Method for replacing natural gas by dynamically tempering byproduct hydrogen-rich tail gas of PDH device

Abstract

The invention provides a method for dynamically tempering substitute natural gas through byproduct hydrogen-rich tail gas of a PDH device. The method comprises the following steps that S1, the standard Wobbe number and the safe combustion potential threshold value of target substitute natural gas are set; s2, collecting flow and component data of a first stream in a hydrogen-rich dry gas pipeline and component data of a second stream in a C2 tail gas pipeline in real time, and reversely calculating theoretical target flow of the second stream; s3, the synthetic combustion potential of the mixed gas is predicted and compared with a safe combustion potential threshold value, if the synthetic combustion potential exceeds the threshold value, intervention adjustment is triggered, and if the synthetic combustion potential does not exceed the threshold value, a flow adjustment instruction is generated; and S4, the flow of the second stream is adjusted, the actually measured Wobbe number of the output mixed gas is monitored in real time, and compensation adjustment is conducted on the flow of the second stream through a feedback compensation algorithm. According to the invention, hydrogen-rich dry gas and C2 tail gas with violent component fluctuation and huge combustion characteristic difference are converted into high-quality fuel gas which is completely standard to industrial natural gas in combustion interchangeability.

Description

Technical Field

[0001] This invention belongs to the field of petrochemical waste gas resource utilization technology, specifically relating to a method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas produced as a byproduct of a PDH unit. Background Technology

[0002] Propane dehydrogenation (PDH) is an important route for propylene production, and it generates a large amount of byproduct gases during the process. These mainly consist of two streams: one is hydrogen-rich dry gas from the cold box and pressure swing adsorption unit, which has a high hydrogen content, typically greater than 40%, and a very small molecular weight, approximately 3-10; the other is C2 tail gas from the top of the ethane removal tower, which mainly consists of ethane and ethylene, has a high calorific value, and a relatively large molecular weight, approximately 26-30.

[0003] Current technologies primarily treat these byproduct gases by burning them in flares, which not only wastes the calorific value of hydrogen but also increases emissions of pollutants such as CO2 and NOx, failing to meet dual-carbon targets. Another approach is to use them as low-grade fuel for simple boiler heating, but this fails to realize their high value.

[0004] There have been attempts to mix the two gases and feed them into the natural gas pipeline network to replace natural gas. However, when trying to use the by-product gas to replace natural gas for downstream heating furnaces, the flow rate and composition of the hydrogen-rich dry gas experience periodic and drastic fluctuations during the adsorption / desorption switching of the PDH unit. This means that simple physical mixing cannot guarantee a stable Wobbe number in the mixed gas, and fluctuations in the Wobbe number lead to fluctuations in the heat load of the downstream heating furnace, affecting product quality. Furthermore, hydrogen burns 7-8 times faster than methane. If only calorific value matching is considered and combustion potential control is ignored, when the hydrogen content in the mixed gas is too high, the flame propagation speed is very likely to exceed the airflow speed in non-dedicated burners, leading to backfire and burning out the burner nozzles. In addition, traditional PID feedback control is difficult to cope with frequent changes in the composition of the feed gas, resulting in regulation lag and failing to meet the stringent requirements for gas quality in industrial parks. Summary of the Invention

[0005] Based on the above background, the purpose of this invention is to provide a method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas produced as a byproduct of a PDH device. By controlling both the Wobbe number and combustion potential, the invention achieves complementarity between high-hydrogen, low-carbon dry gas and high-carbon, high-calorific-value tail gas, thus solving the technical problems of unstable combustion and easy backfire caused by simply mixing hydrogen-rich dry gas and C2 tail gas in the prior art.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit, the method being implemented based on a dynamic conditioning and proportioning system, the dynamic conditioning and proportioning system comprising a feedstock pretreatment unit, a dynamic mixing unit, a buffer homogenization unit, an online monitoring and feedback unit, and a control unit, wherein the feedstock pretreatment unit is connected to both a hydrogen-rich dry gas pipeline and a C2 tail gas pipeline, the method comprising the following steps:

[0008] S1, Reference Setting: Set the reference Wobbe number and safe combustion potential threshold for the target natural gas substitute in the control unit;

[0009] S2. Feedforward calculation: Real-time acquisition of flow rate and composition data of the first stream in the hydrogen-rich dry gas pipeline and composition data of the second stream in the C2 tail gas pipeline. Based on the benchmark Wobbe number, the theoretical target flow rate of the second stream is calculated in reverse using the calorific value balance model.

