Managing moisture in online pellet sampling: moisture removal from continuous pellet streams with low mass flows can be challenging. This new dryer design enables significant moisture removal at short residence time.
For example, in defect inspections, the presence of moisture can have detrimental effects on the film quality, and depending on the extent, can cause erroneously elevated gel counts. In severe cases, the moisture impacts the film coherence to the degree that measurements are no longer possible. As these analyses are often quality-critical measurements, the manufacturing process needs the results for high-quality production.
Various root causes can be responsible for elevated moisture levels, amongst them improper sampling location, insufficient drying capabilities in the overall process such as inadequate spin dryer design, water entrapment during pelletization, and excessive carryover due to high pellet-surface roughness. Further, pellet morphology and environmental temperature, along with the conveying method, play important roles. Pellets with significant melt fracture, for example, are much more difficult to dry than those with a smooth surface.
The actual moisture levels at which problems with the analytical measurements can occur tend to vary. While some measurements such as image analysis on films may tolerate a few hundred parts per million (ppm), fundamental measurements using chemometric models may be impacted by lower levels.
Available options & shortcomings
When dealing with the challenge of excessive moisture in the sample stream, the first factor to evaluate is the criticality of the measurement. If the analytical information does not provide sufficiently valuable guidance, termination of the sampling and analysis process is a viable solution to consider. Another consideration when determining the importance of the measurement is if there is a process condition that can be used to react to a given reading.
If the process control parameters can allow for a reduction of the pellet sample moisture content, this might be a suitable path, e.g., changes in the pelletizing operation, or higher air velocity or temperature in a pneumatic sample line. Another possible solution is to account for any water present in the sample during the analytical measurement, thus removing the impact of varying moisture levels on the results (e.g., some spectroscopic methods allow for this sort of correction). As a final option, one can attempt to dry the sample prior to the analysis.
While drying ofthe sample may sound trivial, the requirements to sustain a continuous pellet stream in an unsupervised fashion poses unique challenges:
* The pellet flow rate for at-line analytical measurements ranges from 5-20 kg/hr, which is a very atypical mass throughput for continuous dryers.
* The inlet moisture content is a critical parameter that needs to be well understood. However, during manufacturing the level can heavily vary and may reach levels of up to 2,000 ppm. Moisture spikes could exceed even those values. A good understanding of the average moisture loading along with maximum spike levels is essential for selecting a successful solution.
* The location of a dryer needs to be identified, weighing factors such as proximity to the sampling point, electrical restrictions in the plant environment, space requirements and accessibility.
* Thermophysical properties of the polymer can pose limitations on drying operations. Materials with lower glass-transition temperatures need to be dried with the addition of little or no heat, while materials exhibiting impact-plasticity may be challenging to handle in high-velocity gas flows.
* For continuous analytical measurements, the dryer needs to operate without supervision and on a continuous basis. Further, it is essential that process irregularities are detected and plant personnel alerted.
* In many cases where the measurement is deemed essential, increases in the analyzer response time must be minimized as much as possible. Longer delays negatively impact prime production. Usually, the maximum tolerable residence time is five minutes, with an aim of one to two minutes.
* Additional criteria that need to be considered for technology selection are: unit cost, implementation cost, space and air requirements, operation complexity, failure risks, dust development, and the ease of cleaning.
Various technical solutions
Drying of solids is an established process requirement and numerous strategies are used to remove moisture from a solid sample or product stream. In the section below, we briefly review the most common methods that can be operated in a continuous fashion, including relevant benefits or limitations for the purpose of discussion. We do not discuss batch-style dryers in this paper due to the requirement of continuous operation.
* Rotary dryer: Water is removed in a high-throughput, rotating drum with strategically positioned flights to lift and shower the product evenly through the heated airstream, which enters the drum countercurrent to the continuously flowing product stream. In addition to conventional convection heating, the dryer can also be operated using infrared heat lamps. Downsides of the dryer are high energy consumption, a large spacial footprint, and potentially long residence times.
* Belt dryer: The product is spread out on a moving belt moving continuously through a drying chamber, and dry, heated air is pushed through the pellet layer from underneath. This operation is applicable only for low moisture levels, as otherwise the space requirements become prohibitively large.
* Vacuum dryer: The moisture is removed through a combination of vacuum and heat. The drying chamber has to be enclosed tightly for the operation to work effectively, thus preventing continuous operation. Commercial models work in a semi-batch fashion. To achieve complete drying, the residence times can be significant, and the continuous maintenance of a vacuum is energy intensive.
* Fluidized bed: Dry air is provided from the bottom of the unit through a perforated plate, suspending the solids until the bed resembles fluid behavior. Directional holes allow for the fluidized bed to be operated continuously with a controllable rotational motion. The operation of a fluidized bed can require an elaborate air-handing system and fairly substantial amounts of dryer air, which is often cost prohibitive.
* Packed bed: Dry and heated air enters a packed bed of the moist solid in a countercurrent manner. The amount of moisture removed in a given time depends on the air volume provided to the unit. While the dryer benefits from a simple design, it can require long residence times, that leads to long delay times between the analytical measurement and potential process controls or adjustments.
* Pneumatic drying: Using dry and heated air during pneumatic transportation can provide necessary driving force to separate the water from the solids. Generally, this is considered a viable option only for streams with moisture levels that are consistently below 1,000 ppm.
