Olefin plants ethylene

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. 

Since the early 1980s olefin plants in the United States were designed to have substantial flexibiHty to consume a wide range of feedstocks. Most of the flexibiHty to use various feedstocks is found in plants with associated refineries, where integrated olefins plants can optimize feedstocks using either gas Hquids or heavier refinery streams. Companies whose primary business is the production of ethylene derivatives, such as thermoplastics, tend to use ethane and propane feedstocks which minimize by-product streams and maximize ethylene production for their derivative plants. 

Ethylene Stripping. The acetylene absorber bottom product is routed to the ethylene stripper, which operates at low pressure. In the bottom part of this tower the loaded solvent is stripped by heat input according to the purity specifications of the acetylene product. A lean DMF fraction is routed to the top of the upper part for selective absorption of acetylene. This feature reduces the acetylene content in the recycle gas to its minimum (typically 1%). The overhead gas fraction is recycled to the cracked gas compression of the olefin plant for the recovery of the ethylene. 

The solvent is 28 CC-olefins recycled from the fractionation section. Effluent from the reactors includes product a-olefins, unreacted ethylene, aluminum alkyls of the same carbon number distribution as the product olefins, and polymer. The effluent is flashed to remove ethylene, filtered to remove polyethylene, and treated to reduce the aluminum alkyls in the stream. In the original plant operation, these aluminum alkyls were not removed, resulting in the formation of paraffins (- 1.4%) when the reactor effluent was treated with caustic to kill the catalyst. In the new plant, however, it is likely that these aluminum alkyls are transalkylated with ethylene by adding a catalyst such as 60 ppm of a nickel compound, eg, nickel octanoate (6). The new plant contains a caustic wash section and the product olefins still contain some paraffins ( 0.5%). After treatment with caustic, cmde olefins are sent to a water wash to remove sodium and aluminum salts. 

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. 

The pyrolysis reactor is an important processing step in an olefin plant. It is used to crack heavier hydrocarbons such as naphtha and LPG to lower molecular weight hydrocarbons such as ethylene. The pyrolysis reactor, in this study, consists of two identical sides each side contains four cracking coils in parallel (see Fig. 2). 

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. 

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. 

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. 

The chemical uses for ethylene prior to World War II were limited, for the most part, to ethylene glycol and ethyl alcohol. After the war, the demand for styrene and polyethylene took off, stimulating ethylene production and olefin plant construction. Todays list of chemical applications for ethylene reads like the WTiat s What of petrochemicals polyethylene, ethylbenzene (a precursor to styrene), ethylene dichloride, vinyl chloride, ethylene oxide, ethylene glycol, ethyl alcohol, vinyl acetate, alpha olefins, and linear alcohols are some of the more commercial derivatives of ethylene. The consumer products derived from these chemicals are found everywhere, from soap to construction materials to plastic products to synthetic motor oils. 

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. 

Another route to ethylbenzene is available for those remote places where olefin plants or refinery crackers are not nearby but a supply of ethane is— catalytic dehydrogenation of ethane to ethylene followed by its reaction with benzene to produce EB. The first of two steps in Figure 8-4 use a gallium zinc zeolyte catalyst that promotes ethane dehydrogenation to ethylerie at 86% selectivity and up to 50% conversion per pass. 

A typical worldscale olefins plant producing a billion pounds a year of ethylene from heavy liquids can also yield up to 50 million pounds of styrene. Since the styrene is a coproduct, and the extraction costs are modest, the economics are very attractive compared to on-purpose styrene. 

The process achieves about 90% conversion of ethane to VC. With the elimination of so many intermediate steps compared to the traditional EDC route, this process could achieve VC production cost savings of up to 35% anywhere an adequate supply of ethane can be found. That could even include the recycle stream from a heavy liquids olefins plant. If these killer economics persevere, this technology could grab all the growth in VC capacity and even replace most of the conventional VC capacity in a couple of decades. That s what happened to the acetylene-based route to VC when the ethylene-based route came on stream in the mid-20th century. 

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... 

Figure 8.4 Distillation columns used in a large olefin plant. The middle one is the C2 splitter and the highest at 200 ft, separating ethane and ethylene. (Courtesy of BP Chemicals, Alvin, Texas)...

Figure 3.2. Flowsketch of an olefins plant and specifications of the ethylene product. AR designates a composition analyzer and controller (after Skrokov (Ed.), Mini- and Microcomputer Control in Industrial Processes, Van Nostrand I Reinhold, New York, 1980).

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. 

Petrochemical plants, especially olefins plants that can manufacture numerous products in different proportions from the same feedstock, have had probably the greatest success at delivering value from sophisticated online plant-wide models Cutler and Perry, 1983 Rejowski et al., 2009 Paules and Meixell, 1994 Fatora and Ayala, 1992 Fatora et al., 1992a Fatora et al., 1992 Houk et al., 1992 Kelly et al., 1991. Over 50 ethylene RTO applications have been deployed, as well as several others on nonolefin petrochemical processes. 

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