21

+ CH3OH

+ O2

+ CH4/O2

CH3NH2

(CH3)2NH

(CH3)3N

Methyl

amines

Acrylo

nitrile

Hexa methylene tetramine

Di-isopropyl amine

Nitric acid

HCN

+ CO2 + HNO3 + H3PO4 + H2SO4

Urea NH2CONH2

NH3 from synthesis gas (CO + H2)

Chemicals and fertilizers from ammonia (Moulijn et al.)

1. Name at least two chemicals that are produced from synthesis gas?

2. Most of the synthesis gas today is produced from natural gas by steam reforming it.

What is natural gas? What are its components?

Synthesis gas is produced from natural gas by reacting it with steam. This reaction is called

steam reforming reaction. Although natural gas contains many chemical compounds, we will

assume that it consists of only CH4. Steam reforming reactions of CH4 occur over a Ni-based

catalyst. The reactions are as follows:

Main reaction: 4 + 2 + 3 2 = +206 /

+ 2 2 + 2 = 41 /

4 + 2 2 + 2 = +247 / Side reactions:

Ammonia NH3

MEA, DEA, TEA

Ammonium Nitrate, NH4NO3

Ammonium Phosphates, NH4H2PO2 (NH4)2HPO4, (NH4)3PO3

Ammonium Sulphate, (NH4)2SO4

22

4 + 22 = 75

20 + 2 = 173 /

Natural gas also contains small amounts of H2S (< 1%). For the Ni catalyst, H2S is a poison

and needs to be removed down to few hundred parts per million. This removal is called

sweetening of natural gas. Which unit operation is recommended for removing H2S from a

concentration of about, say, 10,000 ppm in natural gas to, say, 100 ppm?

__________________.

Which liquid do you recommend for the unit operation?

A) Water [low solubility for H2S]

B) Adsorption

C) Ethanol amines [basic in nature; reacts reversibly with H2S; heating liberates H2S]

(Sourced from: Wikipedia)

Explain the working of the above natural gas sweetening plant.

Coking

reactions:

23

Lets return to the steam reforming reactions.

Thermodynamics favors ____________ pressures (high/low) and _________ temperatures

(high/low) for the main reaction. N2 and H2 react at 100-250 bar pressure to produce

ammonia. We have two options for the choice of pressure:

1. Produce synthesis gas at low pressures and compress it before it enters NH3 reactor

2. Compress the natural gas, produce synthesis gas at high pressures and feed it to the

NH3 reactor.

Obviously CO is removed before the reactor. Discuss the implications of the above choices

on capital and operating costs.

Were in a similar situation as sulphuric acid manufacture: we choose conditions NOT

favoured by thermodynamics.

In actual practice, excess steam, three times the theoretical amount is added to natural gas for

steam reforming. By adding more steam,

CH4 conversion _____________________

Carbon formation ___________________

As steam reforming reactions are endothermic (net), heat needs to be supplied. These

reactions are carried out at high temperatures, about 800 900 oC. Even at such

temperatures, a catalyst is required because CH4 is a stable gas.

Study the flow sheet below and comment on the energy recovery and energy integration. The

reformer actually consists of several tubes, shown as a dotted line here. The tubes are heated

by a fuel supplied to the outside of the tubes. .

What is the fuel in the above figure?

24

CH4 + steam

Synthesis gas

Fuel + air

Flue gas

CH4 + steam

Synthesis gas

Fuel + air

Flue gas

Air

Boiler water

Steam

Natural gas

Superheated steam

Exhaust gas to stack

Synthesis gas

Select one of the options for synthesis gas reactor. Justify your selection with cogent

arguments.

A) Multi tubular reactor 500-600 tubes, each tube 7 to 12 meters long, 3 to 5 inches in

diameter.

B) Several huge bed of catalyst (in parallel) heated outside by a fuel. Only 1 bed is

shown below. Diameter = few meters; height = 10-15 meters

Schematic diagram of reformer (adapted from Felder and Rousseau)

25

Typically, about 90-95% conversion of methane is achieved by steam reforming. Ideally, we

want all the methane to be converted. For close to 100% conversion, what can be done?

A) Increase temperature [high temperature may be harmful to the catalyst]

B) Have longer catalyst filled tubes [its an equilibrium limited reaction]

C) Use higher H2O/ CH4 ratios

D) Reform the remaining CH4 by adding air to a second reformer to which effluents of

primary reformer are fed. [ammonia needs N2]

In addition to reaction in previous discussion, the following reactions will also occur when air

is introduced.

