A. S. Ekambikar and V. R. KARRI, Saudi Aramco, Al Khobar, Saudi Arabia
With the global increase in plastic consumption, demand for linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE) has increased exponentially. This has led to multiple projects to increase production capacity in East Asia and the Middle East. While existing producers seek to increase production rates by debottlenecking or adding new trains, new grassroots facilities are being announced, and many are in the design and construction phases. This article highlights and compares the various technology platforms available to provide technical insight into the selection of the best LLDPE/HDPE platform for a given facility. A high-level overview of the various technology platforms is provided based on open-source available data and the authors’ years of technical and operating experience.
LLDPE and HDPE are thermoplastics that are widely used for a variety of applications ranging from films and coatings to containers, tanks, pipes, crates, etc. LLDPE possesses a branched structure usually produced with comonomers like butene-1, hexene-1 or octene-1; in comparison, HDPE has a less or not-branched structure with a very crystalline nature. Little to no comonomer is used to produce HDPE.
Various technologies are available globally to produce LLDPE and HDPE. Generally, LLDPE is produced in swing technologies, wherein HDPE products can also be produced in the same unit.
AVAILABLE TECHNOLOGIES
Three types of LLDPE/HDPE swing technologies are available in the market: gas phase, solution phase and slurry phase. The technologies are differentiated based on the type of reaction process. Of these three technologies, the gas phase reaction process covers almost 85% of worldwide capacities.
Gas phase technology. The gas phase LLDPE/HDPE process utilizes fluidized bed gas phase reactors. All polymerization processes are exothermic in nature, and heat evolved during the reaction must be removed constantly or there are chances of a runaway reaction or chunk formation. The heat from the polymer particles is removed using circulating hydrocarbon gas that keeps the polymer bed in a fluidized condition, thereby avoiding particle-particle fusion.
The actual fluidization velocity—which is 3–5 times the minimum fluidization velocity—is maintained using a centrifugal gas compressor that recycles the gas from the reactor. The circulating gas from the cycle gas compressor is then cooled using either demineralized water or cooling water in a shell-and-tube-type heat exchanger.
The catalyst used to produce LLDPE/HDPE is either Ziegler–Natta (traditional and advanced) or metallocene. The catalyst used in gas phase processes contains either titanium, chromium or other transition metals that are activated using metal alkyls as co-catalysts. Typical metal alkyls used are triethylaluminum (TEAl), triisobutylaluminum (TIBAl), tri-n-hexylaluminum and diethylaluminum chloride (DEAC). Catalysts are supported on traditional magnesium chloride (MgCl2) or on silica as per recent trends. The catalysts are either pre-polymerized or fed directly to the reactor based on catalyst activity, reactor cooling effectiveness, catalyst type, etc. A typical block flow diagram for the gas phase PE process is depicted in FIG. 1, and a typical process flow diagram for the gas phase polyethylene (PE) process is depicted in FIG. 2.
The gas phase process offers a broad/complete portfolio of products (LLDPE/HDPE, unimodal metallocene LLDPE) coupled with huge capacities in a single-reactor PE plant. Proven single-reactor capacities up to 650,000 tpy have been demonstrated.
The main features of the process include:
Lower capital expenditures (CAPEX)
Lower operational expenditures (OPEX)
High operational stability
Wide product and catalyst portfolio
Proven metallocene capability
The latest development includes the capability to produce bimodal HDPE products in a single reactor.
Concerns of the gas phase process include:
Monitoring and control of reactor static generation
Requires seedbed management for startup
Requires periodic cleaning of the reactor distributor plate, associated piping and exchangers due to circulating fines in the loop and, at times, due to carryover excessive fines.
Solution technology. In the solution phase technology, the reaction takes place in a solution phase and the PE formed remains in the solution, which is then separated from the solvent and molten polymer is directly fed to the extruder. To keep the polymer in the solution phase, the reaction is operated at higher temperatures and pressures. Typical solvents used are isopar, hexane, cyclohexane, etc. Proven capacities of up to 450,000 tpy have been demonstrated. A typical block flow diagram of the solution process is shown in FIG. 3, a typical process flow diagram of the solution technology is shown in FIG. 4.
The main features of the solution process include:
Well-proven technology with benchmark LLDPE and HDPE products
Proven for metallocene catalyst-based LLDPE grades
Reduced residence time (often a few minutes)
Grade campaigns as low as 200 tonnes (t)–500 t are possible and the transition time usually varies between 30 min and 1.5 hr
Able to produce plastomers with a density range of 0.9 gm/cm3–0.91 gm/cm3 and elastomers with a density range 0.8 gm/cm3–0.9 gm/cm3, unlike in the gas phase and slurry processes.
