Thiết kế qui trình công nghệ sản xuất vinyl axetat

Reaction of methyl acetate or dimethyl ether with carbon monoxide and hydrogen in the liquid phase in the presence of homogeneous catalysts, e.g., rhodium salts or noble metals of the pl

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ĐỒ ÁN CHUYÊN NGÀNH

ĐỀ TÀI 1: Thiết kế qui trình Công nghệ sản xuất Nitrobenzen.

HỌ TÊN SINH VIÊN:

1:………

2:………

ĐỀ TÀI 2: Thiết kế qui trình Công nghệ sản xuấn Ankyl benzensunfonic.

1:………

2:………

ĐỀ TÀI 3: Thiết kế qui trình Công nghệ sản xuất Axetanilit.

1:………

2:………

ĐỀ TÀI 4: Thiết kế qui trình Công nghệ sản xuất Etyl axetat.

1:TRẦN MINH THIỆN

2:VŨ THỊ VƯƠNG

ĐỀ TÀI 5: Thiết kế qui trình Công nghệ sản xuất Vinyl axetat.

1:NGÔ THANH TRÍ………

2:NGUYỄN MINH TRUNG………

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ĐỀ TÀI 6: Thiết kế qui trình Công nghệ sản xuất Formandehit từ metanol.

1:………

2:………

ĐỀ TÀI 7: Thiết kế qui trình Công nghệ sản xuất Axit axetic.

1:VÕ PHƯỚC TUYỂN………

2:NGUYỄN HỮU VƯƠNG………

ĐỀ TÀI 8: Thiết kế qui trình Công nghệ sản xuất Anilin.

1:………

2:………

ĐỀ TÀI 9: Thiết kế qui trình Công nghệ sản xuất Phenol.

1 NGUYỄN VĂN VIỆT

2 LÊ TRÍ

ĐỀ TÀI 10: Thiết kế qui trình Công nghệ sản xuất Etyl benzen.

1:ĐOÀN DUY TÙNG

2:TRỊNH ĐÌNH TUYỀN

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ĐỀ TÀI 11: Thiết kế qui trình Công nghệ sản xuất Etyl Acrylat.

1:………

2:………

ĐỀ TÀI 12: Thiết kế qui trình Công nghệ sản xuất MTBE.

1:PAHN VĂN TRUNG

2:NGUYỄN VĂN TRƯỜNG

ĐỀ TÀI 13: Thiết kế qui trình Công nghệ sản xuất ETBE.

1:HUỲNH KIM Ý

2:PHẠM THANH TÙNG

ĐỀ TÀI 14: Thiết kế qui trình Công nghệ tách Hydrocacbon thơm.

1:………

2:………

ĐỀ TÀI 15: Thiết kế qui trình Công nghệ tách n-parafin.

1:………

2:………

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ĐỀ TÀI 16: Thiết kế qui trình Công nghệ sản xuất Dicloetan.

1:LÂN QUỐC VIỆT

2:ĐẶNG MINH VƯƠNG

ĐỀ TÀI 17: Tìm hiểu Công nghệ tái sinh dầu nhờn.

1:NGÔ BÁ THÙY TRANG

2:LÊ THỊ MỸ VÂN

ĐỀ TÀI 18: Công nghệ thu hồi và xử lý dầu loang.

1:HÀ ANH TUẤN

2:NGUYỄN ĐÌNH VŨ

ĐỀ TÀI 19: Tổng quan về phụ gia cho xăng Etanol.

1:NGUYỄN HỮU VƯƠNG

2:VÕ PHƯỚC TUYỂN

ĐỀ TÀI 20: Tìm hiểu qui trình tổng hợp nhiên liệu sạch từ nguồn nguyên liệu

biomass Việt Nam bằng công nghệ tổng hợp F -T ở áp suất thường

1:NGUYỄN MINH TRUNG

2NGÔ THANH TRÍ

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ĐỀ TÀI 21: Tìm hiểu công nghệ sản xuất ethanol từ xenlulo.

ĐỀ TÀI 23: Tìm hiểu qui trình công nghệ tổng hợp mỡ nhờn canxi.

1:………

2:………

ĐỀ TÀI 23: Chưng cất dầu thô bằng phần mền hysys.

1:NGUYỄN THỊ KIM YẾN

2:ĐINH VĂN YÊN

2 Vinyl Acetate

2.1 Properties

Physical Properties Vinyl acetate [108-05-4], CH3CO2CH=CH2, Mr86.09, is a colorless,

flammable liquid with a characteristic, slightly pungent odor, bp 72.8 °C, density at 20 °C 0.932 g/mL, mp – 93.2 °C, viscosity 0.43 mPA · s, vapor pressure 12 kPa at 20 °C, 42.6 kPa

at 50 °C, coefficient of cubic expansion 0.0014 K–1, flashpoint – 8 °C, ignition temperature

385 °C Lower/upper flammability limits in air 2.3/13.4 vol %, ignition group (VDE 0165)

G 2, specific heat 1.926 kJ/kg; heat of evaporation 379.3 kJ/kg at 72.7 °C, heat of combustion2082.0 kJ/mol, refractive index 1.3956, heat of polymerization 1035.8 kJ/kg, solubility of water in vinyl acetate 0.9 wt % at 20 °C, solubility of vinyl acetate in water 2.3 wt % at

20 °C, azeotrope with water bp 66 °C/100 kPa, water content 7.3 wt %.

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Chemical Properties The chemical property which is exploited almost exclusively is the capacity to polymerize (see Section Use, Economic Importance).

Other reactions of vinyl acetate: halogens give 1,2-dihaloethyl acetates [6], hydrogen halides give 1-haloethyl acetates [7], acetic acid gives ethylidene diacetate (see Section Production), hydrogen cyanide gives 2-acetoxypropionitrile [8], hydrogen peroxide gives

hydroxyacetaldehyde [9], and dienes, such as butadiene or cyclopentadiene, give

Diels – Alder products [10]

Transesterification with carboxylic acids produces the corresponding vinyl carboxylate and acetic acid [11] (see also Section Quality Specifications, Analysis,Storage, Transport, and Toxicology) and with alcohols gives the corresponding acetate and acetaldehyde Thermal cleavage gives ketene and acetaldehyde [12] Acid-catalyzed [13] or thermal [14] hydrolysis produces acetaldehyde and acetic acid Vinyl acetate can be epoxidized with peracetic acid (84 % yield) [15] It undergoes addition of H-active compounds, e.g., dimethyl phosphite gives dimethyl acetoxyethylphosphonate [16]

2.2 Production

There are various possible routes for vinyl acetate production:

1 Addition of acetic acid to acetylene:

a in the liquid phase in the presence of homogeneous mercury salt catalysts

b in the gas phase in the presence of heterogeneous catalysts containing zinc salts

2 Addition of acetic anhydride to acetaldehyde giving ethylidene diacetate, and subsequent cleavage of the latter to form vinyl acetate and acetic acid

3 Reaction of ethylene with acetic acid and oxygen

a in the liquid phase in the presence of palladium/copper salts as homogeneous catalysts

b in the gas phase on heterogeneous catalysts containing palladium

4 Reaction of methyl acetate or dimethyl ether with carbon monoxide and hydrogen in the liquid phase in the presence of homogeneous catalysts, e.g., rhodium salts or noble metals

of the platinum group, giving ethylidene diacetate; cleavage of the latter giving acetic acid and vinyl acetate

Acetylene, which is expensive, has mostly been replaced by the cheaper alternative, ethylene;

ca 80 % of the available capacity is used for process 3 b and ca 20 % for process 1 b

Processes 1 a, 2, and 3 a are no longer used, while process 4, although allegedly developed to

an industrial level, has not yet been used industrially It may become more important because the starting materials can readily be produced from coal or naphtha

