Maximising ethane in liquids crackers R ecent advances in the recovery of gas from shale reserves in the US have resulted in a shift in the economics of US ethylene plant feedstock. The American Chemistry Council estimates that US shale deposits contain 100 years of natural gas supply, a “game changer” that could rejuvenate America’s chemical industry. Strong ethane supplies are positioning the US as the most competitive, low-cost ethylene producer, resulting in increased investments in ethane recovery and pipelines. As a result, several companies have already announced their plans for major investments in the US ethylene sector. At present, many ethylene producers currently cracking liquid feedstocks such as naph- tha or gas oil are either maximising or considering maximising the cracking of lighter feeds such as ethane. Producers who designed plants to crack ethane and/or propane and butane feeds have inherent advantages since their plants require minimal and/or no modifications. However, plants that were configured for heav- Economics for energy and feedstock supply favour ethane feed cracking, but plant constraints must be examined rigorously before maximum ethane feeds are pursued MUHAMMAD IMRAN Technip Stone & Webster Process Technology www.digitalrefining.com/article/1000712 PTQ Q1 2013 1 ier liquid feedstocks such as naphtha and gas oil will be forced to consider the impact on their units if they are to process the ethane feed. With shale gas advances in other parts of the world, this trend may also propagate in other regions in the coming years. The following factors need to be considered before deciding to shift to ethane feedstock. Ethane availability and price Shale gas recovery by horizon- tal drilling and the use of fracking technology has resulted in the US having an abundance of natural gas. Once natural gas is available, it is fractionated to separate ethane from the rest of the natural gas. The separated ethane is then fed into the pipeline. The US has a good pipeline infrastruc- ture to supply ethane from the source to the ethylene produc- ers. In addition, new pipelines are being constructed to meet the increased demand of consumers. Depending on the availability of shale gas reser- voirs and the availability of fracking technology, ethane cracking may also become economically attractive in other parts of the world in the coming years. The desire to develop shale gas reserves outside of the US is strong. As per the International Energy Agency’s World Energy Outlook 2011, 2 China has already auctioned shale gas exploration rights but with participation limited to Chinese companies. International companies willing to participate have therefore sought to enter into partnership with Chinese companies. Other countries becoming active or considering becoming active in shale gas exploration include India, Poland, Germany, Spain, the UK and Ukraine. Liquids cracker configurations Ethylene plants are typically designed with the demetha- niser, deethaniser or depropaniser tower sequenced at the front end of the recovery section. Each scheme will present unique challenges for maximising ethane feed flexi- bility. Depending on the plant configuration, the cracked efflu- ent’s flow rate and composition will vary in each section of the
Transcript
Maximising ethane in liquids crackers
Recent advances in the recovery of gas from shale reserves in the US have
resulted in a shift in the economics of US ethylene plant feedstock. The American Chemistry Council estimates that US shale deposits contain 100 years of natural gas supply, a “game changer” that could rejuvenate America’s chemical industry. Strong ethane supplies are positioning the US as the most competitive, low-cost ethylene producer, resulting in increased investments in ethane recovery and pipelines. As a result, several companies have already announced their plans for major investments in the US ethylene sector.
At present, many ethylene producers currently cracking liquid feedstocks such as naph-tha or gas oil are either maximising or considering maximising the cracking of lighter feeds such as ethane. Producers who designed plants to crack ethane and/or propane and butane feeds have inherent advantages since their plants require minimal and/or no modifications. However, plants that were configured for heav-
Economics for energy and feedstock supply favour ethane feed cracking, but plant constraints must be examined rigorously before maximum ethane feeds are pursued
MUHAMMAD IMRANTechnip Stone & Webster Process Technology
ier liquid feedstocks such as naphtha and gas oil will be forced to consider the impact on their units if they are to process the ethane feed. With shale gas advances in other parts of the world, this trend may also propagate in other regions in the coming years.
The following factors need to be considered before deciding to shift to ethane feedstock.
