Olefins plants, ethylene, and propylene

Olefins plants, for the most part, all have the same basic technology, but the process flows differ with the varied feedstocks that can be used. This chapter will cover in some depth the feeds, the hardware, the reactions, and the variables that can be manipulated to change the amount and mix of products. The physical properties of ethylene and propylene, which present some unique handling problems, will be covered also. 

A handful of plants with completely different technologies have been built and are described in a section at the end of this chapter. They contribute only a minor amount of olefins to the marketplace, and their economics and outlooks are still under the microscope, so most of this chapter will concentrate on the popular design. 

Traditional olefin plants have more than one alias. One is even fraudulent. They are variously called ethylene plants after their primary product steam crackers because the feed is usiuilly mixed with steam before it is cracked or whatever aacker, where whatever is the name of the feed (ethane cracker, gas oil cracker, etc.). Olefin plants are sometimes referred to as ethylene crackers, biit only those who don t know any better, use that misnomer. Ethylene is not cracked but rather is the product of cracking. 

The one aspect of ethylene manufacture that sets it apart from most other petrochemicals is the wide range of alternate feedstocks that can be used. Most others are limited to one or a few commercial alternatives. Ethylene is a simple molecule, CH2=CH2, and if you thought that lots of hydrocarbons could be cracked to form it, you d be right, as you can see in Table 5—1- 

Ethane and propane produce a high yield of ethylene. Propane also gives a high yield of propylene. The earliest commercial olefin plants of any size were designed to use these two feeds, and they dominated U.S. plant designs in much of the 20th century. 

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Metathesis of ethylene and butylenes to propylene. Another on-purpose route to propylene is metathesis, a chemical reaction that starts with two compounds, involves the displacement of groups from each and produces two new compounds. The application in this case converts ethylene and mixed butylenes to propylene and butene-1. This route could appeal to a company with refinery or olefins plant ethylene and butylenes that both have market values less than propylene, which could be the case in some local markets. 

In Chapter 4 you II find a complete discussion of the manufacture of ethylene and propylene by cracking naphtha or gas oil in an olefin plant. One of the by-products of cracking those feedstocks is benzene. The term by-product may not be appropriate anymore, since about a third of the benzene supply in the United Stares now comes from olefins plants. 

Methanol dehydrogenation to ethylene and propylene. In some remote ioca-tions, transportation costs become very important. Moving ethane is almost out of the question. Hauling propane for feed or ethylene itself in pressurized or supercooled vessels is expensive. Moving naphtha or gas oil as feed requires that an expensive olefins plant with unwanted by-products be built. So what s a company to do if they need an olefins-based industry at a remote site One solution that has been commercialized is the dehydrogenation of methanol to ethylene and propylene. While it may seem like paddling upstream, the transportation costs to get the feeds to the remote sites plus the capital costs of the plant make the economics of ethylene and its derivatives okay. 

Cracking large hydrocarbons usually results in olefins, molecules with double bonds. Thats why the refinery cat crackers and thermal crackers are sources of ethylene and propylene. But the largest source is olefin plants where ethylene and propylene are the primary products of cracking one or more of the following ethane, propane, butane, naphtha, or gas oil. The choice of feedstock depends both on the olefins plant design and the market price of the feeds. 

The base-load supply of butadiene is from olefins plants simply because butadiene is coproduced with the other olefins. There s not much decision on whether or not to produce it. It just comes out, but in a small ratio compared CO ethylene and propylene. Cracking ethane yields one pound of butadiene for every 45 pounds of ethylene cracldng the heavy liquids, naphtha or gas oil, produces one pound of butadiene for every seven pounds of ethylene. Because of the increase in heavy liquids cracldng, about 75% of the butadiene produced in the United States is coproduced in olefin plants. 

As chemical companies rely more heavily on ethane and propane feeds to their olefins plants to generate their ethylene and propylene supplies, the coproduction of butadiene in olefins plants has not kept up with demand. Industry has resorted to building plants that make on-purpose or swing supply butadiene. The processes involve catalytically dehydrogenating (removing hydrogen from) butane or butylene. 

Stanley SJ, Sumner C. Catalytic distillation and hydrogenation of heavy unsaturates in an olefins plant in the manufacture of ethylene and propylene. WO 9909118, ABB Lummus Global Inc., 1999. 

Petrofin [Process enhancement through recovery of olefins] A process for recovering olefins (ethylene and propylene) from polymerization processes by adsorption on zeolite 4A. Developed by BOC and used at Montell s polypropylene plant at Lake Charles, LA. First demonstrated in 1997. 

The synthesis of olefins from methanol using aluminophosphate molecular sieve catalysts was studied [76], Process studies were conducted in a fluid-ized-bed bench-scale pilot plant unit utilizing small-pore silicaluminophosph-ate catalyst synthesized at Union Carbide. These catalysts are particularly effective in the catalytic conversion of methanol to olefins, when compared to the performance of conventional aluminosilicate zeolites. The process exhibited excellent selectivities toward ethylene and propylene, which could be varied considerably. Over 50 wt% of ethylene and 50 wt% propylene were synthesized on the same catalyst, using different combinations of temperatures and pressures. These selectivities were obtained at 100% conversion of methanol. Targeting light olefins in general, a selectivity of over 95% C2-C4 olefins was obtained. The catalyst exhibited steady performance and unaltered... 

