Olefin plants ethylene/propylene

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. 

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 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. 

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. 

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. 

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. 

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. 

The petrochemical products from olefins plants are ethylene, propylene, C4 s (butanes, butylenes, and butadiene) and a stream containing the BTXs, Refinery cat.crackers produce propylene and C4S. They produce some ethylene, but often it is not recovered. 

For most of olefin plant history, there was plenty of propylene around, especially in refineries, so olefin plants didn t really have to be built to make propylene, only ethylene. But a petrochemical product that has... 

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. 

Recent awards for world-scale ethylene plants for Borouge in Abu Dhabi, Optimal in Malaysia, Amir Kabir in Iran and Marun in Iran. The latter plant represents the largest olefin plant with a production of 1.1 million mtpy ethylene and 200,000 mtpy propylene. 

The most important monomers for the production of polyolefins, in terms of industrial capacity, are ethylene, propylene and butene, followed by isobutene and 4-methyl-1-pentene. Higher a-olefins, such as 1-hexene, and cyclic monomers, such as norbornene, are used together with the monomers mentioned above, to produce copolymer materials. Another monomer with wide application in the polymer industry is styrene. The main sources presently used and conceivably usable for olefin monomer production are petroleum (see also Chapters 1 and 3), natural gas (largely methane plus some ethane, etc.), coal (a composite of polymerized and cross-linked hydrocarbons containing many impurities), biomass (organic wastes from plants or animals), and vegetable oils (see Chapter 3). 

Propylene. Unlike ethylene, propylene production does not represent the requirement for propylene derivatives. With few exceptions, propylene is not made on purpose but is obtained as a by-product of other processes. More specifically, large quantities of relatively low purity (40-70%) propylene are produced in refineries as a by-product of gasoline manufacture. Additionally, significant quantities of higher purity propylene originate in olefins plants, where ethylene is the primary product. However, only polymer-grade propylene (>99% pure) can in any way be considered an on-purpose product. To better understand... 

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. 

Physical, chemical and biochemical conversion of plant-based oligo- and polymers into industrial bulk products as well as into specialties is a well established technology, which favourably extends the product spectrum obtained through petrochemical synthesis. Volumewise, less than SO million tons p.a. of chemical intermediates and end-products are derived from renewable plant-based raw materials and constitute less than 15% of about 400 million tons provided by petrochemistry. This imbalance is not so much a result of a relative shortage or lack of availability of renewables compared to petrochemical feedstocks, rather, it reflects the versatility of ethylene, propylene and olefin based chemical synthesis in meeting the product and product range requirements of an industrialized world. 

Ethylene. The largest potential chemical market for n-butanc is in steam cracking to ethylene and coproducts. n-Butane is a supplemental feedstock for olefin plants and has accounted for 1 to 4 percent of total ethylene production for most years since 1970. It can be used at up to 10 to 15 percent of the total feed in ethane/propane crackers with no major modifications. n-Butane also can be used as a supplemental feed at as high as 20 to 30 percent in hea y naphtha crackers. The consumption of C s has fluctuated considerably from year to year since 1970, depending on the relative price of butane and other feedstocks. The yield of ethylene is only 36 to 40 percent, with the other products including methane, propylene, ethane, butadiene, acetylene, and butylenes. About 1 to 2 billion lb of butane are consumed annually to produce ethylene. 

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