Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes

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Urea possesses a unique property of forming adducts with n-paraffins.

This is used in separating C

-C

n-paraffins from kerosines for deter-

gent production (Chapter 6).

Nitric Acid (HNO

)

Nitric acid is one of the most used chemicals. The 1994 U.S. produc-

tion was approximately 17.65 billion pounds. It is a colorless to a yellow

liquid, which is very corrosive. It is a strong oxidizing acid that can

attack almost any metal. The most important use of nitric acid is to pro-

duce ammonium nitrate fertilizer.

Nitric acid is commercially produced by oxidizing ammonia with air

over a platinum-rhodium wire gauze. The following sequence represents

the reactions occurring over the heterogeneous catalyst:

4NH

(g) + 5O

(g)

4NO(g) + 6H

O(g) ∆H° = –226.4 KJ/mol

2NO(g) + O

(g)

2NO

(g) ∆H° = –56.5 KJ/mol

3NO

(g) + H

O(1)

2HNO

(aq) + NO(g) ∆H° = –33.4 KJ/mol

The three reactions are exothermic, and the equilibrium constants for the

first two reactions fall rapidly with increase of temperature. Increasing

pressure favors the second reaction but adversely affects the first reaction.

Chemicals Based on Methane 147

Figure 5-3. The Snamprogetti process for producing urea

: (1) reactor, (2,3,4) car-

bonate decomposers, (5,6) crystallizing and prilling.

Chapter 5 1/22/01 11:01 AM Page 147

For this reason, operation around atmospheric pressures is typical. Space

velocity should be high to avoid the reaction of ammonia with oxygen on

the reactor walls, which produces nitrogen and water, and results in lower

conversions. The concentration of ammonia must be kept below the

inflammability limit of the feed gas mixture to avoid explosion. Opti-

mum nitric acid production was found to be obtained at approximately

900°C and atmospheric pressure.

Uses of Nitric Acid. The primary use of nitric acid is for the production

of ammonium nitrate for fertilizers. A second major use of nitric acid is in

the field of explosives. It is also a nitrating agent for aromatic and paraf-

finic compounds, which are useful intermediates in the dye and explosive

industries. It is also used in steel refining and in uranium extraction.

Hydrazine (H

N-NH

A colorless, fuming liquid miscible with water, hydrazine (diazine)

is a weak base but a strong reducing agent. Hydrazine is used as a

rocket fuel because its combustion is highly exothermic and produces

620 KJ/mol:

N-NH

+ O

+ 2H

O + 620 KJ

Hydrazine is produced by the oxidation of ammonia using the Rashig

process. Sodium hypochlorite is the oxidizing agent and yields chlo-

ramine NH

Cl as an intermediate. Chloramine further reacts with ammo-

nia producing hydrazine:

2NH

+ NaOCl

N-NH

+ NaCl + H

Hydrazine is then evaporated from the sodium chloride solution.

Hydrazine can also be produced by the Puck process. The oxidizing

agent is hydrogen peroxide:

2NH

+ H

N-NH

+ 2H

Uses of Hydrazine. In addition to rocket fuel, hydrazine is used as a

blowing agent and in the pharmaceutical and fertilizer industries. Due to

the weak N-N bond, it is used as a polymerization initiator. As a reduc-

ing agent, hydrazine is used as an oxygen scavenger for steam boilers. It

is also a selective reducing agent for nitro compounds. Hydrazine is a

148 Chemistry of Petrochemical Processes

Chapter 5 1/22/01 11:01 AM Page 148

good building block for many chemicals, especially agricultural prod-

ucts, which dominates its use.

METHYL ALCOHOL (CH

OH)

Methyl alcohol (methanol) is the first member of the aliphatic alcohol

family. It ranks among the top twenty organic chemicals consumed in the

U.S. The current world demand for methanol is approximately 25.5 mil-

lion tons/year (1998) and is expected to reach 30 million tons by the year

2002.

The 1994 U.S. production was 10.8 billion pounds.

Methanol was originally produced by the destructive distillation of

wood (wood alcohol) for charcoal production. Currently, it is mainly pro-

duced from synthesis gas.

As a chemical compound, methanol is highly polar, and hydrogen bond-

ing is evidenced by its relatively high boiling temperature (65°C), its high

heat of vaporization, and its low volatility. Due to the high oxygen content

of methanol (50 wt%), it is being considered as a gasoline blending com-

pound to reduce carbon monoxide and hydrocarbon emissions in automo-

bile exhaust gases. It was also tested for blending with gasolines due to its

high octane (RON = 112). During the late seventies and early eighties,

many experiments tested the possible use of pure (straight) methanol as an

alternative fuel for gasoline cars. Several problems were encountered,

however, in its use as a fuel, such as the cold engine startability due to its

high vaporization heat (heat of vaporization is 3.7 times that for gasoline),

its lower heating value, which is approximately half that of gasoline, and

its corrosive properties. The subject has been reviewed by Keller.

