Energy-saving tail wash system for phosphorus-ammonium plant

Abstract

The application discloses an energy-saving tail washing system for a phosphorus-ammonium workshop and relates to the technical field of tail gas treatment of phosphorus fertilizer production. The existing phosphorus-ammonium tail gas washing system adopts a two-open-one-backup operation mode, and problems of double-pump parallel pressure blocking and high energy consumption exist. The application comprises a tail gas absorption tower, a circulating washing main pipe and three washing pumps, the outlets of the three washing pumps are connected in parallel to the circulating washing main pipe, and the three washing pumps are configured as a pipeline structure in which only a single washing pump is connected to the circulating washing main pipe for operation, and the other two washing pumps are used as backup pumps; a backflow pressure stabilizing pipeline is arranged on the outlet pipeline of each washing pump, and an adjusting valve is arranged on each backflow pressure stabilizing pipeline. The application eliminates the problem of double-pump parallel pipeline network pressure blocking through structural modification, reduces the vibration of equipment and the risk of mechanical seal leakage, prolongs the service life of equipment, simultaneously reduces the power consumption of the washing process, and has good economic and environmental protection benefits.

Description

Technical Field

[0001] This invention belongs to the field of phosphate fertilizer production tail gas treatment technology, and in particular relates to an energy-saving tail gas washing system for use in ammonium phosphate workshops. Background Technology

[0002] During the production of diammonium phosphate (DAP), the neutralization reaction of phosphoric acid and ammonia produces a large amount of ammonia- and dust-containing exhaust gas. The exhaust gas scrubbing system is a core facility ensuring that exhaust gas emissions meet standards and comply with the environmental control requirements of GB31573 "Emission Standards for Water Pollutants and Air Pollutants for Phosphate Fertilizer Industry." It directly determines the environmental compliance of the enterprise's production. Most domestic DAP enterprises use a multi-pump parallel operation mode for their diammonium phosphate exhaust gas scrubbing systems, with the mainstream being a three-pump system with two pumps in operation and one on standby. However, due to the characteristics of parallel operation of centrifugal pumps, when two pumps of the same model are running synchronously in parallel, the total system flow is only 1.8 to 1.9 times the rated flow of a single pump. Pipeline friction and local resistance increase quadratically with the flow rate, leading to a series of abnormal problems such as pipeline pressure buildup, pump outlet pressure misalignment, and equipment operating condition deviations. This breeds multiple process and equipment safety hazards. Under this operating condition, the scrubbing... The pump has been operating outside its high-efficiency range for an extended period, with persistently high and periodically fluctuating outlet pressure. The pump shaft, impeller, and mechanical seal are continuously subjected to alternating high pressure, leading to increased equipment vibration and excessive bearing temperature. Coupled with the highly corrosive properties of the dilute phosphoric acid scrubbing solution, mechanical seal leakage is frequent. Simultaneously, high pipeline pressure causes overpressure atomization and tearing of the tower top spray nozzles, resulting in scrubbing solution splashing and uneven spray distribution. This leads to poor gas-liquid mass transfer, reduced ammonia and dust absorption efficiency in the exhaust gas, frequent fluctuations in exhaust gas emission indicators, and increased pressure to meet environmental standards. Furthermore, the pump outlet check valve is repeatedly opened and closed due to pressure counteraction, causing frequent damage and increasing the risk of scrubbing solution backflow and pump cavitation. This further exacerbates system instability. Additionally, the long-term overpressure of pipelines and flange welds, combined with the corrosive effects of the medium, significantly increases the risk of scrubbing solution leakage, posing a dual safety hazard of personnel burns and environmental compliance violations.

[0003] The parallel operation of dual pumps not only causes various process equipment problems, but also has prominent issues of serious energy waste and poor operating economy. The actual spray volume required for production in this exhaust gas scrubbing system only matches the rated flow of a single pump. Starting a second pump does not generate any process gain and only causes ineffective energy consumption. Under abnormal conditions of pipeline pressure buildup, both scrubbing pumps are out of their high-efficiency operating range, and the hydraulic efficiency is greatly reduced. A large amount of electrical energy is not used for exhaust gas scrubbing production, but is converted into heat energy loss inside the pump body and pipeline, resulting in serious waste of electrical resources. Long-term inefficient operation makes the overall energy consumption of the unit remain high, directly pushing up the unit power consumption of diammonium phosphate, increasing the company's production and operating costs, reducing the overall economic benefits of the phosphate production unit, and hindering the achievement of the company's production quality improvement, efficiency enhancement, and energy conservation goals.