[0010] S3. Safety constraint verification: Based on the flow rate of the first stream and the theoretical target flow rate of the second stream, predict the synthesis combustion potential of the mixture, compare the synthesis combustion potential with the safety combustion potential threshold, and if it exceeds the threshold, trigger intervention adjustment; if it does not exceed the threshold, generate a flow adjustment command.

[0011] S4. Execution: Adjust the flow rate of the second stream according to the flow rate adjustment command, so that the two streams of gas from the first stream and the second stream enter the dynamic mixing unit for mixing, and output the mixture after being stabilized by the buffer homogenizing unit; and monitor the measured Wobbe number of the output mixed gas in real time, calculate the deviation between the measured Wobbe number and the reference Wobbe number, and adjust the flow rate of the second stream by means of a feedback compensation algorithm.

[0012] Preferably, in step S2, the control unit calculates the theoretical target flow rate QB of the second stream based on the following Wahbe number coupling formula:

[0013]

[0014] In the formula, The baseline Wobbe number for the target natural gas replacement is given, QA is the real-time flow rate of the first stream, LHVA is the lower heating value of the first stream, LHVB is the lower heating value of the second stream, dA is the relative density of the first stream, and dB is the relative density of the second stream.

[0015] Preferably, in step S3, the intervention and regulation specifically include:

[0016] When the calculated synthetic combustion potential is greater than the safe combustion potential threshold, the control unit forcibly executes a hydrogen reduction strategy, which includes limiting the flow input of the first stream until the recalculated synthetic combustion potential is less than the safe combustion potential threshold.

[0017] Preferably, in step S4, the feedback compensation algorithm employs PID control logic. The control unit calculates a correction amount u(t) based on the deviation between the measured Wobbe number and the reference Wobbe number, and adds this correction amount to the flow regulation command generated in step S2. The formula for calculating the correction amount u(t) is:

[0018]

[0019] In the formula, E(t) is the real-time deviation between the baseline Hua Bai number and the measured Hua Bai number, Kp is the proportional coefficient, Ki is the integral coefficient, and Kd is the differential coefficient.

[0020] Preferably, the dynamic mixing unit includes a Venturi jet mixer and a static mixer arranged sequentially along the gas flow direction. The first stream enters the Venturi jet mixer as an ejector fluid, and the second stream enters the suction chamber of the Venturi jet mixer as the sucked-in fluid. After the two fluids are initially mixed at the throat of the Venturi jet mixer, they enter the static mixer for secondary shear mixing.

[0021] Preferably, the buffer homogenizing unit includes a buffer tank, which has baffles inside, and the volume of the buffer tank is configured such that the average residence time of the mixed gas in the tank is maintained between 30 seconds and 60 seconds.

[0022] Preferably, the first stream is hydrogen-rich dry gas from the cold box and PSA unit of the propane dehydrogenation unit, with a hydrogen volume content of 40% to 90% and an average molecular weight of 3 to 10; the second stream is C2 tail gas from the top of the ethane removal tower of the propane dehydrogenation unit, with a total volume content of ethane and ethylene of 80% to 95% and an average molecular weight of 26 to 30.

[0023] Preferably, the target Wobbe number for the natural gas to be replaced in step S1 is set within the range of 46.5-50 MJ / Nm³. 3 .

