1.0 introduction downstream processing refers to the processing of the product from wells,...
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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 – 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 CHCH
• product of combustion rather than natural
• ring – naphthenes or cycloalkanes (CnH2n)
• aromatics (arenas) - compounds that have at least one benzene ring as part of their chemical structure
nonhydrocarbons influence product qualityS – 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
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
Series of unit operations where process variables are specified:
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
1.2.1 Process Flow Diagram
Mixer
100 moles/h C2H6
T=320oCP=1.4 bar
2000 moles/h air0.21 O2 0.79 N2
T=320oCP=1.4 bar
0.0476 C2H6
0.2 O2
0.752 N2
2100 moles/h
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
1.2.2 Degree of Freedom (DOF) Analysis
DOF = independent variables – independent equationsDOF = 0 problem completely specifiedDOF < 0 over specified, some of equations are either redundant or inconsistentDOF > 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
1.2.3 Material Balancesdmi/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 Ci
x
where Ci - is concentration or partial pressure of species “i”
k – is rate constant = Ae-Ea/RTe.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
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/continuousdetermines form of mass balance equationA B + Cstart with mass balance on component “A”: dmA/dt = mA,in – mAiout ri
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 dmA/dt = rA (in reactors usually use mole
balance so ni)
ii. Continuous Flow Reactora) CSTR
nA0 = nAi,in – nAiout rA
b. PFR – fluid flows as a “plug”
nA
dnA
0= nA|V – (nA +dnA)|V+dV rA
0 = dnA/dV rA
3. Mixing Pattern 4. Feed Composition5. 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)
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 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…
• 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
d.) Phase EquilibriumMost chem. processes material is transferred from one phase to anotherSingle component phase diagram:
P
T
Multi-component phase diagramMixture of natural gas
0
2000
4000
6000
8000
10000
-160 -110 -60 -10 40
Temperature (C)
Pre
ssur
e (k
Pa)