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Improve Steam Cracking Furnace Productivity and Emissions Control Through Filtration and Coalescence

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Improve Steam Cracking Furnace Productivity and Emissions Control Through Filtration and Coalescence Scientific & Technical Report GDS138 Improve Steam Cracking Furnace Productivity and Emissions Control through Filtration and Coalescence Mark Brayden, Process Research Leader, The Dow Chemical Company Thomas H. Wines, Ph.D., Senior Marketing Manager, Pall Corporation Ken Del Giudice, Senior Project Manager, Pall Corporation The impact of feedstocks on the operation of an ethylene plant April 26, 2006 Presented at the American Institute of Chemical Engineers (AIChE) Spring National Meeting, Ethylene Producers’ Conference (EPC) Orlando, Florida, April 23 – 27, 2006 Abstract Hydrocarbon streams feeding ethylene steam culty in maintaining the optimum furnace tem- cracking furnaces often contain significant lev- perature and steam/hydrocarbon feed ratio. This els of corrosion products, water, and salts. This can lead to poor yield of ethylene by the crack- is especially true when naphtha is supplied by er and undesirable by-products. marine vessels. In these cases, high efficiency Installation experience at The Dow Chemical liquid–liquid coalescers and filters are recom- Company (Dow) in Freeport,Texas is presented mended to condition the inlet feed stream. for the use of high efficiency liquid–liquid coa- Contaminants in the inlet hydrocarbons can lescers and filters to extend the steam cracker adversely affect ethylene production in a num- service life between decokings. The naphtha ber of ways. Sodium and iron oxides are known feed was supplied by marine transport and con- to be coke promoters, and their presence can tained significant salt water contamination. An reduce the run time of the ethylene furnaces economic evaluation of the savings due to before decoking is required, and in some improved operation efficiency and the payback instances reduce the life of the furnace tubes by period for the coalescer system is provided. as much as one third. Unscheduled or frequent The installation of the high efficiency decoking cycles lead to a loss in ethylene pro- coalescer–filtration system was found to have a duction, shortened furnace tube life, and create payback of less than ten months based on extend- higher maintenance costs. Frequent decoking ed furnace run times alone, assuming that will also result in increased particulate release to ethylene production is limited by furnace avail- the atmosphere and can create environmental ability. concerns over excessive emissions. Fouling of flow meters and control valves can lead to diffi- Introduction Rising costs for raw materials and ever-increas- The cracking of LCN or FCC gasoline is a recent ing competition have made margins in ethylene trend, due to the new 2005 specifications on production slim in recent years. Many produc- refining products.While all feed stocks can bring ers are looking for ways to reduce costs, includ- solids due to pipe corrosion, LCN or FCC gaso- ing using alternate feed stock supplies such as line has additional sodium contamination due to those shipped in by marine vessels,or the use of previous treatment to remove sulfur compounds. Light Catalytic Cracked Naphtha (LCN) or The desulfurization process includes a caustic Fluidized Catalytic Cracked (FCC) gasoline. wash step that results in high sodium concen- While alternate steam cracker feed stock sources trations that should be removed prior to the fur- can reduce raw material costs, they also pose naces. new challenges to ethylene producers. Contamination in the feed stocks can include corrosion products,water,and salts. Marine trans- port vessels may also use the same tanks for sea- water ballast as they use for hydrocarbon feed stocks, thus creating a greater risk of contami- nation to steam cracker furnaces. 1 Contaminants in the inlet hydrocarbons can • During decoking cycles,significant amounts of adversely affect ethylene production in a num- particles are released to the atmosphere: ber of ways: The total annual emissions can be reduced as • Sodium and iron oxides are known to be coke the number of cycles is decreased. promoters [1,2] and their presence can reduce Separation of the harmful contaminants can be the run time of the ethylene furnaces before a difficult task especially when the salt water is decoking is required. Unscheduled or frequent emulsified with the hydrocarbon feed stock. decoking cycles can lead to a loss in ethylene Fortunately, new liquid–liquid coalescer tech- production, shortened furnace tube life, and nology has been developed to separate even the higher maintenance costs. most difficult emulsions. • Fouling of flow meters and control valves can Experience with high efficiency liquid–liquid lead to difficulty in maintaining the optimum coalescers and filters at The Dow Chemical furnace temperature and steam/hydrocarbon Company (Dow) in Freeport,Texas is presented. feed ratio. This can result in a poor yield of The naphtha feed is marine transported,and both ethylene by the cracker and