Ethane availability and priceShale gas recovery by horizon-tal drilling and the use of fracking technology has resulted in the US having an abundance of natural gas. Once natural gas is available, it is fractionated to separate ethane from the rest of the natural gas. The separated ethane is then fed into the pipeline. The US has a good pipeline infrastruc-ture to supply ethane from the source to the ethylene produc-ers. In addition, new pipelines are being constructed to meet the increased demand of consumers. Depending on the availability of shale gas reser-voirs and the availability of fracking technology, ethane
cracking may also become economically attractive in other parts of the world in the coming years. The desire to develop shale gas reserves outside of the US is strong. As per the International Energy Agency’s World Energy Outlook 2011,2 China has already auctioned shale gas exploration rights but with participation limited to Chinese companies. International companies willing to participate have therefore sought to enter into partnership with Chinese companies. Other countries becoming active or considering becoming active in shale gas exploration include India, Poland, Germany, Spain, the UK and Ukraine.
Liquids cracker configurationsEthylene plants are typically designed with the demetha-niser, deethaniser or depropaniser tower sequenced at the front end of the recovery section. Each scheme will present unique challenges for maximising ethane feed flexi-bility. Depending on the plant configuration, the cracked efflu-ent’s flow rate and composition will vary in each section of the
uneconomical to separate the heavier products formed by ethane cracking. This product shift can have an impact on the local market and can result in shortages, price hikes and possibly even result in the import of heavier products. There are technologies availa-ble for converting lower-chain olefins to higher-chain olefins, but these processes require additional capital, equipment and utility consumption. These processes can be used to offset the shortage of heavier co-products due to ethane feed cracking. Details of these proc-esses are outside the scope of this article.
Ability to maintain liquids cracking capabilityEthane pricing and ethane cracking economics may change in the future. It is therefore important to maintain flexibility so that, where possible, owners can cost effectively shift back to liquids cracking when desirable.
Ethane cracking and productsdistributionFigures 1, 2 and 3 explain the change in product make when gradually shifting the feedstock from 100% naphtha to 100% ethane. How much ethane can be cracked in a particular naph-tha or gas oil liquids cracker depends on many factors: avail-able design margins on equipment/piping, plant configurations, compressor performance curves and quench system design. The overall extent of the required modifica-tions will have to be determined on a case-by-case basis. It should be noted that product yields are strongly dependent
plant. Various sections of the ethylene plant may thus be more than adequate, with other sections requiring possible modifications.
Capacity increase in addition toflexibilitySome ethylene producers may well target increasing plant capacity in parallel to achieving maximum ethane cracking flex-ibility. Capacity increases will require additional capital spending, particularly in areas where existing equipment is already tight and flow rates and compositions are changing.
Available equipment margins incurrent operationThe extent of modifications required will depend on whether the existing equipment has any remaining margin. For maximum ethane cracking, flexibility will depend on the available margins in the exist-ing equipment.
Products shift and marketeconomicsIn addition to ethylene, a typical liquids cracker produces byproducts includ-ing hydrogen, fuel gas, propylene, butadiene, pyroly-sis gasoline, fuel oil, C4s and C5s. With ethane cracking, there is a significant shift in the overall product slate. Ethane cracking yields for ethylene are higher than those from liquid feed cracking; however, being a lighter feed-stock, its cracking yields are significantly lower for the heavier products. Often, due to the small make, it becomes
Feedstock T/T of ethyleneEthane 1.25Propane 2.3Naphtha 3.0Gasoil 3.85
Feedstock (tons/ton of ethylene) based on ultimate yield
Table 1
60
50
30
30
20
10
0% e
than
e
100%
nap
htha
20%
eth
ane
80%
nap
htha
40%
eth
ane
60%
nap
htha
60%
eth
ane
40%
nap
htha
80%
eth
ane
20%
nap
htha
100%
eth
ane
0% n
apht
ha
Furn
ace e
fflu
ents
, w
t%
0
PropyleneEthaneEthylene
28% ethylene**
43% ethylene50% ethylene*
Ethylene yield will be slightly higher depending on ethane % conversion Ethylene yield will be higher depending on saturates % and cracking severity
on cracking severity and ethane conversion per pass. Actual product yields will be depend-ent on the cracking conditions in a particular plant. Figures 1, 2 and 3 should be used only for explaining the effect of increas-ing ethane cracking on product distribution and should not be used for design work.
