Application of air pollution control technology and thermal energy utilization technology in coating industry

introduction

The PSA industry is very dependent on fossil fuels, oil, coal, and natural gas as energy and production feedstocks for numerous process functions. Historically, in the United States, fossil fuels have been available almost unlimitedly at reasonable prices, so energy costs have been a smaller proportion of overall product costs and people have been less concerned about the availability of sufficient energy, which has been the reason for some choices in the development of production technology. However, with the rapid economic growth of China, India, and Southeast Asia, the situation is changing. As the development of new oil resources has not kept pace with the pace of consumption, the demand for global energy supplies has increased greatly. As a result, the cost of fossil fuels has grown significantly and people have become more aware of limited supplies. There is no reason to think that this situation will change in the future. In order to remain competitive, PSA manufacturers have begun and must continue to focus on reducing energy consumption, improving production processes to increase efficiency, and utilizing renewable resources in their production facilities to minimize the energy requirements of products and production processes to achieve significant cost benefits.

1Coating and Discharge

During the adhesive application process, solids are diluted with different solvents to achieve the proper rheology to meet the requirements of the chosen application method. There are several factors to consider when choosing between water-based and hydrocarbon solvent-based adhesives, including product performance requirements, product cost, and return on investment.

Traditionally, waterborne coatings have not provided ideal bonding properties in many applications, and although waterborne coating formulations are constantly improving, hydrocarbon solvents still have a significant market share, and waterborne coating formulations also generally require longer (i.e. more expensive) drying machines. However, waterborne coatings have the advantage of meeting most emission standards without the use of additional environmental control equipment, and the economics of additional control equipment are also evolving through the use of advanced energy efficiency and solvent recovery technologies.

From a process perspective, the coating is applied to the web and then passed through a dryer, which evaporates the solvent, leaving behind the active coating. Any hydrocarbon solvent molecules present in the air are called volatile organic compounds (VOCs), and the resulting VOC fumes are then exhausted from the dryer. However, this hot air cannot be discharged into the atmosphere because it is a significant source of air pollution and health hazards. Once released, VOCs react with nitrogen oxides in the air and form ground-level ozone due to the presence of sunlight. Ground-level ozone is toxic and is a possible carcinogen in addition to being an irritant and smog contributor. VOCs themselves can also cause many health problems. Many VOCs are irritating to the skin, eyes, lungs, and throat, and are known to cause neurological damage and birth defects. Therefore, the VOCs exhausted from the dryer exhaust must be destroyed through thermal oxidation or recycled and reused as much as possible during the production process.

2 Emission Control

There are many different ways to treat waste gas in hydrocarbon-based PSA coating operations. This article will discuss the two most widely used technologies: activated carbon-based adsorption oxidation and thermal oxidation.

2.1 Activated carbon-based emission control adsorption system

The oven concentration of many PSA coating production lines is 10%~15% of the lower flammable limit (LFL), and the volume fraction of VOCs is 4×10-4~8×10-4. Under such conditions, solvent recovery is possible and economical.

Principle: The working principle of the adsorption system is the "Van der Waals force" principle, that is, due to the weak intermolecular force formed between the solvent molecules and the carbon surface, the solvent molecules adhere to the surface or active sites of the carbon, and the regeneration of the active sites is achieved by providing energy slightly greater than steam or inert hot gas. Factors affecting adsorption: (1) exhaust flow rate: (2) exhaust temperature: (3) exhaust pressure: (4) ambient air relative humidity: (5) solvent concentration in the exhaust: (6) solvent type: (7) particles.

Process description: The air discharged from the dryer enters the pretreatment unit after pretreatment, and the temperature and relative humidity are controlled to make it suitable for adsorption (this unit is usually part of the system), and then the air passes through adsorber No. 1 (vertical or horizontal container filled with adsorbent, such as activated carbon, etc.), the solvent molecules are adsorbed by the carbon (attached to the surface), and the clean air is discharged to the atmosphere. Once the carbon bed is saturated with solvent molecules, the sLA (solvent-containing air) flow is transferred to adsorber No. 2, and the saturated adsorber No. 1 is regenerated using steam or hot nitrogen. The specific process is shown in Figure 1.