[0024] Compared with the prior art, the present invention has the following advantages:

[0025] This invention provides a method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas produced as a byproduct of a PDH unit. This method overcomes the limitations of traditional chemical byproduct gases, which can only be simply physically mixed or downgraded for utilization. Through dual coupling control of Wobbe number calorific value anchoring and combustion potential safety constraints, hydrogen-rich dry gas and C2 tail gas with drastic component fluctuations and huge differences in combustion characteristics are transformed into high-quality fuel gas that is completely comparable to industrial natural gas in terms of combustion interchangeability. This not only solves the risks of unstable combustion and backfire explosion caused by fuel changes in downstream user heating furnaces and realizes efficient cascade utilization of energy, but also significantly reduces the carbon emission level of chemical industrial parks. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0027] Figure 1 This is a schematic diagram of the dynamic conditioning and proportioning system on which the method of the present invention is based;

[0028] Figure 2 This is a schematic flowchart of a method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas produced as a byproduct of a PDH device according to the present invention. Detailed Implementation

[0029] The technical solution of the present invention will be further described in detail below through specific embodiments and in conjunction with the accompanying drawings. It should be understood that the implementation of the present invention is not limited to the following embodiments, and any modifications and / or alterations made to the present invention will fall within the protection scope of the present invention.

[0030] In this invention, unless otherwise specified, all parts and percentages are by weight, and the equipment and raw materials used are commercially available or commonly used in the art. Unless otherwise specified, the methods in the following embodiments are conventional methods in the art. Unless otherwise specified, the components or equipment in the following embodiments are general standard parts or components known to those skilled in the art, and their structures and principles can be learned by those skilled in the art through technical manuals or conventional experimental methods.

[0031] The embodiments of the present invention will now be described in detail with reference to the accompanying drawings. In this detailed description, numerous specific details are set forth to facilitate explanation and provide a thorough understanding of the embodiments of the present invention. However, one or more embodiments may be practiced by those skilled in the art without these specific details.

[0032] Embodiments of the present invention disclose a method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit. This method is based on... Figure 1 The dynamic conditioning and proportioning system shown is executed.

[0033] The dynamic conditioning and proportioning system includes a raw material pretreatment unit, a dynamic mixing unit, a buffer homogenization unit, an online monitoring and feedback unit, and a control unit. The raw material pretreatment unit is connected to both the hydrogen-rich dry gas pipeline and the C2 tail gas pipeline. To ensure accurate proportioning, flow control valves and online component analyzers are installed on each pipeline before it enters the dynamic mixing unit.

[0034] To address the challenge of uniformly mixing hydrogen-rich dry gas and C2 tail gas, this embodiment employs a multi-stage series mixing structure in its dynamic mixing unit. Specifically, a Venturi jet mixer and a static mixer are sequentially arranged along the gas flow direction. During operation, the high-pressure hydrogen-rich dry gas acts as the ejector fluid, passing through the Venturi tube at high speed. A negative pressure zone is created at the throat, entraining the C2 tail gas as the drawn-in fluid. The two fluids achieve primary mixing under turbulent conditions. Subsequently, the mixed gas enters the static mixer, where it undergoes secondary shearing and segmentation through its complex internal flow channel structure, achieving uniform mixing and preventing stratification of the gas over long distances.

[0035] The buffer homogenization unit, located downstream of the mixing unit, includes a buffer tank. The buffer tank contains baffles. Because the upstream PDH unit's pressure swing adsorption unit generates periodic pressure and component pulses when switching adsorption towers, this embodiment configures the buffer tank's volume to maintain the average residence time of the mixed gas within the tank between 30 and 60 seconds. This time window smooths out instantaneous fluctuations from upstream, ensuring a stable gas pressure output to downstream users.

[0036] Based on the above system, the method flow of the present invention is as follows.

[0037] S1, Baseline Setting

[0038] The control unit sets the baseline Wobbe number and safe combustion potential threshold for the target natural gas replacement. In this embodiment, to ensure complete compatibility with existing natural gas heaters in the industrial park, the baseline Wobbe number is set at 46.5-50 MJ / Nm³. 3 Meanwhile, considering that the combustion rate of hydrogen is 7-8 times that of methane, a safe combustion potential threshold is set to prevent backfire.

