The nature of the problem
The major air pollution problem in the textile industry occurs during the finishing stages, where various processes are employed for coating the fabrics. Coating materials include lubricating oils, plasticizers, paints and water repellent chemicals -- essentially, organic (usually hydrocarbon) compounds such as oils, waxes or solvents.
After the coatings are applied, the coated fabrics are cured by heating in ovens, dryers, tenter frames, etc. A frequent result is the vaporization of the organic compounds into high molecular weight volatile organic (usually hydrocarbon) compounds (VOCs).
In terms of actual emissions, the industry must also deal with larger particles, principally lint.
The nature of smoke
The main air pollution problem in the textile industry is the creation VOCs, which take the form of visible smoke and invisible but objectionable odor.
Smoke is basically made up of tiny solid or liquid particles of VOCs less than one micron -- 1/100 the diameter of a human hair -- in size. The VOCs are suspended in the gaseous discharge. When the smoke/gas mix goes up the stack, the Environmental Protection Agency (EPA) measures the density or opacity of the emission using an official transparent template, and labels the opacity as ranging from 0 to 100%. The more opaque the smoke, the more visible it is.
Opacity of smoke is actually related to the quantity, rather than the weight, of particles present in the gas.
For a given particle size and stack diameter, percentage of opacity is a function of the grain loading or weight of particles, measured as grains per cubic foot (1 lb = 7,000 grains).
A tenter frame, for example, might have a typical exhaust of 20,000 acfm at 320 F, with opacity greater than 20%. This amounts to a total emission of 17 lb per hr, or 62 tons per year. To reduce exhaust-stack discharge from an opacity of 20% to the EPA-mandated 'zero opacity,' an air pollution control system has to provide a removal efficiency above 95% for particles below one micron in size.
The nature of odor
The problem with odor is that it isn't practically measurable. For our purposes, 'smell' is a sensation originated by interaction between molecules of hydrocarbons and ten million nerve fibers in the human olfactory membranes. The human nose, nature's sophisticated detection system, can differentiate between 4,000 different odors.
The odors associated with textile plant emissions are usually caused by hydrocarbons with molecular weight less than 200, and fewer than 15 carbon molecules (C1 to C14).
These odorous molecules attach themselves to the particles of smoke and can be carried -- and cause complaints -- great distances from their point of origin.
When air pollution equipment has abated the smoke, the odor molecules have no vehicle to carry them. That is why savvy plant managers find that the installation of smoke abatement equipment sometimes solves the odor problem, too. Relatively inexpensive equipment is also available to address the problem directly.
Best Available Control Technology
Table I shows principle process requirements for air pollution control in the textile industry, and available technologies to meet those specific needs. We will consider the various solutions in reverse order, from least to most effective. The best available technology isn't necessarily essential for every situation, but it is usually advisable for difficult cases. Abatement strategies
Selection of an air pollution control (APC) system should involve several orderly steps:
- Analysis of chemicals involved -- the exact nature of the VOCs to be abated -- is necessary so that the system can be fine-tuned to deal with them.
- Evaluation of existing exhaust system.
- Consideration of exhaust stream precooling.
- Consideration, comparison and selection of optimum APC equipment.
Analysis of existing exhaust system
It is important to evaluate the mechanical parameters of the existing exhaust system to be sure it will properly complement the new APC equipment. These are some major considerations:
- In order to keep smoke out of the working area, pressure inside the heating equipment should be negative. Investigate all openings in the body of the equipment and eliminate any that are not necessary for the process.
Exhaust gas volume should be adequate to absorb moisture evaporated from the textiles (and also, of course, to bring the total VOC content below 25% of Lower Explosive Limit -- an essential consideration, but not directly related to pollution control).
- Exhaust system ductwork from heating equipment to cooling section of the APC requires special attention. The ductwork should be solid steel construction, welded and tested for gas and liquid tightness. The welded seams should be located at the top of the duct.
