Process overview HDPE

Traditionally, chromium catalysts have been used in single loop and gas phase reactor processes to produce type 2 HDPE products (broad molecular weight distribution), while Ziegler catalysts have been used for products with a narrow molecular weight distribution (type 1 HDPE). 

However, in processes with at least two reactors in cascade, it is possible to control (broaden) the molecular weight distribution and the comonomer distribution of the final product by operating each reactor under different conditions. The products produced in this way are normally referred to as bimodal and are generally considered to have a better performance than unimodal products of the same density and molecular weight. Especially stirred tank reactors, but also loop reactors, are used for production of bimodal HOPE. 

A dual reactor process consisting of a loop operated in supercritical propane followed by a fluidised bed gas phase reactor has been developed especially for the production of bimodal polyethylene. This reactor combination gives the flexibility to both broaden the molecular weight distribution and to produce low density PE thus expanding the feasible operating window in terms of density of the final bimodal product to cover the whole range from LLDPE to HOPE. 

A disadvantage with the slurry processes is that the diluent contained in the slurry from the reactor has to be separated Ifom the polymer powder and purified before it is recycled back into the reactor. This process step is more complicated and more expensive than the corresponding recycling system for the gas phase process. Using a light diluent (isobutane, propane) makes it possible to separate most of the diluent by a direct flash of the slurry Ifom the reactor, which is not feasible with heavier diluents due to the higher boiling point. 

Over the years, several suspension process variants have been developed based on the use of Ziegler catalysts. In early plants, a cleaning stage was needed to remove the residues of the catalyst from the product. Owing to the increased activity of catalysts, it has, for quite a few years now, been possible to dispense with this. The modem stirred tank slurry plants usually use hexane as the diluent. Some old plants use heavier hydrocarbons, which often require steam stripping for removal of the diluent from the pol5mier. 

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Figure 23.9. PLM photographs of the longitudinal cross-section of SCORIM-processed HDPE (a) overview (b) central core (c) transition layer (d) skin...

An overview of HDPE processes and parameters is shown in Table 3.5. 

Table 3.14 shows an overview of the production costs for the processes described for the production of polyethylene. All data have been standardised for the different processes by using USD 600/t as the feedstock price for both ethylene and butene-1. As can be seen for all processes, the impact of the feedstock price is about 80 %. All data used are based on ChemSystem (1996/97 for LDPE and LLDPE, 1999/2000 for HDPE) data for new large scale plants. 

The main three polyethene classes are low-density polyethene (LDPE), high-density polyethene (HDPE), and linear low-density polyethene (LLDPE) (Scheme 6.20.1). Table 6.20.1 gives an overview of these classes and indicates the differences in density, production processes, and chemical structure. 

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