Concerns with the solution process include:
Typically uses multiple reactors in series to achieve the requisite properties
High operating temperature and pressure for conventional LLDPE/HDPE
As a liquid-based technology, this process generates more wastes (e.g., waxes) compared to gas phase processes
Few technologies use solvents operating at an auto ignition temperature
Energy-intensive compared to gas phase technology
Limited licensing options.
Slurry technology. Slurry phase technologies operate at lower operating pressures and temperatures than solution processes, but higher than gas phase processes. Like solution processes, the slurry-type processes are good for catalysts that are prone to static, as slurry and solution processes dissipate local heat effectively and avoid static formation. Typical technologies use a slurry loop reactor and a gas phase reactor in a series or use a single-loop reactor/dual-loop reactor in a series based on product requirements. Proven capacities of 550,000 tpy have been demonstrated, and designs indicate the potential for 700,000 tpy–1 MMtpy. A typical block flow diagram of the slurry process is depicted in FIG. 5, while a typical process flow diagram of the slurry process is shown in FIG. 6.
The main features of the slurry process include:
Proven process for benchmark HDPE unimodal and bimodal grades
Bimodal HDPE products with good environmental stress cracking resistance (ESCR)
Ability to produce metallocene-based LLDPE and Ziegler–Natta-based LLDPE products without static issues
Superior in terms of producing HDPE grades, especially bimodal covering blow molding and pipe applications
Capable of producing broad molecular weight distribution (MWD) LLDPE
Ability to produce grades with densities as low as 0.916 gm/cm3.
Concerns with the slurry process include:
Higher CAPEX due to multiple reactor requirements
Wax generation
Full range of LLDPE is difficult at times due to the solubility of PE in solvents and the softening of PE at the operating temperatures.
Comparison of technologies. A key technology parameter comparison across the gas phase, solution phase and slurry processes is presented in TABLE 1.
Technology selection. A market survey must be conducted to decide PE grades for various applications and regions. A market survey report helps narrow down the technology bid slate and the selection of the best available technology, which can be selected based on the criteria below.
Technology capability or experience:
Total reference list of commercial units
Market capitalization
Reference of similar capacity and similar grade production
Acceptance of grades in the market by customers
Experience in similar geography
Safety records of the technology.
Process performance:
Catalyst and other co-catalysts performance
Length of campaign/onstream hours
Turndown ratio
Specific raw material consumption
Specific energy consumption
Capability to cover a wide portfolio of applications with proven market acceptance
Product quality
% prime grade production
Startup and grade transition time.
Commercial aspects:
Fixed cost
Variable operating cost, catalyst sourcing, proprietary, toll manufacturer, use of third-party catalysts
Basic engineering fees, licensing fees
Technical support
Liability
Scope of work
Possibility of third-party licensing or JV.
Takeaways. The technology platform should be selected based on the required products grade slate (technological capability) and the ability to produce the required quantity (nameplate capacity). Once this is done, a techno-economic analysis based on the various parameters highlighted above must be completed. Specifications such as greater market capitalization, proven large capacity units, lower CAPEX and OPEX, and the ability to produce a wide range of products provide operators a pathway to select the best technology for their operational needs. Issues such as commercial terms, licensing requirements and limitations must also be considered. Where additional units are added to an existing facility, issues such as ease of operations, reliability and inventory optimization are also factors. HP
Ajay Ekambikar is a Senior Engineer working within the Saudi Aramco Project Management Department. He has more than 22 yr of experience in the petrochemicals industry working with operating and technology companies. Ekambikar has been involved with the development, commissioning, operating, troubleshooting and optimizing new/revamp of polyolefins units in the Asia-Pacific region. He holds a B.Tech degree in chemical engineering from Dr. Babasaheb Ambedkar Technological University, Maharashtra, India, and is a certified Project Management Professional from PMI.
Venkata Rajesh Karri is a Senior Engineer within the Saudi Aramco Project Management Department. He has more than 17 yr of experience in the petrochemicals industry working with operating and technology companies. He has been involved with the development, commissioning, operating, troubleshooting and optimizing new/revamp of polyolefins units in the Asia-Pacific region. Karri holds a B.Tech degree in chemical engineering from Jawahar Lal Nehru Technological University, Hyderabad India, and is a certified Project Management Professional from PMI.