- It is important to prevent condensation of VOCs in the exhaust system, and subsequent dripping of condensate back into the oven. Velocity in the duct should be sufficient to keep the droplets moving; gas temperature should remain high until the gas enters the cooling section of the APC system; and the ductwork should be kept as short as possible. It often pays to insulate the entire run of exhaust system ductwork.
- If the heating equipment has several gas-collection zones with separate exhaust blowers, ductwork design should incorporate a manifold and provisions for pressure adjustment to ensure adequate exhaust from each zone. A separate 'zero pressure' point, located between discharge of the zone blower and the manifold, will prevent the central blower from influencing exhaust rates from individual zones.
Consideration of exhaust stream precooling
For the APC system to capture VOCs, the vapors must be condensed. Cooling the airstream is therefore an important first step in the capture process. The efficiency of the cooling/condensation process depends on the boiling temperatures of the specific VOCs, their concentration in the exhaust gas, and operating temperature of the APC device.
As a rule of thumb, the amount of condensed VOC is doubled for every 30 F temperature drop from the initial condensation point in the cooling section of the APC. On the other hand, excessive cooling is costly. The operating temperature of the APC cooling section should be selected to ensure effective condensation while minimizing capital and operating costs of the cooling system.
In textile applications, most VOCs condense at the saturation temperature of the exhaust stream, which usually ranges between 110 and 160 F. Sometimes -- as in a textile printing operation with paint based on a low molecular weight VOC -- it is necessary to reduce gas temperature to 100 F or below to achieve reasonable efficiency of condensation.
Cooling can be accomplished indirectly with fin and tube heat exchangers, or directly by injecting cooling liquid in the prescrubbing section of an APC system. The appropriate cooling design should be selected on an individual basis, taking into consideration the composition of the exhaust, amount of solids, and the plant's operating experience with similar systems.
Tubular heat exchangers are rarely the optimum selection, because of the difficulty of cleaning if tubes become clogged. For instance, a high concentration of 'sticky' oil in combination with particles of lint definitely precludes the use of a tubular heat exchanger; so do the oils which have a tendency to polymerize on the surface of the tubes. If the exhaust contains relatively small amounts of easily removed condensing oil, the heat recovery efficiency of a tubular exchanger can effect meaningful savings. But these are quickly wiped out by greater than expected cleaning costs.
In most textile industry applications, a simpler solution is preferred. An evaporative cooling system provides effective cooling with minimum maintenance, simply by spraying cooling water into the exhaust gas. This method, however, produces water droplets contaminated by lint and oil, consequently adding to total inlet loading of the system.
Solving a typical air pollution problem
A variety of APC methods is available to meet textile industry requirements. To define and illustrate selection of Best Available Control Technology (Table I), let's consider a typical textile industry air pollution problem. These are typical parameters for textile finishing:
- Volume of exhaust gas ranges from 10,000 to 20,000 acfm for a single stack, with a total system of four stacks.
- Temperature of exhaust gas ranges from 300 to 400 F. Moisture content is 20 to 40% or higher.
- Liquid particles of condensed VOCs can amount to 0.2 grains per cubic foot of gas with particle size from 0.01 to 0.8 micron.
APC candidate: Fiber filter
The fiber filter, or fiber bed mist eliminator, is seldom recommended for textile industry applications, because the high flammability of VOCs and dry lint in the gas stream usually precludes the use of dry filtration devices. There are other disadvantages as well. Fiber filter usage is limited to rare applications where the product of VOC condensation is a free-flowing oil, and no waxes or lint are present.
A fiber filter typically consists of glass or polyester fibers from 5 to 10 microns in diameter, packed between flat or cylindrical screens. Particles of condensed VOCs are collected on the surface of the fibers by inertial impaction, direct interception and Brownian movement or diffusion.
Removal efficiency of fiber filter rises with density and thickness of the fiber bed. Unfortunately, pressure drop through the bed, and probability of plugging, also rise. So for a typical exhaust of 20,000 acfm at 320 F, the exhaust blower will require a 100 hp motor. In addition, replacement and disposal of clogged filters (classified as toxic waste) is a high annual expense. A cooling section acting as a prescrubber adds to the efficiency -- and expense -- of a filter.