2.2.1 Addition of Acetic Acid to Acetylene

Liquid-Phase Process The addition of carboxylic acids to acetylene using mercury salts as thecatalyst [17] is now of historical interest only For more details see [18], [19]

Gas Phase Process

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The first process was developed in Munich by Consortium f Elektrochemische Industrie [20] It was further developed and used industrially by Wacker Chemie in Burghausen Until

1965, almost all vinyl acetate was produced by the acetylene gas-phase process Only two smaller plants used the ethylidene diacetate process [21] Zinc salts on activated charcoal have proved to be effective catalysts The development of suitable types of activated charcoalhas improved the process [22]

Most of the industrial development work was concerned with carrying out the reaction In theshaft furnaces used initially as reactors, controlling the heat of reaction was difficult An occasional runaway of the reaction led to baking of the catalyst Exothermic

autodecomposition of the acetylene could not always be avoided For better heat removal, other types of reactor employing a cooling medium were used At Hoechst, Fischer furnaces were initially used, and at Wacker-Chemie tube furnaces

As far as is known, all producers using the solid bed catalysts have started to use tube

reactors because of the defined gas flow, the easier charging and discharging of the catalyst, and the good temperature control Only at Kurashiki in Japan, Du Pont in the United States, and in some plants in the former Soviet Union have fluidized-bed reactors been used Solid bed and fluidized bed processes are considered of equal value

Process Description The modes of operation of individual producers no longer differ

significantly [23] The process used by Hoechst until 1975 is described as an example (Fig 1) Process data are given later in this section

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The mixture leaving the reactor is cooled in stages The last cooling stage takes place in the quenching tower (d) In this packed column, the gas mixture is cooled to 0 °C The

condensate itself is used for cooling It is circulated from the bottom to the top of the

quenching tower via a brine – cooled condenser The liquid product stream is removed; excess acetylene is recycled via a circulating gas blower (e)

The crude vinyl acetate produced at the bottom of the quenching tower is distilled giving acetic acid, which is recycled, and pure vinyl acetate Some 90 % of the circulating gas is acetylene; the remainder is CO2, CO, and methane formed from thermal decomposition, acetaldehyde, N2, and other inert substances To prevent enrichment of the impurities in the circulating gas, a small portion is removed behind the quenching tower (d) and then purified.The acetylene from the gas stream, which has been brought to ca 100 kPa overpressure by means of a liquid ring pump (f), is extensively absorbed in a washing column (g) containing brine-cooled vinyl acetate The inert substances are led as waste-gas to flare The sump product from (g) is freed from dissolved acetylene in a regeneration column (h) by boiling The acetylene is recycled The crude vinyl acetate formed in the bottom of the quenching tower (d) contains ca 62 – 63 wt % vinyl acetate and 30 –35 % acetic acid It also contains dissolved acetylene, acetaldehyde, crotonaldehyde, acetone, methyl acetate, ethylidene diacetate, and acetic anhydride

In the lightends column (i) acetaldehyde, acetone, methyl acetate, dissolved acetylene, some vinyl acetate, and water (originating from the starting materials) are first distilled overhead The sump product is fractionated in distillation columns ( j – m) Pure vinyl acetate is

removed as the top product of the column ( j) In (k) crotonaldehyde distills at the head as an azeotrope after addition of water The bottom of the acetic acid column (l) also contains, besides acetic acid, the high-boiling ethylidene diacetate and acetic anhydride (unless they have already been hydrolyzed in the crotonaldehyde column), and very small quantities of polymers Column (m) is operated under vacuum, and to some extent batchwise; the residual acetic acid distills so extensively that the liquid sump product which remains can

subsequently be incinerated The distillate from the lightends column (i) contains mainly acetaldehyde, acetone, methyl acetate, vinyl acetate, and water It is purified in distillation columns (o – q)

Stabilizers To avoid polymerization during the distillative work-up of the crude vinyl

acetate, polymerization inhibitors are added The preferred stabilizer is hydroquinone Copperresinate, phenothiazine, or methylene blue are also used

Materials and Environmental Aspects The plant is made from mild steel in the hot area of thereaction section and in places where no liquid acetic acid is present These areas include the

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reactor, the gas – gas heat exchanger, and the circulating gas blower For the distillation section and the equipment in the reaction section which comes into contact with liquid product, stainless steel 316 L is used The process has almost no polluting waste streams All gaseous or liquid byproducts (high-boiling, crotonaldehyde, and acetone – methyl acetate fractions) can be incinerated Waste circulating gas is passed to the excess gas burner.

Process Data The catalyst consists of zinc acetate on activated charcoal (particle size 3 –

4 mm) as the carrier material The zinc content is 10 – 15 wt % The catalyst is produced by dipping As the activated charcoal can contain traces of copper, small quantities of other components are added to the catalyst to prevent the formation of cuprene, which can block the tubes

The operating time is ca 5000 – 7000 h, depending on the type of activated charcoal used It also depends on the purity of the acetylene and the circulating acetic acid When using acetylene produced from carbide, the type of carbide is important Because of the varying contents of phosphorus hydrides, hydrogen sulfide, arsine, and ammonia in the acetylene, it sometimes has to be further purified before being used in vinyl acetate production This involves several steps

Acetylene produced from petrochemicals does not contain these components The decrease incatalyst activity is caused to a small extent by migration of zinc acetate out of the catalyst, but

to a greater extent by formation of byproducts or by foreign components, which either act as catalyst poisons or adhere to the catalyst surface and pores [24], [25] Vinyl acetate itself does not appear to contribute to the deactivation of the catalyst under the reaction conditions.The space – time yield for the vinyl acetate is normally 60 – 70 g per liter catalyst per hour.The reaction temperature is 160 – 170 °C, depending on the type of activated charcoal used

as catalyst, but increases to 205 – 210 °C as the catalyst activity decreases At the elevated temperature in the reactor, more byproducts are formed

The pressure can increase a little during the operating time of a catalyst charge This is caused by a slight increase in the flow resistance of the catalyst The maximum pressure is ca

40 kPa overpressure Plants are protected against overpressures greater than 40 – 50 kPa because acetylene can undergo exothermic autodecomposition at high pressures and

temperatures:

The acetylene pipes leading to the plant therefore contain barriers which inhibit the acetylene autodecomposition The plants are also provided with equipment for automatic flushing with inert gas The molar ratio acetic acid : acetylene is normally between ca 1 : 4 and 1 : 4.5 Theload per m3 catalyst is ca 135 m3 (STP) recycle gas per hour and 76 kg acetic acid per hour The recycle gas consists of ca 90 % acetylene Acetylene conversion is ca 15 % and acetic acid conversion ca 55 % The yields based on acetic acid can reach 99 %, or 98 % based on acetylene, if the acetaldehyde formed (2 – 3 kg per 100 kg vinyl acetate) is included in the yield

2.2.2 Ethylidene Diacetate Process

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The process was developed by Celanese Corporation of America [26] and was operated industrially in the United States from 1953 to 1970, and then replaced by the ethylene gas-phase process A small plant (Celmex) operated in Mexico until 1991 Acetic anhydride is converted to ethylidene diacetate with acetaldehyde in the liquid phase using catalysts such asiron(III) chloride The ethylidene diacetate is then cleaved thermally to vinyl acetate and acetic acid, using catalysts such as toluenesulfonic acid Some of the ethylidene diacetate is converted back to acetic anhydride and acetaldehyde

2.2.3 Reaction of Acetic Acid with Ethylene and Oxygen

Liquid-Phase Process The formation of vinyl acetate from ethylene and acetic acid in the presence of palladium chloride and alkali acetate in glacial acetic acid was first described by

MOISEEW [27]:

Addition of benzoquinone to the reaction mixture was said to reoxidize the palladium to palladium chloride The reaction corresponds to the Wacker – Hoechst process, in which acetaldehyde is obtained from ethylene and water in the presence of palladium chloride:

