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ICP final > C3 - Steam cracking (ethylene focus) = pyrolysis > Flashcards

C3 - Steam cracking (ethylene focus) = pyrolysis Flashcards

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1
Q

What is the goal of steam cracking?

A

turn alkanes into reactive alkenes

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2
Q

name 3 exemples of feedstock for this process

A

naphta
ethane
steam (inerte)

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3
Q

Name the products

A

ethene
propene
H2
CH4
BTX
C4H6

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4
Q

With ethylene as feedstock, what products can you create

A

Polymerization (PE)
Oxidation (PU)
Halogenation (PVC)
Alkylation (PS)

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5
Q

Why is ethylene important?

A

Serves as building blocks for petrochemical industry
Low cost for ethane cracking –> shale gas

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6
Q

What are the steps to shale gas production?

A

Sour gas (acid gasses) –> gas sweetening –> dehydration –> hot gas liquid recovery –> fractionation (produce ethene)

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7
Q

Why is shale gas becoming more popular?

A

makes ethane cracking easier, cheaper, more ethylene produced > propylene

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8
Q

Simple formula of ethane cracking

A

C2H6 –> H2 + C2H4

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9
Q

What are the 3 steps to the free radical mechanism?

A
  1. initiation
  2. propagation
  3. termination
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10
Q

Name 3 mechanisms possible during propagation

A

H-abstraction
B-scision
addition reaction

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11
Q

What is termination?

A

H° + H° –> H2
H3C-C°H2 + H3C° –> H2C=CH2 + CH4

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12
Q

What is initiation

A

H3C-CH3 –> H3C° + H3C°

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13
Q

Is it endo or exo?

A

endo

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14
Q

Process at high or low T?

A

high T

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15
Q

Equilibrium indicates high or low P?

  • C2H6 –> H2 + C2H2
A

Low P, but not vacum for safety reason

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16
Q

kinetics indicates high or low P

A

High P, but side product at low P

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17
Q

What is the operating P, and why?

A

Atmospheric pressure as a compromise for both kinetics and equilibrium

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18
Q

Name the 8 process requirements

A
  1. Endothermic
  2. Low P equilibrium
  3. High P kinetics
  4. Separation due to a lot of products formed
  5. Acetylene removal
  6. Short t –> reactive intermediates
  7. Quench reactor = fast step
  8. Coke formation = regeneration unit
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19
Q

What is the solution for the many products needing seperation?

A

C2, C3 splitter –> need to isolate ethane

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20
Q

What is the solution to acetylene (ethyne) removal?

A

Present in low amounts –> selective hydrogenation (H2)
catalyst reduces the triple bond to keep ethene

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21
Q

What is the solution to reactive intermediates in the reactor

A

Low residence time (mili s)

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22
Q

What is the solution to reactive intermediates after reactor, we want to stop it

A

TLE : transfer line exchange = cools down/huge heat exchanger
or
Quench (direct water injection) = better for heavier fractions

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23
Q

What to do for coke?

A

Decoking every x month, only run at 80% capacity, many reactors in parallele
+ heavy = + coke
decoking