APC candidate: Wet scrubber
Wet scrubbers have been used in the industrial environment since the early 1800s. Like fiber filters, they utilize impaction, interception and diffusion as collection techniques. The important difference is that the filtering role is performed by a scrubbing liquid rather than fibers, removing the potential for fire or explosion.
The scrubbing liquid is usually water, which is sprayed as droplets which become targets for precipitated particles and absorbed gases. The droplets are larger than the particles, and become heavier and heavier as they collect impinged particles, so they are relatively easy to remove.
In textile applications, simple wet scrubbers are successfully used to remove lint and other solids above about 5 microns in size, but not for VOC smoke abatement.
APC candidate: High energy venturi scrubber
Condensed VOC droplets are typically smaller than one micron. To collect them from the exhaust gas stream, the optimum target droplets must be in the range of 10 to 15 microns in size -- much smaller than the drops generated in the simple wet scrubber. To generate these tiny droplets in the quantities required for efficient scrubbing, a substantial amount of electric energy is required (Fig. 3). The most common wet scrubber for submicronic particle removal is the high energy venturi scrubber (HEVS).
The HVES is essentially a gas-atomizing device relying on sheering and impaction forces to break water into an evenly distributed cloud of very fine droplets at high density. The density raises the probability of collisions between submicronic droplets of condensed VOCs and the larger -- but still tiny -- droplets of recycling liquid.
The droplets are produced in the venturi throat of the scrubber, where velocity of the moving gas can be as high as 400 feet per second. This is about eight times higher than the velocity of the exhaust gas in the duct, resulting in high pressure losses, and high energy requirements for operating the system. Fig. 3 shows the relationship between particle size, pressure drop and energy requirements.
APC candidate: Packed bed scrubber
Wet scrubbers are more successful in applications where the basic problems are odor, and relatively large particles of lint and other solids, rather than smoke. To provide odor removal in textile operations, basic, acidic or oxidizing agents are added to the recycling liquid. Successful removal agents include potassium permanganate, sodium hypochlorite, chlorine dioxide, ozone and dichromates.
In order to provide intimate contact between scrubbing liquid and odor producing gas, packed bed scrubbers are employed. The packed bed utilizes specially shaped plastic or ceramic elements which are highly permeable to gas flow.
But the conventional packed bed presents its own problem. Solid particles which are not soluble in water -- lint is a major culprit here -- will clog conventional packing. For textile odor remediation, two somewhat specialized scrubbers can be used:
- In a multi-rod deck scrubber, gas and scrubbing liquid compete for space between solid tubular structures. The result is highly turbulent intermixing, with little chance of clogging.
- In a multi-channel scrubber, channels are continually created by interaction of gas, scrubbing liquid and the packing of solid glass or ceramic spheres ('marbles'). The scrubbing action removes not only absorbed gas, but also solid particles trapped in the scrubbing liquid; the rotating marbles are highly resistant to clogging.
In these scrubbers, efficiency of removal of particles above 2 microns in better than 90%, and substantial odor reduction is accomplished through the mass transfer capability of the rods or marbles flooded with scrubbing liquid.
APC candidate: optimized scrubber/quencher
Where the remediation problem is basically odor-producing vapors and larger solid particles like lint, a preferred treatment may be a low pressure-drop scrubbing system combined with a quencher for precooling the gas stream, and arrangements for separating out pollutants from the scrubbing liquid.
For example, one such system utilizes a venturi-rod design with open type spray nozzle. In a typical situation, gas at 300 to 400 F enters the scrubber/quencher where it's cooled to full saturation temperature while particles of lint are trapped in the scrubbing liquid. VOCs condense in the quencher to become submicronic droplets of oil. These droplets, together with larger droplets of scrubbing liquid mixed with solid particles, must be separated from the gas stream.