4 +1

22 + 2 2 = 36 /

4 + 22 + 22 = 803 /

+1

22 2 = 284 /

2 +1

22 2 = 242 /

1. What is a downside of the secondary reformer?

2. Write all the compounds present in gases exiting secondary reformer?

3. What are goals of the next few steps to prepare the feed for NH3 synthesis reactor?

4. CO2 gas can be absorbed in liquid solvents but not CO. Why? (Find an answer to this

question). Refer to the reactions in previous pages and suggest a method to remove

CO.

Draw a block diagram for the ammonia process discussed so far. Apparently this diagram is

incomplete. We will build it further as we move along.

26

Water gas shift reaction:

+2 2 + 2 = 41 /

Low temperatures favor the shift reaction. In the past, catalysts were active > 600 K.

More active catalysts at 500 K were developed. The water-gas shift reaction is carried out in

two adiabatic reactors: in the first high temperature (HT) reactor, ironoxide is used as the

catalyst; in the second low temperature (LT) reactor, copper based catalysts are used.

0

100

200

300

400

500

600

700

400 450 500 550 600 650 700 750 800

Kp

Temperature, K

27

Syn gas

Shifted gas

High temperature reactor

Low temperature reactor

640 K

710 K

490 K

510 K

Why do you think two reactors are used instead of one large reactor with superior LT

catalyst?

Again, note that exothermic reversible reactions are carried out at high temperatures. What is

the implication of high pressure on reactor size? [these reactors are operated almost at the

same pressure as reformers].

Most of the CO is converted to CO2. The exit gas from LT reactor contains about 0.3 mole %

CO. The exit gas from LT reactor is at about 500 K.

What is the unit operation precedes absorption of CO2 for effective absorption?

CO2 is absorbed in hot K2CO3 solution at ~ 70 oC. This solution is regenerated by heating it.

Complete the below flow chart used for absorption of CO2.

28

steam

CO2 lean gas

K2CO3 + CO2

Gas from LT reactor

What are the components of gas stream leaving absorption column?

CO and CO2 are poisonous to ironbased ammonia synthesis catalyst. They need to be

removed to vanishing levels before synthesis gas can be fed to NH3 reactor.

Which of the following is a good choice of CO and CO2 removal from the gas exiting the

absorption system? Why?

A) Additional water-gas shift reaction and absorption

B) Reverse steam reforming reaction

+ 32 4 + 2 = 206 /

+ 42 4 + 22 = 165 /

Water is condensed from the gas exiting the methanation reactor. Now, this gas contains N2,

H2, CH4, Ar. The first two react to make ammonia; the last two are inerts.

NH3 production is a landmark in the manufacture of bulk chemicals. It is the first chemical

produced at high pressure (>100 bar) and high temperature (~ 700 K or 430 o

C). N2 is very

stable and inert, hence suitable catalyst needs to be developed for NH3 synthesis. Haber

developed a commercial process during 1908 1913 in collaboration with scientists and

engineers at BASF. The group at BASF tested a whopping 6500 catalysts before discovering

the correct iron based catalysts. Haber received the Nobel Prize for chemistry in 1919 for

ammonia synthesis.

29

Thermodynamics:

2 + 32 23 = 91.44 /

Clearly ____________ temperatures and ____________ pressures are favorable for NH3

production. However, ___________ temperatures are used in practice because

_______________________

Typical reactor conditions:

Temperatures: 400 oC (inlet), 400 500

oC (outlet); pressures: 100 250 bar

Commercial NH3 synthesis reactors:

Few commercial reactors for NH3 synthesis are shown. Discuss salient features of the

reactors and compare their designs.

0

5

10

15

20

25

30

35

40

45

50

200 250 300 350 400 450 500 550 600

% c

on

ver

sion

Temperature, oC

200

300industrial plants

30

Why the feed gas is flowing through the annulus?

Why is quenching done?

What could be one disadvantage of indirect cooling in separate heat exchanger to

produce the steam?

Typical conversion of N2/H2 is ~15-20 %, why such low conversions?

31

Whats immediately downstream of the reactor?

In H2SO4 production, cooling of exit gas and removal of SO3 ensured a high conversion of >

99.5%.

Could we use same principle in NH3 production? Both are equilibrium limited reactions.

[Hint: feed gas for NH3 synthesis contains an inter gas CH4]

Draw block diagram below for NH3 synthesis.

32

Study the flow-sheet for NH3 synthesis.

Purge gas from the plant contains significant H2. How can we remove this H2?

A) Adsorption

B) Distillation

C) Membrane

D) Absorption