Olefin plants process

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. 

Basic Yield Data. This is a good place to start asking questions. If the process uses a catalytic reaction, do the yields represent new catalyst or catalyst regenerated a number of times For a thermal reaction like an olefin plant steam cracker, questions might be asked about on-stream time between decokings. Therefore, how much contingency is there in the specified number of crackers required ... 

The total plant or train main process bottleneck will probably be identified by the licensor, such as the gasifier for a coal gasification train, the main exchanger for a mixed refrigerant LNG plant train, or the cracked gas compressors for an olefin plant. First and foremost, be sure that the licensor has not made the utility area a bottleneck. This can never be allowed since overloaded utilities could repeatedly shut the entire complex down on a crash basis, adversely impacting economics. 

The licensor s basis for sizing has already been discussed and agreed to or changed. For an olefin plant, the number of steam crackers of the licensor s standard size is firm. For a new process, reactor scaleup methods have been agreed to. For a coal gasification plant, gasifier size. 

Hower, T. C., and Kister H. Z.. Unusual Operating Histories of Gas Processing and Olefins Plant Columns, paper presented at A.I.Ch.E. Annual Meeting, November 2-7. 1986, Miami Beach, Florida. 

There is also a certain amount of statistical information available on the failures of process system components. Arulanantham and Lees (1981) have studied pressure vessel and fired heater failures in process plants such as olefins plants. They define failure as a condition in which a crack, leak or other defect has developed in the equipment to the extent that repair or replacement is required, a definition which includes some of the potentially dangerous as well as all catastrophic failures. The failure rates of equipment are related to some extent to the safety of process items. If a piece of equipment has a long history of failures, it may cause safety problems in the future. Therefore it would be better to consider another equipment instead. It should be remembered that all reliability or failure information does not express safety directly, since all failures are not dangerous and not all accidents are due to failures of equipment. 

DECOMPOSITION/RECONCILIATION IN A SECTION OF AN OLEFIN PLANT 12.2.1. Process Description... 

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). 

The benzene leaves the olefins plant fractionator mixed with the other gasoline components so it is handled the same way as a refinery stream. An aromatics concentrate is made and run through one of the two separation processes you just read about, solvent extraction or extractive distillation. 

Since toluene is nothing more than benzene with a methyl group attached, creating one from another is relatively easy. Benzene, toluene, and for that matter, xylenes too, are coproduced in the processes just described—coke making, cat reforming, and olefin plants operations. The ratio of benzene to the other aromatics production is rarely equal to the chemical feedstock requirements.. fo.r the three. One method for balancing supply and demand is toluene hydrodealkylation (HDA). This process accounts for 10—15% of the supply of benzene in the United States and is a good example of what can be done when one or more coproducts are produced in proportions out of balance with the marketplace. 

The manufacture of the xylenes is a dejk vu story of benzene and toluene—cat reforming, olefin plants, a small amount naturally resident in crude oil, and coke making. A small but rapidly growing amount of xylene comes from catalytic disproportionation, the process described in the ben-... 

With the percentages of the three xylenes from the various sources differing so much, it s not likely that a company, or the industry for that matter, will produce just the amount of the xylene isomer it wants. Para-xylene has the biggest demand and meta- the smallest, but none of the processes, cat reforming, olefins plants, or disproportionation, have commensurate yields. 

Most of the toluene and xylenes have their origin in catalytic reforming or olefins plants. From there, the processing schemes vary widely from site to site. The schematic in Figure 3-6 captures most of the variations, although its hard to portray that some plants separate the BTXs from each other early in the scheme while others do it at varying places downstream of an aromatics recovery unit. 

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. 

In an olefins plant, the feed is subjected to very high temperatures in cracking furnaces for a few moments and then cooled rapidly to stop the cracking. Elaborate separation facilities are necessary to separate the olefins. from the by-products of the cracking process. 

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. 

When butadiene is produced in olefins plants or in refinery crackers, they come mixed with relatively large volumes of the other C4 family. Sometimes the other C4S need not be separated from each other, for example if they are going to be used for allcylation plant feed. In that case, the butadiene can be separated from the other C4S by extractive distillation. This process uses a solvent that will preferentially dissolve butadiene, ignoring the other components in the stream. 

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 source of these compounds is varied. The butanes are found naturally in crude oils and natural gas. They, plus the olefins, are products of various refinery processes and of olefins plants. They are separated by fractionation, except for butadiene and isobutylene, which are sometimes recovered by extractive distillation. They all vaporize at room temperature, so they are handled in closed, pressurized systems.. 

Refinery cat reformers produce a reformate stream with aromatics. That stream, with or-without the benzene-laden scream from the olefins plant, can be split apart in the various processing schemes in the BTX recovery facility. 

The process of dehydrogenation of EB is shown in Figure 8-6. The process is similar to operations in an olefins plant in that dehydrogenation is done by mixing the feed with steam and cracking it in pyrolysis furnaces. However, the cracking products are more limited, primarily because of the use of a catalyst, iron oxide. 

In this process, it is very uncommon to detect any benzene rings brealdng up. As in the olefins plant, the thermal stability of the benzene ring is demonstrated by its survival under these severe operating conditions, especially the high temperatures. 

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. 

One process that capitalizes on butadiene, synthesis gas, and methanol as raw materials is BASF s two-step hydrocarbonylation route to adipic acid(3-7). The butadiene in the C4 cut from an olefin plant steam cracker is transformed by a two-stage carbonylation with carbon monoxide and methanol into adipic acid dimethyl ester. Hydrolysis converts the diester into adipic acid. BASF is now engineering a 130 million pound per year commercial plant based on this technology(8,9). Technology drawbacks include a requirement for severe pressure (>4500 psig) in the first cobalt catalyzed carbonylation step and dimethyl adipate separation from branched diester isomers formed in the second carbonylation step. 

Table 7 shows the yield distribution of the C4 isomers from different feedstocks with specific processing schemes. The largest yield of butylenes comes from the refineries processing middle distillates and from olefins plants cracking naphtha. The refinery product contains 35 to 65% butanes olefins plants, 3 to 5%. Catalyst type and operating severity determine the selectivity of the C4 isomer distribution (41) in the refinery process stream. Processes that parallel fluid catalytic cracking to produce butylenes and propylene from heavy cmde oil fractions are under development (42). 

A manufacturing precast for producing ortho-phthalate otters derived from alkyl acid ortho-phthalatos and olefins has boon developed and demonstrated on the pilot plant scale. Process variables Include choice of reactants, stoichiometry, reaction kinetics, recycle of recovered materials and the fate of the perchloric add catalyst. Seme physical properties of the ortho-phthalate esters have been determined and severed of the esters have been evaluated as plasticizers for polyvinyl chloride. The composite data show that the acid-olefin esterification process provides commercially acceptable plasticizers for polyvinyl chloride. 

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).

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