As Figure 1 shows, ethane cracking results in higher ethyl-ene yields with minimum production of byproducts. Typical once-through ethylene yields from ethane cracking are greater than 50% (depending on ethane once-through per cent conversion). Based on recycle cracking of ethane to extinction, an ultimate yield of ~80% ethylene is achievable. For overall comparison purposes, naphtha cracking gives a 25% to 36% ethylene yield depending on paraffin content and cracking severity.
Due to higher ethylene yields, ethane cracking requires a significantly lower feed rate (tons/ton of ethylene) as compared to other feedstocks. Table 1 shows feedstock tons required per ton of ethylene.
As was explained above, shale gas recovery in the US has resulted in reduced pricing of ethane. The combination of a relatively low ethane price and better ethylene yields can result in improved feed/product margins by shifting from naph-tha to maximum possible ethane cracking plus naphtha. Shifting from heavier to lighter feeds, however, substantially changes plant traffic patterns, decreasing plant loads for systems handling C3+ compo-nents, while burdening systems handling the lighter C2-/C2 fractions. The quench area will
probably be most affected; a number of modifications will most likely be required, but the changes will be a function of the overall feedstock before and after the addition of shale gas.
Figure 1 shows yield patterns for recycle ethane and propyl-ene. There is a significant increase from 5 wt% recycle ethane at 100% naphtha crack-ing to 35 wt% recycle ethane at 100% ethane cracking. Actual
recycle ethane within the system will depend on how much fresh ethane feed can be cracked in a given liquids cracker. Recycle ethane is typi-cally flashed to a pressure to achieve the equivalent of the coldest propylene refrigeration credit (-35°F, -37°C). Thus, a higher recycle ethane yield has the potential for additional refrigeration credit at the cold-est propylene level. This additional refrigeration credit
helps to reduce the required horsepower of a propylene refrigeration compressor. Propylene production, however, drops significantly when operating with maxi-mum ethane cracking. As can be seen in Figure 1, the propylene yield reduces from 14 wt% to 2 wt% when shifting from 100% naphtha to 100% ethane feed.
Ethane cracking also has a significant impact on other, heavier prod-ucts. As shown in Figure 2, with ethane cracking, 1,3 butadiene (13BUTD), butenes, C5-200, BTX and fuel oil production all are reduced. Due to a very low production of C3+ material, in most of the cases it is not economical to separate the C3 and heavier products. Since plants designed for liquids cracking have C3 and heavier product fractionation systems installed, many operators may decide to keep these systems in operation by a combination of external make-up and internal recycles. Quench system opera-tion also requires careful attention with possible modifications.
Figure 3 shows the shift in hydrogen and methane produc-tion with ethane cracking. Hydrogen production increases from 1 wt% to 4 wt% and methane production drops from 13 wt% to 5 wt%. Higher hydrogen, lower methane and higher C2s will lower the molecular weight of cracked gas, impacting the performance of cracked gas compressors and other equipment capabilities. A higher percentage of hydrogen
also results in a high pressure drop in systems handling the separation of hydrogen and methane from other components.
Cracking furnace and utilitiesconsiderationsEthane cracking results in higher ethylene yields than do other feeds; therefore, a similar ethylene capacity can be achieved with a lower through-put of ethane feed. If a coil designed for liquids cracking is employed, run length and operational flexibility will be reduced unless other adjust-ments are made. The number of furnaces that can be econom-ically shifted from liquids to ethane cracking will depend on the design of the downstream recovery section and its capa-bility to handle lighter cracked gas. If operators want to crack more ethane, additional capital spending will be required in the recovery section to achieve more flexibility. A cracking
furnace designed for liquids cracking may have some limitations when cracking ethane; for example: • Reduced run length• Higher than normal steam-to-HC ratio required to minimise coking rate• Capacity limitations in radiant coil• Burners and induced draft fan limitations• Possible metallurgy issues• Convection section high pressure drop due to a higher volumetric flow rate• Primary quench exchanger coking,
resulting in a higher outlet temperature of cracked gas.
