Rayon fibers are dried on both drum dryers and conveyor dryers, but most fiber producers today use conveyor dryers. Contemporary conveyor dryers are large, multi-stage, multi-zone pieces of equipment approximately 160 to 325 feet (50 to 100 meters) long.
Rayon producers can manipulate several process parameters to achieve the final moisture targets. Three of the most direct parameters for producers to manipulate are:
- Process air temperature.
- Product retention time.
- Process air speed.
Each of these parameters can affect other fiber qualities such as color, strength and amount of fiber conditioner remaining on the fiber. Further, because process air temperature is a relatively easy parameter to change, that is often the first change rayon producers make when trying to change the drying rate.
In some cases, however, high process air temperatures can negatively affect the product quality. This article looks at several measures of product quality — color, strength and oil pickup — and shows how they vary with process air temperatures.
Color is an important parameter to rayon producers. Fibers are analyzed for whiteness and yellowness. In general, higher whiteness measurements and lower yellowness measurements are better. Therefore, rayon producers can enhance the quality and value of their fiber by consistently delivering high whiteness values, low yellowness values and stable color levels.
Fiber strength is another important parameter. Fiber strength is measured in both wet and dry conditions. The strength of the fiber affects its ease of processing in each step of the fiber creation, and stronger fibers are more durable in processing.
Oils and similar substances are added to rayon fibers as conditioners to the fibers. These conditioners aid in processing the fibers downstream by reducing static electrical charges and lubricating the individual staple fibers while they are formed into yarn. Rayon producers can enhance the quality and value of their fiber by delivering consistent levels of conditioner on the rayon fiber. Additionally, rayon fiber producers can reduce costs by efficient use of raw materials — adding only the amount of conditioner needed.
Conditioners are added upstream of the drying step in rayon production, and there is concern that the drying process can drive off the volatile conditioners. Rayon fiber manufacturers measure the amount of conditioner on the fiber as “oil pickup,” or OPU. This article looks at parameters of the drying rate — temperature and retention time — to determine if there is a relationship between drying rate and oil pickup.
Drying Rate and Its Influence on Oil Pickup
Tests were conducted in February 2013 using a benchtop dryer. The tests varied process air temperature and drying retention time to determine if these conditions changed the final properties of the fiber.
Test results showed that for a given final moisture target, process air temperature and drying retention time are interrelated. For example, to reach a specified final moisture target, fiber can be dried at:
- High temperatures with relatively short retention times.
- Moderate temperatures and moderate retention times.
- Low temperatures with relatively long retention times.
It is important to consider both the temperature to which the fiber is exposed and the duration for which the fiber is exposed to those temperatures.
Drying Rate. As shown in figure 1, samples were dried to target final moisture of 10 percent dry-weight basis (DWB) using process air at 194, 248 and 302°F (90, 120 and 150°C). As expected, increasing the temperature reduces drying time, with the sample dried at 184°F taking more than double the time of the sample dried at 302°F. If there are no negative effects on the fiber quality, using higher process air temperatures and shorter retention times in the dryer would allow rayon producers to increase capacity through the dryer.
Product Temperature. Product temperature is an important result of the process air temperature, and product temperatures predictably increase with higher process air temperatures. Higher product temperatures can introduce the possibility of negatively affecting the fiber: decreasing the fiber strength, driving off the conditioners and decreasing the whiteness of the fiber. As shown in figure 2, at the 6 minute mark, the product temperature for the 302°F trial is nearly double the temperature of the 194°F trial and trending higher.
It should be noted that rayon fibers enter the dryer with a very high moisture content — in the range of 90 to 120 percent on a dry-weight basis. Because of the high moisture content, the fiber can be protected from increasing temperature during the period that moisture is rapidly evaporating. The heat delivered by the process air is going into evaporating the water rather than into increasing the temperature of the product. Looking at the graphs for 194°F and 248°F, one can see a period where the product temperature does not increase, but drying is still taking place. Rayon producers can use this information to adjust temperatures in the dryer and shield the fiber from increasing temperature.
For example, during the initial phase of drying, while the fiber is very wet, process air temperatures can be higher without necessarily increasing product temperature. Then, as the moisture evaporates, process air temperatures can gradually decrease, protecting the fiber from excess heat and possible thermal damage.
Fiber Whiteness. As process air and product temperatures increase, rayon fibers decrease in whiteness. In general, higher whiteness values represent higher quality fiber. However, there are no absolute whiteness values; the required whiteness is a customer-specific or product-specific value.
Figure 3 can be used to guide rayon producers that are optimizing dryer operation. Higher temperatures mean shorter retention times and, therefore, higher capacity through the dryer — but not without a drop in whiteness. Using customer-specific whiteness values, producers can optimize dryer throughput by maximizing temperatures.
Fiber Yellowness. Rayon producers also measure yellowness in fibers (figure 4). As process air and product temperatures increase, rayon fibers increase in yellowness. This is represented by a yellowness value close to zero.
Similar to the whiteness measurement, there are no absolute values, but less yellow is associated with a higher quality product. Higher temperatures mean shorter retention times and, therefore, higher capacity through the dryer. However, higher temperatures also increase the yellowness value.
Oil Pickup. Rayon fiber manufacturers measure the amount of conditioner on the fiber as oil pickup. Oil pickup does not show a strong correlation with temperatures for the range of temperatures (194 to 302°F) evaluated in this study (figure 5). All products met the specified oil pickup value.
Rayon producers can be confident that for the range of temperatures studied, therefore, process air at the studied temperatures does not remove the conditioners. Also, because drying retention time is inversely related to process air temperatures, the test did not show a correlation between retention time and oil pickup.
It should also be noted that longer retention times do not reduce oil pickup values. Longer time in the dryer removes more moisture but not more conditioners. If producers target higher fiber-whiteness values by using lower process air temperatures and longer retention times, the conditioner will remain on the fiber.
Fiber Strength. Fiber strength does not have a strong correlation with process air temperatures for the range of values studied (figure 6). The products from all temperature trials exceeded the specified values of 2.6 gram/denier.
In conclusion, the experimental results show that for the range of process air temperatures and drying retention times studied, higher temperatures lead to a decrease in the color quality of the rayon fibers. Temperatures above 248°F lead to a decrease in fiber whiteness and an increase in fiber yellowness.
The experimental results also show that, for the range of temperatures studied, oil pickup is independent of process air temperature and retention time. Fiber producers can add the right amount of conditioners and have confidence that the drying process will not remove it. However, fiber whiteness does depend on process air temperature, with lower temperatures producing whiter fibers.
Rayon fiber producers are concerned about the optimum conditions to operate their dryers. The desire is, of course, to produce the highest quality fiber at the maximum output. Process air temperature and retention time are two parameters that rayon producers can adjust to produce the desired fiber. If the product can be dried at higher temperatures and still meet the quality requirements, rayon producers can use higher temperatures to increase capacity with their existing equipment. Alternatively, if producers require a very white fiber, longer retention times and lower temperatures are required.