John W. Doub, Jr.,
Vice President & General Manager
222 E. Thomas Ave. Baltimore, MD 21225
Drying has become a necessary addition to processing plastics, particularly with the increased usage of the so-called “engineered” resins. Unfortunately, many processors concern themselves with drying only after realizing material defects such as splays, etc. in the produced part. Even those processors who realize that drying is a vital link in the chain of proper part production, are often confused and unclear in determining the “how’s and why’s” of good dryer operation. The purpose then, of this paper, is to provide general data concerning moisture in air and how it is removed mechanically, and how to dry plastic resin, by a desiccant hopper/dryer system.
However, before we examine the mechanics of the drying process, a review of a few simple properties of air and moisture measurement are in order:
DRY BULB TEMPERATURE
This is the temperature of the surrounding medium as indicated on a thermometer. Most of us are familiar with this as “temperature”.
WET BULB TEMPERATURE
There are many definitions for this and some of them are quite confusing, but quite simply, “wet bulb” is the temperature reached by a wet surface when exposed to air. In other words, it is the temperature at which water evaporates. Wet bulb is measured with a device called a psychrometer, which is an ordinary mercury thermometer with its bulb covered with a layer of cloth saturated with water. When the cloth is exposed to an air stream, the water is evaporated, and a temperature indicated, referred to as “wet bulb”.
A term familiar to most people, relative humidity is a ratio of the quantity of water vapor present in a given volume of air. It is also the ratio of the pressure of water vapor present to the pressure of saturated water vapor at the same temperature. Most often, weather people refer to it as “humidity”.
The temperature at which the vapor begins to deposit as a liquid or condense, on a surface. Weather forecasters sometimes refer to dew points as the temperature at which we would see fog if the relative humidity remained constant. It is an extremely accurate indication of the moisture content of a gas.
It is generally quite easy to obtain dry bulb and wet bulb temperatures using thermometers and psychrometers, and with these values, the psychometric chart can be used to calculate the remaining properties of the air sample. Those of most interest to us in drying are humidity in terms of percent, dew point temperature, and moisture content. By examining the psychometric chart one can easily see the ratio between moisture content and temperature. As dry and wet bulb temperatures are lowered, so is the amount of moisture that an air sample contains. Conversely, as these values are elevated, the air sample can hold more water.
Perhaps the biggest area of confusion I have encountered since becoming involved in drying plastics is the relationship between percent moisture content and dew point. Many people in the plastics industry refer to moisture content in terms of percent and unfortunately, most of us in the drying industry talk about dew point. This chart demonstrates a comparison between dew point and percent moisture content in air. For example, a .04% moisture content of air represents actually only about -200 dew point. A -400 dew point represents a moisture content of about .013%. In the drying of some plastics, this can be a significant difference.
Generally, plastic resins, which are affected by moisture, fall into two classifications- hygroscopic and non-hygroscopic.
Non-Hygroscopic resins collect moisture on the surface of the pellet only. The origin of the moisture can of course be from several sources, but nevertheless, it can present a problem to processors.
Drying this surface moisture can be accomplished by simply passing warm air over the material. We observed from the psychometric chart that the warmer air has the ability to hold water. When a warm air stream is passed over the resin, the moisture tends to leave the surface of the pellet in favor of the warm air, and dry resin results.
Hygroscopic materials however, collect moisture inside the pellet itself. If this moisture isn’t removed, it can greatly reduce the strength and/or appearance of the finished product. Removal of this moisture requires dry air as well as heat, and so dryers for these materials require proper design and careful machine selection for each application.
There are various ways to remove moisture in an air stream. We saw from the psychometric chart that lowering the temperature of air will also lower the dew point, and at relatively high moisture levels this is a practical approach. Many of you may remember the early days when a refrigerant dryer was considered satisfactory in removing moisture from resins. However, refrigerant-type dryers are in effective in trying to reach dew points below +40°F. Air can also be compressed and then cooled to remove moisture but this is a complicated process, and quite expensive.
The most simple and uncomplicated method of removing moisture at low dew points is with a desiccant.
And what is a desiccant?
Desiccants have a variety of shapes and are of different types, but basically all types are of materials, which have a natural affinity for water. For the most part, they are crystals or spheres, each of which contains tiny pores. As moisture-laden air is passed over the desiccant, water is retained in these pores. This action is called selective adsorption. As stated, it is merely a phenomenon wherein water vapor is extracted, condensed, and held on a surface. There is no physical change or chemical change of the desiccant during this adsorption process. Adsorption will occur within a wide range of dry bulb temperatures to as low as -10°F., or up to +l50°F.
This process is not to be confused with absorption, which is penetration of a substance into the interior of solids or liquids and does involve a chemical and/or physical change of the absorbent. In absorption, regeneration is not possible or practical since a physical change of the absorbent has occurred.
In adsorption however, the water trapped on the surface of the desiccant can be removed without physically changing the adsorbent. Since there is no change in the desiccant during the adsorptive and “regenerative” process, adsorption cycles can be repeated many times utilizing the same material. In this way, drying of the air is accomplished very simply involving no elaborate cooling or compression processes and/or complex controls.