Ethylene producers with the objective of minimal capital spending have an option of using a liquids cracking furnace (see Figure 4) on ethane crack-ing with reduced operational flexibility. The existing furnace liquids feed header and feed control valves, however, will need to be replaced to work effectively with ethane feed. For greater operational flexibil-ity and longer run lengths, an extensive furnace revamp may be required. Ethylene produc-ers can opt for a feed flexible radiant coil such as Technip’s Ultra Selective Conversion (USC) W coil furnace, which will more effectively handle the shift from liquids cracking to ethane cracking. In cases where higher capacity and ethane only cracking are the objectives, Technip’s USC M coil is a better choice. Revamp of the convection section, burners, induced draft fan and primary
quench exchangers may also be required to achieve the required capacity and run length while providing greater operational flexibility.
Another important considera-tion when converting a liquids furnace for ethane cracking is the addition of the secondary quench exchangers down-stream of the primary quench exchangers. Typically, gas crackers are designed to have a lower cracked effluent tempera-ture to the quench section as compared to liquids crackers. In liquids crackers, the temper-ature is kept high to avoid liquid condensation and possi-ble quench exchanger and transfer line fouling. For refer-ence, for gas cracking, the cracked effluent temperature to quench section is ~350°F (177°C) compared to ~700°F (370°C) for liquids crackers. A secondary quench exchanger can therefore be added to produce additional high-pres-sure steam during ethane cracking. Bypass provision can be included to minimise the duty on the secondary quench exchanger during naphtha or gas oil cracking.
With a secondary quench exchanger in service, the combined furnace effluent temperature from liquids and gas cracking furnaces will be reduced. This will lower the bottom temperature of the quench oil tower and may reduce the amount of usable waste heat. Typically, in liquids crackers, quench oil is used for dilution steam generation. A lower cracked gas effluent temperature will have a signifi-cant impact on dilution steam production. Additional medium-pressure steam import
Quench system considerationsFor ethylene producers accus-tomed to cracking naphtha or heavier feed, one of the most challenging systems to main-tain efficient operation when introducing ethane feed is the quench oil system. As ethylene producers consider cracking various feedstocks, they attribute a certain amount of fuel oil make to both naphtha and gas feeds. The net fuel oil make from naphtha cracking is typically within 4 to 6% of the total furnace effluent. Ethane cracking, at the other extreme of the spectrum, makes up less than 0.2%. Therefore, one of the consequences of transitioning to a lighter feed slate is a significant reduction in fuel oil make from the furnaces, thus the need for additional external fuel oil or flux oil to ensure the quench oil circulation system is operational.
The quench oil system thrives on having an abun-dance of heavy and middle distillate oils. The shortage of these components, which is common when cracking prima-rily ethane, leads to inefficiencies and potential plant reliability problems. With ethane cracking, light and middle boiling compo-nents vaporise in the quench oil tower, leaving behind the heavy components. A lower yield of heavy components results in an increase in the residence time in the system, causing the formation of poly-nuclear aromatics (PNA) and tar due to agglomeration. Viscosity therefore increases, causing a drop in the heat transfer coefficient in the heat exchangers. With time, the concentration of PNA and tar
may be required to cover for reduced dilution steam.
