Survey of Industrial Chemestry - chapter 12

7/25/2019 Survey of Industrial Chemestry - chapter 12

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Chapter 12

Chemicals from M ethane

In previous discussions we studied a large percentage of importantchemicals derived from m ethane. Those in the top 50 are listed in Ta12.1 and their syntheses are summarized in Fig. 12.1.

As we learned in Chapters 3 and 4, many inorganic compounds, not justammonia, are derived from synthesis gas, made from methane by steamreforming. In the top 50 this would include carbon dioxide, ammonia, nitracid, ammonium nitrate, and urea. No further mention need be made ofthese important processes. We discussed MTBE in Chapter 7, Section 4, Chapter 10, Section 9, since it is an important gasoline additive and 4derivative. In Chapter 10, Section 6, we presented w-butyraldehyde, madeby the oxo process with propylene and synthesis gas, which is made frmethane. In Chapter 11, Section 8, we discussed dimethyl terephthalate.Review these pertinen t sections. That leaves only two chemicals, methanoland formaldehyde, as derivatives of methane that have not been discussWe will take up the carbonylation of methanol to acetic acid, now the mostimportant process for making this acid. Vinyl acetate is made from acetic

Table 12.1 Methane Derivatives in the Top 50

Ammonia DerivativesMethyl /-butyl ether/7-ButyraldehydeDimethyl terephthalateMethanol

Formaldehyde

Acetic acid

Vinyl acetate


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methanol

catalyst

methane synthesis gas ammonia and derivatives

w-butyraldehyde

formaldehyde

acetic acid vinyl acetate

MTBE

Figure 12.1 Synthesis of methane derivatives.


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acid as seen in Chapter 9, Section 4. We will also discuss an important clof compounds, chlorofluorocarbons, some of which are derived frommethane. Although not in the top 50, these have important uses and making headlines these days.

METHANOL WOOD ALCOHOL, METHYLALCOHOL)

H OH

Before 1926 all methanol was made by distillation of wood. Now it is allsynthetic. Methanol is obtained from synthesis gas under appropriateconditions. This includes zinc, chromium, manganese, or aluminum oxidesas catalysts, 30O0C, 250-300 atm (3000-5000 psi), and most importantly a1:2 ratio of COiH2. Newer copper oxide catalysts require lowetemperatures and pressures, usually 200-30O0C and 50-100 atm (750-1500psi). A 60% yield of methanol is realized. As seen in Chapter 3, masynthesis gas systems are set to maximize the amount of hydrogen in themixture so that more ammon ia can be made from the hydrogen reacting withnitrogen. The shift conversion reaction aids the attainment of this goal.When synthesis gas is to be used for methanol manufacture, a 1:2 raCO:H2 ratio is obtained by adding carbon dioxide to the methane and wate

3CH 4 + 2H 2O + CO 2 ^ 4CO + 8H 2

CO + 2H 2 A v CH 3OHpressuremetal oxides

Thus methanol and ammonia plants are sometimes combined sincarbon dioxide, which must be removed from hydrogen to use it forammonia production, can in turn be used as feed to adjust the CO:H2 ratio to1:2 for efficient methanol synthesis. The methanol can be condensed andpurified by distillation, bp 65 0C. Unreacted synthesis gas is recycled. O therproducts include higher boiling alcohols and dimethyl ether.

Table 12.2 gives the uses for methan ol. The percentage of methanol uin the manufacture of formaldehyde has been fluctuating. It was 42% in1981. It has decreased in part because of recent toxicity scares offormaldehyde. The percentage of methanol used in acetic acid manufactu

is up from 7% in 1981 because the carbonylation of methanol has becomethe preferred acetic acid manufacturing method. MTBE is the octaenhancer and is synthesized directly from isobutylene and methanol. It was


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Table 12.2 Uses of M ethanol

MTBE 40%Formaldehyde 24

Acetic acid 12Solvents 6Chloromethanes 3Methyl methacrylate 3Methylamines 2Dimethyl terephthalate 2Miscellaneous 8

