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High-purity propylene from refinery LPG

A case study and financial analysis show that investment opportunities for refiners exist with the expected future high price of propylene

Mustang Engineering
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Article Summary
In a petroleum refinery, propylene is a by-product of fluid catalytic cracking (FCC), coking and visbreaking. The FCC unit is by far the largest source of this material. Propylene is used internally in a refinery to manufacture gasoline via alkylation or polymerisation or, in some cases, as fuel. The whole propylene/propane (PP) stream may also be sold as a chemical feedstock. There are several large, centralised facilities on the US Gulf Coast that process mixed PPs gathered from surrounding refineries and petrochemical facilities into high-purity propylene that is used for feedstock to various chemicals and polypropylene production facilities. There are also refineries that have 
onsite separation facilities to 
recover high-purity propylene. Although conventional distillation equipment, including a reboiler utilising low-pressure steam, may be used to separate propylene and propane, a number of PP splitter designs have used a heat pump concept, where tower overhead vapours are compressed to a pressure with a corresponding condensing temperature suitable for reboiling the tower. This allows operation at pressures lower than that required to condense the reflux/product stream with water or air, which in turn requires fewer fractionation stages and/or lower energy input to achieve the required propylene product recovery and purity.

This article considers the results of a study that was prepared to assess the financial viability of an onsite PP splitter in a refinery. First, the various uses and the current demand for high-purity propylene are discussed. Next, the interactions of certain process variables are considered, followed by an analysis of the auxiliary facilities required for feed pre-fractionation, product treating and storage. Finally, the results of a financial analysis to determine the rate of return for in-plant recovery facilities processing a range of refinery PP production rates are presented.

This study includes the optimisation of process variables such as operating pressure, number of trays and reboiler temperature difference. Financial variables that are considered include the value of propylene as a chemical feed and as a feed to a refinery alkylation unit, utility costs, and capital costs for a new PP splitter facility. The value of the raw feedstock is also calculated as a function of the high-purity (polymer-grade) propylene product price, with a project rate of return calculated with the value of propylene being either a feedstock to an alkylation unit or sold as refinery-grade propylene included as a parameter.

Propylene is sold in the merchant market as refinery, chemical or polymer grade. Refinery-grade propylene specifications are as negotiated between the buyer and seller. The propylene content is usually 65-75%, while propane is 20-30%, with the balance being butane/butylene (BB) and ethylene/ethane. In the US, the price is typically calculated based on a posted price with adjustments for actual composition. Chemical-grade specifications usually require a propylene purity of 92%+, while polymer grade is 99.5 wt% minimum propylene, with additional limitations on ethylene/ethane, BB and other contaminants such as dienes, sulphur and arsine.

Worldwide consumption of propylene in 2011 was about 79 million tonnes. World demand is expected to reach about 97.5 million tonnes in 2015, with a majority of the increase in demand occurring in Asia. Worldwide, non-gasoline demand for propylene is distributed to various petrochemical products, as shown in Table 1.1

Roughly 57% of the worldwide production was from steam crackers in 2011, which has decreased from 2008 values of 65%. Some 30% of the remaining demand currently is from petroleum refineries, while 13% is from on-purpose supply. Additionally, North America traditionally has 
a larger contribution of 
propylene supply provided by refineries when compared to other regions of the world.1
It is expected that market demand for propylene will continue to increase at rates that will outpace worldwide supply. On the supply side, the main contributors to this trend are steam crackers processing lighter feedstocks and decreasing refinery rates.2 As a result, propylene prices that were high in 2010 (approximately $0.60/lb) have continued to increase and are projected to remain between $0.70 and $0.80/lb in North America. In addition, IHS is projecting that the price spread between polymer- and refinery-grade propylene is expected to increase from $0.05 to $0.08/lb. This result provides opportunities for refiners to consider investments that can achieve attractive rates of return.1,2

PP splitter design
As noted previously, most of the larger PP splitter facilities utilise a heat pump concept in lieu of a conventional distillation scheme with a low-pressure steam reboiler and a condenser. Figure 1 shows a simplified process flow diagram of a single heat pump configuration. Overhead vapours are compressed and then a portion is condensed in the reboiler from where the condensate is returned to the top tray for reflux. Net product plus additional reflux are condensed in a parallel exchanger. This type of design works for towers that have a column operating pressure of over 125 psig. For towers that operate at pressures lower than 125 psig, a second compressor stage is typically required to increase the pressure of the net product vapours so they may be condensed with air or cooling water (see Figure 2). There are numerous other variations where separate cooling is used to remove the heat of compression and/or interchange heat between other warm and cold streams in the system.

There are a number of facilities that use various low-level heat mediums (normally low-pressure steam) for reboiling instead of a heat pump. These PP splitters need to operate at a pressure of approximately 265 psig in order to be able to use cooling water for the overhead condenser. Even though most refineries have excess low-pressure steam or other low-level heat sources available, the magnitude of the requirements for reboiling as well as the cooling water demand for the overhead condensers usually favour the heat pump design. For example, a splitter operating at 265 psig with 210 trays, processing 6000 b/d of FCC PPs (which corresponds to an FCC capacity of around 50 000 b/d) and designed to produce polymer-grade propylene and HD-5 propane would require almost 80 000 lb/h of 20 psig steam and 10-15 000 gal/m of cooling water. In most refineries, this would require new infrastructure facilities to produce these quantities of utilities, increasing the supply costs significantly. In contrast, a comparable heat pump design operating at 85 psig would have a compressor power requirement of around 3000 HP, which in most cases would be significantly less expensive to accommodate.

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