[0039] S2, Feedforward Calculation

[0040] The control unit collects the flow rate and composition data of the first stream in the hydrogen-rich dry gas pipeline and the composition data of the second stream in the C2 tail gas pipeline in real time. Based on the benchmark Wobbe number, the theoretical target flow rate of the second stream is calculated in reverse using the calorific value balance model.

[0041] Since hydrogen-rich dry gas is typically a waste gas discharged from production, its flow rate often fluctuates and is uncontrollable depending on operating conditions. Therefore, the system treats it as the primary variable and C2 tail gas as the regulating variable. The control unit uses a calorific value balance model to calculate the theoretical target flow rate of the second stream. Specifically, the theoretical target flow rate QB of the second stream is calculated based on the following Wobbe number coupling formula:

[0042]

[0043] In the formula, The baseline Wobbe number for the target natural gas replacement is given, QA is the real-time flow rate of the first stream, LHVA is the lower heating value of the first stream, LHVB is the lower heating value of the second stream, dA is the relative density of the first stream, and dB is the relative density of the second stream.

[0044] The physical significance of the Wobbe number coupling formula lies in using the high calorific value and high relative density of C2 tail gas to neutralize the low calorific value and low relative density of hydrogen-rich dry gas, so that the Wobbe number of the final mixture is anchored to the benchmark value.

[0045] S3, Safety Constraint Verification

[0046] Based on the flow rates of the first and second streams and their theoretical target flow rates, the synthesis combustion potential of the mixture is predicted. The synthesis combustion potential is then compared with a safe combustion potential threshold. If the threshold is exceeded, intervention regulation is triggered. If the threshold is not exceeded, a flow regulation command is generated.

[0047] Before issuing a command, a safety verification must be performed. The control unit predicts the synthesis combustion potential of the mixed gas based on the current operating conditions. If the predicted synthesis combustion potential exceeds the set safe combustion potential threshold, it indicates that the hydrogen content in the mixture is too high. Direct supply to downstream systems could easily burn out the burner nozzles. In this case, the control unit triggers intervention regulation, forcibly implementing a hydrogen reduction strategy. Specific hydrogen reduction strategies include: limiting the flow input of the first stream, or introducing a reserved inert gas or methane stream into the system for dilution, until the recalculated synthesis combustion potential returns to a safe range. Only after the verification passes will the control unit generate the final flow regulation command.

[0048] S4, Execute

[0049] The flow rate of the second stream is adjusted according to the flow rate adjustment command, so that the two streams of gas from the first and second streams enter the dynamic mixing unit to mix, and are output after being stabilized by the buffer homogenizing unit; in addition, the measured Wobbe number of the output mixed gas is monitored in real time, the deviation between the measured Wobbe number and the reference Wobbe number is calculated, and the flow rate of the second stream is compensated and adjusted through a feedback compensation algorithm.

[0050] According to the generated flow regulation command, the control unit drives the regulating valve on the C2 tail gas pipeline to operate, and the two gases enter the dynamic mixing unit according to the calculated ratio. To eliminate the deviation between the theoretical calculation model and actual operating conditions, a feedback compensation mechanism is introduced. The online monitoring feedback unit monitors the measured Wobbe number of the final output mixture in real time and calculates its deviation from the reference Wobbe number. The control unit calculates the correction amount u(t) based on the deviation between the measured Wobbe number and the reference Wobbe number, and adds this correction amount to the flow regulation command generated in step S2; the formula for calculating the correction amount u(t) is:

[0051]

[0052] In the formula, E(t) is the real-time deviation between the baseline Hua Bai number and the measured Hua Bai number, Kp is the proportional coefficient, Ki is the integral coefficient, and Kd is the differential coefficient.

[0053] This control mode, which combines feedforward coarse adjustment with feedback fine adjustment, ensures both the system's ability to respond quickly to upstream fluctuations and the control accuracy of the final product.

[0054] This article uses specific examples to illustrate the principles and implementation methods of the present invention. The descriptions of the above embodiments are only for the purpose of helping to understand the method and core ideas of the present invention. It should be noted that those skilled in the art can make several improvements and modifications to the present invention without departing from the principles of the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention.

