March 2021

Special Focus: Petrochemical Technology

How high can polyethylene and polypropylene plant capacities rise?

Polyethylene (PE) and polypropylene (PP) plants are the popular downstream derivatives of ethylene crackers or petrochemical fluidized catalytic cracking (petro-FCC).

Polyethylene (PE) and polypropylene (PP) plants are the popular downstream derivatives of ethylene crackers or petrochemical fluidized catalytic cracking (petro-FCC). Refineries have been diversifying into petrochemicals to improve upon the gross refining margin (GRM) and flexibility in product mix. This has led to the evolution of the configuration of integrated refinery and petrochemicals complexes. Building such large complexes requires high CAPEX, so a rise in the demand to optimize cost economics is also seen.

Consequently, ethylene cracker plants with capacities exceeding 1 MMtpy of ethylene have become the norm. Ethylene plant capacities have soared close to 2 MMtpy, which was unthinkable two decades ago.

As the ever-growing thermoplastic, PE has been riding the wave and sharing the bulk of the ethylene pie. It is not uncommon to see one, two or even three PE plants in the configuration.

Similarly, at least one PP plant always fits in the configuration. The integration of petro-FCC with a PP plant has become the norm in many refineries that do not want to venture into other petrochemicals. Similarly, the combination of propane dehydrogenation (PDH) and PP plants has also risen in last two decades. This has increased the trend of one or two large PP plants with the ever-expanding PP market.

With the inclusion of multiple PE/PP plants to match the olefins balance, the single-line capacities of PE/PP plants have risen to 450,000 tpy–550,000 tpy. Until about 2010, PE/PP plant capacities of 300,000 tpy–400,000 tpy were considered standard for a large plant. However, PE/PP technology licensors have kept up the pace by offering capacities in the range of 450,000 tpy–650,000 tpy, depending on the product mix.

This article will explore the determining factors for different premium technologies that might limit single-line capacities, as well as potential challenges for future capacity increases.

PE/PP CAPACITY PERSPECTIVES

PE/PP plant capacities must be looked at from two different perspectives: one perspective is the technology licensors’, and the other is the viewpoint of owners-producers that do not license technology.

Owners-producers develop, build and operate their own plants, which can be in one or many locations and are either owned outright or part of a JV. These PE/PP producers do not license the technology. Capacities of these plants have mainly been built in a modular way and are not necessarily comparable with other plants; therefore, these plants are outside the purview of this article.

Two types of technology licensors operate: those that develop, build and operate their own plants, either at their own locations or at a JV’s locations; and those that, at this stage, do not operate their own plant or do not possess their own plants. Obviously, either they or their predecessors of these technologies have pioneered, developed, built and operated their own plants in the past.

When licensors license the technology know-how to licensees, they design the plants to meet the performance guarantees as per license agreements. Under such situations, the licensors will design the plants with reasonable design margins of typically 10%–15%. Therefore, if a plant is guaranteed for 100% capacity, the licensor will design the plant for 110%–115 % of that capacity.

This article addresses licensed technologies about which some information is available in public domain.

PE technologies: Present and emerging scenarios

PE has been the leading thermoplastic resin in the market for the last seven decades. While the low-density polyethylene (LDPE) produced by high-pressure technology emerged in the 1930s, the real breakthrough took place in 1950s with the advent of high-density polyethylene (HDPE) produced by low-pressure processes using Ziegler catalysts or chromium-based catalysts.

Since then, evolving HDPE technology has brought radical changes to the way PE has been used for diverse applications. HDPE, with a broad range of applications, is normally produced either by a slurry process or by a fluidized-bed gas phase process. The slurry process is further divided in two processes: continuous flow stirred tank reactor (CSTR) or loop slurry. Main licensors in this category include LyondellBasell (CSTR)¹, shown in FIG. 1; Mitsui Chemicals (CSTR)², shown in FIG. 2; and Chevron Phillips Chemicals (loop slurry)³, shown in FIG. 3, using one or multiple reactors. The single-train capacities with these technologies have settled in the 400,000 tpy–500,000 tpy range to produce especially niche applications in the high molecular weight (HMW) range.

