V. K. Tiwari and S. GUHARAY, Bechtel Energy, New Delhi, India
Product pellets produced in polyolefin plants—such as polyethylene (PE) or polypropylene (PP)—require transportation from the unit to the end users that process these pellets to manufacture different end-use plastic products. It is critical to design pellet handling systems to ensure smooth continuous operations that meet the demands of downstream users. Products produced up until the extrusion section should be moved to blending and storage silos from where they are loaded in transport containers or moved to bagging silos for bagging. The design of this area requires interaction with the owner to understand the owner’s operational philosophy and dispatch modes at the location where the plant will operate. The following sections will discuss a stepwise approach to finalize the conceptual design of this area.
For illustration purposes, PE pellets will be considered in this article. In addition, the PE plant has a capacity of 450,000 tpy, with onstream hours of 8,000/yr [56.25 tons per hour (tph) of pellet production]. The objective is to develop a specification that clearly outlines the requirements for the package supplier. Therefore, all necessary inputs must be comprehensively listed to ensure a well-defined specification.
Pneumatic conveying of pellets from the extruder to blending and storage silos. Pellets produced in the extruder are continuously pneumatically transported to blending and storage silos. Several silos for product blending will achieve uniformity of pellets per product specifications.
Pellet properties. To prepare the process specification, pellet properties are an important input. Typical properties for PE pellets are listed in TABLE 1, which can be confirmed with technology licensors.
Resins formed in the reactor are mixed with additives and extruded in the extruder to form pellets. Plants operate to produce different types of polyolefin grades. Based on the reactor operation, three types of resin grades—and therefore pellets—are produced in the plant:
Pellet transportation: Operation description. Transport consists of one open-loop transfer line with air as the conveying medium (FIG. 1). PE pellets discharged from the extrusion unit are continuously transferred by means of the pneumatic transport package from a feeding hopper to the blending silos. A cooler is provided at the compressor discharge to remove the heat of compression. The conveying mode can be a dilute (lean) or dense phase, which should be finalized by the package supplier. The required pellet flowrate for pneumatic conveying should be specified in the document.
Capacity of pellet feeding hopper and pellet conveying package: Extrusion section-blending silos. Pellets from extrusion sections must be stored in a pellet feeding hopper for a duration that will permit the cleaning of conveying lines through a sequence during silo switching (FIG. 1). The minimum residence time is 5 min–10 min at the design capacity of the extruder. During normal operation, this hopper is nearly empty. This hopper operates at atmospheric pressure with an operating temperature of approximately 80°C.
The minimum horizontal slope for the bottom conical part should be 30°. The hopper should be designed to avoid product stagnation and the discharge nozzle should completely drain the product pellets. The bottom of the hopper should allow gravity discharge to a rotary feeder.
The minimum flow of pellets depends on the turndown capacity of the extruder—typically at 55% of the extruder’s design capacity. Depending on the selected extruder design capacity, this flow can be obtained from the extruder supplier for the smooth operation of the plant.
Maximum flow is selected based on the design capacity of the extruder, which must have some margin over the hourly plant design capacity to cater to the removal of any accumulated resin due to unplanned extruder shutdown without impacting the reactor production rate. The industry norm is 11% (10% margin over plant capacity and 1% for additive), though a higher margin can also be taken.
The design flow should have some margins over the maximum flow. Typical margins are 10%–15%. For this example (a PE plant with 56.25 tph), the extruder’s design capacity was selected as 63 tph—an 11% margin over 56.25 tph. These flows are detailed in TABLE 2.
The system should be capable of operating from 35 tph–70 tph, and the supplier should consider this to properly design the system.
Pellet pneumatic transport compressors/blowers (extrusion section to any of the blending or storage silos). Pellets are pneumatically transported from the extrusion section to any of the blending/storage silos. Depending on the type of conveying, this is achieved with air compressors or blowers. Two electric motor-operated compressors/blowers are selected as a minimum for this operation, one operating and one on standby.
