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Preliminary study for the production of propylene and ethylene in Nova Scotia.

Preliminary Study for the Production of Propylene and Ethylene in Nova Scotia

The preliminary economic study was used to determine the feasibility for the coproduction of ethylene and propylene in Nova Scotia. The analysis used the rate-of-return on equity (ROE) as an evaluation criterion with a minimum attractive rate-of-return (MARR) set at 20%. Three integrated plants were examined: the C-E Lummus thermal cracking process and UOP's Oleflex process (T&O), the C-E Lummus thermal cracking process and the Catofin process developed by Air Products (T&C) and the C-E Lummus thermal cracking process and the Triolefin process of Phillips Petroleum (P). Each integrated plant would be designed for 180,000 MTA of ethylene and 46,000 MTA of propylene.

The proposed plants were assumed to have a project life of 20 years excluding the construction period. The plant will take three years to build, beginning production in 1991. The taxable lifetime is 20 years starting in 1991; a constant tax rate of 40% and the straight line depreciation method is used. The base cost analysis case for the T&O plant showed a $117-million capital investment and a 11.7% ROE, a $120-million capital investment and a 11.0% ROE for the T&C plant, and a $114-million capital investment and a 11.9% ROE for the P plant. Each base case assumes 70% of the plant's cost was based on equity and 30% paid by bonds. The sensitivity analysis indicated that the ROE is significantly affected by equity capital and product price, less by feedstock price and utility costs, and minimally by plant products rates.

According to the previous feasibility study, the perspective for the coproduction of ethylene and propylene does not look very promising unless there is a dramatic change in the product market. This result, however, suggests that it may be worthwhile to study the production of just one product. Since the propylene market has a reasonably bright future and the capital cost for a propylene plant is comparatively low, production may seem profitable.

A feasibility study using the UOP Oleflex process designed to produce 46,000 MTA of propylene gave a 15.6% ROE. The ROE could reach 20% if the market price of propylene increased to $0.20/lb, which is not impossible.

The engineering study is for the production of 40,000 MTA of polymergrade propylene by catalytic dehydrogenation of a propane feed at a plant in Nova Scotia. The process is based on UOP's Oleflex process, especially the reactor section. The propylene plant consists of three main units: conditioning, reaction and separation.

Unit Selection

The conditioning unit is needed for preheating the feed gas to 720 [degrees] C before it enters the reactor. There are no process streams in the plant that would have a temperature higher than 720 [degrees] C. Therefore the only method to heat this stream to 720 [degrees] C is with the use of a fuel-fired heat exchanger. The reaction section is used for the conversion of propane to propylene and a tubular reactor is needed. Appropriate reactions must occur within this reactor so the effluent stream will mainly consist of propylene, hydrogen and a minimum amount of by-products.

The last section is for the separation of the reactor effluent into appropriate component streams. These streams are: a hydrogen-rich recycle stream, off gases which are to be used as fuel and the propylene product. The first operation here is the removal of the lowest molecular weight component (hydrogen) from the effluent stream. A flash separator was used as the hydrogen stream does not have to be very pure and the use of a distillation column meant an increase in capital and operating costs. Methane and ethane were the next components to be removed from the effluent stream and a deethanizer distillation column was implemented. A deethanizer was used because a multiple equilibrium stage process was needed so the loss of propane and propylene in the overhead could be minimized. The final separation was for the removal of propane from the deethanizer bottoms so a polymer-grade propylene product could be produced. A distillation column (propane-propylene splitter) was used because the separation needed a multiple equilibrium stage operation.

Optimization of the Flowsheet

All main units were simulated using the Process flowsheet package. The reactor section used in this process was developed by UOP and includes a series of three to four adiabatic reactors with a continuous slow circulating catalyst bed. The catalyst used in the reactor is a Pacol type and is continuously regenerated. Many revisions were necessary to optimize energy recovery within this plant. Heat exchangers and turbo expanders were implemented to reduce operating expenses.

Conclusions A final economic analysis, based on the preliminary engineering study, gave a ROE of 20.6% for a 40,000 MTA polymer grade propylene plant. This indicates that a plant designed for the dehydrogenation of a propane feed can produce an attractive rate of return if the price of propylene increases to $0.20/lb and the price of propane is approximately $0.06/lb.

In conclusion, it appears that the future production of propylene can be made profitable. The strong growth of the plastics industry is likely to raise the market price of propylene and will, therefore, offer a reasonable ROE. It is recommended that detailed studies should be done to guarantee profitable results before such a plant is constructed.

PAUL A. BROWN, PAK KAI YUET AND LISA CHAN Technical University of Nova Scotia, Halifax
COPYRIGHT 1989 Chemical Institute of Canada
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:winning paper from the Canadian Society for Chemical Engineering's plant-design competition
Author:Brown, Paul A.; Yuet, Pak Kai; Chan, Lisa
Publication:Canadian Chemical News
Date:Oct 1, 1989
Previous Article:Parylene at the Canadian Conservation Institute.
Next Article:It's time to take science and technology seriously.

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