Chlor-alkali chemical industry, no matter what kind of process method is used to make alkali, it is inevitable to encounter a nasty product - chlorine gas, and it is precisely because of the existence of chlorine gas that the increase in caustic soda production is limited. The physical and chemical properties of chlorine gas determine that it is easy to be liquefied, the chlorine gas liquefaction is not a means of chlorine treatment, liquid chlorine is more convenient than gaseous chlorine storage and transportation. Chlorine in such industries as synthetic fibers, pharmaceutical industry, leather industry, paper industry, printing and dyeing industry, as well as hospitals, water disinfection, oil, electricity, detergents and other widely used, but also most of the use of liquid chlorine cylinders transport use. Improper filling operation of liquid chlorine makes its toxicity proliferation, and doubly concerned.

I. Process Introduction

Our company adopts low-pressure production process for liquid chlorine production, the dry chlorine gas sent from chlorine-hydrogen process enters into tube-type liquefier, exchanges heat with calcium chloride brine at -35℃ to liquefy the chlorine gas at low temperature, the liquid chlorine flows into liquid chlorine tank or vaporizer from the bottom of the liquefier, and the tail gas which has not been liquefied returns to the chlorine-hydrogen tail chlorine distribution table from the top of the liquefier. Liquid chlorine filling using a vaporizer pressurized method of liquid chlorine storage tank pressure out, weighed and measured pressed into the liquid chlorine cylinder for packaging. After the cylinder is packed to the specified filling weight, the liquid chlorine in the packing line is pumped to the secondary sodium absorption.

Second, the problem

With the expansion of the enterprise production scale year by year, the pressure of liquid chlorine production is also increasing. The original 4 sets of filling equipment to 6 sets, the operator's labor intensity increased. In order to ensure that the filling process is strictly controlled without the phenomenon of overfilling, the number of operators increased accordingly. However, because the filling control is manually read by the operator of the six electronic weighing instruments, judging the manual shut-off valve closed and open, the filling weight control accuracy is not high, overfilling is inevitable, bringing hidden dangers to the safety of production.

Structure and working principle of liquid chlorine cylinder scale system

Liquid chlorine cylinder scale controls the opening and closing of the pneumatic valve to achieve automatic control of liquid chlorine cylinder filling by comparing the detected weighing signal with the given weight by PL.

The specific control process is that the empty cylinder to be filled is placed on the electronic scale, the secondary instrument of the electronic scale displays the weight of the empty cylinder and transmits the weight to the PLC controller with a 4-20mADC signal, and the same weight is displayed on the LCD touch screen connected to the PLC through the communication line. Operator will need to fill the weight through the touch screen input to confirm that there is no error click "open", the corresponding pneumatic shut-off valve to open, start filling. When the volume of empty bottles reaches the set value, the PLC automatically outputs the pneumatic valve to close and complete the filling process.

There is a manual/automatic switching device in this weighing control system, and the process operator can choose the control mode according to the actual situation on site. In addition, it can also output sound prompts to remind the operator of the end of filling or abnormal filling, which is convenient for the operator to find and solve the problem in time.

Fourth, the advantages and recommendations

PLC control is simple and due to the measurement point is not much general S7_200 can meet the control requirements. Liquid chlorine cylinder scales in most companies will be supporting the weighing module, weighing instrumentation, weighing platform, and weighing instrumentation are mostly 4 ~ 20mADC output function, direct access to the PLC can be, without the need to add too many intermediate conversion links. Relatively speaking, the selection of pneumatic shut-off valve to consider a little more, but according to the liquid chlorine process control requirements of chlorine in the water content of ≤ 0.04% corrosive has not been strong, only need to consider the low-temperature conditions and leakage levels on it, the investment will not be too large, but can receive a good control effect.

Suggested with the PLC system at the same time supporting UPS uninterruptible power supply, in order to prevent the system from sudden power failure pneumatic valve closure of the entire liquid chlorine system affects the normal production.

V. Conclusion

Through the automatic control of liquid chlorine cylinder scale, it reduces the labor intensity of operators, improves the filling accuracy, greatly reduces the possibility of overfilling, and provides guarantee for the safe, smooth and reliable operation of the whole production equipment.