As indicated previously, there are different desiccants, and since they all have an affinity for water, the question arises as to which one to employ to meet our requirement. Besides cost, several factors influence our decision:
Inlet conditions -Some desiccants will not adsorb water at elevated temperatures and are therefore ineffective in warm air streams.Effluent requirements enter in as well -Different desiccants adsorb moisture in different volumes.
The remaining three factors -chemical reactions, molecular size and co-adsorbable components -gain more significance in removing moisture from gas streams than in removing moisture from air streams to dry plastics, and are only worth a brief mention here.The two most commonly used desiccants are silica gel and molecular sieves. When inlet concentrations of moisture in an air stream are high, silica gel will remove more moisture by weight. That is, a fixed amount of gel will be more effective than the same amount of sieve. However, with lower inlet conditions, molecular sieve is the desired adsorbent choice. This, plus the fact that the incoming air stream to the desiccant bed in a plastics dryer is warm (generally above 1000 F.), and silica gel begins to loose it’s useful capacity at higher temperatures, the choice is obvious for us in drying plastics -we must use molecular sieves.
Molecular sieves are round spheres or cylindrical extrusions, with internal pores of various diameters. A desiccant “bed” is merely many of these individual spheres placed together in a chamber. An often asked question is, how do we know how much desiccant is required to effectively remove moisture from the air stream?
Several factors go into sizing a desiccant bed in dynamic adsorption. Obviously, enough desiccant must be used to meet the moisture removal requirement! Also, it must be deep enough to allow sufficient time for the air stream to pass over the sieve so moisture can be adsorbed. Pressure drop must not be too great for the fan selected. Actual bed design is a subject all in its own, and beyond the scope of this paper -but you can see many criteria must be considered in order to provide a machine capable of meeting the exact requirements.
Now, let’s take a look at the desiccant bed and see exactly what does occur as the air passes over it. Transfer of the moisture to the desiccant begins as soon as the wet air contacts it. Initially, the bed contains little moisture and as the air stream enters, moisture is collected in the pores of the desiccant -actually “transferred”. This area where the exchange occurs is referred to as a “Mass Transfer Zone”. The mass transfer zone is a length of packed adsorbent through which the water content in the air is reduced from the inlet condition to the outlet condition. The length of this mass transfer zone depends on the velocity of the air and the exit concentration of moisture in the air stream.
As the air stream is fed into the desiccant bed, this mass transfer zone is continuously displaced from the inlet to the outlet of the bed, and eventually moves to the bottom of the bed as the cycle time progresses. When the leading edge of the mass transfer zone reaches the bottom portion of the bed, the bed becomes saturated. Before saturation “break-through” occurs, the bed must be regenerated to remove the moisture in preparation for the next adsorption step. (Normally, the main process airflow will be shifted to a second tower or bed for adsorption while the regeneration process goes on in this tower or bed.)
There are several methods of accomplishing regeneration, but the most convenient is the use of heat. Generally, heat is applied to the bottom of the bed and excess moisture is driven off. One can see that regeneration flow counter-current to the adsorption flow is highly desirable for two reasons:
The portion of the bed with less moisture can be heated more rapidly.
The relatively dry portion of the bed assists in driving off moisture from the top of the bed.
Regeneration is a most important factor in the adsorption process, and proper ‘regeneration is a necessity in obtaining satisfactory dryer performance. With other factors being equal, the dew point produced by a molecular sieve dryer will be lower as the regeneration temperature is increased. Generally, temperatures to 6000 F. are required for proper regeneration. Besides proper inlet temperature, regeneration heat must be employed long enough to raise the temperature of the desiccant sufficiently to vaporize the liquid and drive off the excess moisture. Thus, the period of heating as well as temperature of the bed must be considered in establishing good regeneration. In proper machine design, the regeneration air temperature is approximately 500 F. higher than that desired for the desiccant.
This then, provides a very simple explanation of how water is removed from an air stream. The desiccant “traps” water in adsorption and releases it in regeneration.
Now let’s take a look at a dryer and see how it is physically constructed.
From the above discussion, the most important part of the machine is the desiccant bed. Once it is designed, the other components are merely supplemental to it to enable it to function satisfactorily between adsorption and regeneration. A fan is employed to move the air across the surface of the desiccant at the proper velocity to effect a satisfactory transfer of moisture. The inlet air stream must be filtered to prevent contamination of the desiccant bed. In the case of plastics, fines and exceptional amounts of dirt may be present and contact the desiccant. Since the desiccant life can be greatly reduced when the pores become clogged, a substantial filter must be employed. (A two-stage filtration system that will remove particles down to 1 micron in size with a 99.9% efficacy such as used on Novatec dryers, is highly desirable.) Inlet moist air then enters the dryer through the filter and passes over the desiccant bed where moisture is removed. In the case of plastics, this dry air then is reheated to a pre-determined temperature depending on the material in the hopper. While this is occurring on one tower or chamber, the other is being regenerated. Ambient air is drawn in through a filter, by a second fan, into a single regeneration heater and up over the desiccant bed. Moist ail: then is driven off to the outside. This can either be ducted directly into ambient air or it can be ducted outside if desired. Four-way valves transfer the air from one tower to the other. Total cycle time of this machine is eight hours for one tower; that is, four hours of adsorption of process drying and four hours of regeneration. Once the bed is heated to the temperature required for effective regeneration, it must be cooled before it returns to the process air stream. The reason “longer” cycles are used with Novatec dryers is to provide sufficient cooling of the desiccant bed. When outside air is used to remove regeneration heat, the bed eventually cools to a point where it starts adsorbing moisture again. This creates a problem when the valves shift and the tower goes on stream to begin it’s adsorption cycle, since moisture adsorbed during regeneration cooling is then driven off into the process air.