Source: Chemical Profiles

the fastest growing use for methanol for many years but it will dropdramatically if MTBE is banned. Many other important chemicals are mfrom methanol, although they do not quite make the top 50 list. Some these can be found in Chapter 13 where the second 50 chemicals aresummarized. With a U.S. production of 2.9 billion gal and a price of

470/gal, the commercial value of m ethanol is $1.4 billion .Not mentioned in the table is the direct use of methanol as fuel for

automobiles. It is added in small amounts to gasoline, sometimes as a blendwith other alcohols such as f-butyl alcohol, to increase octane ratings andlower the price of the gasoline. Experimen tation is even being done onvehicles that burn pure methanol. This fuel use is usually captive but a goodestimate is that it may account for almost 10% of the methanol produced.

2. FORM ALDEHYDE METHANAL)

O

H C H

Formaldehyde is produced solely from m ethanol. The process can be airoxidation or simple dehydrogenation. Since the oxidation is exothermic andthe dehydrogenation is endothermic, usually a combination is employedwhere the heat of reaction of oxidation is used for the dehydrogenation.

oxidation

dehydrogenation


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Table 12.3 Uses of Formaldehyde

Urea-formaldehyde resins 23%

Phenol-formaldehyde resins 19Acetylene chemicals 12 oly cet l resins 11MDI 6Pentaerythritol 5Urea-formaldehyde concentrates 4HMTA 4

Melamine resins 4Miscellaneous 12


Various metal oxides or silver metal are used as catalysts. Temperaturesrange from 450-90O0C and there is a short contact time of 0.01 sec.Formaldehyde is stable only in water solution, commonly 37-56%formaldehyde by weight. Methanol (3-15%) may be present as a stabilizer.Formaldehyde in the pure form is a gas with a bp of - 210C but is unstableand readily trimerizes to trioxane or polymerizes to paraformaldehyde.

paraformaldehyde

trioxane

Table 12.3 summarizes the uses of formaldehyde. Two importantthermosetting plastics, urea- and phenol-copolymers, take nearly one half theformaldehyde manufactured. Urea-formaldehyde resins are used inparticleboard, phenol-formaldehyde resins in plywood. 1,4-Butanediol is

made for some polyesters and is an example of acetylene chemistry that not yet been replaced. Tetrahydrofuran (THF) is a common solvent thamade by dehydration of 1,4-butanediol.

THF


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Polyacetal resins have a repeating unit of -Q-CH 2-. They are strong,stiff polymers for valves, hoses, and tube conn ectors. Pen taerythritol findsend-uses in alkyd resins and explosives (pentaerythritol tetranitrate). Toappreciate this synthesis, the student should review two condensationreactions, the crossed aldol and the crossed Cannizzaro. Acetaldehyde reactwith 3 mol of formaldehyde in three successive aldol condensations. Thiproduct then undergoes a Cann izzaro reaction with formaldehyde.

pentaerythritol

Hexamethylenetetramine (HMTA) has important uses in modifyingphenolic resin manufacture and is an intermediate in explosive manufacture.

Although it is a complex three-dimensional structure, it is easily made by thecondensation of formaldehyde and ammonia.

Debate is continuing on the safety and toxicity of formaldehyde and itsproducts, especially urea-formaldehyde foam used as insulation inconstruction and phen ol-formaldehyde as a plywood adhesive. Presently theTLV-STEL of formaldehyde is 0.3 ppm . Formaldehyde is on t Reasonably A nticipated to Be Human Carcinogens list.

3. ACETIC ACID

Two manufacturing methods and the uses of acetic acid were discusin Chapter 9, Section 3, since it is made from ethylene and the C4 stream.


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However, since 1970 the preferred method of acetic acid manufacture iscarbonylation of methanol (Monsanto process), involving reaction ofmethanol and carbon monoxide (both derived from methane) with rhodiumand iodine as catalysts at 1750C and 1 atm. The yield of acetic acid is 99%based on methanol and 90% based on carbon monoxide.

The mechanism is well understood, involving complexation of

rhodium with iodine and carbon monoxide, reaction with methyl iodi(formed from the methanol with hydrogen iodide), insertion of CO in therhodium-carbon bond, and hydrolysis to give product with regeneration ofthe complex and more hydrogen iodide.