Typically, liquids crackers have hydrogen recovery and purification sections. As was explained above, ethane crack-ing generates additional hydrogen, which can cause limitations in the hydrogen recovery area. There is a possi-bility of increased hydrogen content in the residue gas, but cracking furnace burners must be designed to operate within a certain percentage range of hydrogen in residue gas. The fuel gas balance must be checked with consideration of natural gas import to maintain a reasonable residue gas composition. Modifications in the hydrogen recovery area may be required to maximise hydrogen recovery and limit hydrogen content in the resi-due gas. In the extreme case, furnace burner modifications or replacement may be required. With no modifica-tions or replacement, existing furnace burners can potentially set the limit on how much ethane can be cracked in a liquids cracker.
Residue gas will have higher hydrogen and lower methane contents because of a high ethylene yield and a significant drop in methane yields with ethane cracking. Typically, liquids crackers have a hydro-gen recovery system installed as part of the process scheme. There will be a reduction in the residue gas make if hydrogen recovery is still a requirement during majority ethane crack-ing. An increased import of natural gas therefore may be required while transitioning from liquids to majority ethane cracking.
increases due to polymerisa-tion, resulting in a further increase in viscosity. A stage is reached when it is no longer feasible to maintain quench oil circulation at the required temperature and the system has to be shut down for cleaning.
To maintain efficient and reli-able operation, producers shifting to majority ethane feedstocks need to properly evaluate the quench system and may need to consider importing external oil (flux oil) to maintain adequate quench oil quality. Proper evaluation and selection of flux oil is required to ensure compatibil-ity with the fuel oil produced in the system. Incompatible flux oil can lead to precipita-tion of asphaltenes in the system. Flux oil characterisa-tion should be done to confirm compatibility with the system. The following tests are recommended:• Aromatic carbon• Unsubstituted aromatic carbon• Substituted aromatic carbon• Average chain length.
Most favourable flux oils are heavy catalytic naphtha (HCN) and heavy cycle oil (HCO). These are produced in refiner-ies. However, these are not readily available, as refiners prefer to produce diesel. The second best choice for flux oil is light cycle oil (LCO) and it is generally available from refineries.
There must be extensive changes in the quench system in case of limitations in flux oil availability. Depending on how much ethane is cracked, the quench oil tower may need to be bypassed, with possible
modifications to the quench water tower to provide addi-tional heat transfer capability. Additional equipment may need to be provided to handle tar removal. A benefit of bypassing the quench oil tower is reduced pressure drop between furnaces and the cracked gas compressor. A higher cracked gas compressor suction pressure can therefore be employed.
Existing quench oil and quench water towers may be limiting if capacity increase is another objective in addition to feedstock flexibility. Technip’s Ripple trays have been used successfully for numerous revamp applications for quench oil and quench water towers (see Figure 5).
Cracked gas systemCracked gas compressor performance needs thorough evaluation. With a higher ethane feed diet, molecular weight reduces from ~30 to ~19 with an increase in volumetric
flow rate. One of the important parameters to consider when shifting from liquids cracking to majority ethane cracking is polytropic head. Polytropic head is inversely proportional to molecular weight. As Equation 1 shows, a lower molecular weight gas will require a higher polytropic head for the same compression ratio:
From the above equations, it is clear that, for the same mass flow rate and compression ratio, a lighter cracked gas from majority ethane cracking will require a higher shaft horse-power as compared to heavier cracked gas. However, as Figure 1 shows, majority ethane cracking results in a higher
ethylene yield and therefore a lower mass flow of cracked gas will be required to produce the same quantity of ethylene as compared to liquids cracking. An important variable to over-come potentially higher horsepower requirements is to increase first-stage suction pres-sure. As Equation 1 indicates, increased compressor suction pressure will reduce the compression ratio and poly-tropic head requirements. On the negative side, a higher first-stage suction pressure results in a higher radiant coil outlet pressure. Since a higher pres-sure at the radiant coil outlet degrades the ethylene yield, increasing the first-stage suction pressure tends to lower ethyl-ene make. On the positive side, the liquids cracking ethylene yield is a stronger function of the coil outlet pressure as compared to gas cracking. With a small sacrifice in ethylene yields, gas cracking furnaces can therefore be operated at a higher coil outlet pressure. There will be a reduced impact on coil outlet pressure in case the quench oil tower is bypassed.