The palladium formed is reoxidized to Pd2+ with copper(II) chloride The copper(I) chloride formed is reoxidized with oxygen (→ Acetaldehyde) Production of vinyl acetate by a similarroute has been widely investigated [28-30] Corresponding production plants were

commissioned by ICI in England, Celanese in the United States, and Tokuyama

Petrochemical in Japan [31], but later shut down Now only the ethylene gas-phase process uses ethylene as the starting material (see below)

In the liquid-phase process, a recycle ethylene gas stream is passed through a reaction

solution containing acetic acid, water, high-boiling byproducts analogous to those of the acetaldehyde process, PdCl2, and CuCl2 Oxygen is passed into the reaction solution at the same time to reoxidize the palladium and the CuCl The reaction and regeneration of the catalyst take place in one step:

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A certain ratio of palladium ions to copper ions and of copper ions to chloride ions is

necessary for reoxidation of the palladium Chlorine, which is lost through formation of chlorinated byproducts, is replaced by hydrochloric acid To maintain the necessary quantity

of chloride ions in solution, alkali metal chloride must be added A typical reaction solution contains ca 30 – 50 mg/L palladium ions and 3 – 6 g/L copper ions

Byproducts include CO2, formic acid, oxalic acid, oxalic acid esters, chlorinated compounds, and butenes The pressure is 3 – 4 MPa, and the reaction temperature 110 – 130 °C The ratio

of acetaldehyde to vinyl acetate can be controlled by adjusting the water concentration and the residence time [32]

Gas-Phase Reaction The process was developed to an industrial scale only slightly later than the ethylene liquid-phase process, and has been used in industry since 1968 Currently, 80 %

of world vinyl acetate capacity uses the ethylene gas-phase process There are two variants: one developed by National Distillers Products (United States) [49], and the other

independently by Bayer in cooperation with Knapsack and Hoechst (Germany) [33-35] Mostplants employ the Bayer – Hoechst variant, of which there are several versions The original process has been further developed by various operators

In the ethylene gas-phase process, ethylene reacts exothermically with acetic acid and oxygen

on solid bed catalysts, giving vinyl acetate and water:

All catalysts used in industry contain palladium and alkali metal salts on carrier materials, e.g., silicic acid, aluminum oxide, lithium spinel, or activated charcoal Additional activators can include gold, rhodium, platinum, and cadmium

The reaction mechanism is assumed to involve either pure metal catalysis [36] or a reaction sequence according to the following equations:

The finely divided palladium on the catalyst is thought to be oxidized to divalent palladium For reoxidation of the palladium, a redox reaction analogous to the liquid-phase process is assumed Copper, manganese, and iron are cited as redox elements [37]

In the process, which operates above 140 °C and at overpressure 0.5 – 1.2 MPa, practically

no acetaldehyde is formed, even if the acetic acid used as starting material contains water Byproducts are water, CO2 and small quantities of ethyl acetate, ethylidene diacetate, and glycol acetates

Process Description: Reaction Section (Fig 2) The recycle gas stream, which consists

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mainly of ethylene, is saturated with acetic acid in the evaporator (a) and is then heated to thereaction temperature The gas stream is then mixed with oxygen in a special unit

The allowed oxygen concentration is determined by the flammability limits of the

ethylene – oxygen mixture The flammability limit depends on temperature, pressure, and composition It is shifted by additional components, such as acetic acid, nitrogen, and argon, which are brought in with the oxygen, or by CO2 In general, the oxygen concentration at the entry to the reactor is ≤8 vol %, based on the acetic-acid-free mixture It is essential to avoid gas mixtures capable of igniting; great care is taken in mixing in oxygen and measuring the oxygen concentration If the oxygen stream is switched off, the inlet line must be flushed with nitrogen immediately to avoid back-diffusion of the circulating gas The mixing

chamber is usually installed behind concrete walls The heat of reaction is removed in the form of steam via (c) from the tube reactor (b) by evaporative cooling in the shell side of the reactor

The reaction temperature is adjusted by the pressure of the boiling water The steam formed can be used within the plant itself in the work-up section The heat of reaction is ca

250 kJ/mol based on vinyl acetate, because of the simultaneous formation of CO2

Pressurized water in a circulation loop is used for cooling in some plants

Ethylene conversion is 8 – 10 %, and that of acetic acid 15 – 35 % Oxygen conversion can

be up to 90 % As small quantities of alkali metal salt on the catalyst migrate under the reaction conditions, traces of alkali metal salt are mixed with the gas at the entry to the reactor [38]

The gas mixture leaving the reactor is first cooled in (d) in countercurrent with the cold recycle gas, which is thus warmed There is virtually no condensation of the acetic acid, vinylacetate, or water The dew point is generally not reached The gas mixture is then led into the

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predehydration column (l) and then cooled to about room temperature in (e) The liquid product consists of an acetic-acid-free mixture of vinyl acetate and water The mixture is separated in a phase separator (m) into an aqueous phase, which is removed, and an organic vinyl acetate phase, which is recycled to the head of the predehydration column.

Between 40 and 50 % of the water formed in the reaction is removed in this way without the need to supply extra energy; this quantity of water does not need to be removed in the

subsequent distillation of the crude vinyl acetate Most of the energy consumed in the

distillation is used for water removal Crude vinyl acetate, which is low in water, collects in the sump of the predehydration column Older plants do not have this column [39] The noncondensed vinyl acetate fraction is washed out of the circulating gas with acetic acid in column (f) [40] The remaining gas is recycled via compressor (g), after addition of fresh ethylene

To remove the CO2 formed in the reaction, a partial stream of recycle gas is first washed withwater in column (h) to remove the remaining acetic acid The CO2 is then absorbed with potash solution in column (i) The potash solution is regenerated by depressurizing to normal pressure and boiling ( j) Depending on the quantity of CO2 formed in the reactor, the desired CO2 content of the circulating gas can be adjusted by altering the quantity of circulating gas present in the circulating gas wash, and the degree of absorption in the potash wash The CO2 concentration is generally 10 – 30 vol % [41], [42] It is also possible to perform water and CO2 washes in the main gas stream [43]

To remove inert gases (nitrogen, argon) mainly brought in with the oxygen, a small quantity

of waste-gas is removed before the CO2 absorption column (i) and then incinerated In some plants, part of the ethylene contained in this waste-gas is recovered by additional purification

to reduce ethylene loss

The liquid products formed, i.e., the condensate from the sump of the predehydration column (l) and the sump of the circulating gas wash, are depressurized to almost normal pressure and drained off into a collector for crude vinyl acetate (k) The circulating gas portions dissolved under the pressure of the circulating gas system are degassed and recycled to the circulating gas system after compression

Process Description: Distillation Section Distillative work-up of the liquid products to give acetic acid (which is recycled) and pure vinyl acetate is carried out in various ways,

depending on the location of the plant and on the relative importance of energy consumption and investment costs Besides the systems shown in Figures 3 and 4, combinations of both versions are used

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a second distillation (c) from dissolved water, other volatile products, and acetaldehyde, formed by vinyl acetate hydrolysis The sump product is dry vinyl acetate, which is distilled

in a third distillation column (d) to give pure vinyl acetate as the top product

To remove polymers, a small partial stream is removed from the sump of the third distillationcolumn and fed back to the first column Thus all nonvolatiles and polymers produced in the distillative work-up are contained in the sump of the first column, together with the recycled acetic acid To remove the polymers and nonvolatiles, a partial stream is removed from the acetic acid evaporator in the reaction section of the plant From this, the acetic acid for recycling is distilled so that the residue, which is still flowable, can be incinerated

The small quantities of ethyl acetate formed are removed through a side exit in the first column (a) as a mixture with acetic acid, water, and vinyl acetate [44]

If an additional column is used for the work-up, only the dissolved water is distilled over in the second distillation, in the third the light ends, and in the fourth the pure vinyl acetate [45]