The time required to switch from one operating compressor/blower to the standby compressor/blower is important and should be done as quickly as possible. A cooler and a filter are provided to remove the heat of compression and dust before the conveying air is supplied to the pick-up point below the feeding hopper. Maintaining the correct air velocity at the pick-up point is important.
Determining the number of blending and storage silos. Silo capacity is determined based on various options—e.g., to store one lot of production (based on the production of a plant for 8 hr–9 hr) or the number of railcars that will be filled by using one silo. Dispatch by railcar is more common for plants in certain locations.
For this example, a lot size of 8 hr is assumed. TABLE 3 is used to determine the capacity and number of silos.
The number of blending silos is determined based on silo sequencing, which is described in a later section of this article. At this stage, a preliminary number can be determined based on the following requirements:
These six blending silos should have internals to blend pellets while pellets are being filled or recirculated with the help of blending compressors/blowers to achieve a uniform property for a lot/batch.
The non-aim grade pellets should be stored in a dedicated blending silo that is equipped with additional small rotary feeders. The purpose of these feeders is to simultaneously feed non-aim grade pellets as slip stream to the recirculating stream from the selected blending silo (FIG. 1). The advantage of this system is that the non-aim grade pellets can be blended quickly.
Pellet pneumatic transport compressors: Blending silos to the bagging silos or loading area. Once the capacity and number of blending silos are determined, the next step is to determine the number of air compressors/blowers and their capacity. The first step is to understand the requirements (TABLE 4).
Owners often prefer to reprocess the off-spec pellets or non-aim grades in the extruder. This requires a conveying line from the blending silos to the extruder feed hopper. Such a facility may require an additional set of blowers/compressors (reprocessing air compressors as described TABLE 4). It is possible to optimize the other blowers/compressors for multi-purpose usages, and process engineers can discuss value-added engineering opportunities with pneumatic conveying package suppliers.
Silo vents are routed through a bag filter house to collect pellet fines generated during conveying and blending operations. The vent from this bag filter is sent to the atmosphere or thermal oxidizer depending on the volatile organic compound (VOC) content in the vent stream.
Bagging silos and elutriators. Bagging silos supply pellets to the bagging machines. Note: These bagging silos are not required when railcar loading is the only option for product dispatch. In this case, the product is directly transferred to railcars using pellet transfer compressors/blowers.
Bagging silos have a minimum storage capacity of 1 hr of pellet production, and their number should be decided so that each bagging silo is connected to two bagging machines. Bagging machine capacities and numbers are discussed in subsequent sections.
When pellets are pneumatically transferred at high velocities through a piping system, heat is generated from the friction between the pellets and the pipe’s surface. This causes the pellets to warm to their softening point to form a skin along the pipe’s surface. Further transfer of the pellets down the pipe causes the skin to peel off, resulting in strands of polymers of varying lengths. These strands are commonly referred to as streamers, angel hair, snakeskin or fines. These can be prevented by using shot-peened pipes.
The typical fine content in the final pellet product should not exceed 200 ppm wt. Fines are classified as particles with sizes between 50 µm and 425 µm. If the fine content formed during pneumatic conveying is higher than 165 ppm wt, an elutriator system is required. To prevent fines in the bagging silos, elutriators are installed upstream of the bagging silos.
The elutriator system consists of air fans, a dust collector bag house and a rotary feeder at the bottom of the bag filter. Airflow to the elutriator system is generated via a radial fan that creates a counterflow vs. the pellet conveying system (FIG. 3). The air entrains the lighter fine particles and streamers and carries them out of the elutriator to the cyclone separator. A dust collector rotary feeder removes the separated material from the cyclone into a disposal fines dumpster or large bag. The typical expected value of fines in pellets at the elutriator outlet should be less than 40 ppm wt. Each bagging silo should be equipped with an elutriator system. Silo purging and elutriation helps to control the dust/fines in the product pellets and limits dust and VOCs in the emissions stream.
Blending and pellet transfer capacity. Product blending rates are supplier specific and are generally completed in 4 hr–6 hr. Once the silo is filled to 20%–30% of its capacity, blending operation can be started simultaneously with the silo receiving product pellets from the extrusion section. This minimizes the blending time and reduces the number of blending silos.