A higher coil outlet pressure also lowers the volumetric flow rate, which is helpful in avoid-ing potential casing or suction nozzle limitations. If feed flexi-bility is the only target, with no increase in ethylene produc-tion, it is sometimes possible that a cracked gas compressor driver designed for liquids cracking furnace effluents can work for cases with majority ethane cracking effluents. In some cases, especially where both feed flexibility and an increase in ethylene production are targets, it may not be possi-
ble to operate by just increasing the first-stage suction pressure. Possible solutions for this situ-ation are:• Adding a booster stage• Adding a parallel stage with a dedicated driver • Adding a new driver for one of the stages to avoid limita-tions on the existing driver• Adding wheels on stages requiring higher polytropic head.
In case the quench oil tower is bypassed to address quench area issues, there will be a reduced pressure drop between furnaces and the cracked gas compressor. For the same coil outlet pressure, it will therefore be possible to increase the compressor suction pressure.
Another important considera-tion for the cracked gas compressor is to confirm that a sufficient operating margin is kept between operating point and surge limit. This operating margin tends to be narrowed due to a higher operating speed and polytropic head required for lighter cracked gas opera-tion. Again, a combination of increased suction pressure and a lower mass flow required for lighter cracked gas operation can help to stay away from the surge limit. Compressor performance curves need to be studied carefully by use of a process simulator to confirm compressor performance for lighter cracked gas effluent.
Chilling train andexpander/recompressorAs Figure 3 shows, switching from naphtha cracking to majority ethane cracking results in an increase in hydrogen production from ~1 wt% to ~4
wt% and a lower methane production from ~13 wt% to ~5 wt%. Higher hydrogen production may result in a higher volumetric flow to the chilling train (depending on composition) and an increased pressure drop. Replacement of some piping and exchangers in the hydro-gen system may help to lower the pressure drop. In the case of no modifications, this addi-tional pressure drop has to be compensated by additional discharge pressure from the cracked gas compressor. This is not always possible due to potential limitations in cracked gas compressors designed for operating with heavier cracked gas.
Lower methane and C3+ production may reduce the demethaniser warmer level feeds, but can add a burden to colder level feeds. If hydrogen is being recovered in the origi-nal liquids cracking scheme, with no slip stream to the expander, there will be an increase in the hydrogen stream flow to the hydrogen recovery core exchanger, with potential hydraulic and surface area limitations. There will also be reduced flow to the expander from the demethaniser over-head due to a lower methane content in the cracked gas. A reduced flow to the expander will lower the expander effi-ciency. The combined effect of lower efficiency and a reduced flow is a reduction in refrigera-tion available to the chilling train from the residue gas expander. The expander feed system can potentially be modi-fied to add load and recover more refrigeration (see Figure 7). Expander internals or the
expander itself may need to be replaced to make it suitable for the new set of operating flow rate and composition. Additional expander stages are sometimes needed to process the lighter molecular weight gas.
Ethylene losses from the demethaniser overhead will also increase due to the lower methane-to-hydrogen ratio. Modifications can be made to the overhead system to reduce ethylene losses. These modifi-cations should be flexible enough to allow reverting back to naphtha or gas oil cracking, if required.
Another important considera-tion is the percentage of hydrogen in the residue gas. An ethane cracker results in
lower molecular weight residue gas due to the increase in hydrogen content. Since resi-due gas is used as a regeneration gas for dryers, there is the potential for an increased pressure drop in the regeneration gas system. It is, however, possible to reduce the regeneration gas flow and therefore reduce the pressure drop due to the improved specific heat of lighter regener-ation gas.