In another version (Fig 4) the water contained in the crude vinyl acetate is removed as an azeotrope with the vinyl acetate together with volatile products, e.g., acetaldehyde, in a first distillation column (a), which operates at increased pressure The dry bottom product, which contains vinyl acetate, acetic acid, polymers, and nonvolatiles, is fractionated in a second column (b) into pure vinyl acetate as the top product, and acetic acid and nonvolatiles The last two are recycled Ethyl acetate is removed through a side exit in the second distillation column as a mixture with acetic acid and vinyl acetate [46], [47]

To avoid polymer formation during the distillative workup of the crude mixture,

polymerization inhibitors, e.g., hydroquinone, benzoquinone, or tert-butylcatechol, must be

added Passing in oxygen-containing gases is also said to inhibit polymerization [48]

Process Data The catalysts used always contain palladium as the metal or a salt, alkali metal

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salts, and additional activators, e.g., metallic gold or alkali acetoaurate, cadmium acetate, or noble metals of the platinum group Silicic acid of various structures, aluminum oxide, spinel,

or activated charcoal are mainly used as the carrier material [49-54], [55]

The space – time yield for vinyl acetate is 200 g L–1h–1, in older plants up to more than

1000 g L–1h–1, depending on the catalyst and the plant layout The life time of the catalyst is

≤ 4 a

The reaction pressure is 0.5 – 1.2 MPa overpressure The space – time yield for vinyl acetate increases with the reaction pressure and with the oxygen concentration in the reaction gas However, an increase in pressure shifts the flammability limits of ethylene – oxygen to lower oxygen concentrations, reducing the quantity of oxygen available, and consequently the quantity of vinyl acetate formed, so pressure limits are set; higher pressures also raise

equipment costs

The reaction temperature is generally > 140 °C It increases to > 180 °C towards the end of the catalyst life A lower reaction temperature results in the formation of less CO2, but then the heat produced in the reactor can no longer be used in the plant

The gas loading of the catalyst is 2 – 4 m3 (STP) per liter catalyst per hour The gas mixture contains 10 – 20 mol % acetic acid, 10 – 30 % CO2, and ca 50 % ethylene The maximum oxygen content is ca 1.5 % below the flammability limit, which varies with the composition

of the gas mixture and the reaction conditions The nitrogen and argon contents are adjusted according to the quantity of waste gas They are generally ca 10 %, but depend on the purity

of the oxygen used

For old plants, energy consumption is ca 3 t of heating steam per tonne of vinyl acetate produced As a result of process improvements, modern plants have a heating steam

consumption of 1.2 t per tonne of vinyl acetate

The yields are up to 99 % based on acetic acid, and up to 94 % based on ethylene, if the acetaldehyde, formed in small quantities by hydrolysis of vinyl acetate during the distillative work-up, is included in the yield

The process has not yet posed any environmental problems Volatile and nonvolatile liquid products are incinerated The water produced in the reaction can contain traces of acetic acid formed by hydrolysis of vinyl acetate in the wastewater column It is subjected to biological wastewater treatment To remove inert gases brought in with the oxygen, some of the waste-gas containing nitrogen and argon is burned after partial recovery of the ethylene it contains There are small quantities of residual ethylene in the CO2, formed in the regeneration column

of the potash absorber Removal of ethylene, e.g., by subsequent catalytic incineration, is now necessary in Germany because of more stringent regulations In addition, special

measures are required for waste-gas incineration

Plants are generally made from stainless steel 316 L, apart from the potash washer, which is made from normal steel or stainless steel 321

Proposals for Process Improvement Since the first plants for the ethylene gas-phase process were commissioned, numerous suggestions have been made for improving the process; some

of these are now being introduced industrially Proposals regarding the catalyst include: simplification of production methods [56]; activity improvement (increase in vinyl acetate space – time yield [57-60], [55], [61]; improvement in selectivity (less CO2 formation) [62], [63]; increase in life span [64-66]; reduction in flow resistance in the reactor (form of carrier material) [67]; better utilization of the noble metal content (distribution on the carrier) [68];

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regeneration of spent catalysts [69-74].

Proposals regarding the mode of operation include: increasing the vinyl acetate space – time yield by raising the oxygen concentration in the gas at the entry to the reactor by adding desensitizers [75]; facilitating separation of ethyl acetate from vinyl acetate [76-79];

increasing the selectivity of the reaction by adding other components to the reaction gas [80]; carrying out the reaction on a fluidized bed [81]; lowering the energy consumption, e.g., by making use of the heat of condensation of the gas coming out of the reactor for removal of the water formed (see Section Production) [39]; simplified work-up of the polymer-

containing residue [82]

2.2.4 Reaction of Methyl Acetate with CO and H 2

Several companies have attempted to develop a reaction which, in principle, has been known for a long time It involves the conversion of methanol, acetic anhydride, dimethyl ether, or preferably methyl acetate to ethylidene diacetate with carbon monoxide and hydrogen [83-95], [96-98]:

The reaction takes place in the liquid phase at ca 7 MPa and ca 150 °C Rhodium salts together with methyl iodide and amines can be used as catalyst The ethylidene diacetate formed can be cleaved to vinyl acetate and acetic acid, using the process previously

employed by Celanese (see Section Production) As the acetic acid formed in the process can

be reesterified to methyl acetate using methanol, the process essentially involves production

of vinyl acetate from methanol and synthesis gas:

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2.3 Quality Specifications, Analysis,Storage, Transport, and Toxicology

Quality The specifications of individual producers often differ only slightly with regard to the content of impurities One specification requires: vinyl acetate ≥ 99.9 wt %, acid (as acetic acid) ≤ 0.005 wt %; carbonyl (as acetaldehyde) ≤ 0.02 wt %, water ≤0.04 wt %; distillation range 95 % at 72 – 73 °C within 0.5 °C; peroxide free; polymer free; capacity for polymerization (see Section Use, Economic Importance)

Analysis The following methods are used for quality testing:

Carbonyl groups: titration of the hydrochloric acid formed after conversion to the oxime

using hydroxylamine hydrochloride solution

Water content: titration with Karl-Fischer solution

Acid content: neutral to litmus

Peroxide: test with potassium iodide solution

Polymer content: mixtures with petroleum ether must not produce turbidity or precipitation

of solids

Polymerization test: carried out differently by individual producers; the time taken for the

onset of polymerization following the addition of a defined quantity of dibenzoyl peroxide and warming to a defined temperature

Storage As a flammable liquid, vinyl acetate is assigned to VbF, group A, class 1, and ignition group T 2 according to VDE 0165 It can be stored in steel, aluminum, or stainless steel containers under nitrogen It is not necessary to add stabilizers at lower temperatures If the vinyl acetate is to be warmed, stabilizers, such as hydroquinone, hydroquinone

monomethyl ether, or diphenylamine are added The quantity of stabilizer used is small, e.g.,

3 – 20 ppm hydroquinone, so that it does not generally need to be removed during the later polymerization

Transport Vinyl acetate is transported in practically all quantities: small containers, drums, tankers, tank cars, and ships Stabilizers are usually added for transportation

Safety Data

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Toxicological Data

Protection at Work

Compatibility with water

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2.4 Use, Economic Importance

Vinyl acetate is used mainly for the production of polymers and copolymers, e.g., for paints (mainly dispersions), adhesives, textile and paper processing, chewing gum [→ Poly(Vinyl Esters), and for the production of poly(vinyl alcohol) (→ Paints and Coatings – Poly(Vinyl Alcohol)) and polyvinylbutyral (→ Paints and Coatings – Poly(Vinyl Acetals))

Vinyl acetate – ethylene copolymers (→ Poly(Vinyl Esters) – Properties) are processed to give resins, paints (→ Paints and Coatings – Poly(Vinyl Esters)), and sheeting Floor

coverings and gramophone records are made from vinyl acetate – vinyl chloride copolymers Vinyl acetate is also used in small quantities as a comonomer in polyacrylic fiber production.Uses differ according to the region In Japan, ca 70 % of vinyl acetate is used in the

production of poly(vinyl alcohol), while in the United States and Europe more than half is processed to give poly(vinyl acetate)

The total capacity for vinyl acetate production was < 106 t/a in 1965 In 1994 it was ca 3.8×106 t/a The rapid increase has been promoted significantly by the development of the ethylene gas-phase process After ethylene had become one of the cheapest raw materials in the chemical industry, it was obvious that vinyl acetate should be produced from ethylene instead of acetylene The vinyl acetate capacities of individual countries are listed in Table 1

Table 1 World vinyl acetate capacities

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A plant with a capacity of 150 000 t/y is to be commissioned by Hoechst Celanese in South East Asia in 1997.