[0004] To address these issues, we provide an energy-saving tail washing system for phosphate fertilizer workshops. Summary of the Invention

[0005] The purpose of this invention is to provide an energy-saving tail washing system for ammonium phosphate workshops. By configuring three washing pumps with only one pump connected to the circulating washing main pipe for operation and the other two as backup pumps, and by setting a return pressure stabilizing pipeline and regulating valve at the outlet of each pump, the problem of pipeline pressure buildup, equipment damage and high energy consumption caused by parallel operation of two pumps in the prior art is solved.

[0006] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0007] This invention relates to an energy-saving tail gas washing system for a phosphate fertilizer workshop, comprising a tail gas absorption tower, a circulating washing main pipe, and three washing pumps. The outlets of the three washing pumps are all connected in parallel to the circulating washing main pipe, which is connected to the spray layer of the tail gas absorption tower. Each washing pump has a T-connector on its outlet pipe, which has a first port, a second port, and a third port. The first port is connected to the outlet of the washing pump, the second port is connected to the circulating washing main pipe, and the third port is connected to a return pressure stabilizing pipeline. Each return pressure stabilizing pipeline is equipped with a regulating valve.

[0008] Of the three washing pumps, only one is in operation and is transporting the washing liquid to the spray layer via the circulating washing main pipe. The other two washing pumps are in standby mode, and the branches of the circulating washing main pipe corresponding to the two standby washing pumps are in an open circuit state.

[0009] The present invention is further configured such that the return end of each of the return pressure stabilizing pipelines is connected to the washing liquid storage tank.

[0010] The present invention is further configured such that the regulating valve is an electric regulating valve or a pneumatic regulating valve.

[0011] The present invention is further configured such that the rated output flow of the single washing pump meets the total circulating flow requirement of the tail gas washing spray within the rated load range of 70% to 110% of the diammonium phosphate unit.

[0012] The present invention is further configured such that each of the three washing pumps is provided with a check valve at its outlet, and each check valve is located between the outlet of the corresponding washing pump and the connection point of the return pressure stabilizing pipeline.

[0013] The present invention is further configured to include a DCS controller, wherein the DCS controller is connected to the drive motors of each of the regulating valves and each of the washing pumps respectively.

[0014] The present invention has the following beneficial effects:

[0015] 1. The present invention has a pipeline structure in which only one of the three washing pumps is connected to the circulating washing main pipe for operation, and the other two are used as backup pumps. This eliminates the problem of pressure build-up in the pipeline network with two pumps in parallel, reduces pump vibration, mechanical seal leakage and pipeline pressure risks, and extends the service life of the equipment.

[0016] 2. Each pump outlet of this invention is equipped with a return pressure stabilizing pipeline and a regulating valve. When a single pump is running, part of the washing liquid can be diverted through the return pressure stabilizing pipeline to stabilize the pipeline pressure, avoid large fluctuations in spray pressure, and ensure constant tail gas washing and absorption efficiency.

[0017] 3. This invention eliminates the pressure buildup problem in the parallel pipeline of dual pumps, solves the impact damage problem caused by repeated opening and closing of the pump outlet check valve due to pressure backflow, avoids backflow of washing liquid and pump body cavitation failure, improves the reliability of system operation, and at the same time, the pipeline and flange welds no longer bear excessive pressure for a long time, reducing the risk of leakage of dilute phosphoric acid washing liquid, and eliminating the hidden dangers of personnel burns and environmental leakage. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of an energy-saving tail washing system used in a phosphate fertilizer workshop.

[0020] The attached diagram lists the components represented by each number as follows:

[0021] 100. Tail gas absorption tower; 200. Circulating washing main pipe; 300. Washing pump; 400. Return pressure stabilizing pipeline; 500. Regulating valve; 600. Washing liquid storage tank; 700. DCS controller; 800. Check valve. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] Example 1: A company's diammonium phosphate tail gas scrubbing system is equipped with three scrubbing pumps 300 (tag numbers P0404A, P0404B, and P0404C). The original design was for a two-on-one standby operation mode, with the three pumps connected in parallel to the same circulating scrubbing main pipe 200 to undertake the task of transporting the circulating scrubbing liquid from the tail gas absorption tower 100.