In the 1960s, Union Carbide introduced a fluidized-bed gas phase process (now popularly known as Unipol PE™, shown in FIG. 4) to produce HDPE focussing mostly on the lower end of applications in terms of molecular weight. Its successor Univation⁴ is presently the only licensor in this technology segment after Ineos discontinued its licensing business. Unipol PE™ offers either Ziegler-Natta (ZN) catalysts, chromium-based catalysts or speciality catalysts to cover the entire range of product applications.

In the 60s and 70s, two processes evolved to pioneer a linear low-density polyethylene (LLDPE) solution and fluidized-bed gas phase process. However, this solution technology that revolutionized LLDPE is practically extinct due to higher capital cost and cumbersome operations compared to competing technologies and does not appear to be prospective for any new plant. No new plant based on this technology has emerged in the last decade.

Univation, successor of Union Carbide, has meanwhile maximized the concept of “swing” operation to produce LLDPE and HDPE in a single reactor. Here again, after the exit of Ineos from the licensing scene, Univation is enjoying a virtual monopoly in this category. The main benefit of Unipol PE™ is its capability to offer large single-line capacities up to 650,000 tpy, and potentially even beyond. The technology offers ZN catalysts, chrome-based catalysts and metallocene catalysts for LLDPE variety, thereby promising everything from LLDPE to high-end HDPE grades all in one reactor.

Main polyethylene technology licensors have been summarized in TABLE 1.

PP technologies: Present and emerging scenarios

PP is the second most popular thermoplastic resin in terms of volume and applications, and is manufactured mainly by loop slurry, fluidized-bed gas phase and stirred-bed gas phase processes.

Loop slurry technology is offered by LyondellBasell (Spheripol™)⁵, shown in FIG. 5, and Mitsui Chemicals (Hypol™)⁶; fluidized-bed gas phase technology is offered by W. R. Grace and Co. (Unipol PP™)⁷; and stirred-bed gas phase technology is offered by Lummus Novolen (Vertical Stirred Gas Phase)⁸, shown in FIG. 6. Ineos offered a horizontal-stirred gas phase process until it exited the licensing business in 2016–2017. With the exit of Ineos, the competition has been reduced mainly to LyondellBasell, W. R. Grace and Co., Lummus Novolen and Mitsui Chemicals. Except Mitsui Chemicals, all other licensors offer single-line capacity in the range of 450,000 tpy–600,000 tpy, depending on product mix. Mitsui Chemicals is expected to offer competing single-line capacities in the future.

Homopolymer PP, which constitutes the bulk of volume (approximately 70%–75%), is produced in a single-stage reactor or two parallel reactors, depending on the technology. Random copolymers PP, which constitutes about 5%–10% of volume, is also produced in a single-stage reactor or two parallel reactors like homopolymer PP using ethylene as co-monomer.

Impact copolymers using ethylene as co-monomer, constituting 20%–25% volume, are produced in series or cascade mode with the second reactor being a fluidized-bed gas phase reactor, as in the case of LyondellBasell, W. R. Grace and Co. and Mitsui technologies, and a vertical-stirred bed reactor for Lummus Novolen. All of these technologies offer their ZN-based proprietary catalyst systems with either proprietary external donors or commercially available donors. Though homopolymer PP dominates the volume, impact copolymers are making strong inroads to newer and innovative applications. Ter-polymers are also emerging for specialized applications like low-temperature heat sealing.

PP technology licensors have been summarized in TABLE 2.

