On-purpose propylene production
Propane dehydrogenation plays an increasingly key role in closing the global shortage in traditional propylene supplies.
Michael Marsh and Jeffrey Wery
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Global demand for propylene continues to rise and the market is projected to enjoy solid growth rates for the foreseeable future. With this increase, however, traditional sources of propylene no longer meet demand. On-purpose propylene solutions are necessary to fill the propylene gap. This article will overview the market drivers for on-purpose propylene production. It will provide insight into the advantages of propane dehydrogenation (PDH) as the most efficient propylene production technology in general, and Honeywell UOP’s Oleflex process specifically as a high performing process technology. To demonstrate the benefits of the technology, two examples are described.
The propylene gap
Increasing opportunity exists for profitable production of propylene. In 2018, 68% of the end-use demand for propylene was for polypropylene. Production of this plastic is increasingly important because the growing middle class around the world, especially in developing nations, needs more plastic for consumer goods ranging from kitchen utensils and food containers to apparel, area rugs, and even car batteries. Polypropylene is a plastic that is widely considered one of the most versatile, and producers of propylene as a byproduct have found no end to the demand for propylene, driven by the demand for polypropylene.
Traditional sources of propylene as a by-product have been steam cracking and fluid catalytic cracking (FCC). The growing gap between the market demand for propylene and the ability of these traditional sources to produce propylene has been widening since 2011 and is projected to continue widening. On-purpose propylene production technologies are filling the gap (see Figure 1).
Propane dehydrogenation is the primary on-purpose technology in use, contributing 22% of the propylene production that is filling the supply/demand gap in 2018. By 2027, the percentage is expected to grow to 32%. Other on-purpose propylene technologies include methanol to olefins (MTO), methanol to olefins plus olefin cracking (MTO + OC), and methanol to propylene (MTP). All of these options produce propylene yields that are at least double those of the traditional technologies. PDH has the highest propylene yield of all, at 85%.
Propane dehydrogenation is a simple process with one feed (propane) that is converted to one primary product (propylene) with the option to use the by-product (hydrogen) for fuel or export for other uses (see Figure 2). A PDH unit is easily integrated at a propane source or at a downstream polypropylene production plant. Honeywell UOP’s Oleflex process is a leading PDH process and will continue to hold that place due to UOP’s investment building on its expertise.
The Oleflex process
A complete Oleflex unit consists of a fractionation section and reaction section. The fractionation section consists of a depropaniser that purifies propane feed to the reaction section, a deethaniser that removes lighter components from the reactor section product stream, a propane-propylene splitter, and a small selective hydrogenation reactor that removes diolefins from the propane, which is recycled to the depropaniser.
The Oleflex reaction section, which converts propane to propylene and hydrogen, is noteworthy for its steady-state operation design and small footprint. These are achieved by using an independent reactor section consisting of four heater cells with four vertical reactors, followed by an independent regeneration section. The high yield, high activity platinum-containing catalyst slowly circulates through the reactors and enters the continuous catalyst regeneration (CCR) section, utilising UOP’s CCR technology and enabling steady-state catalyst regeneration (see Figure 3).
Key benefits of the process are lower capital expenditure, lower operating expenditure, better economy of scale, higher plant productivity and dependability, smaller environmental footprint, and UOP’s project execution, technical services, and continuous innovation. Overall, the financial benefits can add up to $20 million/y lower net cost of production, an additional $80 million of net present value and an additional 3% return on investment compared to other PDH technologies.
The streamlined design of UOP’s PDH technology offers several advantages for capital expenditure. One of the main reasons the Oleflex process requires significantly lower capital expenditure is the highly active and stable nature of the platinum-containing Oleflex catalyst. With high activity, a smaller volume of catalyst is required for the same design production. With high stability, the unit can be designed to tighter parameters because catalyst deactivation is highly predictable. Both of these factors contribute to the smaller size of Oleflex reactors when compared to other PDH technologies. Additionally, the process is designed with positive reactor pressures. This results in a smaller, simpler reactor effluent compressor as well as smaller piping and equipment throughout the entire unit when compared to systems operating under vacuum pressure. Another key feature of the process – the independent regeneration section – comes with the inherent benefit to capital expenditure of avoiding expensive equipment such as large isolation valves, large air blowers or drivers, a selective catalytic reduction system (SCR), or a wastewater stripping system. All of these would be required for a design that relies on cyclic catalyst regeneration rather than steady-state CCR technology. Finally, Oleflex reactors are oriented vertically, resulting in a smaller plot plan that requires only two-thirds of the footprint that other PDH units require. The advantages of UOP’s PDH technology can result in a 15-20% saving in capex.
Similarly, its simplicity of design and high performance make Oleflex units simpler and less expensive to operate. The stability of the catalyst results in lower consumption of propane for the same propylene production over the catalyst life. At 90%, the lion’s share of operating expenditure goes to the propane feed, so the high product yield of Oleflex units compared to competing PDH technologies gives operators an advantage with operating costs. Furthermore, utility costs are minimised due to the lower compressor duty associated with positive reactor pressure, the absence of a need for nitrogen purge to seal valves, the ability to regenerate without large utility air compressors, and the absence of a steam purge. Oleflex technology has lower coke generation, which results in the benefits of reduced fuel or electric power requirement since no large air blowers are needed for catalyst regeneration, and more light ends (C2-) by-product recovery for fuel or export. These opex advantages can add up to a $10/t propylene production saving for a 600 000 t/y propylene unit.
Better economy of scale
As the propylene supply/demand gap continues to widen, companies are interested in commissioning larger and larger PDH units. Whether a unit is designed for 450000 t/y or 750000 t/y, the Oleflex process utilises four vertical reactors, incrementally sized for the plant’s designed production. Competing PDH technologies, with larger reactors, scale up by increasing by as many as double the number of reactors. For small or large applications, the Oleflex process maintains the same configuration, offering the same benefits of a simple, streamlined design.
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