However, methanol is a potential fuel for gas turbines because it burns

smoothly and has exceptionally low nitrogen oxide emission levels.

Due to the high reactivity of methanol, many chemicals could be

derived from it. For example, it could be oxidized to formaldehyde, an

important chemical building block, carbonylated to acetic acid, and

dehydrated and polymerized to hydrocarbons in the gasoline range

(MTG process). Methanol reacts almost quantitatively with isobutene

and isoamylenes, producing methyl t-butylether (MTBE) and tertiary

amyl methyl ether (TAME), respectively. Both are important gasoline

additives for raising the octane number and reducing carbon monoxide

and hydrocarbon exhaust emissions. Additionally, much of the current

work is centered on the use of shape-selective catalysts to convert

methanol to light olefins as a possible future source of ethylene and

propylene. The subject has been reviewed by Chang.

Chemicals Based on Methane 149

Chapter 5 1/22/01 11:01 AM Page 149

Production of Methanol

Methanol is produced by the catalytic reaction of carbon monoxide

and hydrogen (synthesis gas). Because the ratio of CO:H

in synthesis

gas from natural gas is approximately 1:3, and the stoichiometric ratio

required for methanol synthesis is 1:2, carbon dioxide is added to reduce

the surplus hydrogen. An energy-efficient alternative to adjusting the

CO:H

ratio is to combine the steam reforming process with autothermal

reforming (combined reforming) so that the amount of natural gas fed is

that required to produce a synthesis gas with a stoichiometric ratio of

approximately 1:2.05. Figure 5-4 is a combined reforming diagram.

an autothermal reforming step is added, pure oxygen should be used.

(This is a major difference between secondary reforming in case of

ammonia production, where air is used to supply the needed nitrogen).

150 Chemistry of Petrochemical Processes

Figure 5-4. A block flow diagram showing the combined reforming for methanol

synthesis.

Chapter 5 1/22/01 11:01 AM Page 150

An added advantage of combined reforming is the decrease in NO

emis-

sion. However, a capital cost increase (for air separation unit) of roughly

15% is anticipated when using combined reforming in comparison to

plants using a single train steam reformer. The process scheme is viable

and is commercially proven.

The following reactions are representative

for methanol synthesis.

(g)

+ 2H

2(g)

→ CH

(1)

∆H° = –128 KJ/mol

+ 3H

→ CH

(l)

+ H

Old processes use a zinc-chromium oxide catalyst at a high-pressure

range of approximately 270–420 atmospheres for methanol production.

A low-pressure process has been developed by ICI operating at about

50 atm (700 psi) using a new active copper-based catalyst at 240°C. The

synthesis reaction occurs over a bed of heterogeneous catalyst arranged

in either sequential adiabatic beds or placed within heat transfer tubes.

The reaction is limited by equilibrium, and methanol concentration at the

converter’s exit rarely exceeds 7%. The converter effluent is cooled to

40°C to condense product methanol, and the unreacted gases are recy-

cled. Crude methanol from the separator contains water and low levels of

by-products, which are removed using a two-column distillation system.

Figure 5-5 shows the ICI methanol synthesis process.

Methanol synthesis over the heterogeneous catalyst is thought to occur

by a successive hydrogenation of chemisorbed carbon monoxide.

Chemicals Based on Methane 151

Other mechanisms have been also proposed.

Uses of Methanol

Methanol has many important uses as a chemical, a fuel, and a build-

ing block. Approximately 50% of methanol production is oxidized to

Chapter 5 1/22/01 11:01 AM Page 151

formaldehyde. As a methylating agent, it is used with many organic acids

to produce the methyl esters such as methyl acrylate, methylmethacry-

late, methyl acetate, and methyl terephthalate. Methanol is also used to

produce dimethyl carbonate and methyl-t-butyl ether, an important gaso-

line additive. It is also used to produce synthetic gasoline using a shape

selective catalyst (MTG process). Olefins from methanol may be a future

route for ethylene and propylene in competition with steam cracking of

hydrocarbons. The use of methanol in fuel cells is being investigated.

Fuel cells are theoretically capable of converting the free energy of oxi-

dation of a fuel into electrical work. In one type of fuel cells, the cathode

is made of vanadium which catalyzes the reduction of oxygen, while the

anode is iron (III) which oxidizes methane to CO

and iron (II) is formed

in aqueous H

The benefits of low emission may be offest by the

high cost. The following describes the major chemicals based on methanol.