LDPE technologies: Present and emerging scenarios

Demand for LDPE products, once thought to be fading, has re-emerged strongly in the last two decades. LDPE is manufactured either by a high-pressure tubular process or high-pressure autoclave process. Technology licensors for LDPE include LyondellBasell, ExxonMobil and Sabtec. In plants built over the last two decades, LDPE is usually manufactured exclusively by a tubular reactor process. Autoclave technology is used to produce only co-polymers like ethylene vinyl acetate (EVA), which has been growing in importance for speciality applications. Design capacities of tubular or autoclave processes are driven solely by high-pressure engineering capability. The tubular process offers up to 450,000-tpy plants and the autoclave process offers up to 150,000-tpy capacity for EVA. The tubular process can also provide up to 20% EVA copolymers, while the autoclave process can produce up to 40% EVA. No further advancement in single-line capacity, either by tubular or by autoclave processes, is expected in near future.

DRIVING REACTOR CAPACITIES

Two primary areas of continuous innovation and advancement have driven the capacity of the single-reactor line. One area is specific to process technology and the other is common for all technologies.

Reactor technology

The specific area pertinent to each technology or each licensor is reactor technology. Regardless of technology, the common area is extruder or pelleting that converts resin powder to product pellet through melting, which is more rheological scale-up and machinery advancement in the pelleting equipment.

Ethylene and propylene polymerization reactions are highly exothermic. The reaction demands heat removal as fast as possible to control the reactor temperature and avoid “hot spots” or localized reactions. Reactor pressure is usually held constant and temperature is controlled, depending on the type of polymer being produced, within +/–1°C. The heat is removed either by jacket cooling or by external coolers, or a combination of both, and the mechanism varies from technology to technology.

The role of the catalyst system is unique in polymerization in that catalyst enables the reaction and dictates the polymer characteristics or polymer product properties. This mandates certain minimum residence time in the reactor, which again varies for each technology. Therefore, reactor size or volume is primarily determined considering the aspects of heat transfer, mass transfer and reaction kinetics.

Catalyst yield or catalyst productivity—as defined by tons of polymer produced per kg of catalyst (metric t/kg)—has been the single most important factor driving PE/PP technology advancement. Though this is not the theme of this article, it should be noted that catalyst productivity through continuous research and innovation has advanced multi-fold to a sufficiently high level to enable the large single-reactor line capacities that are now prevalent. Key factors contributing to reactor capacity differ for each type of technology.

CSTR technologies for PE

Two main factors ae involved: reactor volume and reactor agitator. The reactor is a pressure vessel where pressures are moderate, so design for a given volume is not a controlling factor. It is the agitator system design for large vessels that is critical.

Ethylene polymerization reaction is heterogeneous, involving a combination of gas (ethylene, hydrogen and co-monomer), liquid (co-catalyst and hexane) and solid (catalyst particles and resin). The agitator design must meet the following minimum criteria:

• Fast or almost instantaneous dispersion of gas into the reaction medium
• Fast reaction velocities
• Solid-liquid suspension consistency
• Heat transfer or heat removal as fast as possible for close temperature control.

Intelligent balance of radial and axial agitation pattern is the key to agitation design. The agitator scale-up has become more predictable and accurate with the advent of computational fluid dynamic (CFD) analysis; nevertheless, experience-based empirical scale-up along with CFD analysis dominates the design. The established vendors have lived up to the challenge to scale up to large capacities, including designing robust agitator sealing systems. A minimum of two reactors are in operation either in parallel or cascade mode. Hostalen ACP technology offered by LyondellBasell employs three reactors.

Multiple reactors are intended for manipulation of product properties, but also to assist in reducing the throughput per reactor. Individual jacketed reactors have reached the size of approximately 350 m³–400 m³. Most of the heat is removed by external coolers by circulating slurry pumps.

