In this article you are going to be presented to a polypropylene production process similar to UNIPOL licensed by Dow Chemicals.
Apart from a historical perspective of this process technology, operating plants, block flow diagram and process description you will have access to a detailed analysis of one of its process areas.
Extending the detailed analysis to the whole process we are able to evaluate its economic perfomance based on a historical chart which compares EBITDA margins and average chemical sector profitability.
The main reaction that occurs in the polymerization of propylene to polypropylene (PP) is shown in the following:
A Ziegler-Natta catalyst is utilized to achieve this. The continuous back-mixed reactor operates at about 33 – 35 bara and contains a fluidized bed of granular polypropylene with a trace of catalyst. Temperature is mild (65 – 80ºC) and is controlled by adjusting the temperature of the cycle gas returned to the reactor. An overall yield of about 99+ wt% of propylene is expected.
In the 1960s, BASF developed a gas phase, mechanically stirred polymerization process for making PP. In that process, the particle bed in the reactor was either not fluidized or not fully fluidized. In 1968, the first gas phase fluidized-bed polymerization process, i.e., the UNIPOL process, was commercialized by Union Carbide to produce polyethylene. In the mid-1980s, the UNIPOL process was further extended to produce polypropylene.
The features of the fluidized-bed process, including its simplicity and product quality, made it widely accepted all over the world. As of today, the fluidized-bed process is one of the two most widely used technologies for producing PP.
Plants and Projects
Click on the icons for further details on the plants presented. The map data is available for download at the bottom of the present page.
Note: This map is illustrative only. Location is not exact, It just indicates the city or province where the plant is located.
In terms of raw materials, polypropylene is the largest downstream derivative made from propylene. Typically, PP manufacturers use polymer grade (PG) propylene, with 99.5 wt% purity, as feedstock. Due to the high cost related to transport of highly pressurized or refrigerated liquids,
propylene is usually produced or purchased from local steam crackers, fluid catalytic cracking (FCC) units or on-purpose plants.
In some cases, propylene is refined to achieve a purity compatible with the sensitivity of the catalyst system and/or to avoid the accumulation of inert substances. The major PG propylene feed impurity is propane. Similar to other inert components such as methane, nitrogen, ethane and other higher alkanes, propane works as a diluent to reduce polymerization rate, not having any other adverse effect.
Block Flow Diagram
Source: Dow UNIPOL TM prospect, Intratec analysis
The process is separated into three different sections: purification & reaction; resin degassing & pelleting; and vent recovery.
Fresh propylene and the other raw materials fed to the unit are passed through the purification facilities, in which trace quantities of impurities are removed. The purified raw materials are then fed to the reaction system.
Only one reaction system, consisting of a fluidized bed reactor, a cycle gas compressor and cooler, and product discharge tanks, is required to produce homopolymer and random copolymer. The raw materials and a recycle stream from the vent recovery system are fed continuously to the reactor. The cycle gas compressor circulates reaction gas upward through the reactor, providing the agitation required for fluidization, backmixing, and heat removal. No mechanical stirrers or agitators are needed in the process reactors. The cycle gas leaving overhead from the reactor passes through the cooler that removes the heat of reaction. Catalyst is continuously fed to the reactor.
Resulting granular polypropylene is removed from the reactor by the discharge tanks and sent to a purge bin where residual hydrocarbons are stripped with nitrogen from the resin and are sent to the vent recovery system. The purged resin is sent to the pelleting system.
Detailed Analysis of the Vent Recovery Area
This section will focus on describing in further details the product recovery area, providing a detailed flow diagram of the process area, its full material balance, the equipment list and utilities consumption, as well as a brief economic analysis. For a similar analysis on the complete process check the Polypropylene via Gas Phase Process, one of Intratec's Publications.
Detailed Flow Diagram
This process area comprises the equipment used to purify the vent stream from the reactor recycling propylene and recovering other byproducts, such as fuel.
Main Streams Material Balance
This material balance presents the compositions and operating conditions for the main streams in the product recovery area.
|Major Equipment List ||Utilities Consumption|
|This list summarizes all major pieces of equipment contained in the product recovery area along with their construction materials.||This table provides detailed utilities consumption in the product recovery area|
In this section operational costs and the total fixed investment are calculated to evaluate the economic performance of the product recovery area. The economic analysis was based on the following assumptions:
Total Fixed Investment Breakdown
In this section the total fixed investment for the Vent Recovery area is calculated. Direct costs are the total direct material and labor costs associated with the equipment (including installation bulks). Indirect costs are defined as the "costs which do not become a final part of the installation but which are required for the orderly completion of the installation."
Operational Expenditures are composed of two elements: a fixed and a variable cost. In this analysis only the variables costs associated to utilities production or consumption were considered.
The fixed cost assumptions Plant Overhead and General and Administrative (G and A) expenses were not considered since they can not be associated to a single process area.
Historical EBITDA Margins
Using a similar methodology as to that presented for the product recovery area, all other process areas were analysed so that the whole process could be evaluated. Comparing the process' EBITDA margins against Intratec's Profitability (IP) Index for the Chemical Sector it is possible to evaluate the economic performance of the technology under analysis.
In the overall, this chemical process technology exhibits an economic performance quite below the average Chemical Sector, the major driver for such result is the elevated market price for propylene in the US. An integration with a propylene plant could use propane feedstock to produce propylene via propane dehydrogenation. Propane prices are low and its availability has been rising, since shale gas began to be exploited in the United States.
Gas Phase Polymerization Polypropylene Units