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1.0 Introduction

1.0 Introduction. downstream processing refers to the processing of the product from wells, compressor stations and oil batteries purpose is to refine the crude oil or gas to a saleable commodity  refineries, upgraders, gas processing plants and petrochemical facilities

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1.0 Introduction

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  1. 1.0 Introduction • downstream processing refers to the processing of the product from wells, compressor stations and oil batteries • purpose is to refine the crude oil or gas to a saleable commodity  refineries, upgraders, gas processing plants and petrochemical facilities • in this class we will focus on gas processing, refineries/upgraders • review of chemistry of petroleum, crude oil and gas

  2. 1.1 Hydrocarbons • petroleum (crude or gas) is made up of various types of HC: • alkanes/paraffins – CnH2n+2 saturated • C1-C4 are gases at STP, C5-C17 liquids, C18+ wax solids (produce anomalous evaporation, dispersion, emulsification, and flow behaviours) • can have n-alkanes (straight chains of HC) or iso-alkanes (branched) • olefins – double bonded HC ethylene CH2=CH2 • unsaturated, more chem. reactive than sats, not usually found in raw gas or crude  product of processing • acetylenes – triple bond CHCH • product of combustion rather than natural

  3. ring – naphthenes or cycloalkanes (CnH2n) • aromatics (arenas)- compounds that have at least one benzene ring as part of their chemical structure • nonhydrocarbons • influence product quality • S – 0.65-6% by wt  free S, H2S (in gas 50%), mercaptans (C2H2SH), thiols ((C2H5)2S), thoiophenes, low API contain more • N – 0.1-2%, reduces heat value, pyridines, quinolenes, indoles, largely unid’ed in crude • oxygen – free O2/CO2, alcohols, esters, phenols, fatty acids, decompose to naphthenic acids on distill • CO2 – common gases/cond, corrosive probs (carbonic acid), dehyd important to prevent corrosion • Vd, Ni, Cu, Zn, Fe

  4. 1.2 Introduction Process Flow • All processing plants are made up of a series of unit operations • solids/liquids/gases must be moved • energy must be transferred • drying, size reduction, distillation, reactions • brief definitions • basis of process flow calculation – flow rate or quantity that indication of size of process (e.g. flow rate of feed or product) • unit operations or system and streams in process flow calcs

  5. Splitter Mixer Mixer Reactor Separator collection of unit ops unit operation

  6. 100 moles/h C2H6 T=320oC P=1.4 bar 0.0476 C2H6 0.2 O2 0.752 N2 2100 moles/h 2000 moles/h air 0.21 O2 0.79 N2 T=320oC P=1.4 bar 1.2.1 Process Flow Diagram Series of unit operations where process variables are specified: Mixer Specifications – stream specs and system specs (conversions etc…) Mass fractions – xA = mass of A/total mass of system Mole fractions – yA = moles of A/total moles in system

  7. e.g. A gas mixture has following composition by mass: • N2 = 0.03 • CH4 = 0.85 • C2H6 = 0.08 • C3H8 = 0.03 • CO2 = 0.01 • Calculate molar composition

  8. 1.2.2 Degree of Freedom (DOF) Analysis DOF = independent variables – independent equations DOF = 0 problem completely specified DOF < 0 over specified, some of equations are either redundant or inconsistent DOF > 0 underspecified, need some more equations Equation sources: • mass/material balances - for nonreactive process no more than ni material balance equations may be written where “i” is number of species • energy balance • process specifications – how several process variables are related (e.g. percent recovery or degree of conversion) • Physical properties and laws – equations of state or other equilibrium relations • Physical constraints – for example mass fractions must add up to 1 • Stoichiometric reactions

  9. 1.2.3 Material Balances dmi/dt = mi,in – mAiout ri rate accumulation of “i” = rate in of “i” – rate out of “i”  rate of consumption of “i” where ri – rate of consumption or production of “i” -form of ri depends on reaction, in general: ri = k Πi=0n Cix where Ci - is concentration or partial pressure of species “i” k – is rate constant = Ae-Ea/RT e.g. global reaction is as follows: CH4 + 2O2 CO2 + 2H2O (irreversible reaction at 800oC and 1 atm) So reaction rate may be = rCH4 = k PCH4PO22 H2S  H2 + ½ S2 So reaction rate may be = rH2S = kf PH2S - krPH2PS21/2

