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

1.1 Hydrocarbons petroleum (crude or gas) is made up of various types of HC: alkanes/paraffins – C n H 2n+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 CH 2 =CH 2 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

ring – naphthenes or cycloalkanes (C n H 2n ) aromatics (arenas) - compounds that have at least one benzene ring as part of their chemical structure nonhydrocarbons  influence product quality  S – % by wt  free S, H 2 S (in gas 50%), mercaptans (C 2 H 2 SH), thiols ((C 2 H 5 ) 2 S), thoiophenes, low API contain more  N – 0.1-2%, reduces heat value, pyridines, quinolenes, indoles, largely unid’ed in crude  oxygen – free O 2 /CO 2, alcohols, esters, phenols, fatty acids, decompose to naphthenic acids on distill  CO 2 – common gases/cond, corrosive probs (carbonic acid), dehyd important to prevent corrosion  Vd, Ni, Cu, Zn, Fe

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

Mixer Reactor Splitter Separator Mixer unit operation collection of unit ops

Series of unit operations where process variables are specified: Specifications – stream specs and system specs (conversions etc…) Mass fractions – x A = mass of A/total mass of system Mole fractions – y A = moles of A/total moles in system Process Flow Diagram Mixer 100 moles/h C 2 H 6 T=320 o C P=1.4 bar 2000 moles/h air 0.21 O N 2 T=320 o C P=1.4 bar C 2 H O N moles/h

e.g. A gas mixture has following composition by mass: N 2 = 0.03 CH 4 = 0.85 C 2 H 6 = 0.08 C 3 H 8 = 0.03 CO 2 = 0.01 Calculate molar composition

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 n i 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

1.2.3 Material Balances dm i /dt = m i,in – m Aiout  r i rate accumulation of “i” = rate in of “i” – rate out of “i”  rate of consumption of “i” where r i – rate of consumption or production of “i” - form of r i depends on reaction, in general: r i = k Π i=0 n C i x where C i - is concentration or partial pressure of species “i” k – is rate constant = Ae -Ea/RT e.g. global reaction is as follows: CH 4 + 2O 2  CO 2 + 2H 2 O (irreversible reaction at 800 o C and 1 atm) So reaction rate may be = r CH4 = k P CH4 P O2 2 H 2 S  H 2 + ½ S 2 So reaction rate may be = r H2S = k f P H2S - k r P H2 P S2 1/2

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

2.Type reactor – batch, semi-batch, or flow/continuous determines form of mass balance equation A  B + C start with mass balance on component “A”: dm A /dt = m A,in – m Aiout  r i i.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 dm A /dt =  r A (in reactors usually use mole balance so n i ) ii.Continuous Flow Reactor a)CSTR nAnA 0 = n Ai,in – n Aiout  r A

b.PFR – fluid flows as a “plug” nAnA dn A 0= n A | V – (n A +dn A )| V+dV  r A 0 = dn A /dV  r A 3.Mixing Pattern 4.Feed Composition 5.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 (E a )

energy reaction extent E3E3 E2E2 E1E1 before catalyst E a =E 3 -E 1 after catalyst E a =E 2 -E 1 reactants products

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

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

c.) combustion reactions rapid reaction of fuel with oxygen e.g. 1 CH 4 + 2O 2  2 H 2 O + 1CO 2 1 C 8 H 17 S+ 35/2O 2  17/2 H 2 O + 8 CO SO 2 since O 2 source is usually air (21% O 2 and 79% N 2 ) have to account for N 2 content if need 1 mole O 2  1/0.21  need 4.76 moles air so for CH 4 example need 9.5 moles of air (stoichiometric air) as impurities increase so does O 2 demand, also H 2 O content in fuel or air increases then more O 2 must be added (as temperature increases H 2 O content of air) stoichiometric air – amount of air required to convert all of fuel to CO 2, H 2 O, SO 2  but to account for impurities in air and water often use excess air Usually complete combustion is not possible: C 8 H 17 S+ nO 2  H 2 O + CO 2 + SO 2 + CO + SO etc…

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=Σx i H i HHV - amount of heat released during complete combustion w/ stoichiometric air if include latent heat of vaporization of H 2 O or if H 2 O in stream is condensed Fuel + O 2  CO 2(g) +H 2 O (l) LHV - amount of heat released during complete combustion w/ stoichiometric air if H2O in steam is NOT condensed Fuel + O 2  CO 2(g) +H 2 O (g) HHV=LHV+n H2O ΔH H2O vap (Tref) usually reference temperature is 15C which why latent heat not included in LHV

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

Multi-component phase diagram Mixture of natural gas

Phase Diagram with multiple liquid phases L 1 and L 2 v v+L 2 v+L 1 L1L1 L2L2 T x,y0 1

T-xy methanol phase diagram for water-methanol mixture