1. Introduction of the technology

There are three procedures mainly in the production of the synthetic ammonia by the natural gas. The first one is gas making, which includes the compression of the natural gas, sulfur removal, primary conversion, secondary conversion, high temperature shift, low temperature shift. The second part is purification, which includes decarbonization and methanation. The last one is synthesis, which includes the compression of the synthesis gas and ammonia synthesis, and liquid ammonia is separated out after the refrigerating procedure.

The brief diagram of the process flow is shown below. 

2. Control Scheme

Advanced HOLLiAS DCS is used for the instrument and control system of the synthetic ammonia plant 300kt/a. By the powerful functions such as feedback control, logic sequential control and various computation, multiple complex control system of the synthetic ammonia plant large-scaled and energy-saving is realized, for example, the control system of the ratio of the water and carbon, the control system of the ratio of the hydrogen and nitrogen, the 3 impulse control system of the boiler barrel, the anti-surge control system of the compressor.

The typical control system is shown in the table below.

A. The control system of the ratio of the water and carbon

The two-way cascade control system is used to control the ratio of the water and carbon FL0209, and with the function of logical addition and subtraction. The main regulated quantity is the duty FL0215, and the minor regulated quantity is the feed gas flow FI0001 and the steam flow FI0002.

The main merits include:

1. Regulate the steam flow automatically to ensure the controlled ratio of the water and carbon according to the analysis of the total carbon FL0201.
2. When the system add and subtract the duty, it can be ensured that the stream is added firstly and the natural gas is added secondly, else the natural gas is subtracted firstly and the stream is subtracted secondly, which can quality the catalysis can not be coked.

3. Two-factor control can avoid the mishandling.

B. The control system of the ratio of the hydrogen and nitrogen

The two-stage cascade system is used to control the ratio of the hydrogen nitrogen. The main parameter is the ratio of the hydrogen and nitrogen of the synthetic gas AIC411 from the inlet of the synthesizing tower. The first minor parameter is the ratio of the hydrogen and nitrogen of the live gas AIC410 entered the synthesizing system, and the second minor parameter is the feedforward signal FX0204 come from the ratio system of the water and carbon.

The main merits include:

1. Overcoming the large time lag of the technological process.

2. The feedforward signal led in ensure that the air capacity can increase gradually and decrease rapidly when the duty vary, so as to avoid the temperature of the secondary conversion exceed the allowable value.

C. The control system of the outlet temperature of the primary conversion

The main loop of the cascade system is the outlet temperature of the primary conversion TIC117, and the minor loop is the flow of the fuel gas FIC063. The total carbon flow of the feed gas is introduced to overcome the time lag, and the pressure of the fuel gas PIC029 is introduced to be used as alternative control to avoid the flame being blow off at low pressure.

D. The level control system of the 3 impulse boiler barrel

The design purpose of the 3 impulse control system is to overcome the false level; with the main parameter is level and the reference parameter is steam quantity produced and the feed water quantity. The main regulator is LIC001 and the minor regulator is BEWC01, whose measured value is the difference between the feed water quantity FI0006 and the steam quantity FI0033, and the system is used as 3 impulse systems or single impulse is determined by the switch SW0006.

E. The anti-surge control system of the compressor 101-J

An anti-surge system is supplied to the control system by the technology personnel according to the theory and the practical situation. The anti-surge quantity is computed by the equation with the outlet pressure PI0048 of 101-J, which is treated by linearization so that the emptying quantity FIC004 can not below a certain reliable value at any time. Thus, the control system is an anti-surge system following-up and boundary variable.

F. The combustion control system of the subsidiary boiler

The combustion control system of the subsidiary boiler is a cascade control system, whose main regulator is the pressure of the high-pressure steam PIC018, and the minor regulator is the fuel gas FIC301 and the combustion air FIC302. The ratio of the air and the fuel gas is regulated by the analyzer of the fume oxygen contentAIC302. The main functions of the control system are:

1. The pressure of the high-pressure steam PIC018 can be controlled stably by regulating the fuel gas of the subsidiary boiler FIC301.
2. The operating procedure is that the air is added firstly and the fuel gas is assed secondly, and the gas is subtracted firstly and the fuel gas is subtracted secondly.
3. When the pressure of the fuel gas is low, it is controlled by the loop PIC029 to avoid the flame blow off.

4. The output of AIC302 is polygonal treated according to the practical situation to keep the fume oxygen content appropriately even the duty is low.

G. The interlock system

Multiple safety and reliable interlock system is designed to ensure the production run stable. When the emergency accident occur, the interlock system can bring the system to rest smooth and steady and avoid the accident extended, so as to avoid the heavy accident and malignant accident of the plant and the equipment.

Process Control Scheme for 12kt/a Ion-Exchange Membrane Project

Electrolyzing technology includes mainly about the technology of Chlorine Engineering Co Ltd., Asahi Kasei Co Ltd. and Uhdnora Co Ltd., although their processes flow are different from the others , they have some similarities as follows.

Control

Control process includes flow control on purified brine of electrolyzing cell, liquid level control on negative pole and positive pole, circulation reflux control on liquid level of negative pole and positive pole, temperature control on circulation liquid of caustic soda, and the adjustment of pure water according to current flow. The key of them is the control on differential pressure between chlorine and hydrogen.