[0024] The specific implementation method of this embodiment is as follows:

[0025] (I) System Structure Modification

[0026] The three washing pumps 300 are configured such that only one washing pump 300 is connected to the circulating washing main pipe 200 for operation, and the other two washing pumps 300 are used as standby pumps. During normal stable production (the unit load ranges from 70% to 110% of the rated load), only one washing pump 300 (such as P0404A) operates continuously, and the other two (P0404B and P0404C) are used as standby pumps. The standby pumps are only switched briefly under extreme conditions (the unit is at full load and the instantaneous concentration of dust and ammonia in the exhaust gas reaches its peak, or a single pump needs to be switched for maintenance). It is prohibited for two or more washing pumps 300 to operate in parallel synchronously for a long period of time.

[0027] In practical implementation, each of the three washing pumps 300 is equipped with a check valve 800 at its outlet. Each check valve 800 is located between the outlet of the corresponding washing pump 300 and the connection point of the return pressure stabilizing pipeline 400. The check valve can effectively prevent backflow of washing liquid, avoid pump body cavitation failure, and ensure that the standby pump can be reliably started when needed. The check valve should be selected from materials resistant to dilute phosphoric acid corrosion, such as 316L stainless steel or duplex stainless steel, to ensure long-term reliable operation in corrosive media.

[0028] (II) Setting of Reflux Voltage Stabilizer

[0029] A return pressure stabilizing pipeline 400 is installed at the outlet of each washing pump 300, with the return end connected to the washing liquid storage tank 600. An electric regulating valve 500 is installed on the return pipeline. Each regulating valve 500 independently and automatically adjusts the return opening according to the pressure signal at the outlet of the corresponding washing pump 300. A portion of the washing liquid is diverted through the return pipeline to stabilize the pipeline pressure. The return flow rate is controlled within 5% to 15% of the rated flow rate of the operating pump to maintain the pressure of the circulating washing main pipeline 200 within ±3% of the design working pressure.

[0030] The 500 regulating valve can be either an electric or pneumatic regulating valve. The specific choice depends on the air supply conditions of the field instruments and the control accuracy requirements. Electric regulating valves have the advantages of fast response and high control accuracy, and are suitable for applications requiring high pressure stability. Pneumatic regulating valves have the advantages of simple structure and good explosion-proof performance, and are suitable for chemical sites with corrosive media. The diameter of the return pressure stabilizing pipeline should be determined by hydraulic calculation based on the return flow range, and is generally taken as 1 / 4 to 1 / 3 of the outlet diameter of the washing pump to ensure that the regulating valve has good regulating characteristics within the operating opening range.

[0031] (III) Motor replacement

[0032] The original Y2-315L2-4-WF1 type obsolete motors will be replaced with YE4-355M2-4GWF1 type high-efficiency energy-saving motors, with a rated power of 250kW, a rated voltage of 380V, and an energy efficiency rating of no less than IE4. The efficiency of the YE4 series motors fully complies with the GB 18613-2020 Level 2 energy efficiency or IEC 60034-30 IE4 level energy efficiency requirements. This series of motors are high-efficiency energy-saving and environmentally friendly motors, featuring low operating costs, low vibration, low noise, and high reliability.

[0033] When selecting a motor, it's important to note that although the rated power of the YE4 motor (250kW) is greater than that of the original Y2 motor (185kW), the actual operating power is much lower than the rated power under single-pump operation conditions. The motor always operates within a light-load, high-efficiency range, preventing additional energy consumption. Conversely, due to the significant efficiency improvement of the YE4 motor, even at the same output power, its input power is lower than that of the Y2 motor. Taking this modification as an example, the efficiency of the YE4-355M2-4GWF1 motor at the corresponding load point is about 3 to 5 percentage points higher than that of the Y2-315L2-4-WF1 motor, which alone can achieve an energy saving effect of about 3% to 5%.

[0034] (iv) Control system connection

[0035] The DCS controller 700 is connected to the drive motors of each regulating valve 500 and each washing pump 300. The DCS controller 700 collects the pressure signal and temperature signal at the outlet of each washing pump 300, as well as the current signal and vibration signal of the motor in real time. When the DCS controller 700 detects that the outlet pressure fluctuation of a washing pump 300 in operation exceeds the set threshold, it automatically adjusts the opening of the regulating valve 500 on the corresponding return pressure stabilizing line 400 to stabilize the system pressure.

