the same scale, preventing any discrepancies. Second is the representation of the batch operation unit. If there are batch operation units throughout the process—such as batch reactors or batch crystallizers used in certain fine chemical processes—it is necessary to clearly show on the PFD the transitional connection between the batch units and the continuous units. The material is discharged from the batch reactor into a buffer tank, and from there it is sent downstream continuously; the size of this buffer tank as well as the residence time must be indicated on the PFD. Third are the valves and instruments on public utility pipelines. It is not necessary to draw every valve and instrument on the PFD—that is the responsibility of the PID. However, the PFD needs to show the key control schemes and utility connection points, so that readers can understand the overall logic of process control. Preview for the next issue: Issue 41 – PFD milestone review: the most important checkpoint. The PFD has been prepared, but this is not something that can be considered complete just by having the process engineering team take a look at it. PFD requires a formal review to be organized. There is only one primary purpose for this review: to ensure that every piece of data on the PFD has a reliable source, and to confirm that the process design is technically sound and feasible from an engineering perspective. Once approved, the PFD is frozen at this point. What does this mean? To be continued in the next issue.