After they leave a refinery, most petrochemicals are used as feedstock for petrochemical plants. Among olefins, ethylene and propylene are building blocks for plastics and specialty chemicals. The olefin butadiene is processed to eventually form synthetic rubber. Among aromatics, benzene is used in the production of synthetic dyes, detergents, and such innovative products as polycarbonates that form light hard plastic shells in many electronic items such as mobile phones. Toluene and xylene are used for solvents or as building blocks for other chemicals, creating, for example, polyester fibers. 

Cryogenic distillation has been nsed for over 70 years for the recovery of ethylene and propylene from olefin plants, refinery gas streams, and other sources (Keller et ah, 1992). These separations are difficnlt to accomplish because of the close relative volatilities. The ethane/ethylene distillation is performed at about -25 °C and 320 psig in a colnmn containing over 100 trays. Propane/propylene distillation is performed at abont —30 °C and 30 psig. These are the most energy-intensive distillations in the chemical and petrochemical industry (Safarik and Eldridge, 1998). 

Gas recycle hydroformylation processes have been licensed worldwide and operate for ethylene and propylene hydroformylation. Butene hydroformylation has been demonstrated in a pilot plant but it was found that problems linked to the formation and removal of heavies make the process in fact technically unfeasible for all olefins heavier than propylene. 

MTO [Methanol To Olefins] A catalytic process for converting methanol to olefins, mainly propylenes and butenes. Developed by Mobil Research Development Corporation and first demonstrated in 1985. Another version of this process was developed by UOP and Norsk Hydro and has been run at a demonstration unit at Porsgrunn, Norway, since Jun 1995. It is based on fluidized bed technology using a SAPO molecular sieve catalyst. It converts 80% of the carbon in the feed to ethylene and propylene. The first commercial plant built in Nigeria in 2008. 

Since that time, a major milestone for advanced MTO commercialization was the startup of a semicommercial, fully integrated MTO demonstration unit in Feluy, Belgium, on Total Petrochemicals premises in 2009. The plant has a capacity exceeding 10 tons methanol/day more than 13 times the capacity of the first smaller demo unit built at Hydro in Norway. The process demonstration unit (PDU) includes a complete MTO process with product recovery and purification as well as integration with ATOFINA/UOP olefin cracking process (OCP) to maximize the yields of ethylene and propylene. Following some initial work by Total Petrochemicals (formerly Atofma) in the mid-1990s. Total Petrochemicals and UOP formed a joint-development alliance in late 2000 [3,45]. 

The latest of three ethylene recovery plants was started in 1991. Sasol sold almost 300,000 t of ethylene in 1992. Sasol also produces polypropylene at Secunda from propylene produced at Sasol Two. In 1992 Sasol started constmction of a linear alpha olefin plant at Secunda to be completed in 1994 (40). Initial production is expected to be 100,000 t/yr pentene and hexene. Sasol also has a project under constmction to extract and purify krypton and xenon from the air separation plants at Sasol Two. Other potential new products under consideration at Sasol are acrylonitrile, acetic acid, acetates, and alkylamines. 

Olefins are produced primarily by thermal cracking of a hydrocarbon feedstock which takes place at low residence time in the presence of steam in the tubes of a furnace. In the United States, natural gas Hquids derived from natural gas processing, primarily ethane [74-84-0] and propane [74-98-6] have been the dominant feedstock for olefins plants, accounting for about 50 to 70% of ethylene production. Most of the remainder has been based on cracking naphtha or gas oil hydrocarbon streams which are derived from cmde oil. Naphtha is a hydrocarbon fraction boiling between 40 and 170°C, whereas the gas oil fraction bods between about 310 and 490°C. These feedstocks, which have been used primarily by producers with refinery affiliations, account for most of the remainder of olefins production. In addition a substantial amount of propylene and a small amount of ethylene ate recovered from waste gases produced in petroleum refineries. 

The following information was used in olefin plant case studies to determine if the ethylene/propylene cascaded refrigeration systems had enough horsepower for various plant operations. The propylene was condensed against cooling water at 110°F and the ethylene was condensed against propylene at -20°F. For comparison, the horsepower requirements for each refrigerant alone are also shown. 

Why start out with benzene The obvious answer is that benzene is one of the handRil of basic building blocks in the petrochemicals industry along with ethylene, propylene, and a few others. The more subde reason is that benzene, more than any of those other chemicals, comes from a broader b e- steel mill coking, petroleum refining, and olefins plants. For that reason, the benzene network, the sources and the uses, is more complex than any of the others. 

In the chapter on olefms plants, in the section on propylene, a route to making propylene involved butene-2. In this process, called metathesis, ethylene and butene-1 are passed over a catalyst, and the atoms do a musical chair routine. When the music stops, the result is propylene. The conversion of ethylene to propylene is an attraction when the growth rate of ethylene demand is not keeping up with propylene. Then the olefins plants produce an unbalanced product slate, and producers wish they had an on-purpose propylene scheme instead of just a coproduct process. The ethylene/butene-2 metathesis process is attractive as long as the supply of butylenes holds out. Refineries are big consumers of these olefins in their alkylation plants, and so metathesis process has, in effect, to buy butylene stream away from the gasoline blending pool. 

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