Pellet transfer capacity is dependent on how the product is being dispatched from the unit. If it is bagged (Note: Bagging operations are not usually run during nighttime), this provides approximately 16 hr to complete the bagging for the entire day’s production. For this example, the hourly PE pellets production is 56.25 tph; therefore, a day’s production would be 1,350 t. With a 16-hr bagging rate, the required pellet transfer capacity would be 85 tph (TABLE 5).
It is economical to maintain the same transfer and blending rates so that the same set of compressors/blowers can be utilized for both the operations. This can reduce the number of compressors/blowers in the process. This operation can be discussed with the supplier to optimize the number of compressors/blowers.
Revisit the selected number of blending silos and silo sequencing. At this stage, silo sequencing is performed to check whether the transfer rate from the extruder to the blending silos, the blending rate, the number of selected silos, and the transfer rate from the blending silos to the bagging silos or railcar loading area are adequate for continuous plant operation (FIG. 4). This sequencing analysis helps to optimize the number of silos. Owner feedback on the selection of the number of silos is critical. Basic assumptions to perform sequencing are:
For this article’s example of 56.25 tph, it will take approximately 8 hr to fill 1,000-m3 blending silos up to 90% of their volume capacity. The blending rate is 6 hr (TABLE 5), with an additional 6 hr to empty the silos to the bagging or loading area.
From silo sequencing, the number of selected silos for this example are adequate—five silos are engaged in operation, while one silo remains empty for off-spec pellets storage. During product grade changes, a transition time of up to 8 hr can be expected where reactor operating conditions and dosing rates of catalyst and chemicals to the reactor are changed (a higher transition time is possible if the type of catalysts to the reactor is also changed). During these transition periods, dedicated off-spec and other available silos can be used to store non-aim pellet grades. These non-aim pellet grades can then be blended with aim pellet grades during blending/transport operations.
Determining the number and capacity of bagging machines. Bagging operations typically run for 16 hr, as bagging machines are maintenance prone, which reduces availability. For this article’s example, a day’s production is 1,350 t.
Form fill seal (FFS) bagging machines are available for 25-kg bags at the high capacity bagging rate of 2,800 bags/hr. Such high-capacity FFS bagging machines are prone to overheating during operations and are more vulnerable to breakdown at sites that experience high ambient temperatures of 40°C–45°C. Frequent breakdowns result in lower bagging rates. Therefore, it is recommended to select high-capacity machines; however, for system design purposes, consider a lower effective capacity. For example, 2,500-bags/hr machines can be selected with an effective bagging rate of 2,200 bags/hr.
Considering an effective bagging rate of 2,200 bags/hr, two 55-tph bagging machines should be adequate to bag a day’s production in 12.5 hr, providing > 20% margin on daily available bagging time. Some owners may also require a facility for jumbo bagging (1,000-kg bags) or bulk bagging. A recommended capacity for such lines is 20 tph–30 tph. For this example, two 200-m3 bagging silos are considered, each connected to a 2,500-bags/hr bagging machine and a common machine for jumbo bags. The system capacity based on this scenario is detailed in TABLE 6.
Washing and drying of the blending and bagging silos and railcars. Silo washing should be considered to prevent cross grade contamination when a unit is producing multiple grades (FIG. 5). Product grades with different visual appearances may stick to the silo’s wall and contaminate the other grades after grade transition. Potable water should be used to wash these silos, and this may require a set of pumps. Purge air fans can be used to dry the silos. However, for quick drying, additional equipment may be needed—blowers, filters and steam heated exchangers can be used to dry silos. Silo washed water can be routed to a wash water tank where fines and pellets are separated from the wash water. The loss of wash water is compensated for by the addition of make-up potable water. The inclusion of silo washing requirements should be included in process specifications and wash water (potable water) requirements in the utility summary. The typical flowrate required for washing silos is 40 m3/hr–50 m3/hr. Silos and steel structures are not usually designed to be full of water; therefore, provisions must be taken to avoid this.
