Hydroprocessing : Hydrocracking & Hydrotreating

Transcription

1 Purpose Hydroprocessing : Hydrocracking & Hydrotreating Hydrotreating [1,2] Remove hetero atoms and saturate carbon carbon bonds Sulfur, nitrogen, oxygen, and metals removed Olefinic and aromatic bonds saturated Reduce average molecular weight & produce higher yields of fuel products Minimal cracking Minimal conversion 10% to 20% typical Products suitable for further processing or final blending Reforming, catalytic cracking, hydrocracking Hydrocracking Severe form of hydroprocessing Break carbon carbon bonds Drastic reduction of molecular weight 50%+ conversion Products more appropriate for diesel than gasoline Hydroprocessing Trends [1 6] Hydrogen is ubiquitous in refinery and expected to increase Produces higher yields and upgrade the quality of fuels The typical refinery runs at a hydrogen deficit As hydroprocessing becomes more prevalent, this deficit will increase As hydroprocessing progresses in severity, the hydrogen demands increase dramatically Driven by several factors Heavier and higher sulfur crudes Reduction in demand for heavy fuel oil Increased use of hydrodesulfurization for low sulfur fuels More complete protection of FCCU catalysts Demand for high quality coke Increased production of diesel Sources of Hydrogen [2 4,6 15] Catalytic Reforming from steam reforming of NG and Naptha reforming methods and also from the catalytic reformer one at Al 2 O 3. Cryogenic Pressure swing adsorption Membrane separation. Steam Methane Reforming Most common method of manufacturing hydrogen 90 to 95 vol% typical purity Synthesis Gas Gasification of heavy feed Hydrogen recovery pressure swing adsorption or membrane separation More expensive than steam reforming but can use low quality by product Steam Methane Reforming. Endotherm catalytic reaction C. Steam Methane Shift conversion. Exothermic fixed bed catalytic reaction 650 C. 1/14

2 Gas Purification:Absorption of CO 2 in amine or hot K 2 CO3 solution. Methanation. Trace amounts of carbon monoxide & carbondioxide removed. Exothermic fixed bed catalytic reactions, C. Hydrogen (H2) is identified to be an ideal energy carrier with high efficiency, since it possesses LHV of 121 MJ/kg compared to any other known fuels and burns cleanly, without generating any environmental pollutants. H 2 production is an attractive subject of current interest for fuel cell applications which are considered to have the potential to provide a clean energy source for automobile as an alternative to gasoline or diesel engines. H 2 is also one of most important chemicals and is widely used for ammonia production, oil refineries, and methanol production etc. In view of growing environmental concerns, such as global warming and the depletion of fossil fuel, major efforts are being dedicated to develop the utilization of renewable energy sources. Bio oil produced from biomass has been proposed as an alternative to produce hydrogen; because this renewable rich energy resource does not contribute to a net increase in atmospheric CO2. Bio oil is a dark brown organic liquid. Bio oil exhibits a complex composition with more than 200 different compounds including acids, alcohols, aldehydes, ketones as well as lignin derived oligomers, which are emulsified with water hence difficult to reform. In general, the steam reforming of bio oil is simplified in terms of the oxygenated organic compounds (CnHmOk) by the following reactions: The above reaction is followed by the water gas shift reaction: The overall process can be represented as followes: Steam reforming of the main component in bio oil such as acetic acid (12 14 by wt %) can offer the information for active catalyst preparation and the knowledge about the potential catalytic behaviour of bio oil in steam reforming. Reaction involved in steam reforming of acetic acid Reforming of acetic acid for H 2 production can be summarized as followes: First, Steam reforming of acetic acid reaction has discussed as stated below: Followed by water gas shift reaction The overall steam reforming reaction of acetic acid is The maximum stoichiometric yield of H2 for acetic acid reforming is 2 mole to mole of carbon. However some undesired reactions take place inside the reactor. Hence there is large possibilities of decomposition of acetic acid to lower molecular weight oxygenates, lighter hydrocarbons (CH4, C2H4) and coke. Some amount of coke can also be formed via Boudouard reaction. 2/14

3 The desired gaseous product i.e. hydrogen is lost through methanation reaction and reverse water gas shift reaction. Small amount of acetone may be formed by ketonization reaction of acetic acid. The terms % H2 yield, % acetic acid (acetic acid) conversion and % selectivity of gaseous product distribution were used to describe the catalytic results in reforming of the oxygenate. The definitions are given below. Where RR is H2/CO2 reforming ratio and RR=4/2. 3/14

