Introduction

Reactor is one of the key dosing equipments in the chemical production process, due to the complexity of the chemical reaction mechanism inside the reactor, the external conditions, changes in the amount of raw materials added, changes in the temperature of the heating and cooling water, etc. have a greater impact on the system, so that the system itself has a greater time-varying, non-linear and time-lagged, so it has been subjected to the attention of research institutes in the field of process control. Here, with a Sino-Japanese joint venture rust preventive oil (wax) project as the background, combined with the current domestic and international advanced automatic dosage system technology, in the reaction sign of the feed control, put forward the control scheme of quality measurement, and achieved the ideal control effect.

1 The determination of control program

1.1 Process requirements

The process provided by the Japanese company for a typical fine chemical process, raw material ratio > reaction process > measurement and packaging, automatic dosage control system process involves a number of control circuits, including raw material ratio control circuit, reaction process temperature control circuit and finished product measurement and packaging control circuit. The project uses steam-heated reactor to complete the process reaction. The order of adding raw materials to the process is as follows: first add the liquid reaction materials to the reactor, then heat the medium temperature to slightly higher than the reaction temperature or the melting point of the solid reactants; slowly add the solid reactants under constant stirring, so that it can continue to react or dissolve to avoid precipitation. Due to the different formulations, each product requires about 4 kinds of liquid raw materials and 2 kinds of solid raw materials, and the dosing ratio is required to reach 0.1%.

1.2 Analysis of the original control program

The original control program for the Japanese company for four tanks through the pump to the reactor to add material, the main ratio through the gear flow meter flow measurement, temperature compensation, density calibration to achieve the measurement of raw material quality, in each pipeline into the reactor to set up a cut-off valve and flow for interlocking, each reactor flowmeter using a 4 ~ 20mA standard signals into the control room of the PLC, the PLC in the control room according to set the proportion of formula control to achieve the feed automatically. The PLC in the control room realizes the automatic metering of the feed material according to the set formula ratio control.

The study found that the original control program has the following shortcomings:

(1) because of the transport of raw materials need steam accompanied by heat (raw materials with low condensation point), the transport pipeline is long, containing large solid particles, resulting in unstable density of the liquid flowing through the flowmeter, the flowmeter internal components of the wear rate is large, and can not achieve the process of the 0.1% accuracy requirements;

(2) tank area to the production of wide room has a certain distance, in the northern region of the cold season steam accompanied by heat can not make the pipeline medium temperature uniformity, can not determine a representative temperature detection location, this situation flow measurement of the control program derived from the quality of the error is greater;

(3) The routine maintenance of the flow meter requires more labor hours.

1.3 Improved control scheme

In view of the shortcomings of the above scheme, on the basis of the demonstration of the process and the comparison of program investment, the control scheme of mass measurement is proposed, i.e., the application of weighing and dosing system.

The weighing and dosing system is mainly realized by weighing instrument. As the weighing instrument with fixed value comparison function, can be directly used for automatic material sorting, screening, bulk quantitative packaging, production batching, filling, logistics in and out of goods and other process control, has been widely used in medicine, raw materials processing, petroleum, chemical and other fields.

2 Weighing system program implementation

2.1 Hardware composition of weighing control system

2.1.1 Weighing module

In order to accurately reflect the changes in quality, the improvement program weighing module adopts a company's high-end process FWC series metering module. The sensor consists of a pressurized head and a field junction box. Pressure head is installed on the side of the reactor into a 120 ° distribution, the weight of the reactor is uniformly pressed on each pressure head. 3 sensors in the form of MV into the field junction box, the field junction box will be converted into the 3 sensor signals are summed into a standard 4 ~ 20mA standard current signals sent to the weighing instrument. 2.1.2 Weighing Instrumentation

In order to meet the requirements of the harsh industrial environments, the weighing instruments are equipped with PANTHER weighing terminals, which are equipped with the patented TraxDspTM digital filtering technology, which allows to obtain stable weight values in real time, and with a metal casing, which is safe for use in reactors and meets the requirements for on-site installations. The weighing terminal can be used on site to perform zero, full and tare functions. In addition, the instrument is equipped with Profbus-DP fieldbus interface, which can communicate with PLC in the control room at high speed and send the weight data to PLC to participate in the measurement control.

2.1.3 Transfer pump and metering control room

The conveying pump adopts airtight motor, and the start and stop of the conveying pump is automatically controlled by PIC. The metering control valve selects regulating valve, and the PLC sends out 4-20mA standard current signal to control the opening degree of regulating valve. When the signal is 4mA, the valve position is fully closed; when the signal is 20mA, the position is fully open. The opening control design of the valve position adopts an adaptive control strategy according to the size of the feeding margin. When the feeding is close to the end, the valve position can be appropriately closed, and when the set quality is reached, it is all closed to ensure the accuracy of feeding.

2.1.4 Programmable controller (PLC)

Mitsubishi's 0 series medium-sized PLC is used to complete the feeding and weighing control of the kettle. 4 kinds of PLC modules are used: switching input module, to receive on-site weighing start command, emergency stop command and pump open status; switching output module, to send out the start of the transfer pump command and on-site metering instructions; analog output module, to regulate the metering valve openness; Profibus-DP communication module, to communicate with the weighing instrument, to collect the instrumentation and to control the weighing accuracy; PLC is used to control the weighing of the kettle. communicates with the weighing instrument and collects the quality of the instrument.

2.1.5 Industrial computer

The industrial computer communicates with PLC through RS232 port to complete the setting of weighing formula, monitoring the status of the whole dosage system, saving and printing the metering data, and realizing the automatic metering of feeding materials in the control room.

2.2 Software realization of weighing control system

2.2.1 PLC software programming

PLC through the special programming software GXDeveloper on its programming control logic with ladder diagrams written in the PLC for arithmetic control, its logical structure: start > load recipe > set the feed value > set the value of zero > press the feed control knob > open the feed pump > delay open the feed pump > to close the feed pump value > close the feed pump > to close the feed valve > delay waiting for the end of the discharging > write down the actual Discharge value.

2.2.2 Profibus-DP Communication Software

Profibus-DP communication protocol is characterized by fast communication speed, strong anti-interference ability and error correction ability, and both Mitsubishi PLC and PANTHER weighing terminal have interfaces in accordance with Profbus-DP communication protocol. There are 5 PANTHER weighing terminals on site, set 1~5 address number respectively, set 0 address in PLC for Profbus-DP interface, communication baud rate bit 1.SMBP, to ensure that the data refresh cycle is less than 0.1s.2.2.3 Friendly interface programming.

(1) industrial computer through the form of RS-232 port and PLC exchange of data including calculations to read the PLC in the control status of field devices, the process of feed metering, fault alarms, etc.; computer to control data to the PLC, such as metering quality, valve shutdown quality, pump shutdown quality.

(2) Field equipment status display. Field operation status using dynamic time tracking, you can directly monitor the quality of the reactor, the operating status of the feed pump, metering control valve opening, has been feeding conditions, etc., Figure 5 for the field display schematic.

(3) Recipe editing. Workers can complete the new recipe, modify, delete, transfer people and other operations, set the quality value of the measured material. Each recipe can include the name of the material and set the quality, that is, it is convenient for workers to set the quality and not easy to operate error.