On the air separation plant automation _5265

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Automation of Air Separation Unit
 
 
Abstract: This paper describes the status of the air separation unit, outlined Hong Kong and Guangdong 6000Nm3 / h oxygen plant in the MS process, and the automation system configuration and typical control loop is analyzed.    Keywords: air separation plant distributed control system, anti-surge     1 Overview   With the industrialization process in the world the continuous development, metallurgy, chemical industry, shipbuilding, machinery and other traditional industries, an increasingly large scale, and on oxygen, nitrogen, argon and other gases required by increasing the depth of frozen liquid air separation method has been to adapt to large-scale industrial production of the preferred method . Its main principle is that air is compressed, the use of air cooling and liquefaction of oxygen, nitrogen and other gases of different boiling point, several times with evaporation, condensation methods repeatedly separated by distillation products, then the product for different purposes pressure storage and delivery to the user. In this paper, Hong Kong and Guangdong 6000Nm3 / h oxygen plant as an example to talk about the characteristics of the control system.   2 air separation process and its automatic control system features   now commonly used pressurized oxygen and medium-sized molecular sieve purification of the whole low pressure process, that is, MS process. Heat than switching (REVEX process) to simplify a lot. MS process only bear the primary heat exchanger cooling process of air, while re-heat reflux of gas, does not assume the task of water and carbon dioxide removal, and thus do not need to switch. To Hong Kong and Guangdong 6000Nm3 / h oxygen plant, for example, purified air all the way through the cold box heat exchangers to lower column oxygen and nitrogen fractionation fractionator; another pass booster pressurized air to the heat exchanger and then through the cooling expander expansion, and finally to oxygen and nitrogen fractionation tower fractionation tower to get oxygen, nitrogen, oxygen and argon distillate products. After the argon fraction out of the cold box after argon purification system, cold box and then return the liquid argon argon distillation column fractionation. Waste nitrogen produced by the cold box sent to purification systems for adsorbent regeneration. Key equipment of the project (including the equipment for instrumentation single equipment) are imported, such as oxygen machine SULZER company, the ice machine is JOY company, the air compressor is COPPER company. The system of control points are 148 points of analog input, analog output 6l point, RTD input 87 points, digital inputs (24VDC) 148 points, digital output (220VAC, 50Hz) 101 points, instrumentation and control system produced by HONEYWELL The MincroTDC-3000X-based distributed control system, shown in Figure 1.   UCN is a universal control network protocol consistent with MAP, with the characteristics of high-speed communications network redundancy, scalability APM Advanced process management station. Because the system is not linked to the Internet LM UCN logic management station. Therefore, similar molecular sieve in the preparation of the order of switching control program, the alone to complete the APM, the efficiency to lower, not easy, as easy on the LM fast interlocking control functions and the general conventional PLc. ICN can achieve local control network wide data collection, and by US centralized monitoring. HM Historical modules Winchester hard drive to ensure during power outages, data loss, while HM run time create a system standby database, you can download data to enable continuous control data.   system I / O, including HLAI high level analog input module, LLADMUL (RTD) module, AO analog output module, DO (220VAC, 50HZ) module, DI (24VDC) module. Total system I / O load accounted for the maximum allowable amount is less than 85%. Loading rate is reasonable. To improve system security and reliability, UCN, LCN, NIM, and power modules are redundant structure.   need to mention that the APM and between NIM communication cable length is generally less than 10m, then we must increase if more than two connectors, so the control room layout should take this into account, the APM as close as possible with the NLM.   some complete sets of air separation equipment, come with a control panel, mainly in situ oxygen compressor cabinets, counters can be completed in place of conventional control of the oxygen compressor.   gas project in Guangdong and Hong Kong, in order to facilitate centralized control of the oxygen compressor of some important parameters such as pressure oxygen compressor entrance display and return valve adjustment, complete control by the DCS, which makes the oxygen regulating the flow of monomer into the press the total logistics control system. Place in the field cabinets and the control room can be carried out on the oxygen compressor start / stop operation.   order to make accurate and reliable monitoring and measurement requirements, site selection HONEYWELL transmitter's ST3000, 900 series of smart transmitters, temperature control valve select U.S. VALTCK valves, mass flow FISH-selected ROSMENGT products, motor bearings, motor stator, the cold box and some other important places difficult to maintain or double vessel thermal resistance are used. Another analysis instrument room equipped with surface analysis instrument cabinet 6, a total of 12 (including 9 units of imports), respectively, for the analysis of product purity, liquid air, oxygen content of the crude argon, nitrogen, oxygen and other pollution process parameters, such analysis useful instrument for the study of fractionation conditions.   for liquid oxygen, liquid nitrogen, liquid argon mass flow measurement commonly used (about 2.0% accuracy), but the need for good thermal insulation pipe, or vaporization of the liquid will affect the measurement accuracy. For the production of small liquid argon has the essence of argon in the cold box to set the buffer tank level gauge (accuracy of about 3.0%) indirect measurement of liquid production, but taking into account the liquid level fluctuation will affect measurement accuracy. 