Loop slurry reactor technologies for PE

Again, reactor volume and a loop circulation pump are the two main factors. The reactor is in the form of long vertical vessels called reactor loops or legs. The heterogeneous reaction includes gas (ethylene, co-monomer and chain terminating agent), liquid (iso-butane) and solid (resin and catalyst). The residence time or volume and heat transfer aspects are determined by L/D ratio. This has given rise to multiple legs or loops operated in series or parallel with multiple circulation pumps. Therefore, increasing the diameter and L/D ratio per loop combined with multiple loops has enabled the reactor scale-up to a higher capacity. More than the reactor volume, the scale-up of the slurry circulation pump has been the challenge for speciality pump manufacturers. The circulation pump must have a large flowrate operating with low ΔP—but at high suction pressure—to maintain high velocity to sustain high slurry consistency or high solid-to-liquid ratio. A mechanically challenging task has been to provide a sealing system to handle ethylene and light hydrocarbons like isobutane, in addition to scale-up of pump impeller design.

The established vendors have overcome this problem gradually to supply the required large-capacity pumps that operate at severe conditions.

Loop slurry reactor technology for PP

The previous discussion also holds true for PP technology, in principle. The difference lies in the fact that reactant propylene acts as a diluent for the slurry. The fluidized-bed gas phase reactor is used in cascade mode to produce impact copolymers.

Fluidized-bed gas phase reactor for PE

Here, the reaction is carried out in a fluidized bed and the resultant heat of the reaction is carried to the external cooler by carrier gas or recycle gas via a recycle gas compressor.

Part of the heat of the reaction is also removed by evaporative cooling using inert iso-pentane in the cycle gas, which is also termed as condensing mode. The reactor has two parts: one is the cylindrical vertical shell that holds the fluidizing resin bed, and the second consists of a large dome-shaped vessel on top of the cylindrical shell, which disengages the cycle gas from the resin powder before leaving the reactor.

The volume of the dome is very large compared to the shell to enable separation of cycle gas from any resin particles. The fluidizing bed volume ensures the minimum residence time and acts as a heat dissipating medium. The volume of the cylindrical shell determines the minimum residence time. The fluidizing velocity must be high so that heat dissipation is fast for accurate and close temperature control, and at the same time allowing good mixing of catalyst and reactants in the resin bed so that the fluidizing bed nearly approximates the CSTR, although it is not comparable to a CSTR.

Therefore, reactor volumes guided by L/D ratio and fluidizing velocity, as well as the corresponding volume of the dome, have grown multi-fold over the years to match the single reactor capacities. The single reactor is used for producing bi-modal HDPE products using radical innovations in catalyst technology.

Recycle gas compressor capacities have kept pace with the required gas flowrates to match reactor capacities. The established vendors have scaled up compressor performance, which requires very high flowrates at low ΔP but at a high suction pressure and an associated robust mechanical sealing system to handle ethylene, co-monomers, hydrogen, etc.

Fluidized-bed gas phase reactor for PP

The above discussion also holds true for PP technology, in principle, but with some variance. Excess reactant propylene is used for evaporative cooling—termed as condensing mode operation—to enhance the heat removal capacity in the given reactor. While some differences in mechanism of ethylene and propylene fluidization exist, the overall design principles are mostly the same.

Vertical-stirred bed gas phase reactor

Here, the reactor is a CSTR type without any diluent, with the resin bed being kept in motion by a low-carrier gas velocity. The excess reactant propylene is also used for evaporative cooling, thereby serving as evaporating liquid, as well as carrier gas.

The high velocity is not required because agitation has substituted the task of fluidization. The mechanical mixing of catalyst in the resin bed improves the mixing performance, as well as reaction and heat dissipation. The carrier gas flowrates are quite small because there is no fluidization. The minimum residence time is handled by reactor volume guided by L/D ratio.

Again, as with other CSTR technologies, the design of the pressure vessel has not been much of a problem, unlike the scale-up of the agitator design. However, unlike CSTR, this agitator does not require robust mixing by radial and axial movements, but rather a gentle agitation intensity. The agitator design is different here vs. CSTR in the sense that here, primarily gas-solid mixing is involved with a very small liquid phase.

