Seven stages of chemical technology from creativity to industrialization (Issue 39/Total 100)--Momentum Balance

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This post was last edited by xiouxingzhe on 2026-6-10 07:01 The seven stages of chemical technology from creativity to industrialization (Issue 39/100 in total) - Technology finalization: Momentum balance calculation dear friends: Hello everyone! In the last issue, we talked about heat balance calculations, where we learned where heating and cooling are needed in the entire process, and we also have the specifications and usage of public utilities. This issue enters the last item of the "Three Balances" - the calculation of momentum balance. The question to be answered by momentum balance is: Why do materials flow in pipelines and equipment? The answer is pressure difference. Where in the whole process a pump is needed, where a compressor is needed, how to calculate the head and power, these are the things that momentum balance needs to solve. 1. Why do materials flow forward? The ability of materials to flow in pipelines and equipment depends on the pressure difference. Where the pressure is high, flow flows to where the pressure is low. As it flows, the friction resistance of the pipeline and the pressure drop of the equipment will continue to consume this pressure difference. When the consumption is insufficient, a pump or compressor must be used to replenish it. This sentence sounds simple, but when many people do material balance and heat balance, their attention is entirely on the temperature and composition, and the pressure is just marked casually. It was only during the detailed design stage that I discovered that the pressure here was not enough and a pump needed to be added. ； The pressure drop there is too great, and the pipe diameter needs to be enlarged - it will be very expensive to make up for it later. The core task of momentum balance is to systematically analyze the pressure distribution of the entire process based on the operating pressure of each equipment that has been determined by the material balance and heat balance, find out where the pressure needs to be boosted, and determine the parameters of the pressure boosting equipment. The job itself is not complicated, but it requires a global perspective. The selection of each pump will affect the upstream and downstream pressure distribution, and the determination of each pipe diameter will affect the resistance and energy consumption of the system. Any omission or miscalculation in any link may cause trouble on site. When I was working on a project in the early years, there was one time because the elevation of the tower bottom pump was not calculated correctly, and the NPSH was not enough. The pump was evacuated as soon as it was turned on, so I had to modify the pump foundation. After that, I had to personally review the NPSH calculation for each pump. 2. Pressure distribution analysis The first step in momentum balance is to sort out the pressure distribution of the entire process. Starting from the entrance of the boundary area, along the direction of the process flow, the pressure changes are analyzed piece by piece, equipment by piece, and pipe section by section. What is the pressure of the raw materials entering the boundary area, how much is left after the first stage preheating, how much pressure is required to enter the reactor, and whether there is enough pressure difference from the reactor outlet to the downstream equipment to promote the flow of materials. During the analysis process, we focused on several issues. Where does pressure boosting equipment need to be installed - pumps for liquids and compressors for gases. Where will the pressure drop significantly? Heat exchangers, filters, and regulating valves are all major sources of pressure drop. Which locations need to maintain a certain back pressure - to prevent liquid vaporization, prevent backflow, and meet the inlet pressure requirements of downstream equipment. There is a basic physical law worth remembering here: It is relatively easy to increase the pressure of liquids, but it is relatively difficult to increase the pressure of gases. Therefore, when designing many processes, if it does not cost too much, the gas is usually liquefied and then pumped to increase the pressure. Because gas transportation and pressure boosting require larger pipelines and equipment, its investment, land occupation, electricity consumption, etc. are higher than those for liquid pressure boosting. You can think about it, except for special circumstances that require electric heating, are all the electricity used in the system centered on how to achieve momentum balance? All momentum lost is simply replaced by a pump or compressor. 3. Pressure optimization: The coupling pressure with materials and heat is not only a matter of rising and falling, but also has room for optimization. There is a coupling relationship between pressure, material balance, and energy balance, and the three balances cannot be treated separately. Give a typical example. The multi-component material system passes through the first rectification tower to remove most of the light components, and then passes through the second tower to refine and remove the light components. The overhead stream of the second tower returns to the first rectification tower. If the operating pressure of the second tower is higher than that of the first tower, then the top of the second tower can be directly returned to the first tower through gas phase discharge without the need for condensation and then pumping. In this way, the first tower saves heating heat, and the second tower saves condensation cooling capacity, killing two birds with one stone. This example looks simple, but behind it is an important design thinking: Do not set the pressure of each unit operation in isolation, but match and optimize the pressure distribution from the perspective of the entire process. The choice of a tower's operating pressure not only affects its own separation effect and energy consumption, but also affects the design pressure and delivery method of its upstream and downstream equipment. When doing stress optimization, I usually ask a few questions. Is there a gap that can be exploited to achieve gravity flow delivery, eliminating the need for a pump? Is there any way to reduce the number of phase changes by adjusting the operating pressure, thus eliminating the need for heating or cooling utilities? Is there any link that causes additional pressure increase or pressure relief due to pressure mismatch? Can the operating pressure of upstream and downstream equipment be adjusted to match? 