During the regeneration process, the solvent vapor passes through the condenser and cooler together with the steam, and then enters the demixer, where sufficient residence time is provided to separate the solvent and water. If the solvent is insoluble in water, the pure solvent remains in the demixer. If the solvent is soluble in water, further treatment, such as distillation, is required to obtain a reusable solvent. The drying and cooling cycle is carried out after each regeneration of the carbon bed to remove the condensed moisture on the carbon surface and cool the carbon bed for the next adsorption cycle. Air is used to dry and cool the carbon bed. When using an adsorption system, the recovered solvent can be returned to the coating process, thereby reducing the cost of one of the consumables. A properly designed adsorption oxidation system can achieve a VOCs reduction rate of more than 99%, and the emission level depends on the solvent concentration, web width and line speed in the coating.

2.2 Thermal oxidation and heat recovery

Since the amount of solvent in the exhaust gas is less than the economic requirement for adsorption recovery (generally volume fraction <1×10-4), and the diluent is a mixture of different solvents, adsorption and distillation recovery may be uneconomical, and there are toxic compounds, adsorption is not feasible. At this time, another option is thermal oxidation, which can convert VOCs into less harmful compounds.

Principle: VOCs react with oxygen at high temperature to generate carbon dioxide and water vapor. Oxidation is an exothermic reaction. The heat generated by the reaction can be used for drying process or other heating needs of equipment. The thermal oxidation formula is as follows:

The three "t"s for VOCs removal efficiency are: time>0.5s, temperature>850℃, and turbulence to ensure good mixing (turbulence can be achieved through high-speed airflow, mechanical mixing devices, and changes in airflow direction). The destruction efficiency of a regenerative thermal oxidizer (RTO) can exceed 99%. It usually includes multiple layers of ceramic media to collect and store energy between cycles of the oxidation system. The ceramic media in the regenerative system is contained in multiple towers or tanks, which are interconnected by a combustion chamber and pipes at the top of the tank and a valve system at the bottom of the tank, as shown in Figure 2. The valve system directs the incoming exhaust gas flow to each ceramic tower. By switching from one ceramic tower to another, one tower will give up its energy while the other tower absorbs energy.

Figure 2 Regenerative thermal oxidation furnace process flow

The operating temperature of RTO is 815~980℃, and most systems on the market today are designed to achieve 85%~95% thermal efficiency, and even 97%, depending on the amount of ceramic media used. Depending on the size of the unit and the solvent load, the amount of energy recovered can be very significant. The ceramic heat recovery media and insulation materials used in these systems are designed for continuous operation under such extreme conditions. The robust design, coupled with the use of a hot gas bypass system, enables the regeneration system to operate efficiently over a wide range of air flows and VOCs concentrations from 0%~25% LFL. The energy required to keep the process running is either provided by the combustion of the solvent (in the exhaust gas) or by the auxiliary fuel provided to the burner. If the exhaust volume is at 5% or higher, the burner is not required (after startup) and the system will operate in a "self-sustaining" mode. As the LFL increases, additional energy is released and available for use. When there is excess energy available, it makes significant economic sense to perform secondary heat recovery. This recovered energy can be used for combustion air preheating, dryer air heating, process makeup air heating, hot water heating, and low temperature steam heating.

Heat recovery systems are becoming very popular as a means of reducing overall plant operating energy costs. Heat recovery can be achieved using various types of heat exchangers - air to air, air to water, air to hot oil.

3 Application Examples

A large electronic materials company has a PSA coating production line. Its VOCs emission waste gas treatment device adopts a two-chamber RTO structure. Due to the high concentration of VOCs and the large amount of extra energy released, two sets of secondary heat recovery systems were applied at the beginning of the design: (1) Air and hot oil exchange system. The waste heat generated by the combustion exhaust gas in the combustion chamber is used to heat the hot oil through the hot oil exchanger. According to the system requirements, it is heated to 250℃, and then the hot oil brings the heat to the dryer for use. (2) Air and air exchange system. The hot air at about 150℃ at the RTO outlet is used to heat the high-concentration air extracted from the glue coating room through the hot air exchanger, and then sent to the air supply system of the dryer. The above design not only greatly reduces energy consumption, but also reduces the operating costs of the enterprise.