Gas-solid mixing has been more empirical than the CSTR agitator. Again, the advent of CFD analysis has helped perfect the proprietary agitator design. Lummus Novolen has entered the licensing business relatively late but has quickly accelerated the capacity scale-up to the global standard with the assistance of established agitator vendors.

The essence of reactor scale-up challenges are summarized in TABLE 3.

Extruder or pelleting package

An extruder is an essential unit in any PE and PP plant, irrespective of technology or licensor. The polymerization reaction system is followed by resin degassing, resin powder treatment, polymer additive mixing and, finally, an extruder or pelleting unit. The extruder is either resin powder fed or resin melt fed. Except LDPE and solution PE technology, all other technologies have resin powder fed extruders. Similarly, all these extruders are twin-screw extruders. An extruder unit is a package item that consists of several components, including:

• Main motor and main gearbox
• Processing section or main extruder consisting of a combination of several proprietary screw elements, gear pump, die plate, underwater pelletizer, pellet dryer, etc.

When scale-up of extruder capacities are discussed, all associated components are automatically included. Each of the components must match the overall extruder capacity. In fact, when the main processing section is specified, the capacity-controlling downstream units (e.g., gear pump, die plate, pelletizer, pellet dryer) must be 10%–15% higher capacity than the main processing section.

Using the same analogy, the extruder package must be 10%–20% higher capacity than the reaction section. This ensures that a minor maintenance shutdown in the extruder area of 4 hr–8 hr does not affect reactor operation; when the extruder is restarted, it can operate at a higher capacity than the reactor to bring down upstream resin inventory to normal levels and set the plant again on steady-state operation. As a guiding principle of plant design, any equipment downstream of the reaction area is not expected to become capacity controlling or limiting.

TABLE 4 illustrates how the extruder packages and pellet conveying (or pneumatic conveying packages) are designed based on plant capacity or reactor capacity. It should be noted that extruder and conveying packages are typically designed for either reactor:extruder:conveying = a 100:120:140 or 100:110:120 capacity ratio with respect to the reactor.

100:120:140 is a more conservative approach and is usually desired and practiced to its fullest extent for any plant. However, the 100:110:120 pattern is sometimes selectively chosen to optimize the economics of the plant or keep the initial CAPEX to a lower level for a higher capacity of > 500,000 tpy, considering product mix.

Extruder capacity

The extruder package is the single costliest item in the whole PE/PP cost estimate. An extruder package occupies a large footprint and adds complexity to the plant design in addition to higher plant CAPEX. The trend is to install only one extruder of equivalent capacity rather than dividing it into two smaller extruder packages.

An exception is made only in those PE plants where a high proportion of pipe grades are to be made (e.g., PE 100 pipes or black pipes), thus justifying the installation of a dedicated extruder for pipe grades.

An extruder with a motor or drive of 15 MW–20 MW is considered very large and are typically single-speed extruders because variable-speed extruders with such large motors are extremely expensive. Additionally, the specific energy index of an extruder package defined as Kwh/kg (kilowatt hour/kilogram) of capacity becomes high and cost prohibitive due to energy or electricity costs.

The extruder capacity is nominally specified by screw diameter. Established extruder package vendors have successfully demonstrated operation with 380-mm–450-mm screw diameters with improved filling efficiency. Mega compounders, as they are called, have nearly reached a torque limit for standardized throughput rate with respect to specific energy, especially for LLDPE and HDPE for certain applications at approximately 100 t/hr.

A single extruder capacity up to 100 t/hr is considered a proven and reliable scale-up. This would imply that components downstream of the processing section are designed for a capacity higher than 100 t/hr. Therefore, it is logical that a reactor or plant capacity of 80 t/hr–85 t/hr (or the equivalent to 700,000 tpy) is achievable with operational reliability if an extruder capacity of 100 t/hr is considered a proven scale-up.