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24
Q

What is decoking

A

burning off the cpke with air
not too fast, not too high T

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25
What kind of reactor is used?
Tubular reactor
26
What is the formula to calculate the energy of the reactor
É = m*Cp*(T2-T1) = U*D*l*deltaTln É = M*H_delta
27
What is the mass flow formula
m*t = D^2 *l
28
What is the residance time = to?
t = H_delta*D/U
29
What is the solution to wanting to produce a ^T with short t
^U = radiation = main heat transport phenomenon lower D doesn't work bc of coking t = 0.1 s
30
Why high partial P of reactants
higher conversion, ^T
30
Higher conversion and higher reactivity with high or low T?
high
31
Is conversion = reaction rate
no
32
Why use steam dillution if it reduces the partial pressure :)
+ ensures heat input in feedstock + partially removes coke (C+H2O<-> CO +H2)
33
inconveniant of using steam dillution
- higher V - difficult to seperate form HC's
33
Why use steam in steam cracking? (x3)
1.steam = diluent mixed with incoming ethane 2. Plays with kinetics and thermo = heat transfer 3. steam balance: reactor has cte V, but high steam ration (reacts with coke on walls = prevention) --> P tot constant + dilluted feedstock with ^Steam:HC ration+ lower partial pressure of HC= less coke
34
Does steam contribute to the reaction?
no, its an inerte
35
What does havin Ptot = 1 atm do?
lowers C2H6 partial pressur = lowers reaction rate = less coke = favors products (reduces kinetics but better conversion)
36
What is fracking
hydraulic fracturing --> explosion / fracturing fluid
37
What is the first step
HC and steam is passed through furnace tubes heated = combustion of fuel = combustion gas = convection to preheat feedstock
38
What is the second step
HC --> pyrolysis (steam cracking)
39
What happens right after pyrolysis
quenche = stop rxn of byproducts = preserve composition
40
From table 4.2 what can you say on product distribution
it depends on T, P, t, steam: ethane ratio
41
Why is naphta no general product
it is not a single compund, its composition varies (C3-C15), the % of alkane, aromatics, naphtalene present affect the cracking yield
42
Why does the composition of naphta affects the cracking yield
different H2 : C ratio different reactivity
43
Describe alkane cracking
easy conversion high yields of light (ethene, propene) bc of hig steam ratio
44
Describe aromatics conversion/cracking
stable low steam to carbon ratio = no yield of lights
45
Describe naphtalene cracking
intermediate
46
Do the reaction condition depend on feedstock and desired product
yes it is critical
47
Describe the ethane --> ethene slectivity
>70 wt%
48
Describe the naphta--> ethene slectivity
low ^CH4 and propene production
49
is ethane more or less stable than naphta
more (at high T, low t)
50
why is a high steam to ethane ratio good
= higher ethene yield, lower propene yield
51
A high partial pressure of the HC will favor the side reactions?
yes coke formation
52
How do you remove sulfur in feed?
easily removed with CO2 by selective extraction process
53
Name 2 product treatment
desulphurization gas dried to residual moistur <5ppm
54
Why is gas dried to residual moisture < 5 ppm
prevent ice crystals in future cooling stages
55
Dram the box / line diagram
TO DO
56
Cons of 1. 1. seperation in order of lower --> ^volatilities
CHECK**
57
What are the 3 different seperation train approaches?
1. seperation in order of lower --> ^volatilities 2. front-end hydrogenation 3. naphta cracking as 1st step
57
PRO's of 1. seperation in order of lower --> ^volatilities
CHECK**
58
Con's of 2. front-end hydrogenation
high H2 content = $$$
59
Pros of 2. front-end hydrogenation
if ethyne/prpyne content is high to have less coke
60
Pros of 3. naphta cracking as 1st step
Removing the least interesting fraction first = smaller P in other columns (smaller size and energy needed = $$:))
61
Cons of 3. naphta cracking as 1st step
CHECK**
62
why is the product in top streams for seperation train?
purity in top stream = easier to increase by playing with reflux
63
Negative impacts of coke formation during pyrolysis on the walls of furnace (x5)
1. reduces cross section of tube 2. ^P drop 3. change in product selctivity (more aromatics) 4. higher T required 5. carburization of metal alloy
64
What is catalytic coke?
due to catalytic action of alloying metal of tubes
65
what is pyrolitic coke? / gas-phase coke
product of thermal pyrolysis
66
What is a solution to coke formation (other than decoking) (x3)
changing reactor design use of feed aditives surface technology
67
what are changes made for the reactor design
chaotic mixing
68
what are con of drawbacks?
^P grad= effect on selectivity of ethylene
69
What is the most common additive in the reactor to reduce coke
S-containign
70
name 2 surface technology
1. barrier coatings that passivates the inner wall(only for cat coking) 2. cat coating that converts coke to CO2 (pyrolysis cracking)
71
how to help of cat coating
adds S can help
72
CON's of cat coating
^CO and CO2 emissions
73
What are the new trends in olefin production
ethylene ^^^ propylene drop in ethane cracker = need alternative routes
74
How to lower CO2 in steam crackers?
1.supplement fossil fules w/ biomass and waste streams 2. passivate burner walls = prevent cat coking 3. Carbon capture + storage 4. Oxyfuel comb + post combustion process 5. Developpment of coal-based techs
75
Name the 4 alternative routes in the production of olefins.
1. MTO (methanol to olefin), From methane 2. Fischer-Tropsch (syngas), from Methane 3. OCM (oxidative coupling of methane) 4. Catalytic dehydrogenation alkanes (light alkenes)
76
Explain MTO - tech
syngas production starting from CH4 Cat, coke formation varianle ethene/propene ratio
77
PRO's of MTO
high selectivity for ethylene/propylene Process conditions can be altered to favour propylene
78
CON's of MTO
rapid cat deactivation due to coke $$$ lots of steps
79
Describe Fisher-Tropsch
based on syngas issues with selectivity = ^$ sep train ^exo
80
PRO's of F-T
mature techno good when crude oil is scarce
81
CON's FT
difficult heat removal lower selectivity = $ seperation S poisoning
82
Describe OCM
no syngas req :) low yield, ^exo :( challenging issues in cat :(
83
describe cat dehydrogenation alkanes
endo coke formation enhance equilibrium conversion : H2 removal membrane
84
PRO of at dehydrogenation alkanes
^selectivity towards single olefins = good bc low prices of propane
85
CON at dehydrogenation alkanes
energy efficicency cat deactivation side rxn at ^T
86
**revoir propylene side note page 15
TO DO
87

ICP final (10 decks)

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