Selecting a separator
Separation is, of course, required for all particle collection systems, whether designed for submicronic or larger particles, or both.
In selecting the separation method for a textile plant APC system, gravity-based settling chambers are immediately ruled out. They are seldom effective with particles under 50 microns, and almost all the polluting particles emitted by textile finishing processes fall into this range.
Cyclonic separators are suitable for larger particles and scrubbing liquid. However, the inlet velocity required for separating particles under 5 microns often results in excessive energy consumption and wear on the equipment.
Since tiny particles usually respond well to electrical forces, electrostatic precipitation is particularly effective in removing submicronic particles. For instance, electrical force exerted on an electrically charged particle of 0.1 micron can be more than a million times the force of gravity on that particle. So where the hydrocarbon aerosol droplets of VOCs are concerned, electrostatic separation often follows the scrubber stage in an APC system for textile plant exhaust gas.
Electrostatic precipitators (ESPs) offer high capture efficiency of submicronic particulate by using electrical forces to remove suspended particles from the gas stream. Because an ESP uses electricity only to charge particles in a passive air stream, it requires much less energy than a Venturi scrubber.
Three steps are involved in electrostatic precipitation: charging, collection and removal. In all cases, a corona discharge electrically charges the suspended particles, which are collected on electrodes. Removal strategies vary.
APC candidate: Dry electrostatic precipitators
In a dry precipitator, the electrodes are periodically washed off to remove accumulated particles for disposal. Despite that washing, dry ESPs are prone to particle buildup on the walls of the collection chamber. The buildup acts as insulation, reducing the overall efficiency of the equipment.
Dry ESPs can be used alone for removal of free flowing particulates, but must be combined with scrubbers or similar equipment to capture VOCs.
APC candidate: Wet electrostatic precipitators
In a wet electrostatic precipitator (wet ESP,) a liquid, usually water, continuously washes particle buildup off the collection surface. The wet ESP is widely used for applications where the gas to be treated is hot, has a high moisture content and contains sticky particles or submicronic particles that can't be treated by other methods -- typical textile industry conditions. In most cases the gas is pretreated in a scrubber. The wet ESP follows to complete particulate removal with a high level of efficiency.
In a conventional 'upflow' wet ESP the gas stream flows co-current to the water. This requires frequent cleaning because bottom water sprays are not completely effective in reaching the top of the chamber where submicronic particles accumulate. The 'downflow' wet ESP overcomes these problems, but does require mist elimination following treatment.
APC candidate: water cooled jacketed wet ESP for submicronic sizes
One variation on the wet ESP design is well suited for removal of submicronic particles. The jacketed, water chilled vertical-tube wet ESP (Figure 2) increases removal efficiency and reduces operating costs and maintenance, by harnessing temperature differential to create particle movement perpendicular to the direction of the gas flow. The design combines a quencher to cool the gas stream with an upflow, vertical tube wet ESP. It integrates variables of velocity, particle size and temperature to maximize removal of submicronic particles.
Figure 4 shows a Croll-Reynolds brand of water cooled jacketed Wet ESP called Condensing WESPtm employed for air pollution control in a textile system. The hot, contaminated inlet stream is cooled from 300 to 140 F in a quencher. Then a low-energy scrubber removes particles larger than 2 microns.
The pretreated gas stream passes into a multitude of tubes containing ionizing electrodes. Each electrode has a number of sharp points which give off the corona discharge that mobilizes the particles. When the particles are charged to the point of saturation, they migrate toward the walls of the collecting tubes.
Within the double-walled, water cooled collection chamber several processes take place which greatly increase the effectiveness of the device.
1. Thermophoresis (heat transmission). Inside the collection chamber, cold water is circulated in a closed-loop system. When gas temperatures on opposite sides of a particle are different, the gas molecules on the warmer side have a higher kinetic energy than the molecules on the cooler side. The hot, active gas molecules will strike the surface of the particle with more force -- in essence propelling the particle toward the cooler temperature zone. This process is thermophoresis. Particle velocity depends on the temperature gradient, the relative thermal conductivities of the gas and the particle, and the density and viscosity of the gas. Thermal forces for particles of 0.1 micron can be 100 times greater than those for particles of one micron.