Silos and pneumatic conveying

Silos are generally made either of stainless steel or Al/Al alloys. These pressure-less vessels are fabricated onsite. The number and size of silos are decided by the client based on their operating philosophy. However, each silo with a capacity of 700 t–1,000 t is common in the industry with 24 hr–48 hr of combined installed storage space for blending and storage determining the number of silos.

Pellet pneumatic conveying

Product pellets are pneumatically conveyed from the extruder to blending/storage silos and conveyed from silos to warehouse for bagging or packaging.

The pneumatic conveying of pellets is largely dictated by the principle of velocity to prevent solids from settling during transportation by conveying. Consequently, pipeline sizes increase proportionately to capacity and tend to become large. This is not as evident in straight pipelines; it is piping elements like bends, diverter valves, rotary feeders, etc., that really set the limitation on the confidence of the scale-up and the proven-ness of the components. Generally, pneumatic conveying capacity up to 120 t/hr is considered proven due to reliable scale-up, although capacities up to 140 t/hr appear promising in the future.

Single-line capacity challenges ahead

This discussion suggests that single-line capacities for PE and PP plants are expected to range from 500,000 tpy–700,000 tpy at the higher end of the spectrum. Plant capacities will be dictated by product mix and the technology selected. Technology selection is guided by factors that include:

• Flexibility
• Versatility
• Product range
• Catalyst range and productivity
• Product transition time
• Proven capacity of critical equipment, such as the reactor, extruder, etc.

This is of course over and above the usual CAPEX and OPEX parameters.

Takeaway

PE faces a greater challenge due to the nature of a wide spectrum of product application, ranging from LLDPE, MLLDPE, MDPE, MM-HDPE and HM-HDPE to bi-modal HDPE. No single technology is expected to encompass the entire range in equal measure for various reasons. This has necessitated licensees to seek a judicious combination of technologies to cover the entire product range, or to focus on a selective product range. The conceivable combinations can be but are not limited to:

• Fluidized-bed gas phase + loop slurry
• Fluidized-bed gas phase + CSTR
• Two fluidized-bed gas phases with swing capability
• Two-loop slurry: SL + ADL.

The single-line capacity of these technologies can be expected to advance to the range of 550,000 tpy–750,000 tpy soon with an emphasis on energy efficient design.

PP has an advantage over PE in that all PP technologies offer a full product range. If ethylene is unavailable, only a homopolymer PP plant can be installed with the provision to add an impact copolymer PP reactor later—Lummus Novolen in an exception, as its two-reactor technology can produce the entire product range. All PP technologies are expected to advance toward a 550,000 tpy–750,000 tpy range soon with an emphasis on energy efficient design.

It is conceivable that the challenge of large-capacity plants can be partly overcome by installing one or more small-capacity, high-value speciality plants that are ethylene or propylene derivatives other than the usual PE and PP. The new trend of diverting part of the ethylene and propylene to produce more value-added speciality chemicals looms bright on the horizon and will emerge sooner than later. Plant size will be conveniently smaller and, accordingly, CAPEX will decrease with relatively higher returns. Elements of business risks accompanying large single-line capacity will be replaced by opportunities. HP

LITERATURE CITED

1. LyondellBasell, online: https://www.lyondellbasell.com/en/search/?q=hostalen
2. Mitsui Chemicals, online: https://jp.mitsuichemicals.com/en/search/?q=cx
3. Chevron Phillips Chemical, online: https://www.cpchem.com/search?search=martech
4. Univation Technologies, online: https://www.univation.com/en-us/unipol.html
5. LyondellBasell, online: https://www.lyondellbasell.com/en/search/?q=spheripol\
6. Mitsui Chemicals, online: https://jp.mitsuichemicals.com/en/search/?q=hypol
7. W. R. Grace and Co., online: https://grace.com/en-us/capabilities/Pages/unipol-plant-and-process-overview.aspx
8. Lummus Technology, online: https://www.lummustechnology.com/Process-Technologies/Petrochemicals/Polypropylene-Production

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