The economic optimum of individual process variants depends on location Investment costs, material consumption (acetic acid, ethylene, acetylene), and energy consumption need to be compared The weighting depends on the prices of energy and starting materials, and on official regulations specific to a particular country

Figure 1 Acetylene gas phase process for vinyl acetate production

a) Acetic acid evaporator; b) Reactor; c) Heat transfer oil, cooling loop; d) Quenching tower; e) Recycle gas blower; f) Liquid ring pump; g) Washing column; h) Regenerating column; i) Lightends column; j) Pure vinyl acetate column; k) Crotonaldehyde column; l) Acetic acid column; m) Residue column; n) Degassing

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column; o) Acetaldehyde column; p) Acetone column; q) Water removal

Figure 2 Ethylene gas phase process for vinyl acetate production; reaction section

a) Acetic acid evaporator; b) Reactor; c) Steam drum; d) Countercurrent heat exchanger; e) Water cooler; f) Recycle gas washing column; g) Recycle gas compressor; h) Water wash; i) Potash wash; j) Potash

regeneration; k) Crude vinyl acetate collector; l) Predehydration column; m) Phase separator

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Figure 3 Ethylene gas-phase process for vinyl acetate production; work-up of crude vinyl acetate

a) Azeotrope column; b) Wastewater column; c) Drying/lightends column; d) Pure vinyl acetate column

Figure 4 Ethylene gas-phase process for vinyl acetate production; variant for work-up of crude vinyl acetate

a) Dehydration column; b) Pure vinyl acetate column; c) Wastewater column

AXIT AXETIC

1 Introduction

Acetic acid [64-19-7], CH 3COOH, Mr 60.05, is found in dilute solutions in many plant and animal systems Vinegar, an aqueous solution containing about 4 – 12 % acetic acid, is produced by the fermentation of wine and has been known for more than

The major producers of acetic acid, accounting for ca 70 % of total worldwide production, are the United States, Western Europe, and Japan World capacity exceeds 7 × 10 6 t/a [ 1 ] The largest end uses are in the manufacture of vinyl acetate [108-05-4] and acetic anhydride [108-24-7] Vinyl acetate is used in the production

of latex emulsion resins for applications in paints, adhesives, paper coatings, and

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textile treatment Acetic anhydride is used in the manufacture of cellulose acetate textile fibers, cigarette filter tow, and cellulose plastics.

2 2 Physical Properties

Acetic acid is a clear, colorless, corrosive liquid that has a pungent odor and

is a dangerous vesicant It has a pKa of 4.77 [2] It melts at 16.75 °C [3] and

boils at 117.9 °C [4] under 101.3 kPa [5] It has a pungent vinegarlike odor The detectable odor is as low as 1 ppm The acid is combustible with a low flash point of 43 °C The explosion limits of acetic acid vary from the upper explosion limit (UEL) of 16 % at 92 °C to the lower explosion limit (LEL)

of 4 % at 59 °C The liquid is usually available as glacial acetic acid with less than 1 wt % water and over 98 % purity Besides water, the acid

contains traces of impurities such as acetaldehyde, oxidized substances, iron,and chlorides

Occasionally, the acid may be colored due to the presence of ethyl

acetoacetate [141-97-9] The acetate is easily mistaken for formic acid because it reduces mercuric chloride Traces of mercury may cause

extensive corrosion by reaction with aluminum Aluminum is a common material for containers to ship the acid [6]

Glacial acetic acid is very hydroscopic The presence of 0.1 wt % water lowers the freezing point significantly [7] Measuring the freezing point is a convenient way to evaluate acetic acid purity This is shown in Table 1 [8]

5.

Acetic acid forms azeotropes with many common solvents, such as benzene,pyridine, and dioxane Acetic acid is miscible with water, ethanol, acetone, benzene, ether, and carbon tetrachloride However, it is not soluble in CS2 [2]

The physical properties of acetic acid are well documented, and their

accuracy is important for commercial production For instance, design and operation of distillation processes requires precise data High-precision values provide a valuable asset to the chemical industry [9], [10]

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The density of mixtures of acetic acid and water [11-13] is listed in Table 2The density exhibits a maximum between 67 wt % and 87 wt %,

corresponding to the monohydrate (77 wt % acetic acid) The density of pureacetic acid as a function of temperature is listed in Table 3 [14], [15]

11.

Due to the difficulty in eliminating traces of water from acetic acid, the value for the boiling point varies from 391 to 392 K [10] Careful studies prove that pure acetic acid boils at 391.10 K under 101.325 kPa [16] The critical temperature and critical pressure are 594.45 K and 5785.7 kPa [3].Precise data on vapor pressure of acetic acid are available from a regression

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equation (Eq 1) [10], which covers the range from the normal boiling point

to the critical point

The vapor pressure of pure acetic acid is given in Table 4 [17] The density

of the vapor corresponds to approximately twice the molecular mass because

of vapor-phase hydrogen bonding [8] Hydrogen-bonded dimers and

tetramers have both been proposed Reports indicate that in the gas phase, the acid exists mainly in an equilibrium between monomer and dimer (Eq 2)according to vapor density data [18], [19] molecular modeling, IR analysis [20], and gas-phase electron diffraction [21]

18.

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In the gas phase, monomic and dimeric acetic acid undergo extensive hydrolysis [22], [23] In the liquid state, acetic acid equilibrates between monomer and dihydrated dimer or cyclic dimer (Eqs 2 and 3) [24] As the concentration of acetic acid increases, the equilibrium shifts to the right, favoring dimeric acetic acid As the temperature is increased, the system shifts to the left, favoring the monomer In addition, the acid may also form open-chain trimers and higher oligomers However, at about 95 wt %, the acid exists mainly as a cyclic dimer and is no longer associated with water

19.

Up to 32 wt %, mixing acetic acid with water leads to evolution of heat At higher acid concentrations heat is absorbed [25] The measured values of theheat of mixing are consistent with the calculated values based on the dimers and tetramers described above The aqueous mixture of acetic acid forms a eutectic mixture at – 26 °C This eutectic mixture prevented earlier attempts

to concentrate the acid by freezing A way to obtain pure acid is to add urea

or potassium acetate to the acid Then, glacial acetic acid can be distilled.Other physical properties of acetic acid are listed below

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by the thermal decomposition of ammonium acetate Acetic acid can be converted to acetyl

Acetic acid is a raw material for a number of commercial processes It can be converted to vinyl acetate with ethylene and oxygen ( → Vinyl Esters ) Acetic acid is used in the manufacture of acetic anhydride ( → Acetic Anhydride and Mixed Fatty Acid Anhydrides ) via ketene and in the production of chloroacetic acid ( → Chloroacetic Acids ) using chlorine.