A most effective way of cooling the bed is to turn off the fan and allow the desiccant to cool by radiating the heat rather than bringing moisture laden outside air across it. Since moisture is not introduced to the desiccant, the bed remains dry to begin the next adsorption cycle. Because of this cooling cycle, longer overall adsorption cycles are employed, meaning less valve and component wear due to less activity, as well as longer desiccant life.
In addition, regeneration heaters are on for only 25% of total machine operating time, so energy efficiency is realized. The regeneration heater is external to the dryer tower for easy inspection if necessary.
Just as construction of the dryer is important, the plenum hopper must also be constructed with certain design parameters in mind:
The air inlet from the dryer must be positioned so air is deflected in such a manner as to contact all of the material.
A diffuser or spreader cone is utilized to deflect the inlet air, and also to distribute the air evenly throughout the material in the hopper. This is especially important in the “critical drying region”.
A material drain, separate from the slide gate discharge, large sight glass, and large clean out door are important features of any hopper.
How is the hopper sized to hold material? From pre-determined tests proper drying temperatures and exposure times of various resins to the dryer air are recommended.
If the particular requirement calls for an exposure time of four hours, and a process rate of 100 pounds per hour is anticipated, a hopper is selected which will hold 400 pounds of material. Obviously then, if 100 pounds of material are processed per hour, a dryer with 100 CFM is selected. These parameters are recommended for virgin materials which are injection molded. Extruded resins or heavy regrind ratios will alter these parameters.
Different processing rates or “through-puts” of course determine the physical size of machine for the application. For larger processing rates, models, which are mounted on the floor, are utilized. Hoppers can be mounted on stands on the floor, or placed directly on the process machines to eliminate moisture contamination of highly hygroscopic resins. In situations where relatively small, but sophisticated production is required, automatically regenerated dryers may be affixed directly to the drying hopper and the complete assembly mounted directly to the molding machine.
Now that we’ve seen how a dryer is constructed, let’s examine some of the particulars that play an important role in drying the resin itself. Temperatures. dew point, volume of air, initial and final moisture content, as well as material configuration all affect drying of materials.
Temperature is probably the most important factor in drying plastic resin. Moisture content in ABS is noted after the materials’ exposure to air at different temperatures. It can be easily identified that the higher temperature had a marked effect on removing moisture from this material. It takes five times longer to dry ABS at 1600 F., as compared to 2000 F.
It has been noted previously that lower dew point (drier) air will retain more moisture than wet air, so dew point naturally has a significant impact on drying hygroscopic resins. Measuring dew point impact on polycarbonate is noted on this chart and it is evident the difference between -200 F. and -400 F. do not extremely impact this material. However, a relatively low dew point is required to remove moisture. Other resins which are highly hygroscopic, such as polyethylene terephthalate (PET) absolutely require dew points in the -400 to -500 F. range to be adequately dried.
In some production facilities airflow sometimes becomes a minor factor in drying. Everyone assumes (rather incorrectly) that if the temperature is correct for the product, and the dew point is relatively low, the material will be dry. Most resin producers however, agree the volume of air required to dry resins is approximately 1 CFM/lb./material/hour. The results with polycarbonate indicate the effect of proper airflow.
Initial and final moisture content obviously have an effect on the drying time. Nylon 66 was dried with two different initial moisture contents and time required noted. You can observe this has a definite effect. Eventually, however, the two curves would reach the same final equilibrium moisture content.
Configuration of the material plays an important role in drying. Large and small pellets dry differently, as well as thin film vs. thick sheet. ‘
Moisture adsorption capability depends on the type of material as well as the ambient in which it is placed. In some instances, exposure of only minutes can be detrimental.
If the material is exposed to a certain temperature and relative humidity for a period of time, it will reach an equilibrium point, referred to as the equilibrium moisture content. This chart shows this EMC for several plastics @80° F., and because this affects the drying time of the product, it is important for the material to be stored in sealed containers. Incidentally, the time to reach the EMC @80° F. is approximately five to seven days.
You can see a variety of factors influence the proper drying of materials. The material qualities themselves, as well as their handling, are extremely important. I might add too, that proper dryers may be designed and selected to adapt to a particular situation by the manufacturer, but it is most important these machines be maintained correctly. Filters ~ be changed regularly to insure longer desiccant life and hopper and dryer hoses maintained leak tight.
In summary, I hope the above has provided some insight on drying and how we, as dryer manufacturers endeavor to help you, the plastics processor, realize a defect-free and product.