[0036] When the DCS controller 700 detects a malfunction in the operating pump (such as a sudden drop in outlet pressure, motor overload, or excessive vibration), it immediately sends a start command to the standby pump and simultaneously closes the check valve 800 at the outlet of the malfunctioning pump, enabling a non-stop pump switching operation. Once the operating parameters of the standby pump stabilize, the system returns to normal single-pump operation. The DCS controller 700's logic configuration should also include safety interlock functions such as high pressure alarm, high vibration alarm, and motor overload alarm to ensure timely response under abnormal operating conditions and protect equipment and personnel safety.

[0037] (V) Implementation Results

[0038] The equipment configuration before rectification is shown in the table below:

[0039] Exhaust gas scrubbing pump P0404A Y2-315L2-4-WF1 Electricity 380 185 Exhaust gas scrubbing pump P0404B Y2-315L2-4-WF1 Electricity 380 185 Exhaust gas scrubbing pump P0404C Y2-315L2-4-WF1 Electricity 380 185

[0040] The equipment configuration after rectification is shown in the table below:

[0041] Exhaust gas scrubbing pump P0404A YE4-355M2-4GWF1 Electricity 380 250

[0042] The results after implementation are as follows:

[0043] The problem of pressure buildup in the parallel pipeline network of dual pumps was eliminated, pump vibration and mechanical seal leakage frequency were reduced, and power consumption in the washing process was significantly reduced. The power consumption per unit product of diammonium phosphate decreased from 30 kWh / t before the rectification to 29.31 kWh / t. The spray pressure was stabilized within ±3% of the design working pressure, the tail gas washing and absorption efficiency was constant, and the ammonia content in the tail gas was consistently below 20 mg / Nm³. 3 The particulate matter concentration is below 30 mg / Nm³, meeting the control requirements of GB 31573 "Emission Standard for Water Pollutants and Emission Standard for Air Pollutants in Phosphate Fertilizer Industry". The frequency of maintenance for pumps, mechanical seals and bearings has been changed from quarterly to annual, which has significantly reduced operation and maintenance costs. At the same time, the pipelines and flange welds no longer bear pressure exceeding the limit for a long time, eliminating the risk of leakage of dilute phosphoric acid washing liquid. The working environment and safety level of the positions have been significantly improved.

[0044] Example 2

[0045] The same technical solution can be adopted for newly built phosphate fertilizer plants or for the renovation of in-service plants:

[0046] It is equipped with three washing pumps 300. The outlets of the three pumps are connected in parallel to the circulating washing main pipe 200. The three pumps are configured such that only one washing pump 300 is connected to the circulating washing main pipe 200 for operation, and the other two are used as backup pumps. Each pump outlet is equipped with a return pressure stabilizing pipeline 400 and a regulating valve 500. The matching motor is a YE4 series or higher energy efficiency level (such as IE5) high-efficiency energy-saving motor.

[0047] In practical implementation, for newly built equipment, the selection of washing pumps and pipeline design can be carried out in the design stage according to the one-on-two-standby mode to avoid additional investment in later modification. The diameter of the 200mm circulating washing main pipe should be designed according to the rated flow of a single pump, without the need for expansion according to the flow of two pumps in parallel, which can save pipeline investment. The selection of nozzles for the spray layer should also be matched according to the rated pressure and flow of a single pump to ensure the best spraying effect.

[0048] For retrofitting existing equipment, pipeline modifications can be completed without stopping the equipment or with only a short-term shutdown. The return ends of each return pressure stabilizing pipeline 400 are connected to the washing liquid storage tank 600, utilizing the existing interfaces of the washing liquid storage tank without the need for additional large equipment. The selection of the regulating valve 500 needs to be determined based on the return flow rate and control accuracy requirements. The DCS controller 700 can be upgraded using the existing control system through software configuration without the need for additional hardware investment.

[0049] The specific equipment selection can be adapted according to the scale of the plant and the amount of exhaust gas to be treated. As long as the rated flow of a single pump can cover the washing cycle requirements of 70% to 110% of the rated load range of the plant, it is sufficient. For smaller plants, smaller power washing pumps and motors can be selected to further reduce equipment investment and operating costs. For larger plants, the specifications of washing pumps and motors should be increased accordingly to ensure that a single pump can meet the spraying requirements of the full load range.

