The business and process decisions faced by those who apply and dry wet surface coatings to productsThere are several factors to consider, including:
First, product quality requirements
2. Security
3. Regulatory and Air Quality Restrictions
4. Fees
5. Simple and reliable operation
Process alternatives include the following:
1. Reformulate
A. Transition from solvent-based to water-based wet formulations
B. Converted to high-solids formulations with little or no volatile components
2. Steam destruction
A. Thermal oxidation of organic compounds produces carbon dioxide and water vapor
B. Catalytic oxidation
3. Steam recovery
A. Absorption-desorption using activated carbon
B. Direct vapor condensation
Reformulation has been the subject of a great deal of research.There has been a lot of success, but in many cases, the use of VOCs provides exceptional end product qualityor advantages. The safety of handling solvents and costs needs to be checked.
Steam destruction or oxidation typically requires less capital expenditure than steam recovery, butThe additional cost of a recovery facility may be justified if large quantities of solvents can be reused,Reduce the need to purchase new solvents each year. If it is oxidized, some heat energy may be recoveredfrom solvents, but in all cases, additional energy needs to be purchased to operate the vapor oxidizer.
In the case of solvent recovery using carbon adsorption-desorption equipment, the cost of recovering the solvent is sometimes higher than the cost of purchasing a new solvent; This additional cost is justified in order to avoid air pollution.
There is no one-size-fits-all control process. Context-specific considerations need to be consideredto determine what preference is. Typically, however, thermal oxidation is preferred for smaller vapor emission rates or where the solvent cannot be reused, and catalytic oxidation is only considered when the gas stream required for smoke capture results in a relatively dilute vapor concentration. If a large amount of steam can be reused and a new dryer is planned, solvent recovery by direct vapor condensation is preferred.
safe
Process safety when handling VOCs needs to ensure that fire is avoided andExplosion damage.
The usual procedure for using solvent-based paints and drying them is to sweep themThe organic vapor disappears with too much airflow, making the vapor concentration too low to maintaincombustion, the ignition temperature should be reached.
Typically, at least 300 to 400 volumes of air are used to dilute one volume of vapor so that it can'tIt is not uncommon to be ignited, and to use thousands of volumes of air per volume of steam.
However, it should be understood that fire and explosion hazards can also be maintained by passingThe oxygen concentration is too low to sustain combustion. Ordinary solvents do not ignite or maintainCombustion in an atmosphere with less than 10% to 13% oxygen (as opposed to air with 21% oxygen). This oneControl procedures are key to an economical condensate solvent recovery process.
Operating costs
The cost of controlling steam emissions is highly dependent on the amount of air being mixedVapor. Excess air can provide the advantage of more reliable safety and/or a higher percentage of steamCaptured in a fume hood, but excess air greatly increases the cost of removing steam from the air.
In industrial operations that include steam incinerators or vapor recovery systems, it is important:Minimize the flow of dilution air without sacrificing safety. For example, tolueneIf ignited, vapors with a concentration of less than 1.2% in the air are too thin to sustain combustion, andIf ignited, toluene vapor concentrations above 7.1% are too concentrated to sustain combustion. These limitationsKnown as LEL (Lower Explosion Limit) or LFL (Lower Flammability Limit), or UEL or UFL as the upper limitLimit. Other solvents have slightly different limits and apply to "normal" dryer conditions up to 200°F.
It is common practice to ensure that the dryer exhaust flow does not exceed 25% LEL (four times saferfactors) to minimize the risk of combustible mixtures inside the dryer. National Fire ServiceThe association standard will allow vapor concentrations "not to exceed 50% of REL", provided that:Continuous indicators and alarm settings are scheduled to shut down the system at lower humidityPoint of view. However, when the average exhaust gas concentration is in the range of 40% to 50% REL, there is noneThere is no guarantee that the concentration inside the dryer will be high. Relatively few dryers are usuallyOperates at 40% or higher LEL vapor concentrations.
Printing, on the other hand, is applied to a small part of the surface, such as multicolorGravure or flexographic lines typically use more air per unit of solventapplication, and the LEL concentration of the total exhaust mixture is relatively low.Effective vapor capture sometimes requires more airflow than LEL safety considerations.