Interlock

The interlock of electrolyzing cell is divided into public interlock and single cell interlock. Public interlock is described as: power failures of the whole factory, all chlorine pressure plants closing down, ultra high differential pressure between chlorine and hydrogen, ultra low differential pressure between chlorine and hydrogen and instrument wind failure , if a public interlock occurs ,all the interlocks of electrolyzing cell will be off work . Single cell interlock occurs, when the electric pressure of electrolyzing cell exceed the normal highly or the input flow of purified brine is low or quantity of reflux is low, but it influences the tank which it belongs merely.

Electrolyzing technology of ion-exchange membrane

Having been refined twice, crude brine which comes from first brine would become into essential brine. Essential brine is decomposed into chlorine gas, hydrogen gas, sodium hydroxide and effluent brine, after it has been electrolyzed in electrolyzing cell. Effluent brine from positive pole which has been dechlorinated is sent back to first brine pool and recycled to electrolyzing cell partly. Caustic soda from negative pole whose consistency has been adjusted is sent to caustic soda tank and electrolyzing cell in given flow. Hydrogen gas from negative pole is sent to hydrogen gas processing, while hydrogen gas from positive pole is sent to chlorine gas processing first, and then to liquid chlorine section and chlorine products plant, after has been dried.

Main arts and points of the process control

1. The technology on single shutdown ensuring for not all shutdown
2. Automatic processing techniques ensuring for safety and efficiency of fetch protection device
3. Single shutdown interlock and all shutdown interlock
4. Stabilization, safety, and ability of resisting electromagnetic-interference of the control system
5. Eight control loops: loop on flow of purified brine, loop on acidity of light brine, loop on consistency of caustic soda, loop on differential pressure between chlorine and hydrogen, loop on PH value of brine to dechlorination tank entrance, loop on PH value of t brine from dechlorination tank outlet, adjustment on oxidizing potential of dechlorinated light brine and adjustment on deoxidizing potential of dechlorinated light brine

6. Control and interlock on chelating resin of purified brine

Safety and maturity of the technology and control

Application of the HOLLiAS Field Bus in 36kt/a Production Line of Carbon Black

Carbon black, which is used widely in commercial manufacture, is a black pulverous matter, sometimes is graininess. It is used as crude material and addition agent in the fields from aerospace to staple.

The production process of carbon black includes many processes. Firstly, raw oil, burner oil and preheated air are spouted into reacting furnace and burned, through which high temperature carbon black fume is generated, secondly, the carbon black fume is dropped in temperature twice by rapid chilled water. At last, the end product is produced, after the processes of filtration and collection, porphyrization and separation, wet pelleting, drying storage and packing.

The production technology of carbon black is composed of seven systems: reaction and warming-up, main bags collecting and air charging, carbon black collecting and air charging, wet pelleting, drying, end products storing and packing while degraded products recycling, as well as pubic and accessory system.

Introduction of the control scheme

Carbon black is used widely, so how to produce high-quality carbon black efficiently by good process equipment and effective control scheme is important in production process. According to the characteristic of carbon black production and process requirements, a bus-mastering system based on PROFIBUS-DP is used by the HollySys Company, which could realize the electric corresponding period with the frequency-changers of Profibus-PA instrument, Modbus, Profibus and CAN bus, as well as reversible communication with the whole protection system. At the same time, the bus-mastering system leaves the transitions between digital and analog out, advances anti-interference ability of signals and has great advantage of measurement accuracy of technologic parameters compared with simulative instrument. The control schemes include PID, cascade control, ratio control and forced fault handling, interlocked protection, and so on. Control schemes of the seven systems in production process are introduced in the following:

• The reaction and warming-up system: there are many important parameters to adjust in this system. In the burner oil workshop section, it is important to control the burner oil flow and the ratio of wind flow to burner oil flow. In the raw oil workshop section, it is important to control the pressure, temperature and flow of raw oil put into furnace. In the wind-feeding workshop section, it is important to control the fan current, measure the temperature of axis and interlock for protection. In chilled water workshop section, it is important to control liquid level, first rapid chilled water flow, internal chilled water total flow and the temperature of chilled water. PID control is the principal control mean, parameters are set so rationally that the system is stable, responds fast and has high-quality of control.                                    
• The main bags collecting and air charging system: The sequential control and time-oriented sequential control of each electromagnetic valve are achieved by ladder diagram, the time parameter and sequence of control could be set and modified optionally. The follow-up system is used in air charging system to regulate stable, which sets upper and lower limit for charged-variables.
• The carbon black collecting and air charging: The switching sequence and interlock of the filter air evacuation and back-blowing valve for bag-discharging are achieved.
• The wet pelleting system: The temperature of mealiness tank, base weight measurement, the granulator power and granulator flow make up a cascade control system.
• Carbon black drying system: It includes pressure and temperature measurements of the air-out from fire-box dryer fire-box. Off-gas flow and air mass flow make up the ratio control system. The PID parameters are set rationally to overcome the lag of the burning system. So, the drying system could be controlled stable.
• The end products storing and packing while degraded products recycling system: It is used to measure the temperature of granular carbon black in elevating conveyor and the liquid level of end production tank.
• The pubic and accessory system: It includes compressed-air system, boiler stream system, and cooling water system and so on. The control process is mainly composed of pressure measurement of the compressed-air system, temperature and pressure measurements of the stream from boiler, pressure measurement of the cooling water system, liquid level control of water tank and flow control of the tap water.