If rail car loading is employed at a plant, the reclamation of pellets left in the return railcar from the customer should be dealt with accordingly—washing and drying of the rail car should be considered. Reclamation of pellets requires vacuum sucking them into a reclamation silo (for further processing in the extruder or for loading), which requires an additional set of equipment. This vacuum/suction system has the simplest material transfer arrangement among the available types of transfer systems, as well as the best dust and fines control. Major disadvantages of this system are a limited conveying range and lower reclamation capacity.
Other important requirements. The following are additional important requirements for a petrochemical plant’s pellet handing system:
A pellet transfer system uses aluminum and/or stainless steel as the material of construction. Silos, bins and hoppers should be constructed of aluminum to reduce the structural load. Piping should be stainless steel to prevent damage during deplugging in the case of line choking.
All gaskets used in this service should be white neoprene (food grade) or equivalent to avoid product contamination. Any piping in contact with clean air should be comprised of stainless steel. All conveying piping and elbows should be provided with proven internal treatment to prevent the generation of snake skins or angel hairs. Shot peening or other equivalent means should be discussed with prospective suppliers. Piping must be grounded at strategic locations along the pipelines to avoid static charge build-up.
The supplier of these systems should include other components required for smooth operations and should include all necessary specifications in their proposal. Vent streams to atmosphere must ensure the maximum concentration of dust emissions according to local regulations and specifications. A suitable interlock system provides all the interlocks and consents for the safe and easy remote operation of the conveying system. The supplier should include a dedicated programmable logic controller (PLC) for the conveying system with logics and interlocks for machine protection, silo selection, silo overfill protection and the prevention of grade contamination. The package PLC should be interfaced with the primary plant’s distributed control system (DCS) to provide daily production, bagged or loaded products quantity, and silo inventories. Suppliers should also provide pressure and vacuum relief valves for equipment protection.
Takeaways. The design of a pellet handling system for polyolefin plants is a critical aspect that directly impacts the efficiency and productivity of the entire production process and final product quality. By following a systematic approach and considering various factors, such as the plant's operational philosophy, regional market conditions and end user requirements, a well-designed pellet handling system can be installed.
The selection and implementation of a pneumatic conveying system for pellets ensures a smooth and continuous operation of the plant. The careful consideration of the number and capacity of blending silos enables efficient storage, a smooth grade transition and easy retrieval of pellets based on market demand. Moreover, addressing challenges related to fines and dust control in pellets, along with incorporating washing and drying requirements for blending silos, contributes to the overall quality of the final product. Finally, the determination of bagging capacity and the appropriate number of bagging machines ensures efficient packaging and dispatching of the pellets to end users.
By following the guidelines presented in this article and engaging in close collaboration with licensors, contractors and owners, the design of a pellet handling system can meet the demands of downstream users, support sustainable growth and contribute to the success of polyolefin plants in meeting the evolving needs of the market. HP
Vijay Kumar Tiwari is an Engineering Group Supervisor (Process Lead) at Bechtel India Pvt. Ltd. He has more than 18 yr of professional expertise in process design, development, engineering and management, and possesses extensive experience in managing petrochemical projects on a global scale. He earned his BTech degree in chemical engineering from the Indian Institute of Technology (IIT) in Delhi, India. His previous roles included positions at Toyo Engineering Korea and Engineers India Ltd.
Siddhartha Guharay is a Principal Engineer, Petrochemicals, with Bechtel India Pvt. Ltd. He has more than 30 yr of experience in refining, petrochemicals, polymers, design engineering (FEED and EPC projects), operations and technical services. He earned an MS degree in chemical engineering from the University of Missouri-Rolla, and a BE degree in chemical engineering from the University of Roorkee (now IIT Roorkee). His previous roles included positions at Engineers India Ltd., Haldia Petrochemicals Ltd., Quotient Engineering (now Wood India Engineering Private Ltd.) and Mott MacDonald Pvt. Ltd.