4 Production

Vinegar is still made by fermentation (→ Vinegar) However, the most important synthetic routes to acetic acid are methanol carbonylation and liquid-phase oxidation of butane, naphtha, or acetaldehyde Methanol carbonylation has been the method of choice for the past 25 years [38-40] and will likely remain the preferred route for large-scale production.Several new technologies for producing acetic acid are being studied Showa Denko may produce acetic acid by the gas-phase reaction of ethylene with oxygen over a supported palladium catalyst that contains a heteropolyacid or salt [41] Numerous patents and

publications discuss the production of acetic acid directly from ethane and oxygen

Production of acetic acid and acetate salts by microorganisms has also received

considerable attention

4.1 Carbonylation of Methanol

The manufacture of acetic acid from methanol [67-56-1] and carbon monoxide [630-08-0]

at high temperature and high pressure was described by BASF as early as 1913 [42]

In 1941 REPPE at BASF demonstrated the efficiency of group VIII metal carbonyls as

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catalysts for carbonylation reactions, including hydroformylation [43], [44] This led to the development of a high-pressure, high-temperature process (700 bar, 250 °C) with a cobalt iodide catalyst The thrust of this work was to develop an acetic acid process not dependent

on petroleum-based feedstocks The current advantage of the methanol carbonylation route

to acetic acid is the favorable raw material and energy costs The synthesis gas raw materialrequired for this process can be obtained from a variety of sources, which range from natural gas to coal The cobalt-based carbonylation process was commercialized in 1960 byBASF in Ludwigshafen, Federal Republic of Germany [43], [45-47] The initial capacity of

3600 t/a was expanded to 45 000 t/a by 1981 [48] In 1966 Borden Chemical Co started up

a 45 000 t/a acetic acid unit in Geismar, Louisiana, United States, based on the BASF technology [43], [45] The unit was expanded to 64 000 t/a by 1981 before it was shut down in 1982 [1], [48] This unit was brought on stream again in 1988 for one year to meet acetic acid supply shortages in the United States

Monsanto developed a low-pressure acetic acid process in the late 1960s with a rhodium iodide promoted catalyst system that demonstrated significantly higher activity and

selectivity than the cobalt-based process Methanol can be carbonylated even at

atmospheric pressure with yields of 99 % and 90 % with respect to methanol and carbon monoxide, respectively [49] This process was proven commercially in 1970 at Texas City, Texas The initial plant capacity of 135 000 t/a has been expanded to 270 000 t/a since

1975 [1] Operating conditions in the reactor are much milder (3 MPa and 180 °C) than in the BASF process [50] Soon after the Monsanto process was commercialized, the BASF process became uncompetitive, so the Monsanto process is the preferred technology for grass-roots acetic acid units Since the start-up of the Texas City plant by Monsanto, more than ten companies have licensed and operated this technology worldwide

In 1978 at the Clear Lake Texas Plant, Celanese Chemical Company (now Celanese, Ltd.) was the first licensee to demonstrate commercially the rhodium-catalyzed Monsanto

process Initial capacity was 27 000 t/a [51] In the early 1980s, Celanese developed a proprietary low-reaction-water rhodium-catalyzed methanol carbonylation process by modification of the original Monsanto high-reaction-water chemistry Modifications to the catalyst system by the addition of inorganic iodide salts improved catalyst stability and activity significantly [52] This technology improvement enabled the Clear Lake unit to expand by more than three times the original capacity to 900 000 t/a with minor capital cost[53]

In 1986, BP Chemicals purchased from Monsanto the high-reaction-water, low-pressure, rhodium-catalyzed methanol carbonylation technology and licensing rights The acquisition

of the technology did not include the improvements developed by Celanese Chemical Company

Monsanto in the early 1960s also discovered that iridium, like rhodium, is an effective methanol carbonylation catalyst This catalyst system has since been developed

commercially by BP in the early 1990s and is known as the Cativa process [54] This process was utilized to convert the original rhodium-catalyzed methanol carbonylation plant in Texas City to an iridium-based process Several advantages claimed by BP for the Cativa process over the conventional rhodium-catalyzed high reaction water carbonylation process include superior catalyst stability, operation at lower reaction water, and less liquid byproducts [55]

Chemistry and Reaction Conditions The chemistry of the cobalt- (BASF), rhodium- (Monsanto and Celanese), and iridium-catalyzed (BP) processes is similar in requiring promotion by iodide, but the different kinetics indicate different rate-determining steps In all three processes, two important catalytic cycles are common, one that involves the metal

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carbonyl catalyst and one that involves the iodide promoter [56], [57].

The cobalt-catalyzed BASF process uses cobalt(II) iodide [15238-00-3] for in situ

generation of [Co2(CO)8] and hydrogen iodide [10034-85-2] Compared to other methanol carbonylation processes, severe conditions are required to give commercially acceptable reaction rates The rate of reaction depends strongly on both the partial pressure of carbon monoxide and the methanol concentration Acetic acid yields are 90 % based on methanol and 70 % based on carbon monoxide A proposed mechanism for the iodide-promoted reaction is summarized in Figure 1 [43], [56], [57]

(4a)

Subsequently, the methyl iodide formed from hydrogen iodide and methanol undergoes nucleophilic attack by the [Co(CO)4]– anion Iodide facilitates this reaction because it is a better leaving group than OH– The CH3I reacts with a coordinatively saturated d10

complex, which is the preferred electron configuration of cobalt(I) Therefore, the methyl migration to form the acyl cobalt carbonyl complex, [CH3C(O)Co(CO)3], is less favored than the same process for the rhodium(III) species

Once formed, this acyl intermediate cannot undergo simple reductive elimination of acetyl iodide because iodide is not coordinated to cobalt Acetyl iodide is formed from the

reaction of hydrogen iodide with the acyl complex to regenerate the cobalt carbonyl anion Rapid hydrolysis of the acetyl iodide forms acetic acid and hydrogen iodide All of the individual steps involved in the otherwise similar mechanism can be assumed to occur at a lower rate for cobalt than for rhodium and iridium This explains the higher temperature needed for the BASF process In addition, higher carbon monoxide partial pressures are required to stabilize the [Co(CO)4]– complex at the higher reactor temperatures

Byproducts in the BASF process are CH4, CH3CHO, C2H5OH, CO2, C2H5COOH, alkyl acetates, and 2-ethyl-1-butanol [58-60] About 3.5 % of the methanol reactant leaves the system as CH4, 4.5 % as liquid byproducts, and 2 % is lost as off-gas Some 10 % of the

CO feed is converted to CO2 by the water gas shift reaction (Eq 4b)

(4b)

The Monsanto process with rhodium carbonyl catalyst [38255-39-9] and iodide promoter

operates under milder conditions than the BASF cobalt-catalyzed process Methanol and carbon monoxide selectivities of greater than 99 % and 90 %, respectively, are obtained

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[49] The system is not as sensitive to hydrogen as the BASF process, and therefore

reduction products such as methane and propionic acid are comparatively insignificant

The chemistry of the rhodium-catalyzed methanol carbonylation reaction has been studied

in detail [57] Kinetic studies show the reaction to be zero-order in carbon monoxide and methanol, and first-order in rhodium and iodide promoter The carbonylation rate is

strongly affected by the reaction media, but the overall kinetics are unaffected by the solvent, which suggests that it does not participate in the transition state of the rate-

determining step [61-64] Methanol carbonylation in polar solvents generally provides a rate enhancement, especially with the addition of protic solvents An acetic acid/water solvent medium is preferred [65]

Many different rhodium compounds act as effective catalyst precursors for methanol carbonylation at common reaction temperatures of 150 – 200 °C The iodide promoter is normally methyl iodide, but other forms of iodide, such as hydrogen iodide or iodine, can

be used without marked differences in reaction rates

Spectroscopic investigations have shown that rhodium(III) halides can be reduced in aqueous or alcoholic media to [Rh(CO)2X2]– [50] The reaction rate is independent of the rhodium precursor charged to the reaction provided adequate iodide, generally methyl iodide, and carbon monoxide are available Under these conditions, [Rh(CO)2I2]– is the predominant rhodium species in the reaction solution These observations strongly suggest the generation of [Rh(CO)2I2]– as the active catalyst species [50]