[0050] Example 3

[0051] This invention is also applicable to washing pump systems that have already adopted variable frequency speed control. Based on the operation of a single washing pump 300, the motor speed can be further adjusted by the frequency converter to achieve precise control of the output flow of the washing pump, so as to adapt to the real-time changes in the load of the device.

[0052] The specific implementation method is as follows: A frequency converter is configured on the drive motor of a single washing pump 300. The DCS controller 700 calculates the required spray volume in real time based on the ammonia concentration and particulate matter concentration signals in the outlet tail gas of the tail gas absorption tower 100, and adjusts the motor speed through the frequency converter to realize the on-demand adjustment of the output flow of the washing pump. The regulating valve 500 on the return pressure stabilizing pipeline 400 serves as an auxiliary adjustment means to supplement the adjustment when the response of the frequency converter adjustment is lagging or exceeds the adjustment range.

[0053] This implementation method further improves the energy-saving effect and control flexibility of the system, and is especially suitable for phosphate fertilizer production plants with large load fluctuations. Under low load conditions, the frequency converter can reduce the motor speed to 80% to 90% of the rated speed. According to the similarity law of pumps (flow rate is proportional to speed, and power is proportional to the cube of speed), the pump energy consumption can be reduced to 51% to 73% of the rated power consumption. The role of the reflux pressure stabilizing pipeline is more important in this scheme. During the response process of frequency conversion regulation, the reflux regulating valve can quickly respond to pressure fluctuations, play a buffering and stabilizing role, and ensure a smooth transition of spray pressure.

[0054] It should be noted that the selection of the frequency converter in this embodiment should be determined according to the rated power and operating conditions of the washing pump motor. Generally, it should be selected at 1.1 to 1.2 times the rated power of the motor. The frequency converter should have PID regulation function and be able to receive 4 to 20mA analog signals from the DCS controller to realize automatic speed adjustment. At the same time, the frequency converter should be set with reasonable acceleration and deceleration time (generally set to 10 to 30 seconds) to avoid sudden speed changes from impacting the pipeline system and spray layer.

[0055] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0056] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention.

Claims

1. An energy-saving tail gas scrubbing system for a phosphate fertilizer workshop, comprising a tail gas absorption tower (100), a circulating scrubbing main pipe (200), and three scrubbing pumps (300), wherein the outlets of the three scrubbing pumps (300) are all connected in parallel to the circulating scrubbing main pipe (200), and the circulating scrubbing main pipe (200) is connected to the spray layer of the tail gas absorption tower (100), characterized in that: Each of the washing pumps (300) is provided with a three-way connector on its outlet pipe. The three-way connector has a first port, a second port and a third port. The first port is connected to the outlet of the washing pump (300), the second port is connected to the circulating washing main pipe (200), and the third port is connected to a return pressure stabilizing line (400). Each of the return pressure stabilizing lines (400) is provided with a regulating valve (500). Of the three washing pumps (300), only one washing pump (300) is in operation and delivers the washing liquid to the spray layer via the circulating washing main pipe (200). The other two washing pumps (300) are in a standby state, and the branches of the circulating washing main pipe (200) corresponding to the two standby washing pumps (300) are in an open circuit state.

2. The energy-saving tail washing system for phosphate fertilizer workshops according to claim 1, characterized in that: The return end of each of the aforementioned return pressure stabilizing lines (400) is connected to the washing liquid storage tank (600).

3. The energy-saving tail washing system for a phosphate fertilizer workshop according to claim 1, characterized in that: The regulating valve (500) is an electric regulating valve or a pneumatic regulating valve.

4. The energy-saving tail washing system for a phosphate fertilizer workshop according to claim 1, characterized in that: The rated output flow of the single washing pump (300) meets the total circulating flow requirement of tail gas washing spray within the 70%~110% rated load range of the diammonium phosphate unit.

5. The energy-saving tail washing system for a phosphate fertilizer workshop according to claim 1, characterized in that: Each of the three washing pumps (300) is equipped with a check valve (800) at its outlet. Each check valve (800) is located between the outlet of the corresponding washing pump (300) and the connection point of the return pressure stabilizing pipeline (400).

6. The energy-saving tail washing system for a phosphate fertilizer workshop according to claim 1, characterized in that: It also includes a DCS controller (700), which is connected to the drive motors of each of the regulating valves (500) and each of the washing pumps (300).

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