Ethylene splitter, C2
hydrogenation and deethaniser systemsShifting from naphtha or gas oil cracking to majority ethane cracking results in a higher ethane per cent to ethylene splitter feed, that is a lower
purity feed to the ethylene splitter. This will require a higher reflux rate for the same production capacity. If addi-tional capacity is another objective, the required tower internal traffic will be further increased. The existing ethylene tower trays would need to be checked for the extra capacity. Possible solutions to overcome limitations in tray hydraulics are the replacement of existing trays with high-capacity trays, or the addition of another tower in series. The deethaniser and C2 hydrogenation reactor systems also need to be evalu-ated for hydraulic constraints and any possible negative impact due to the potential higher flow rates.
Ethylene and propylenerefrigeration compressorsDepending on composition, available credits and mass flow rate, a higher ethylene refrigera-tion duty may be required for the chilling train during maxi-mum ethane cracking. An ethylene refrigeration compres-sor revamp might be required in this case. Scope of the revamp can be extensive if the objective is to both increase plant capac-ity and add feed flexibility. Overall, the ethylene refrigera-tion compressor may need a higher hydraulic capacity and horsepower to enable increased ethane cracking.
Propylene refrigeration load will increase with two major users during maximum ethane cracking. First, as was explained above, the ethylene splitter will require a comparatively higher reflux rate for majority ethane cracking as compared to naph-tha or gas oil cracking. For non-heat pumped systems, a
higher reflux rate will require a higher duty on the ethylene splitter condenser, which is the largest user of the coldest propylene refrigerant. For low-pressure heat pump systems integrated with the ethylene compressor, there will be an increased load on the heat pumped ethylene compressor. Second, the ethylene refrigera-tion condenser duty will increase due to a higher demand for ethylene refrigera-tion. The ethylene refrigeration condenser is normally the second largest user of the cold-est propylene refrigerant.
On the positive side, there will be two important refriger-ation credits to propylene refrigeration with increased ethane cracking. These credits can be used to reduce the load on the propylene refrigeration compressor. The first credit is the increased recycle ethane production from ~5 wt% to ~35 wt%. Recycle ethane can potentially be used for the coldest level refrigeration credit. A higher recycle ethane refrigeration credit can help to lower significantly the propyl-ene refrigeration compressor horsepower. An additional credit is the fresh liquid ethane feed, which can be flashed to furnace feed pressure to achieve additional refrigera-tion credit at the coldest level. Ethane feed, however, may need to be dried before flash-ing to a lower pressure and temperature to avoid the formation of hydrates. Additional investment in dryers and regeneration systems may need to be considered. Higher duties on the ethylene splitter and ethyl-
ene refrigeration condensers can be balanced by these addi-tional credits during periods of high ethane cracking. The propylene refrigeration compressor must be checked on a case-by-case basis before shifting to higher ethane cracking.
ConclusionWhile current economics for energy and feedstock supply favour ethane feed cracking, plant owners must rigorously examine operational and capac-ity constraints before transitioning to maximum ethane feeds. Detailed feasibil-ity studies need to be undertaken to evaluate the impact of maximising ethane cracking on plants originally designed for naphtha or gas oil cracking. A well-planned feasi-bility study can help improve production margins. Each ethylene plant is unique and needs dedicated investigation for suitability in feedstock transition.
Figure 8 Grassrootsethyleneplant
References1 American Chemistry Council, Shale Gas and New Petrochemicals Investment: Benefits for the Economy, Jobs, and US Manufacturing, March2011.2 International Energy Agency, World Energy Outlook 2011.3 BernardA,PickettTM,ManekBM,FryeD K, Flux oil stream import to quenchsystem risk and impacts, paper number33e,AIChE2011SpringNationalMeeting,Chicago,Illinois,14March2011.
Muhammad Imran is a Lead ProcessEngineer for Technip Stone & WebsterProcess Technology, Houston, Texas.He has 16 years of experience withpetrochemical,refineryandgasprocessingprojects,withthepastsevenyearsfocusedonethylene.Heholdsabachelor’sdegreeinchemicalengineeringfromUniversityofEngineeringandTechnologyLahoreandamaster’sinchemicalengineeringfromtheUniversityofHouston.