The catalytic cycle shown in Figure 2 is based on kinetic and spectroscopic studies [66-78].The complex anion [Rh(CO)2I2]– reacts in the rate-determining step with methyl iodide by oxidative addition to form the transient methylrhodium(III) intermediate Methyl migration gives the pentacoordinate acyl intermediate The acyl intermediate eliminates acetyl iodide and regenerates [Rh(CO)2I2]– The acetyl iodide reacts with water to regenerate hydrogen iodide and produce acetic acid Hydrogen iodide reacts with methanol to form methyl iodide In this way both the original rhodium complex and methyl iodide promoter are regenerated

In the 1980s Celanese made innovative improvements to the rhodium catalyst system of the

Monsanto process with the implementation of the proprietary acetic acid optimization (AO)technology which incorporates catalyst co-promoters Advantages of AO technology are enhanced carbonylation rates and increased carbon monoxide and methanol efficiencies at lower reaction-water concentrations [52] These modifications represent the most

significant development in rhodium-catalyzed methanol carbonylation since the

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development of the Monsanto process The AO technology enabled the Celanese Clear Lake Plant to more than triple unit capacity from 27 000 t/a since start-up in 1978 to

90 000 t/a acetic acid in 1998 with very low capital investment [51], [53]

The increased carbonylation rates at low reaction water concentrations are achieved by catalyst promotion with iodide and acetate anions [38], [70-72] This unique enhancement

in catalyst activity is due presumably to the generation of a strongly nucleophilic coordinate dianionic catalyst species, namely, [Rh(CO)2I2(L)]2– (L = I– or OAc–), which is more active than [Rh(CO)2I2]– toward oxidative addition of methyl iodide This additional reaction pathway is summarized in Figure 2

five-The primary byproducts and major inefficiency with respect to carbon monoxide in the rhodium-catalyzed process are carbon dioxide and hydrogen via the water gas shift reaction(Eq 4) The water gas shift reaction is also catalyzed by [Rh(CO)2I2]– [50], [79-81] The overall two-step reaction is summarized in the following:

The proposed kinetic pathway for the water gas shift reaction is shown in Figure 2, which emphasizes the interrelation between methanol carbonylation and water gas shift reaction

as catalyzed by [Rh(CO)2I2]– The addition of inorganic iodide co-promoters in the

Celanese process reduces significantly the proportion of [Rh(CO)2I2]– that catalyzes the water gas shift reaction As a result, the conversion of carbon monoxide and methanol to acetic acid is improved over the conventional Monsanto process, and the rate of methanol carbonylation is increased [38]

Compared to the cobalt-based process, byproducts derived from methanol such as CH4, CH3CHO, and C2H5COOH are formed in very small amounts, even in the presence of significant amounts of hydrogen in the carbon monoxide feed gas [82] This low methanol inefficiency is associated with occurrence of the rate-determining step prior to formation of

a organometallic compound, the short lifetime of the methylrhodium complex, and the rapid reductive elimination of the acylrhodium complex to form acetyl iodide, which is rapidly hydrolyzed to acetic acid and hydrogen iodide

Similar to the rhodium-based carbonylation processes, the reaction chemistry of the BP

iridium-catalyzed methanol carbonylation is well characterized [50], [56], [57], [78], 88] The iridium-catalyzed reaction proceeds through a series of similar reaction pathways

[85-to the rhodium-catalyzed system but involves a different rate-determining step The

proposed rate-determining step is methyl migration to form the iridium acyl complex This pathway involves the elimination of iodide and the subsequent addition of carbon

monoxide The direct dependence of the reaction rate on carbon monoxide concentration and the inhibiting effect of low concentations of iodide are consistent with the rate-

determining step [55], [83] The proposed catalytic scheme for the reaction is given in Figure 3 [55] Model studies at 25 °C demonstrate that the oxidative addition of methyl iodide to iridium is about 120 – 150 times faster than for rhodium [55], [83] However, methyl migration on iridium is 105 to 106 times slower than for rhodium [83] The main byproducts of the iridium-catalyzed process are also carbon dioxide and hydrogen via the water gas shift reaction (Eq 4b) The mechanism of this reaction is similar to the rhodium-catalyzed process [57] The production of CH4 derived from methanol is higher for iridium

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than for rhodium This observation is consistent with the greater stability of the

methyliridium(III) complex associated with the hydrogenation of the iridium – carbon bond[83]

Other transition-metal complexes have been investigated as promising catalysts for

methanol carbonylation, in particular, nickel complexes by Halcon and Rhône-Poulenc [57], [89], [90]

BASF Process [91] (Fig 4)

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the intermediate-pressure separator (e) The gas released is also sent to the wash column; the liquid from the intermediate-pressure separator is sent to the expansion chamber (f) The gas from the chamber goes to the scrubber (p) The gas from the scrubber and the washcolumn is discarded as off-gas Both scrubber and wash column use the methanol feed to recover methyl iodide and other iodine-containing volatile compounds; this methanolic solution is returned to the reactor The off-gas composition in vol % is 65 – 75 CO, 15 – 20CO2, 3 – 5 CH4, and the balance CH3OH.

The raw acid from the expansion chamber contains 45 wt % acetic acid, 35 wt % water, and 20 wt % esters, mainly methyl acetate The acid is purified in five distillation towers The first column (h) degasses the crude product; the off-gas is sent to the scrubber column The catalyst is then separated as a concentrated acetic acid solution by stripping the volatilecomponents in the catalyst separation column (i) The acid is then dried by azeotropic distillation in the drying column (k) The overhead of the drying column contains acetic and formic acids, water, and byproducts that form an azeotrope with water This overhead

is a two-phase system that is separated in the chamber (g) Part of the organic phase, composed mainly of esters, is returned to column (k), where it functions as an azeotroping agent The remainder of the organic phase is sent to the auxiliary column (n) where heavy ends are separated at the bottom of the column, and light esters from the overhead are recycled to the reactor The aqueous phase and the catalyst solution are returned to the reactor The base of the drying column is sent to a finishing column (l), in which pure acetic acid is taken overhead The bottom stream of the finishing tower is sent to the residue column (m) The overhead of this residue column is sent back to the dehydration column The bottom of the residue column contains about 50 wt % propionic acid, which can be recovered

Monsanto Process [38], [83], [92] (Fig 5)

liquid-to the reacliquid-tor The reacliquid-tor solution is forwarded liquid-to the flasher (b) where the catalyst is separated as a residue stream from the crude acetic acid product and recycled to the reactor.The crude acetic acid, which contains methyl iodide, methyl acetate, and water, is taken overhead in (b) and sent to the light-ends column (c) The light components (methyl iodide,methyl acetate, and water) are recycled to the reactor as a two-phase overhead stream,

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while wet acetic acid is removed as a side stream from (c) and sent to the dehydration column (d) An aqueous acetic acid overhead stream from (c) is recycled to the reactor and

a dry acetic acid product residue stream is forwarded to a heavy ends column (e) As a residue stream in (e), propionic acid, which is the major liquid byproduct of the process is removed with other higher boiling carboxylic acids Product acetic acid is removed in (e) as

a sidestream, and the overhead stream is recycled to the purification section of the process.Since the Cativa process as demonstrated in the Sterling Plant at Texas City, Texas is a retrofit of the original Monsanto process, the overall general process is presumably the same, possibly with some modifications [54], [93], [94]

4.2 Direct Oxidation of Saturated Hydrocarbons

Liquid-phase oxidation (LPO) of aliphatic hydrocarbons was once practiced worldwide [95] Due to competition from carbonylation technology, plants have reduced production

by LPO significantly The process changes depending on the availability of raw materials

Raw materials include n-butane and light naphtha [96] In the United States and Canada,

Celanese employs butane, while BP in the United Kingdom uses light naphtha to produce acetic acid [97] Capacities of related production are shown in Table 5 [99]