  10. a.) Application to reactors • Design variables: • T and P – optimal to max conversion and minimize by-products • V – determines time for reaction(s), also important from cost, weight, space constraints • 1. Residence time – time component stays in reactor = volume of vessel/flow rate

  11. Type reactor – batch, semi-batch, or flow/continuous • determines form of mass balance equation • A  B + C • start with mass balance on component “A”: • dmA/dt = mA,in – mAiout ri • Batch – reactor charged with reactants, allowed to react, then products/unreacted material withdrawn, no flow in or out so t= 0 to reaction time • So dmA/dt =  rA (in reactors usually use mole balance so ni) • Continuous Flow Reactor • CSTR 0 = nAi,in – nAiout rA nA

  12. PFR – fluid flows as a “plug” 0= nA|V – (nA +dnA)|V+dV rA 0 = dnA/dV  rA nA dnA • Mixing Pattern • Feed Composition • Catalyst – speeds up rate of reaction  can be liquid (e.g. acid/base), solid (metal based), biological (enzymes) • not consumed in reaction • act by decreasing the energy required for reaction (Ea)

  13. E3 before catalyst Ea=E3-E1 E2 energy after catalyst Ea=E2-E1 E1 reactants products reaction extent

  14. b.) Chemical Equilibrium • when no changes can occur without outside stimulus – thermodynamic eqm (absence of change thermo properties or tendency to change), chemical equilibrium • chemical kinetics tell us the rate of reaction while chemical equilibrium tells us if reaction will occur at specified T and P and the final equilibrium concentrations (much the same way that thermodynamics tells us direction and quality of energy while heat transfer refers to the rate of energy transfer) • irreversible reaction (reactants  products) where equilibrium composition refers to complete consumption of limiting reactant OR • reversible reactions (reactants  products) where the direction of reaction can shift according to concentration of reactants/products, T and/or P • conversion = (species input – species output)/species input

  15. Thermophysical properties • use correlations (Equations of State, excess Gibbs) to determine behaviour of gases/liquids/solids • P, T, V, and/or n determine the “state” of substance • ideal gas law and more complex EOS (PR, RK, VdW, compressibility factor), Wilson, UNIQUAC

  16. c.) combustion reactions • rapid reaction of fuel with oxygen •  e.g. 1 CH4 + 2O22 H2O + 1CO2 • 1 C8H17S+ 35/2O2 17/2 H2O + 8 CO2 + 1 SO2 •  since O2 source is usually air (21% O2 and 79% N2) have to account for N2 content • if need 1 mole O2  1/0.21  need 4.76 moles air so for CH4 example need 9.5 moles of air (stoichiometric air) • as impurities increase so does O2 demand, also H2O content in fuel or air increases then more O2 must be added (as temperature increases H2O content of air) • stoichiometric air – amount of air required to convert all of fuel to CO2, H2O, SO2  but to account for impurities in air and water often use excess air • Usually complete combustion is not possible: • C8H17S+ nO2 H2O + CO2 + SO2 + CO + SO etc…

  17. the value of fossil fuel as a heating medium is determined by heating value of gas or amount of heat released during combustion • HV – amount of heat released during complete combustion w/ stoichiometric air • HV=ΣxiHi • HHV - amount of heat released during complete combustion w/ stoichiometric air if include latent heat of vaporization of H2O or if H2O in stream is condensed • Fuel + O2 CO2(g)+H2O(l) • LHV - amount of heat released during complete combustion w/ stoichiometric air if H2O in steam is NOT condensed • Fuel + O2  CO2(g)+H2O(g) • HHV=LHV+nH2O ΔHH2Ovap(Tref) • usually reference temperature is 15C which why latent heat not included in LHV

  18. d.) Phase Equilibrium Most chem. processes material is transferred from one phase to another Single component phase diagram: P T

  19. Multi-component phase diagram Mixture of natural gas

  20. Phase Diagram with multiple liquid phases v v+L1 v+L2 T L1 L2 L1 and L2 0 x,y 1

  21. T-xy methanol phase diagram for water-methanol mixture

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