2. Diffusiophoresis (diffusion transmission) occurs when the partial vapor pressure of water in a carrier gas is greater than the water vapor pressure at the surface of the water droplets in a scrubbing liquid. In the Condensing WESPtm, water vapor will condense onto the water droplets, creating a bulk motion of gas and entrained particles toward the droplets. Because the droplets are larger than the particles, and therefore more easily removed by either electrical or mechanical means, diffusiophoresis can improve the efficiency of collecting fine particles, when compared to a conventional wet ESP.
3. Condensing action. In the Croll-Reynolds brand of water cooled jacketed wet ESP Condensing WESPtm, charged particles, both liquid droplets and solids, are subject to electrical forces in addition to thermophoresis and diffusiophoresis. They migrate toward the positive (grounded) wall of the collection tubes. The interior of the tubes is cooled by circulating water, and the cool tube surface condenses the water vapor.
This process deposits a wet film on the collection surface which tends to flush particles down the wall and out of the system. The wet film also discourages sticking of oils and waxes, and reduces the need for system shutdown for cleaning.
By condensing water vapor from the contaminated gas, the Condensing WESPtm design can cut external water needs by thirty percent compared with conventional wet ESPs. The continuous presence of condensate also reduces corrosion.
Corona power in Condensing WESPtm
Corona power of an electrostatic precipitator is the product of the operating voltage and the operating corona current. The efficiency of a wet ESP is directly proportional to its corona power.
In operation, corona power is limited by internal sparking between ionizing and collecting electrodes. Since efficiency is directly proportional to the electrical power conveyed to the moving gas, each time a precipitator sparks, the voltage -- and consequently the particulate collection efficiency -- is reduced.
For a given equipment design, gas flow condition and particle concentration, sparking rate is a function of gas- to-liquid ratio of entrained liquids, quality of the wet ESP components, and the appropriate selection of high-voltage power supply components to match wet ESP geometry and the physical and chemical nature of the suspended particles.
Consequently, the power level and efficiency of a wet ESP can be dramatically increased by proper design of its automatic voltage control system. The industry in general seems satisfied with 40% conduction -- the ability to deliver 40% of available power to the gas. But with proper selection of the transformer rectifier, automatic voltage control and automatic current-limiting power systems can be designed to deliver over 86% conduction -- the maximum theoretically possible.
An integrated approach for the textile industry
The powerful combination of liquid scrubbing, thermophoretic and diffusiophoretic effects and high-voltage electrostatic precipitation in a condensing water chilled jacketed wet ESP is an effective method of submicron particle removal. In the textile industry, a Condensing WESPtm is probably the optimum single selection for obtaining zero opacity, satisfying state and federal regulations for textile exhaust.
The wet ESP is also an excellent candidate for operation as part of an energy-efficient, low-maintenance integrated system. By combining a Condensing WESPtm with a relatively inexpensive, low-energy scrubber which acts as the air distribution device and odor abatement section, large and small particles are effectively removed from textile-plant exhaust. This is an APC solution to keep today's textile plant in environmental compliance well into the 21st Century.
- What to look for in an advanced wet ESP system for textile plant air pollution control
- Wide-spaced electrodes or large-diameter tubes allowing operation at 40,000 volts or higher.
- Rigid ionizing electrodes that ensure system maintainability and increase longevity.
- Continuous cleaning action by liquid film on the walls of collecting electrodes to prevent accumulation of stick oils and waxes, ensuring high removal efficiency with minimum maintenance.
- High-voltage system with solid-state controls capable of delivering maximum available electric power to the exhaust gas.
- Prescrubbing section located in the bottom section of the wet ESP to minimize physical size and capital expense for the APC system.