Table 5 Butane liquid-phase oxidation processes

Reaction Mechanism Oxidation of hydrocarbons follows similar kinetics, both in the gas and liquid phases, especially in slightly polar solvents [98] However, the mechanism of thereaction is very complicated The reaction can be considered roughly as a radical chain reaction [99] For example, the oxidation of butane proceeds through initiation, oxidation, propagation, and decomposition steps [96] The initiation and propagation probably involveradicals abstracting hydrogen from a secondary carbon atom of butane Subsequent

reaction with oxygen yields hydroperoxides These intermediates decomposes to produce acetic acid Catalysts, agitation, and high temperature accelerate the decomposition

Catalysts are not essential for LPO [100] However, metal catalysts may influence the distribution of products, induction period, and operating temperature [101], [102]

In a simple mechanism, the first step of oxidation is the abstraction of a secondary

hydrogen atom (Eq 5) to give alkyl radicals The oxygen in the solvent rapidly converts

these radicals to sec-butylperoxy radicals [103] (Eq 6) In other interpretations, oxygen is

believed to directly react with one or two alkane molecules to form radicals (Eqs 7 and 8) [104], [105], [106] Initiation, especially with catalysts, affects the induction period

significantly [107]

(5)

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(9)

(10)

(11)

(12)

(13)

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The hydroperoxide decomposes to give an alkoxy radical by thermolysis The radical undergoes a bimolecular reaction to terminate the formation of radicals (Eq 11) In the steady state, the termination of radicals is balanced by their production This mechanism is too simple to explain the reaction in detail In real systems, thermolysis may not be the onlycourse of decomposition [108].

Alternatively, hydroperoxides may come from the complexation of metal catalysts with peroxy radicals (Eq 12) Hydroperoxide radicals generate new radicals (Eqs 10, 13) Therefore, the catalyst is important in maintaining a constant flux of radicals However, in certain cases, catalysts may actually inhibit the decomposition [109]

Manganese and cobalt are common catalysts for LPO These metals accelerate the

decomposition of hydroperoxide There is evidence that manganese may enhance the oxidation of ketone intermediates through a mechanism involving enols [110]

The peroxy radicals terminate by forming tetroxides (Eq 11), which decompose to yield alkoxy radicals and oxygen (Eq 14)

If the tetroxide has an -hydrogen atom, the decomposition may follow a Russel

mechanism [111] The products are oxygen, a ketone, and an alcohol (Eq 15) However, the mechanism is controversial Recent reports both support [112] and challenge the mechanism [113]

Besides hydrogen abstraction, alkoxy radicals can participate in -scission (Eq 16)

Products vary depending on the structure of the radicals Primary, secondary, and tertiary alkoxy radicals yield alcohols, aldehydes, and ketones, respectively

The mechanism of butane oxidation is complex However, with detailed understanding of product distribution and rates, a mathematical model was developed [103]

About 25 % of the carbon of consumed butane appears as ethanol in the initial step as the first isolable non-peroxidic intermediate The reaction probably involves decomposition of

sec-butoxy radicals to form acetaldehyde and an ethyl radical (Eqs 16, 17) Another source

of acetaldehyde is the oxidation of ethanol Acetaldehyde then reacts rapidly to produce acetic acid Therefore, acetaldehyde is a major intermediate in butane LPO

(17)

Besides acetaldehyde, 2-butanone or methyl ethyl ketone is another major byproduct The

ketone is the result of the termination of sec-butylperoxy radical by the Russel mechanism

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(Eq 15).

Other impurities include propionic acid and butyric acid secButoxy radicals undergo

-scission to yield propionaldehyde Oxidation of the aldehyde gives propionic acid Butyric

acid is derived from n-butyl radicals.

Higher paraffins are oxidized by similar mechanisms to that of butane However, the products include shorter chain methyl ketones and difunctional intermediates These are theresults of intramolecular hydrogen abstraction European countries usually produce acetic acid from naphtha since naphtha is cheap and available However, the naphtha-based processes yield large amounts of impurities that increase the cost of purification

Industrial Operation [6], [114-116] (Fig 6) Air or oxygen-enriched air can be used as the oxidant Multivalent metal ions, such as Mn, Co, Ni, and Cr, are used as catalysts Some processes, however, are noncatalytic Reaction conditions are 150 – 200 °C for a range of reaction pressures that include 5.6 MPa The reaction pressure for naphtha oxidation is lower The reaction solvent consists of acetic acid, varying amounts of intermediates, water, and dissolved hydrocarbons Control of the water concentration below some

maximum level appears to be critical

is rich in hydrocarbons and is recycled to the reactor The bottom, aqueous layer, is

distilled to recover the hydrocarbon for recycle

The residual, hydrocarbon-free product consists of volatile, neutral oxygenated derivatives (aldehydes, ketones, esters, and alcohols), water, volatile monocarboxylic acids (formic, acetic, propionic, and butyric from butane), and nonvolatile materials (difunctional acids, -butyrolactone, condensation products, catalyst residues, etc)

The volatile neutral substances can be recovered as mixtures or individually They are used for derivatives, sold, or recycled to the reactor Most of these components generate acetic acid on further oxidation

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The separation of water and formic acid from acetic acid involves several distillations (f) Water removal is the most difficult and costly step of the purification process It is

accomplished by azeotropic distillation with entrainment agents, such as ethers, or by extractive methods

Formic acid is separated from the resulting anhydrous acetic acid usually by fractionation with an azeotroping agent The remaining higher boiling acids are separated from acetic acid as a residue stream by fractionation

In some cases, hydrocarbons such as heptane and isooctane can be added to the reactor to improve separation These chemicals form a high-boiling azeotrope with formic acid

In the case of naphtha oxidation, diacids such as succinic acids are isolated for sale [117] The nonvolatile residue can be burned to recover energy

Iniation:

Propagation:

Trang 39

Peracetic acid reacts with acetaldehyde to generate acetaldehyde monoperacetate 0] The acetaldehyde monoperacetate decomposes efficiently to acetic acid by a hydride shift in a Baeyer – Villiger reaction The methyl migration leads to the byproduct methyl formate [118]:

[7416-48-The alkyl migration becomes more pronounced with higher aldehydes, particularly

aldehydes having a branch at the -position

Chain termination occurs primarily through bimolecular reactions of acetylperoxy radicals via an intermediate tetroxide (Eq 22) [119]

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Equations (23) – (25) are the source of most of the carbon dioxide, methanol,

formaldehyde, and formic acid byproducts Uncatalyzed oxidation is efficient so long as the conversion of acetaldehyde is low and there is a significant concentration of aldehyde

in the solvent This keeps the steady state concentration of acetylperoxy radicals low and favors the Baeyer – Villiger reaction over the reactions (23) – (25) Special precautions must be taken in the uncatalyzed reaction to prevent the concentration of acetaldehyde monoperacetate from reaching explosive levels [6]

At low oxygen concentrations another free-radical decomposition reaction becomes

important (Eq 26)

The rate of decarbonylation increases with increasing temperature Decarbonylation

becomes significant when insufficient oxygen is present to scavenge the acetyl radicals

Catalysts can play several important roles in aldehyde oxidations [120] Catalysts

decompose peroxides and so minimize the explosion hazard (Eq 27) In addition,

manganese also reduces acetylperoxy radicals [121] directly to peroxy anions [122]:

Reaction with Mn2+ assists in suppressing the concentration of acetylperoxy radicals The

Mn3+ formed can generate the acyl radical for the propagation step (Eq 28), but does not contribute to inefficiency-generating reactions

Manganese also greatly increases the rate of reaction of peracetic acid and acetaldehyde to produce acetic acid [123] The reaction in the presence of manganese is first-order with respect to peracid, aldehyde, and manganese In addition, the decomposition replenishes thesupply of radicals This is important since the oxidation requires a constant flux of radicals

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