Experience sharing of VOCs waste gas reduction and enrichment in printing and packaging enterprises

When printing and packaging enterprises implement VOCs waste gas treatment, the primary goal is to achieve standard emissions on the basis of safe and stable operation of environmental protection treatment facilities, while taking into account energy consumption, operating costs, maintenance costs and other related links. The implementation of VOCs waste gas treatment projects in most printing and packaging enterprises will involve the reduction of wind and concentration of VOCs waste gas. The company where the author works has completed a number of VOCs waste gas treatment cases in printing and packaging enterprises. This article will share with you the experience of VOCs waste gas reduction and concentration in printing and packaging enterprises, hoping to help printing and packaging enterprises orderly promote VOCs waste gas treatment.
Overview of air reduction and enrichment
Air reduction and enrichment means to increase the carrying efficiency of unit fresh air by using air distribution as an auxiliary improvement means. For printing and packaging enterprises, it is a way to improve the reuse efficiency of hot air in the oven of the gravure printing unit as much as possible under the premise of safety, measurability, controllability and qualified product quality, so as to reduce the air volume of VOCs exhaust gas and increase the exhaust gas concentration and reduce the heat energy consumption.
Necessity of air reduction and enrichment
First, it is necessary to reduce the investment scale of end treatment equipment. If the exhaust air volume of the gravure printing unit is large, it will inevitably lead to a large investment in the end treatment equipment. If the air volume of VOCs waste gas to be treated at the end is reduced to 50% of the original air volume, the investment scale of RTO equipment and supporting pipelines can be reduced by more than 30%. The second is the need to reduce the operating cost of terminal treatment equipment. If the air volume of VOCs input into RTO is large and the concentration is low, it will lead to high fuel consumption and high power consumption during RTO startup and operation. Printing and packaging enterprises can reduce fuel consumption during RTO startup and power consumption during operation by implementing wind reduction and concentration increase; On the other hand, when the concentration of VOCs waste gas input into RTO is not less than 1.8g/m3, RTO operation can be maintained in a self-sustaining state (that is, the normal operation of RTO equipment can be maintained without additional fuel replenishment), which will greatly reduce the cost of RTO operation. Third, it is necessary to reduce the production energy consumption of printing and packaging enterprises. After the printing and packaging enterprises implement the air reduction and concentration increase, the hot air in the oven of the gravure printing unit is recycled efficiently, which greatly reduces the steam or electricity consumption of fresh air heating. If the air volume decreases by 66%, the energy consumption for heating is expected to decrease by more than 50%; When the concentration of VOCs waste gas input into RTO is higher than 4g/m3, the heat energy can be recovered through waste heat recycling device for enterprise production, office and living.
Mode of reducing wind and increasing concentration
When printing and packaging enterprises implement wind reduction and concentration increase, they generally adopt manual adjustment or LEL automatic adjustment. Table 1 shows the comparison of two air reduction and concentration enhancement modes.
1. Manual adjustment mode
The operators of the gravure printing unit manually adjust the valves for fresh air, return air and exhaust air of each color group of the gravure printing unit according to the number of color groups of different printing products, whether they are full, ink solvent consumption, ambient temperature and other relevant conditions, so as to achieve the purpose of reducing air and increasing concentration. The advantage of this mode is that it can make full use of the existing devices of the equipment without additional investment; One of the disadvantages is due to the difference of printing products
The valves of various color groups need to be adjusted frequently. Second, higher requirements are put forward for the skills of operators. Third, there are potential safety risks due to the lack of exhaust gas concentration monitoring.
2. LEL automatic adjustment mode
(1) Introduction to LEL automatic adjustment mode
The lowest concentration of combustible gas in the air when encountering open fire explosion is called lower explosive limit (LEL) for short; The highest concentration of combustible gas in the air when it encounters open fire explosion is called upper explosion limit (UEL) for short. If the concentration (i.e. volume ratio) of combustible gas in the air is between LEL and UEL and there are fire sources and oxygen at the same time, explosion will occur under appropriate temperature and pressure.
LEL automatic regulation mode is to install infrared combustible gas detector, gas integrated control box, automatic positioning damper and other facilities on the pipeline outside the oven of the color deck of the gravure printing unit. Through the LEL central control system, according to the concentration value of VOCs exhaust gas detected in real time, the automatic positioning damper will make safe and energy-saving action instructions to achieve the purpose of reducing air and increasing concentration.

(2) Type of LEL automatic adjustment mode
In terms of design type, different modes such as automatic adjustment of monochrome group, automatic adjustment of multi color group patrol inspection and semi-automatic adjustment of end monitoring should be selected according to the actual situation of printing and packaging enterprises.
① The monochrome group automatic adjustment mode means that each color group of the gravure printing unit is equipped with an infrared combustible gas detector, a gas integrated control box and an automatic positioning damper (fresh air, return air, exhaust air, etc.) to accurately monitor the concentration of VOCs in the exhaust duct of each color group. Based on the monitoring data, the automatic positioning damper is controlled in real time to maximize the return air of the oven and reduce the fresh air and exhaust air under the premise of ensuring safety, Greatly reduce heat consumption. This mode has the advantages of high safety factor, good control accuracy, and good effect of reducing wind and increasing concentration, while the disadvantage is that the investment scale is relatively large.
② Multi color group patrol automatic adjustment mode refers to the installation of infrared combustible gas detector and automatic positioning damper in each color group of the gravure printing unit, equipped with a gas integrated control box less than the number of color groups of the gravure printing machine, and the LEL central control system sends an action command to the automatic positioning damper of each color group according to the VOCs exhaust gas concentration data monitored for each color group in the way of patrol monitoring to achieve the purpose of reducing air and increasing concentration. The advantage of this mode is that the investment quota is relatively small, and the disadvantage is that the VOCs exhaust gas concentration value of each color group cannot be monitored in real time, which has potential safety risks. In particular, if the adjacent color groups of patrol inspection use different solvents, the monitoring may be in a failure state (the common infrared combustible gas detector can only effectively monitor a single solvent).
③ The terminal monitoring semi-automatic regulation mode refers to the installation of infrared combustible gas detector, gas integrated control box, etc. at the main exhaust duct at the end of the gravure printing unit, and the installation of automatic positioning damper at the fresh air, return air and exhaust duct of each color group. During the production of the gravure printing unit, the operator adjusts the automatic positioning damper of each color group according to the VOCs exhaust gas concentration. The advantage of this mode is that the investment quota is low, and the disadvantage is that because of the manual end monitoring, there is a potential risk whether the concentration of VOCs waste gas in the monochrome group oven is safe and controllable. When gravure printing products use multiple solvents, there is a potential risk of failure in monitoring and control. The printing and packaging enterprises should select the mode of reducing wind and increasing concentration of gravure printing unit suitable for their own characteristics on the basis of combining the current situation of equipment, investment scale, product process, etc., to provide a better basis for the enterprise's VOCs waste gas treatment, provide more effective support for the enterprise's cost reduction and efficiency increase, and realize the dual promotion and co development of environmental protection treatment and enterprise operation.