Claims

1. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit, the method being executed based on a dynamic conditioning and proportioning system, the dynamic conditioning and proportioning system comprising a feedstock pretreatment unit, a dynamic mixing unit, a buffer homogenization unit, an online monitoring and feedback unit, and a control unit, wherein the feedstock pretreatment unit is connected to both a hydrogen-rich dry gas pipeline and a C2 tail gas pipeline, characterized in that: The method includes the following steps: S1, Reference Setting: Set the reference Wobbe number and safe combustion potential threshold for the target natural gas substitute in the control unit; S2. Feedforward calculation: Real-time acquisition of flow rate and composition data of the first stream in the hydrogen-rich dry gas pipeline and composition data of the second stream in the C2 tail gas pipeline. Based on the benchmark Wobbe number, the theoretical target flow rate of the second stream is calculated in reverse using the calorific value balance model. S3. Safety constraint verification: Based on the flow rate of the first stream and the theoretical target flow rate of the second stream, predict the synthesis combustion potential of the mixture, compare the synthesis combustion potential with the safety combustion potential threshold, and if it exceeds the threshold, trigger intervention adjustment; if it does not exceed the threshold, generate a flow adjustment command. S4. Execution: Adjust the flow rate of the second stream according to the flow rate adjustment command, so that the two streams of gas from the first stream and the second stream enter the dynamic mixing unit for mixing, and output the mixture after being stabilized by the buffer homogenizing unit; and monitor the measured Wobbe number of the output mixed gas in real time, calculate the deviation between the measured Wobbe number and the reference Wobbe number, and adjust the flow rate of the second stream by means of a feedback compensation algorithm.

2. The method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: In step S2, the control unit calculates the theoretical target flow rate QB of the second stream based on the following Wobbe number coupling formula: In the formula, The baseline Wobbe number for the target natural gas replacement is given, QA is the real-time flow rate of the first stream, LHVA is the lower heating value of the first stream, LHVB is the lower heating value of the second stream, dA is the relative density of the first stream, and dB is the relative density of the second stream.

3. The method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: In step S3, the intervention and adjustment specifically include: When the calculated synthetic combustion potential is greater than the safe combustion potential threshold, the control unit forcibly executes a hydrogen reduction strategy, which includes limiting the flow input of the first stream until the recalculated synthetic combustion potential is less than the safe combustion potential threshold.

4. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: In step S4, the feedback compensation algorithm employs PID control logic. The control unit calculates the correction amount u(t) based on the deviation between the measured Wobbe number and the reference Wobbe number, and adds this correction amount to the flow regulation command generated in step S2. The formula for calculating the correction amount u(t) is as follows: In the formula, E(t) is the real-time deviation between the baseline Hua Bai number and the measured Hua Bai number, Kp is the proportional coefficient, Ki is the integral coefficient, and Kd is the differential coefficient.

5. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: The dynamic mixing unit includes a Venturi jet mixer and a static mixer arranged sequentially along the gas flow direction. The first stream enters the Venturi jet mixer as the ejector fluid, and the second stream enters the suction chamber of the Venturi jet mixer as the sucked fluid. After the two fluids are initially mixed at the throat of the Venturi jet mixer, they enter the static mixer for secondary shear mixing.

6. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: The buffer homogenizing unit includes a buffer tank, which has baffles inside, and the volume of the buffer tank is configured such that the average residence time of the mixed gas in the tank is maintained between 30 seconds and 60 seconds.

7. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: The first stream is hydrogen-rich dry gas from the cold box and PSA unit of the propane dehydrogenation unit, with a hydrogen volume content of 40% to 90% and an average molecular weight of 3 to 10; the second stream is C2 tail gas from the top of the ethane removal tower of the propane dehydrogenation unit, with a total ethane and ethylene volume content of 80% to 95% and an average molecular weight of 26 to 30.

8. A method for dynamically conditioning and replacing natural gas with hydrogen-rich tail gas as a byproduct of a PDH unit according to claim 1, characterized in that: The target Wobbe number range for the natural gas substitution set in step S1 is 46.5-50 MJ / Nm³. 3 .

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