Customer Application Brief

Chemical

Polypropylene and Polyethylene Production IntroductionPolypropylene (PP) and Polyethylene (PE) are used in a wide variety of end-products, many of which depend on visual quality, as well as performance, for their acceptance. Contamination, introduced with raw materials or generated in the production system, adversely affects product quality and is generally reduced by means of filtration.

In this application brief, the use of high efficiency filtration systems in polyethylene and polypropylene production plants is discussed. The brief describes how filtration ensures the production of polymers that consistently meet specifications, result in higher yields, less scrap, and reduce downtime.

The PE and PP Processes Ethylene and propylene hydrocarbon gases are the starting materials for production of PE and PP. Figure 1 shows a generic HDPE solution or slurry process in which the principal monomer, ethylene, is combined with a small amount of a co-monomer, often butene, and a solvent or diluent in a reactor with a catalyst. The solvent, typically cyclohexane or isobuane, is used to prevent the viscosity of the reaction mass from getting too high. The solvent is reduced from the reactor and sent to a solvent recovery unit where it is purified by distillation so it can then be reused in the process. The polyethylene polymer then is sent from the reactor to the extruder and pelletizer where additives, if any, are incorporated and a water bath is used to solidify the molten polymer strands exiting the extrusion die. Typical polyethylene plants produce from 300 million pounds per year to a billion pounds per year of polymer.

Figure 1. – Generic HDPE Solution or Slurry Process

A

A

A

A

B

D C

Cyclohexane

Butene

Catalyst

Additives

Ethylene

HDPE Product

Pelletizer

Reactor

Solvent Recovery & Distillation

SolventReduction

Water Bath

Extrusion

2

Figure 2 shows a generic polypropylene production process. Propylene monomer, solvent and catalyst react in the reactor to form polypropylene. The solvent, typically hexane or isobutane, is reduced in the devolatilizer located downstream of the reactor then re-circulated back to the upstream side of the polymerization process. As in polyethylene manufacturing, the polypropylene produced in the reactor then proceeds to the extruder and pelletizer followed by packaging. Molten polypropylene strands leaving the extrusion die are solidified with filtered water to ensure product cleanliness.

The Problem

Contamination in the form of particles and gels will cause quality issues in the final product. The source of the particles in both PP and PE production can be categorized as introduced to the process and created during the process.

Introduced to the Process — Contamination that are present in monomers, co-monomers, solvents and additives, and agglomerated catalyst particles, can become incorporated into the polyethylene or polypropylene during the polymerization process. Contaminates can also be introduced with additives and water used during the pelletizing process.

Created by the Process — In addition to solid particles introduced in the various process streams, deformable, gel-like particles can be formed and found in monomer storage, the reactor, and the solvent recovery system. Particles can also be carried over in the recycle solvent reduced in the devolatilizer.

The pelletized polyethylene or polypropylene polymer will retain these potentially visible contaminants and result in rejected product, particularly when the polymer is being produced for making blown or extruded thin gauge films. Quality control tests for the presence of these contaminants include examination of compression-molded plaques and laboratory extrusions of the final product pellets. It is especially important to control such contaminants in polymers requiring these stringent quality control examinations in order to minimize potentially high scrap and rework costs.

Another area of concern is that of hazardous materials, such as toxic catalysts, used in some production processes. From additional time required to properly gown workers for exposure prevention during filter change-out, to the disposal of used filters, coping with these materials can be costly.

Figure 2. – Generic Polypropylene Production Process

A

A

B

A

C

Propylene

SolventDevolatilizer

Unfiltered Water

Catalyst

Product Packaging

Pelletizer

Reactor

Catalyst Prep

Solvent

Water Bath

Extrusion

3

The SolutionBetapure™ PK series filters (Figure 3) are well suited for filtration of the major process streams. Contaminants in monomers and recycled solvents are the primary sources of particles that create off specification polymers. The recommended grades of Betapure PK series filters for these applications is a 20 or 40 micron absolute (see Table 1), as this will ensure the reduction of visible particles that will create off-spec product. Associated process streams such as catalyst systems, additives to the polymer at the extruder, and wash water prior to the pelletizer should also utilize cartridge filtration as contaminants in these fluids can also compromise the quality of the polymer produced in the process. Specific filter recommendations are listed in Table 1.