Since acetic acid is used to make vinyl acetate (Chapter 9, Section 4) inlarge amounts, this top 50 chemical is also dependent on methanol amajor raw material.

4. CHLOROFLUOROCARBONS CFCs ANDHCFCs) AND FLUOROCARBONS HFCs)

Because of the growing importance of CFCs in environmental chemisa basic understanding of the chemistry and uses of this diverse chemicalfamily is necessary. Together they represent a production of over 800

million Ib/yr that, at $1.00/lb, is a large commercial value.This industry segment uses common abbreviations and a numberingsystem for CFCs and related compounds. The original nomenclature


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developed in the 1930s at Du Pont is still employed and uses three digits.When the first digit is O it is dropped. The first digit is the number ofcarbons minus 1, the second digit is the number of hydrogens plus 1, and thethird digit is the number of fluorines. All other atoms filling the fourvalences of each carbon are chlorines. Imp ortant non hydrogen -containinCFC s are given below . O riginally these were called Freons.

CCl 2F 2 CCl 3F CCl 2FCClF 2

CFC-12 C F C - I l CFC-113

When some of the chlorines are replaced by hydrogens, CFCs becomHCFCs, the now m ore comm on nom enclature fo r those chlorofluorocarbonscontaining hydrogen. The number ing is the same. W hen m ore than oneisomer is possible, the most symmetrically substituted compound has only anumber; letters a and b are added to designate less symmetrical isomers.

CHClF2 CF3CHCl2 CF3CHClF

HCFC-22 HCFC -123 HCFC -124

CCl2FCH3 CClF2CH3

HCFC-141b HCFC-142b

When there is no chlorine and the chemical contains only hydrogefluorine, and carbon, they are called HFCs.

CF 3CHF 2 CH 2FCF 3 CH 3CHF 2

HFC-125 HFC-134a HFC-152a

Halons, a closely related type of chemical that also contain bromine, areused as fire retardants. N um berin g here is m ore straightforward : first dino. of carbons; second digit, no. of fluorines; third digit, no. of chlorin

and fourth digit, no. of bromines. Com mo n Halon s are the following : Halon1211, CF2BrCl; Halon 1301, CF 3Br; and H alon 2402, C2F4Br2.Most CFCs are manufactured by com bining hydrogen fluoride and either

carbon tetrachloride or chlorofo rm . The hydrogen fluoride comes fromfluorspar, CaF 2, reacting with sulfuric acid. The chlorinated methanes aremanufactured from methane. Important reactions in the manufacture ofCFC-11 and -12 and HC FC-22 are given in Fig. 12.2.

The current use pattern of CFCs is shown in Table 12.4. The classicCFCs that have been used for refrigeration and air conditioning are moCFC-11 and -12, with some -114 and -115. A large portion of this usage is


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Figure 12.2 M an ufacture of Chlorofluorocarbons.

now for automobile air condition ing. Refrigerants and home conditioning are switching to HCFC-22. Foam blowing agents use CFC-11and -12. Solvent use, especially for cleaning of electronic circuit boaremploys CFC -113. A large previous use of CFCs was in aerosols apropellants. This has been outlawed. An estimated 3 billion aerosol cans/yrused CFCs in the early 1970s.

What are the properties of CFCs that make them unique for certaapplications? Propellants for aerosols need high volatility and low boilingpoints. Interestingly, compared to the same size hydrocarbons,fluorocarbons have higher volatility and lower boiling points, unusual fhalides. They are less reactive, more compressible, and more thermallystable than hydrocarbon s. They also have low flammability, toxicity, andodor. They are used in air conditioners and refrigerators because they hhigh specific heats, high thermal conductivities, and low viscosities. Thnonflammability and low toxicity are also attractive in these applications.

What's the problem with CFCs? In the mid-1970s CFCs weredetermined to photodissociate in the stratosphere to form chlorine atoms.These chlorine atoms then react with ozone to deplete this protective layer in

Table 12.4 Uses of Chlorofluorocarbons

Refrigerants/air-conditioning 46%Fluoropolymers 28Foam blowing agents 20Solvent cleaning 3Miscellaneous 3



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our atmosphere. The mechanism is a typical free radical chain process.Initiation in step (1) involves breaking a carbon-chlorine bond, weaker thana carbon-fluorine bond. Two propagation steps then can rapidly depleteozone by reaction with the chlorine atoms.