4. Determination of pump and compressor parameters. After the pressure distribution analysis is completed, the parameters of each booster equipment are calculated. The core parameters that need to be determined for the pump include flow, head, NPSH, and recommended form. The flow rate is taken from the material balance sheet, usually 1.1 to 1.15 times the normal flow rate as the design flow rate - this margin is used to absorb operating fluctuations, but it should not be too large, which will cause the pump to deviate from the high-efficiency zone for a long time. The head is calculated based on the pressure difference, head height difference and pipeline resistance loss between the pump outlet and inlet. When calculating, the most unfavorable working conditions should be considered - such as the regulating valve fully open and the initial pressure drop of the heat exchanger before fouling is minimal - to determine the maximum flow rate that may occur at the pump outlet to ensure that the pump will not exceed power under this working condition. ； At the same time, it is also necessary to consider whether the pressure drop increases after the heat exchanger is fouled and whether the head can meet the requirements when the regulating valve is fully closed. The parameter of NPSH is easily ignored by many young engineers. The effective NPSH at the pump inlet must be greater than the required NPSH of the pump, otherwise the pump will suffer from cavitation - loud noise, large vibration, and short impeller life. This is why the installation height of the tower bottom pump and the height of the tower skirt are so important. They directly determine whether the effective NPSH at the pump inlet is sufficient. The tower skirt is usually not less than 7.5 meters. Behind this data is the NPSH demand of the pump. The specific design considerations for the tower skirt will be discussed in detail later in the equipment selection issue. The compressor mainly provides procurement data sheets in the process package stage. The core parameters include inlet composition, temperature, pressure, and outlet required pressure and temperature. With these data, compressor manufacturers can select appropriate compressor models and stages according to requirements. 5. Frequently Asked Questions and Precautions In the calculation of momentum balance, there are several places where errors are easy to make. One is to ignore the resistance calculation of the piping system. Some people directly take the pump head as the outlet equipment pressure minus the inlet equipment pressure, ignoring the friction loss of the pipeline and the local resistance of the pipe fittings. When the pipe diameter is small, the flow rate is high, and the pipe is long, the pipeline resistance may account for a large proportion of the total lift. If this part of resistance is ignored, the outlet pressure of the pump will be low during actual operation and the flow rate will not reach the design value. Another is the disconnect between equipment placement and momentum balance. The calculation of the pump head requires knowing the elevation difference between the pump inlet and outlet equipment, but this elevation difference is determined during the equipment layout stage. If the momentum balance calculation is not cross-checked with the equipment layout, and the elevation changes after the equipment layout is completed, the pump head may not be enough. Therefore, momentum balance and equipment layout should be an iterative process - first estimate the elevation difference according to the preliminary layout plan, then determine the pump head range through momentum balance, and then check after the equipment layout is deepened. If there is a deviation, adjust the pump selection or equipment layout elevation. Also pay attention to the mutual influence when multiple pumps are connected in parallel. If multiple pumps share a suction pipe or discharge pipe, the inlet pressure and back pressure of a single pump will be affected by the start and stop of other pumps and changes in flow rate. When calculating the pump head, the most unfavorable working conditions under various operating combinations should be considered to ensure that each pump can operate normally within the required working conditions. In addition, special attention is required: If the valve on the outlet pipeline of a multi-stage pump is closed too small at low load, the pump may run under pressure, causing the pump temperature to rise and vibration to be high. This requires minimum backflow or minimum flow protection measures to be considered during the process design stage. Next issue preview No. 40: PFD preparation - The three-balance calculation of the core drawing carrying the three-balance information has been completed, and the data of mass, heat, and momentum are available. The next step is to condense all these results on one drawing - PFD, process material flow diagram. This is the core drawing in the craft package and the most important milestone among the sixteen tasks. What should be marked on the PFD? How deep is it? How to plan the drawing? Expand next issue.