4 TRANSPORTATION FUELS Fig:6.1 Transportation Fuel Evolution [1 2,12 16] Exhaust from automobiles major pollutant Strict emission norms all over the world Corresponding changes in fuel quality specifications Indian fuel quality standards following Euro norms Table:6.1 [1 6] WORLD CRUDE OIL QUALITY C Proprties of Crude Oil S in crude oil(wt%) API Gravity Of Crude Oil Residue in Crude Oil(vol%) Metals in crude oil Residue S in crude oil(wt%) Specification BIS 2000 BS II Table:6.2 Key Specifications of MS [1 6] Euro III Equ Euro IV Equ Regular Premium Regular Premium Sulphur,ppmw(max) RON,Min MON,Min No spec No spec AKI,Min No spec No spec No spec No spec Benzene,Vol%(max) Aromatics,Vol% (max) No spec No spec Olefins,Vol%(max) No spec No spec Table:6.3 DIESEL SPECIFICATION IN INDIA [16 19] Parameter BIS 2000 Bharat II Bharat III EURO IV Sulfur,ppm Cetane Number(*) Density,kg/m % Recovery, o C /14

5 Year of implementation PAH,wt% Metros April 1,2005 April 1,2010 Mega Cities April 1,2003 April 1,2005 April 1,2010 Rest of india April 1,2000 April 1,2005 April 1,2010 Fig.6.2 Future Production of Clean MS [1 6,12 19] 5/14

6 Fig.6.3 Steam Methane Reforming [1 2,11 18] 6/14

7 Catalyst Catalyst is a material that can influence a chemical reaction without changing itself chemicallly It makes reaction faster by reducing activation energywithout affecting equillibrium Can selectively favor a certain over other Key Factors Activity(reaction temperature) Selectivity(desired product) Stability(deactivation rate) Hardness/crushing strength Catalyst Charateristics Fig.6.4 Hydrocracking Catalyst [1 6,15 25] Important characteristics of catalysts which influence its performance are : Partical size Pore volume Size distributor Pore diameter Surface area Shape of catalyst particles Catalyst Catalyst used in hydroprocessing 7/14

8 Typical composition CoO 2 6wt% MoO wt% Al2O3 Factor helping catalyst activity Higher H2 partial pressure Lower liquid hourly space velocity(lhsv) Lower H2 recycle gas Catalyst Loading Fig.6.5 Catalyst [1 5,15 25] Special technique is used to load catalyst into reactor to maximise bulk density.this is callled dense loading Dense loading helps in Liquid distribution in the bed Minimising bed slumping Achieving longer cycle length Maintaining lower operating temperture for same hetero atom removal and saturation Reducing deactivation rate Catalyst Deactivation [1 6,12 16,26 30] Catalyst activity gradually reduce with time If reacting molecules cannot reach all catalyst active sites,rate of reaction decreases.it is called catalyst deactivation. Catalyst Deactivation study necessary at higher reactor temperature for same S,N,O removal/saturation Two types of deactivation temporary and permanent Major cause of temporary deactivation is coke deposition in catalyst due to undesireable side reaction Coke can cover active catalyst sites and block the entrance of pores leading to active sites During plant upsets,coke deposition acclerates When temperory deactivation reaches the limits,catalyst regeneration is necessary. Permanent deactivation is caused due to metal accumulation in catalyst Metals like Pb,Fe,As,P,Na,,Ca,Mg and orgonomettalic compounds are poisonous to catalyst Process parameter which significantly influence catalyst deactivation Reactor temperature Feed boiling range/ API Feed Rate H 2 partial pressure 8/14

9 Recycle gas rate Feed contamination Catalyst Regenration [1 6,20 28] Hydroprocessing catalyst cost is a significant fraction of unit cost processing To reduce operating cost it is better to regenerate Strict environmental norms for disposing spent catalyst also encourage regeneration of catalyst and reuse Normally two regeneration are allowed and its impart total life of 3 5 years to catalyst Catalyst regeneration involves removal of deposited coke in oxidised atmoshere using stem/airs. Diesel Hydrotreating [1 2,24 30] Feed Straight run:srgo(hgo and LGO),VLGO,SRVD,SO Cracked stock VBGO,LCO Typical Operating condition : Temperature C H2 pressure bar LHSV h 1 H2:Oil Ratio Nm 3 /m 3 Catalyst Ni/Mo/Al2O3 CoMoAl2O3 Hydrocracking [1 6,20 28] Feed :VGO,RESID,DISTILLATES Typical operating condition Conversion %(Normally low for once thru and high for recycle configuration) Temperature C H2 pressure bar LHSV h 1 H2 oil ratio Nm 3 Catalyst Ni/Mo or Ni/W on Amorphous Si/O2/Al2O3 or zeolyte Table:6.4 Hydroprocessing Reaction Effects Of Operating Condition 9/14

