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

 

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 – 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)

energy

reaction extent

E3

E2

E1

before catalyst Ea=E3-E1

after catalyst Ea=E2-E1

reactantsproducts

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)

Phase Diagram with multiple liquid phases

L1 and L2

v

v+L2

v+L1

L1 L2

T

x,y0 1

T-xy methanol phase diagram for water-methanol mixture

335

340

345

350

355

360

365

370

375

0 0.5 1

mole fraction of methanol

T (

K)

liquid molefraction

vapour molefraction