* Cyclone: Moisture removal is achieved through the continuous separation of smaller water droplets from the polymer pellets based on mass difference. The dryer does not contain any moving parts, and the design is simple. However, it does not provide removal of the surface water beyond what can be achieved through prior mechanical impact (knocking the water off). Additionally, operation requires a fairly large volume of dry air.
Special dryer design & case study
As outlined in the previous section, continuous analytical measurements require a steady feed stream with minimal delay to ensure timely results, good process control and low energy consumption. In addition, the solution needs to be highly reliable to enable unsupervised operation. Above all, the dryer needs to effectively reduce the moisture content to acceptable levels. When need presented itself, no suitable solution was found in the marketplace; therefore, we designed and built a special dryer.
The dryer introduced in this case study is shown in Figure 1. The design combines a disengaging hopper on the top, which separates the moist particles from the wet conveying air and any entrained droplets, and removes surface water via a cyclone effect, followed by a fluidized/moving bed section on the bottom. The design is specifically aimed at removing maximum moisture levels with the shortest possible residence time when connected to a pneumatic pellet sample line.
Contrary to other concepts in the market, this design operates truly continuously with minimal sample mixing. It is worth noting that the dryer operation requires about 30 scfm at 60 psi of dry air continuously, and flows need to be adjusted individually for the top and bottom section to optimize performance. For materials with sufficiently high glass-transition temperatures, the drying air can be preheated for even faster moisture removal rates and shorter residence ti mes.
The detailed process follows the following steps:
1. Pellets are sampled from the process after the classifier and fall by gravity to the eductor. The eductor accellerates the pellets into the cyclone and begins the first phase of drying.
2. Pellets enter a cyclone separator tangentially. The heavier material (pe ll ets) go to the wall of the cyclone while the moist conveying air and the water droplets move to the vortex of the cyclone and exit at Vent-1. Additional air (Air-1) can be injected into the bottom of the cyclone to help free the water from the pellets and add more dry air capacity to remove water. Air-1 also increases the residence time of the pellets in the cyclone section to improve drying efficiency in the free-water removal step.
3. Pellets slow down in the conical section of the cyclone portion as they lose energy and move into the fluidized bed drier. This is where the fine separation of water occurs. This fluidized bed is more vigorous than packed column approaches and helps to facilitate removal of moisture entrained in rough surfaces. The vent of wet air from the fluidized bed is removed at Vent-2.
4. Residence time of pellets in the fluidizer can be varied by adjusting the height of the standpipe located in the center of the fluidizer. The bed of fluidized pellets moves in a circle around the center exit because the bed plate (Figure 2) is divided into quadrants with 45 degree angled holes. The rotation is created to enable good mixing of the bed without requiring bubbling of the large-diameter particles typical of plastic pellets. An adjustable-height weir acts as a baffle to help move pellets towards the center.
5. Fluidized pellets pass into the variable-height standpipe and are accelerated on to the sampling hopper feeding the analyzer. This is an additional drying step because fresh, dry air (Air-3) is introduced. Note that a single dryer can feed multiple analytical sample hoppers if more than one dry pellet analysis is desired.
The dryer was tested under controlled conditions in the laboratory, and it was found that at 14 kg/hr flow, the moisture content could be removed by about 3,000 ppm. Subsequently, the unit was implemented in the pellet stream feeding an at-line film analyzer for gel measurements, where moisture levels are particularly critical to avoid false detection of defects. We modified the process to test the dryer's abilities and the pellet moisture content reached peak values of up to 1,800 ppm, rendering film casting for image analysis impossible.
During initial field tests, the air flows to the various sections were optimized and a target pellet residence time of 2-3 min was visually estimated. To reduce the residence time even further, to less than 2 min, the air delivered to the fluidized bed section was heated to 60 [degrees]C and the flow increased. The higher airflow increased pellet movement and decreased bed density, thus reducing the residence time as well. Subsequent operation enabled uninterrupted film analysis without any evidence of false defect detection.
Although pellet drying is an established industrial process, moisture removal from continuous pellet streams with low mass flows, such as encountered for process analytical steps, is challenging and remains largely unaddressed in the market. We are pleased to introduce in this report a special dryer design that, through combining a cyclone and fluidized bed concept, enables significant moisture removal at short residence time. This approach successfully addresses moisture problems, especially for applications where a minimal time delay is critical.
By Birgit Braun, Serena Stephenson, Bruce Hook & Shrikant Dhodapkar Dow Chemical Co.
ABOUT THE AUTHORS
All the authors work for Dow Chemical Co. Birgit Braun, Ph.D., is a research scientist in Freeport, Texas. She has a polymer science background and spent five years in the Plastics Process Characterization R&D group before moving to Olefins, Aromatics & Alternatives R&D in 2016. Serena Stephenson, Ph.D., is a senior R&D manager with Formulation Science R&D in Collegeville, Pa. Prior to her current role, she led the Plastics Process Characterization R&D group. Bruce Hook, PhD., is a fellow scientist for Specialties Chemicals R&D. Shrikant Dhodapkar, Ph.D., is a fellow scientist in Plastics Process Fundamentals R&D, specializing in materials handling. Both Bruce and Shrikant are based in Freeport, Texas, have been with Dow for more than 25 years and are AIChE Fellows.
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|Title Annotation:||TECHINCAL ARTICLE|
|Comment:||Managing moisture in online pellet sampling: moisture removal from continuous pellet streams with low mass flows can be challenging.|
|Author:||Braun, Birgit; Stephenson, Serena; Hook, Bruce; Dhodapkar, Shrilkant|
|Date:||Jun 1, 2017|
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