(1) CCl 2F 2 hv

Cl- + -CClF2

or CCl 3F J^ el- + .CCl 2F

(2) Cl- + O 3 > ClO- + O 2

(3) ClO- + O * Cl- + O 2

then (2), (3), (2), (3), etc.

Net reaction, (2) + (3): O 3 + O ^ 2O 2

Long-range effects of having less ozone in the stratosphere involvegreater ultraviolet sunlight transmission, alteration of weather, and anincreased risk of skin cancer. The ozone depletion potential for CFCs other fluorocarbons have been measured and are given below relative CFC-Il and -12. Notice that the HCFCs with lower chlorine content havlower depletion potentials than the CFCs, and the one HFC studied shows nodepletion potential because it contains no chlorine.

C FC -I l 1.0 HCFC-123 0.016CFC-12 1.0 H C F C - H I b 0.081CFC-113 0.8 HC FC-22 0.053CFC-114 1.0 HFC-134a OCFC-115 0.6

HCFCs and HFCs, because of the hydrogen in the molecule, react whydroxyl groups in the lower atmosphere. The HCFCs are being pushed aspossible temporary replacements in some applications of CFCs, thoHCFCs will be phased out early in the 21st century.

The manufacturing picture and the recent past and future of CFCs arerapidly changing. In 1988 annual CFC consumption was 2.5 billion Ib. Inthe U.S. about 5,000 businesses at 375,000 locations produced goods andservices valued in excess of $28 billion. More than 700,000 jobs weresupported by these businesses. In 1999 the consumption of fluorocarbonswas only 800 million Ib. Obviously the CFC phaseout must be done


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properly to minimize the effects on these businesses and individuals. Thefollowing brief chronology w ill give the student an idea of the situation as ofthis w riting.

1978 The EPA outlawed CFC-I l and -12 in aerosol and propellantapplications because of fear of ozone depletion. They were replaced bypropane and butane, highly flamm able hydrocarbons.

1984 An ozone hole over Antarctica was discovered with especially lowconcentrations of ozone above that continent in their spring (NorthernHemisphere's fall). This was linked to CFCs.

1988 Du Pont, the largest producer of CFCs, called for a total CFCproduction phaseout. A possible arctic ozone hole was studied. The EPA

called for a total ban of CFCs.1989 The Montreal Protocol was completed. This asked for

worldwide production freeze at the 1986 levels, a 20% cut by 1993, andanother 30% lowering of production by 1998 for CFC- I l , -12, -113, -114,and-115.

1992 At a meeting in Copenhagen 80 nations set HCFC deadlinincluding a production freeze by 1996, a 35% reduction by 2004, 90%

2015, and 100% by 2030.1993 HCFC-HIb replaced CFC- I l as the blowing agent in insulatingfoams.

1994 HFC-134a replaced CFC-12 in auto air-conditioners for all newcars. CFC-12 was to be used in old cars because of the high cost to switch.HFC-134a and HCFC-123 replaced CFC-12 and CFC-Il in large buildingcooling systems.

1995 The Nobel Prize in Chemistry went to Rowland, Mokina, andCrutzen for linking ozone depletion w ith NOx and CFCs.

1997 Large doses of HCFC-123 were found to possibly cause liverdamage.

1998 HFCs were found to possibly have an effect as long-livedgreenhouse gases, increasing the average atmospheric temperature.

Suggested eadings

Chemical Profiles in Chemical Marketing Reporter, 6-22-98, 10-11-99, 7-31-00, and 8-28-00.

Kent, Riegel s Handbook of Industrial Chemistry, pp. 800-809.Szmant, Orga nic Building Blocks of th e Chem ical Industry, pp. 61-187.Wiseman, P etrochemicals, pp. 148-155.Wittcoff and Reuben, Industrial Organic Chem icals, pp. 294-337.

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Survey of Industrial Chemestry - chapter 12