10 Effect of Process Variables on Cetane Index [17 19] Fig.6.6 Diesel Quality Issues [17 19] 10/14

11 Fig.6.7 Effect of Process Variables on Cetane Index Hydrocracking Feeds [17 19] 1. Hydrocracking does a better job of processing aromatic rings without coking than catalytic cracking. 2. Hydrogen used to hydrogenate polynuclear aromatics (PNAs) 3. Reduces frequency of aromatic condensation. 4. Hydrocracking more severe for resids high in resins, asphaltenes and heteroatom compounds. 5. Heteroatoms and metals prevalent in resins & asphaltenes poison hydroprocessing catalysts. 6. Feedstock selection in much more sophisticated than mare determination of CCR 7. Distribution of aromatic, naphthenic, and paraffinic structures important Hydrocracking Products [1 6,17 18] 1. Hydrocracking primary to make distillates 2. Intent is to minimize the production of heavy fuel oil 1. Lights ends are approximately 5% of the feed 2. Middle distillates<(kerosene,jet fuel,diesel,heating oil) still contain uncracked polynuclear aromatics 3. All liquid fraction are low in sulphur and olefins Hydrocracking Chemistry [1 8] Cracking Reaction 1. Saturated Paraffins cracked to form lower molecular weight olefins and paraffins 2. Side chain cracked off small ring aromatics(sra) and cycloparaffins(napthenes) 3. Side chained cracked off the resins and asphaltenes heaving thermally stable polynuclear aromatics(pnas) 4. But condensation(dehydrogenation) also occurs if not limited by hydrogenation Isomerization Reactions [1 4] Isomerisation provides branching of alkyl groups of paraffins and opening of naphtic rings Condensation Reaction Suppresed by hydrogen Hydrogen H 2 Consumption [7 19] Carbons bonds with hetero atoms and saturated Create lights ends Heavier distillates make more lights from breakings more complex molecules Sulphur converted to hydrogen sulfide Nitrogen converted to ammonia Oxygen converted to water Organic chlorides converted to hydrogen chloride Typical chemical hydrogen consumption 2 3 wt% on feed Hydrcracking catalyst [1 19] Hydrocracking catalyst generally a crystalline silica alumina base with rare earth metal deposited in lattice Acid function is provided by silica alumina base Chloride not required in catalyst formation Feed stock may be hydrotreated Acid function is provided by silica alumina base Chloride not required in catalyst formation 11/14

12 Catalyst suspectible to sulphur poisioning if hydrogen sulphide is present in large quantities Catalyst activity affected by ammonia if present in large quantities Something necessary to remove moisture to protect the crysatlline structure of catalyst Hydrocracking with a mettalic hydrogenation function is sensitive to matal contamination Hydrocracking catalyst Deactivaton & Regeneration [1 19] Catalyst deactive and coke does from even from hydrogen present Hydrocracker reqires peroidic regenration of fixed bed catalyst system.typical catalyst cycle length is 2 years Channeling caused by coke accumulation a moajor concern Can create hot spot that can leads to temperature runaway Continuous withdrawal of catalystfrom an expanded circulating bed for regeneration For use in hydrocracking whole crude or short resid Hydrocracking Effect of Process Variables [7 19,22 30] Crackability of feed desired yields of products determine operating severity.operating Severtiy Catalyst Space Velocity Total Pressure Hydrogen partial pressure Severe operation needed to significantly reduce molecular weight of feed & increase the hydrogen:carbon ratio in product Severity Mild operation for diesel or fuel oil from heavy gas oil Severe operation for kerosene or naphtha from light gas oil Temperature Temperature not used to increase severity Temperature adjusted to offset decline in catalyst activity Consider 343 C to 400 C as a description of mild operation and 400 C to 450 C to severe operation Hydrocracking process Single stage or two stage process Unit size Severity of operation Product desired Nature of feedstock Feed pretreating for conntaminant removal Single stage Hydrocracking [11 27] Simple hydrocracker single reactor combining modest desulphurization with hydrocracking of gas oil to distillates Hydrogen sulfide must be relatively low and not be a problem for these catalysts Desulphurisation catalyst in the top bed and sulphur intensitive hydrocracking catalyst in lower bed Oleffins saturation can be a problem in term of heat release Hydrogen quence Additional quence netween hydrocracking catalyst beds 12/14

13 Single stage Hydrocracking Fresh feed recycle feed,and hydrogen heated in furnace to reactor temperature approx 700&d F.Operating pressure1,200 psig or more.1000 scl/bblor more hydrogen for combined desulphurization and hydrocracking Product Separation Reactor product is flashed to recycle hydrogen as high a pressure as poosible Minimize recompression horsepower Gas from low pressure(50 to 75 psig) flash to gas plant liquid from flash fractionated overhead condition Straight run gasoline Naphtha siuitable for reforming Distillates either jet fuel/kerosene or diesel treating oil Bottom for recycle Some bottom may be pursed to fuel oil,which would reduce severity Two Stage Hydrocracking [1 2] Fig.6.8 Single Stage Hydrocracking with Recycle [1 2] Use separate reactor with desulphurisation and olefins saturation in 1st reactor and hydrocracking in 2nd reactor 1st reactor removes contaiminats and saturate aromatics It Can also do part of th hydrogenation conversion Effluent from 1st reactorsent to fractionator fractionator bottoms sent to 2nd stage hydrocracking reactor.may need a separate internal hydrogen sulphide removal 13/14

14 Fig.6.9 Two Stage Hydrocracking with Recycle & Separator System [1 2] 14/14