152 Chemistry of Petrochemical Processes

Figure 5-5. The ICI low-pressure process for producing methanol:

(1) desulfu-

rization, (2) saturator (for producing process steam), (3) synthesis loop circulator,

(4) reactor, (5) heat exchanger and separator, (6) column for light ends recovery,

(7) column for water removal.

The main industrial route for producing formaldehyde is the catalyzed

air oxidation of methanol.

Chapter 5 1/22/01 11:01 AM Page 152

A silver-gauze catalyst is still used in some older processes that oper-

ate at a relatively higher temperature (about 500°C). New processes use

an iron-molybdenum oxide catalyst. Chromium or cobalt oxides are

sometimes used to dope the catalyst. The oxidation reaction is exother-

mic and occurs at approximately 400–425°C and atmospheric pressure.

Excess air is used to keep the methanol air ratio below the explosion lim-

its. Figure 5-6 shows the Haldor Topsoe iron-molybdenum oxide cat-

alyzed process.

Uses of Formaldehyde. Formaldehyde is the simplest and most reac-

tive aldehyde. Condensation polymerization of formaldehyde with phenol,

urea, or melamine produces phenol-formaldehyde, urea formaldehyde, and

melamine formaldehyde resins, respectively. These are important glues

used in producing particle board and plywood.

Condensation of formaldehyde with acetaldehyde in presence of a

strong alkali produces pentaerythritol, a polyhydric alcohol for alkyd

resin production:

Chemicals Based on Methane 153

Figure 5-6. The Haldor Topsoe and Nippon Kasei process for producing formalde-

hyde:

(1) blower, (2) heat exchanger, (3) reactor, (4) steam boiler, (5) absorber,

(6,7) coolers, (8) incinerator, (9) heat recovery, (10) methanol evaporator, (11)

boiler feed water.

Chapter 5 1/22/01 11:01 AM Page 153

Formaldehyde reacts with ammonia and produces hexamethylenete-

tramine (hexamine):

154 Chemistry of Petrochemical Processes

Hexamine is a cross-linking agent for phenolic resins.

Methyl Chloride (CH

CI)

Methyl chloride is produced by the vapor phase reaction of methanol

and hydrogen chloride:

OH + HCI

CI + H

Many catalysts are used to effect the reaction, such as zinc chloride on

pumice, cuprous chloride, and ignited alumina gel. The reaction condi-

tions are 350°C at nearly atmospheric pressure. The yield is approxi-

mately 95%.

Zinc chloride is also a catalyst for a liquid-phase process using con-

centrated hydrochloric acid at 100–150°C. Hydrochloric acid may be

generated in situ by reacting sodium chloride with sulfuric acid. As men-

tioned earlier, methyl chloride may also be produced directly from

methane with other chloromethanes. However, methyl chloride from

methanol may be further chlorinated to produce dichloromethane, chlo-

roform, and carbon tetrachloride.

Methyl chloride is primarily an intermediate for the production of

other chemicals. Other uses of methyl chloride have been mentioned

with chloromethanes.

Acetic Acid (CH

COOH)

The carbonylation of methanol is currently one of the major routes for

acetic acid production. The basic liquid-phase process developed by

BASF uses a cobalt catalyst at 250°C and a high pressure of about 70

Chapter 5 1/22/01 11:01 AM Page 154

atmospheres. The newer process uses a rhodium complex catalyst in

presence of CH

I, which acts as a promoter. The reaction occurs at 150°C

and atmospheric pressure. A 99% selectivity is claimed with this catalyst:

OH + CO

COOH

The mechanism of the carbonylation reaction is thought to involve a

first-step oxidative addition of the methyl iodide promotor to the Rh(I)

complex, followed by a carbonyl cis insersion step:

Chemicals Based on Methane 155

Carbonylation followed by reductive elimination produces back the

Rh(I) catalyst:

The final step is the reaction between acetyl iodide and methyl alcohol,

yielding acetic acid and the promotor:

Figure 5-7 is a flow diagram showing the Monsanto carbonylation

process.

Acetic acid is also produced by the oxidation of acetaldehyde and the

oxidation of n-butane. However, acetic acid from the carbonylation route

has an advantage over the other commercial processes because both

methanol and carbon monoxide come from synthesis gas, and the process

conditions are quite mild.

Uses of Acetic Acid. The main use of acetic acid is to produce vinyl

acetate (44%), followed by acetic acid esters (13%) and acetic anhydride

(12%). Vinyl acetate is used for the production of adhesives, film, paper

Chapter 5 1/22/01 11:01 AM Page 155

156 Chemistry of Petrochemical Processes

Figure 5-7. The Monsanto methanol carbonylation process for producing acetic acid.

Chapter 5 1/22/01 11:01 AM Page 156