undesirable sodium and iron needed to be removed prior to byproducts. the cracking furnaces. Details of the filtering • Coke fines and sodium are often released inad- and coalescing systems are provided along with vertently into the downstream quench sys- the separation performance, process benefits, tems during decoking cycles and this can lead and payback period. to further complications in the decanter and downstream separation units, including the quench water stripping tower and heat exchangers in the dilution steam system. Solid and Salt Water Contamination in Naphtha Streams Solid contamination Analysis of solid contamination in naphtha Solid contamination in naphtha performed on steam crackers A recent study based on 36 samples of naphtha Analysis of different naphtha feeds was performed cuts from 19 refineries worldwide (11 different on crackers prior to the furnaces and are con- oil companies) showed that the quantity of solids sistent with the study on refineries: in naphtha varied between 1 and 10 ppmw. The Table 1: Naphtha solid contamination: particles were composed of iron oxides and iron measurements of TSS* on naphtha sulfides with a size range between 2 and 70 crackers micron (µm) and an average size of 10 µm [3]. Location TSS value Depending on the naphtha cuts (catalytic cracked USA plant 1 1.2 to 1.5 mg/l naphtha, naphtha straight run as examples), the USA plant 2 4 to 6 mg/l particle content may be significantly different. Filters have already been successfully installed for Japan 1.3 mg/l the protection of naphtha hydrotreaters in refiner- Average 3 mg/l ies. * Total Suspended Solids 2 A photomicrograph of typical solid contaminants 20 dyne/cm indicates that the emulsion is very in naphtha is presented in Figure 1. The stable. Under such conditions,separation in stor- red–orange colors are indicative of iron oxides age tanks is ineffective except for bulk water while the black particles are representative of removal, and a small percentage of emulsified coke fines and iron sulfides. water would be expected to remain in a stable form, requiring the use of high efficiency Figure 1: Photomicrograph liquid-liquid coalescers for separation. of naphtha solid contaminants at Depending on variations in process operations, 100 X magnification interfacial tensions can vary dramatically.At the refinery where naphtha is produced, corrosion inhibitors (filming amine type) are typically inject- ed into the overhead column to minimize cor- rosion in the separation drum and overhead heat 100 µm exchangers.Variations in inhibitor content can sig- nificantly affect emulsion stability of water in Based on the analysis of thirty six different naph- naphtha. tha streams,it can be stated that the typical solid High Solids Loading Naphtha Filter: Naphtha contamination in naphtha is in the range of 1 to feed stocks can contain significant solid partic- 10 ppmw in crackers and refineries.Iron oxides ulate in the form of corrosion products (iron and iron sulfides represent the main part of the oxides,iron sulfides),and in lower quantities silt solid contamination with more than 80% of the (sand,clay) or precipitated salts (sodium chloride, particles having sizes below 70 µm, and an aver- sodium sulfate,etc.).In some cases,aqueous con- age size of 10 µm. taminants are not present, and in others, both The naphtha filter rating: finding the best aqueous and solid contaminants need to be compromise to optimize furnace protection removed. Therefore, in some instances, the feed Typically, large solid particles (>300 µm) are stock may require stand-alone particulate filtra- removed in the storage tanks or by coarse mesh tion,and in others cases,incorporate a coalescer filters installed upstream of the furnaces.However, system with pre-filters. for efficient protection of furnace tubes, the In order to protect the steam cracker, a filter removal ratings of the naphtha filters should be with an absolute rating of 10 µm should be used. lower than 70 µm absolute.Considering absolute Several types of filters are available, including filter efficiencies, and the particle size distribu- string wound,pleated,depth,and advanced high tion in the naphtha, a 10 µm absolute rating is flow capacity designs that use laid over, cres- recommended. Since 1998, the plants having cent-shaped pleats. When the solids loading is such filter installations have reached the expec- high, backwash type filters become more eco- tations in terms of furnace protection and cost nomical than disposable type. Here, the initial operation. capital investment is higher, but the operating Emulsified water costs are much lower since the filters are regen- Laboratory tests determined that the interfacial erable and can last years before requiring clean- tension between water and naphtha at the crack- ing. An example of a high flow capacity horizontal ing plants and in refineries ranged from 1 to disposable filter system is shown in Figure 2: 24 dyne/cm. An interfacial tension below 3 Figure 2: er droplets are easier to separate from the con- High flow capacity pre-filter offering tinuous hydrocarbon phase liquid.The enlarged high surface area.
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