PLASTIC fantastic

The chances are that any polyethylene in your home or office was made using a gas-phase process. The main reason why more gas-phase polyethylene technology (70 per cent market share) is being licensed now than any other type of polyethylene technology is its low cost (capital and operating) and flexibility. In terms of market coverage, a full range of both broad and narrow molecular weight linear low-density PE and high density PE can be produced to cover all film, moulding and extrusion sectors. Of benefit also are low operating temperature (about 90 C), low pressure (about 20 barg), and low hydrocarbon inventory. Over and above these inherent advantages of gas-phase technology, Innovene is different from its competitors.

The full product range is produced within a single reactor using transitions between different reactor conditions, gas feed composition and catalysts which are directly injected into the reactor. Multiple reactors are not required.

Innovene has the unique feature of cyclones in the reactor loop design. These eliminate fouling of the exchanger and compressor, and prevent cross-contamination of products particularly important during catalyst transitions. A clean loop allows longer on-stream times, produces a higher percentage of prime product, and avoids expensive downtime. Downstream of the reactor, the degassing system has been optimised since the early licences were sold, and now achieves very efficient monomer recovery and no hydrocarbon emissions.

Since worldwide licensing of Innovene started in 1984, the technology has evolved continuously. For example, stronger C6 LLDPE for packaging film, and easier to process broad molecular weight blow moulding resins, were major advances in product performance that have become part of the standard Innovene range. The Innovene process is particularly renowned for its exceptionally high quality moulding and film.

MAKING THE GRADE

Since the Innovene process was launched, more than 150 grades of PE have been developed using the process. Moreover, 4 million tpa of annual capacity have been licensed worldwide.

Innovene has continually evolved since 1985, by the increase in reactor scale and additions of new catalyst systems. BP Chemicals aims to keep its excellent reputation as a licensor which is built on successful transfer of the technology to every licensee. The company provides a comprehensive range of services as part of the technology package, extending from customised process design and documentation, through to training and commissioning. Licensees have found that this reduces commissioning times, and allows them to run their plants well above design capacity.

Licensees and BP itself also benefit from a technical exchange programme, which enables them to share knowledge and experiences gained collectively from operating and developing the technology. This is a unique part of the Innovene licensing package and is highly valued by everyone involved.

One such technical development was the removal of heat from the reactor, which is the major limiting factor on reactor capacity in the gas-phase PE process. The breakthrough that solved this problem for BP, known as High Productivity, is now allowing the building of reactors capable of making over 300 000tpa of a full range of products at even lower cost.

The efficient removal of the vast quantities of heat generated by ethylene polymerisation in a world-scale gas-phase reactor presented a significant technical challenge. The way this is done is by injection of cooling liquid hydrocarbon directly into the fluidised bed using a proprietary nozzle system without destabilising the fluidisation.

High Productivity, launched in 1995, with patents granted in the US (1997) and Europe (1998), provides the highest heat removal capacity. This modification is running in full commercial operation at both BP Chemicals Grangemouth and Lavera plants and increased capacity to the new design levels immediately on start-up during 1996.

Enhanced High Productivity optimises the process further by making significant changes to the polymerisation loop design, cutting capital cost reduction for the loop by a tenth, with a similar operating cost reduction.

In August 1996, technical co-operation with Dow Chemical was announced in the field of single-site catalyst technology to meet new customer requirements for even stronger film products. Our partnership with Dow has led to the development of unique composition of matter based on the performance of Dow's 'Insite' metallocene catalyst in the Innovene process. This substantially improves processability of film products, even high strength products with a narrow molecular weight distribution. Normally this is a difficult combination to achieve, because of the trade-off between strength and processability. The reason behind the novel polymer architecture is the constrained geometry within the catalyst itself, which produces polymers with a very even distribution of branching along the carbon chain backbone.

Products that are not only stronger (making downgauging possible) but also more processable than LLDPE have a significant advantage over conventional Ziegler products, and those based on high pressure LDPE technology. Most importantly, there is no output penalty using existing extrusion equipment. This is important if metallocene products are to be widely accepted in volume markets.

A further benefit is the products' sealability in packaging film applications. The seal starts to form at lower temperature and the hot tack strength is greater than even the best LLDPE. This, again, is a result of regular distribution of branching in the polymer backbone. The benefits will be faster packaging line speeds, such as in form-fill-seal applications.

I CAN SEE CLEARLY NOW

Apart from high performance film, other products can be designed having the processing characteristics of LDPE while retaining the advantages in mechanical properties, heat sealing and blocking. Optical properties, particularly transparency, are also good and will enable broad application in many packaging markets. The first product to be launched from the BP/Dow joint development will be a catalyst system to produce such an LDPE type resin.

Three pilot plants have been equipped to produce polymer made with Innovene and Insite technology. The largest of these is used to supply developmental quantities of resin. A commercial scale manufacturing plant has already been established to supply fully formulated catalysts.

BP and Dow are also using the resources of four research centres to develop this program; Freeport, Texas, and Lavera, France, house major production sites and research and development for both companies. Midland, Michigan, and Sunbury, England, house the respective catalyst R&D centres. This approach allows both companies to bring talent and expertise to the project. The team has access to powerful resources that range from the most sophisticated analytical facilities to semi-commercial production units and, of course, product testing facilities.

BP and Dow are working to bring this technology to market as soon as possible, and will have more information to share on this exciting endeavour in mid-1998.

Gas-phase technology is now the industry standard, and is taking the majority of new build capacity. As a licensor, we recognise it as essential to follow through licensee investment in world-scale plant, with future catalysts that are compatible with established Ziegler-Natta and chromium types as well as the reactor hardware and conditions. The goal is to improve performance of products for converters and extend market coverage for the producer.

For existing high volume general purpose grades, there is a constant aim to drive down the costs of production. A future target after high strength, high performance films and LDPE lookalike is to make bimodal HDPE from a single reactor. The properties will be specifically designed for the Asean market which has most demand for this type of product.