The rigid, gradient pore size structure of Betapure PK series filters (Figure 4) is ideally suited for the reduction of both deformable and non-deformable contaminants from the process streams in polyethylene and polypropylene production applications. With an efficiency of 99.9% ( = 1000) at the assigned rating. Betapure PK series will ensure the elimination of particles that can cause off specification product to be produced. Betapure PK series will neither unload previously reduced particles into the effluent stream nor allow deformable particles to pass through the depth matrix of the media and will retain its integrity throughout its service life. Betapure PK series is also fully compatible with the typical solvents and diluents used, as well as with ethylene and propylene monomers.

Some polyolefin manufacturing processes employ toxic catalysts such as TiCl4. Operator exposure to toxic catalyst residues during filter change-out on circulating solvent streams can be reduced by use of the CUNO™ CTG-Klean system (Figure 5). The CUNO CTG-Klean system uses a separate pressure vessel and disposable filter pack to isolate the product to be filtered from the housing. This reduces worker exposure during filter cartridge change-out. In one case, a large volume producer of HDPE resins was able to achieve several thousand dollars in savings by reducing the labor cost and down-time required for filter change-out.

Conclusion3M Purification’s 20 micron Betapure PK series filters are recommended for the primary PE, or PP filtration applications, including monomers, recycled solvents, diluents, and additives. 3M Purification’s 40 micron Betapure PK series filters are recommended for catalyst and additives. 3M Purification’s 40 micron Betapure PK series filters are recommended for the water bath ahead of pelletizing. Final product quality and yields will be substantially improved using 3M Purification Filtration Systems.

FLUID FLUID

GRADED-POROSITYSTRUCTURE

Figure 3. – Betapure™ PK Series Filters

Figure 4. – Graded Porosity Structure

Figure 5. – CUNO™ CTG-Klean System Structure

Table 1. – Specific Filter Recommendations

Schematic Filter Location Product Recommendation

A Monomers and solvent carriers 20 µm absolute Betapure™ PK Series Filters

B Catalysts and catalyst slurries 40 µm absolute Betapure™ PK Series Filters

C Additives before polymer extrusion 20 µm absolute Betapure™ PK Series Filters

D Water used in pelletizing 40 µm absolute Betapure™ PK Series Filters

3M Purifi cation Inc.400 Research ParkwayMeriden, CT 06450U.S.A.Phone (800) 243-6894 (203) 237-5541Fax (203) 630-4530www.3Mpurifi cation.com

Important NoticeThe information described in this literature is accurate to the best of our knowledge. A variety of factors, however, can affect the performance of the Product(s) in a particular application, some of which are uniquely within your knowledge and control. INFORMATION IS SUPPLIED UPON THE CONDITION THAT THE PERSONS RECEIVING THE SAME WILL MAKE THEIR OWN DETERMINATION AS TO ITS SUITABILITY FOR THEIR USE. IN NO EVENT WILL 3M PURIFICATION INC. BE RESPONSIBLE FOR DAMAGES OF ANY NATURE WHATSOEVER RESULTING FROM THE USE OF OR RELIANCE UPON INFORMATION.

It is your responsibility to determine if additional testing or information is required and if this product is fit for a particular purpose and suitable in your specific application.

3M PURIFICATION INC. MAKES NO REPRESENTATIONS OR WARRANTIES, EITHER EXPRESS OR IMPLIED INCLUDING WITHOUT LIMITATION ANY WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE OR OF ANY OTHER NATURE HEREUNDER WITH RESPECT TO INFORMATION OR THE PRODUCT TO WHICH INFORMATION REFERS.

Limitation of Liability3M Purification Inc. will not be liable for any loss or damage arising from the use of the Product(s), whether direct, indirect, special, incidental, or consequential, regardless of the legal theory asserted, including warranty, contract, negligence or strict liability. Some states do not allow the exclusion or limitation of incidental or consequential damages, so the above limitation may not apply to you.

Please recycle. Printed in U.S.A.3M is a trademark of 3M Company.

Betapure and CUNO are trademarks of 3M Company used under license.© 2011 3M Company. All rights reserved.

70-0201-8641-0 REV 1011pb