  3 Typical Control System Analysis   3.1 air compressor control system (Figure 2)     generally constant pressure air compressor and the constant flow in the run, when the pressure or flow is too high, and regulation by adjusting the PV valve venting elution flow control valve FV to maintain a certain degree of pressure and traffic and avoid the surge occurred. When the product requirements change, the need for air compressor flow, pressure, temperature control, the general air compressor to ensure that the liquid yield in the case of constant, variable conditions the range of flow control 108% 75% . The picture is by the air flow into the cold box to adjust the MAC FC2 valve inlet guide vanes, so that the flow of air into the cold box to keep around the set value. At present more practical automatic anti-surge control is when the flow of a centrifugal compressor is too small, the pressure is too high, automatically opens the vent valve designed as a starting point. UY introduction of air compressor discharge pressure, compressor outlet flow, with air compressor surge curve provided by the manufacturer to control the venting valve to prevent the surge occurred. For safety considerations, relief valves should be selected FO valve FV. When surge occurs,You are not allowed to view links. Register or Login, to ensure rapid evacuation, and easy maintenance in the field consider the case of relief valves should be as close to the head of FV, Bleeding from the pipe to the muffler is also the best short. Bleeding control while also considering some bold as to facilitate the road.   3.2 booster anti-surge control   surge phenomenon is the structural characteristics of a centrifugal compressor caused. When the air flow through the booster flow of traffic reduced to a minimum value, the compressor performance becomes unstable. Surge, the pressure and flow fluctuations will increase the vibration of the booster should be avoided. Load reduction is the main reason for the compressor surge, requiring an anti-surge control system to ensure that any speed limit flow Ql compressor surge limit line will not be less than the limit corresponding to flow Qp, and would like to boost machines as far away from the normal operating point when the operating point surge, which can guarantee a greater range of compressor flow control. So take circular flow method, can prevent access to the surge area. Let the flow through the compressor is always greater than a certain value traffic, when the supercharger intake volume is greater than Q1 Qp, the bypass valve is shut; when Q1 is less than Qp, the bypass valve is opened, so that through the booster gas flow rate increases to greater than Qp, the results of gas supply to the pipeline system will be reduced to meet the needs of the load decreases.   Qp is a certain value, the correct selection is the key Qp, Qp usually on a maximum speed limit traffic surge when the given value as an Fc. But there is a problem is low speed when the booster, while booster is not entering the surge area, but the entrance of gas may also be less than the set value (set value is set by the maximum speed of flow), the result is bypass valve is opened, part of the gas back, resulting in energy waste. Therefore, frequent changes of load, this method is not for you. Also sometimes not always in the booster inlet throttling device installed on the pipeline, as measured in the inlet pipe flow can cause pressure drop, so the export pipeline to install the supercharger throttle device, shown in Figure 3  This project corresponds to the flow of traffic Ql Qp than slightly larger than the surge limit of 5% to 10%. 3.3 Hydrogenation ratio control system in oxidation furnace comes from the crude argon column reaction of the crude argon and hydrogen to remove oxygen in the crude argon. Oxidation furnace imported crude argon flow with the oxygen content detected by multiplying the crude argon in order to calculate the actual volume flow of oxygen, taking into account after the oxidation reaction of excess hydrogen gas furnace (about 1%) can be calculated adding the actual volume flow of hydrogen.   reaction, the process is less than the oxygen content in argon 3ppm, excessive hydrogen content of less than l%. If the crude argon column is greater than the oxygen content in the crude argon 4.5%, or lower inlet temperature, stop the crude argon compressor. Because of high crude argon oxygen, indicating a problem fractionation process, the production of oxygen will be affected, through oxidation furnace will be more difficult to reduce the oxygen content.   this control system, the furnace into the oxidation of hydrogen and argon, respectively, have their own crude flow control loop. Hydrogen from the hydrogen and oxygen flow ratio set loop calculator. At the same time when the main load of the crude argon put down, the amount of hydrogen input to mention tracking down, and caused both the ratio unchanged. Hydrogen flow measurement accuracy is more important here, the project board with holes measuring.   3.4 Regeneration of waste nitrogen temperature control system   waste nitrogen in the system and steam into the steam heater. Heated waste nitrogen into the molecular sieve purification system, regeneration of molecular sieve adsorbent is used, the gas project in Guangdong, by adjusting the steam flow to stabilize the temperature of outgoing waste nitrogen regeneration. But we can see that this control system is defective, if the waste nitrogen flows into the heater changes, will affect the export of waste nitrogen in the temperature of the heater; while the amount of change is slow because of temperature measurement, and use it to control the amount of steam into a lag problems. So if using feedforward control, waste nitrogen flow changes will interfere with the introduction of the pilot control loop, can enhance the response speed, stable temperature. However, in order to ensure the accuracy of feed-forward compensation, the requirements for more stringent control valve, through the output of the feedforward controller and temperature controller's output, after superposition of the steam flow controller as a given value, which constitutes a feed-forward. Cascade control, the effect will be better.   4 Conclusion   Today, more and more towards the air separation technology specialization, scale, standardization of the direction, to ensure the maximum benefit under the premise is that efforts to reduce energy consumption The technology faces a major work. Good at practical work in the problems found in the system, improve process control solutions, will further enhance the automation and control of air separation technology.   References   l Wei Chang. Air compressor applications. Machinery Industry Press. 1987   2 Age of Mu. Fractionation. Chemical Industry Press. 1971.