phenothiazine

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Phenothiazine as Stabilizer for Acrylic Acid

For the safe and trouble-free operation of a manufacturing plant and the safe sto-rage of acrylic monomers a sufficiently effective polymerization inhibitor is ne-cessary. As stabilizers different radical interceptors are used. Hydroquinonemonomethyl ether (MeHQ) and phenothiazine (PTZ) are the standard stabili-zers. In this paper, the decomposition kinetics of PTZ was investigated under pro-cess conditions. In acrylic acid a linear PTZ consumption was detectable underprocess conditions, whereby five different decomposition products were formed.The PTZ consumption was caused by thermal decomposition and by radical andoxidation reactions. With increasing temperature the portion of the total PTZconsumption caused by radical reactions decreased rapidly (68% at 60 C 38% at 90 C) and the part of the oxidation reactions of the inhibitor increased(27% at 60 C 54% at 90 C). Comparative investigations in air and nitrogenatmosphere resulted in different values for PTZ consumption and radical forma-tion rates. Measurements of oxygen and PTZ consumption in air atmosphereshowed a ratio of 2:1 molmol1 (60 C) which increased with temperature up to4:1 molmol1 (90 C). The data showed that in acrylic acid stabilized with PTZthe oxygen consumption could not be totally prevented. This indicates that onepart of the oxygen is consumed by the oxidation of PTZ while another part reactsdirectly with the primary radicals which are not trapped by the inhibitor. Withthe results of this work it is possible to optimize the PTZ stabilization of acrylicacid under process conditions in the presence and absence of oxygen.

Keywords: Acrylic monomers, Decomposition, Polymerization

Received: February 6, 2006; revised: March 28, 2006; accepted: April 9, 2006

DOI: 10.1002/ceat.200600056

1 Introduction

Acrylic acid, the simplest unsaturated monocarbonic acid, isan intermediate produced on an industrial scale. This mono-mer is extremely reactive and easy to polymerize so that, dur-ing synthesis, storage, transport and reprocessing, many safetyguidelines must be followed [1]. Acrylic acid is mainly usedfor the production of acrylic esters (53%), superabsorberpolymers (31%) and washing agents (6%). The rest is usedfor a wide range of special applications (10%) [2].During the production of acrylic acid the inadvertent poly-

merization in the chemical reprocessing via rectification is animportant problem. The polymerization is caused by radicalsformed by impurities, UVor cosmic radiation. The exothermalpolymerization with a reaction enthalpy of 76 kJmol1 re-

flects a significant security risk during the handling of thismonomer. On the one hand, the liberation of reaction enthal-py can cause deflagrations and explosions and, on the otherhand, the polymers can lead to blockages causing interrup-tions, higher costs and loss of production. Therefore, acrylicacid is produced in the presence of polymerization inhibitors(PTZ, MeHQ and/or oxygen) in different plant units [3]. Un-der production process conditions PTZ is used. It can react di-rectly with primary radicals or with peroxide radicals formedduring the reaction of primary radicals with molecular oxygen.The presence of oxygen is not essential for PTZ inhibition.In this work, the consumption of PTZ and molecular oxy-

gen during the inhibition period was investigated. The con-sumption rates were measured in different solvents (acrylicacid, acetic acid) and under different atmospheres (air, nitro-gen). By comparing the results obtained the portion of the dif-ferent decomposition reactions of PTZ (radical, oxidative andthermal decomposition) can be calculated. With the consump-tion kinetics of PTZ and oxygen the start of polymerizationcan be predicted. In addition, a more precise dosage of PTZ ispossible, which reduces the raw material costs for PTZ.

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com

Holger Becker1

Herbert Vogel1

1Department of Chemistry,Ernst-Berl-Institute ofTechnical Chemistry andMacromolecular Science,TU Darmstadt, Darmstadt,Germany.

Correspondence:Dr.-Ing.H.Vogel ([email protected]),TU Darmstadt, Department of Chemistry, Ernst-Berl-Institute ofTechnical Chemistry and Macromolecular Science, Petersenstr. 20,D-64287Darmstadt, Germany.

Chem. Eng. Technol. 2006, 29, No. 8, 931936 931

In the absence of inhibitors the copolymerization (acrylicacid monomer + O2) and the normal polymerization (polymer+ monomer) are opposing reactions [4]. If the oxygen concen-tration is high enough the primary radicals react with oxygento peroxide radicals and a copolymer with alternating mono-mer/oxygen units is formed [57]. In the presence of PTZ theradical chain is interrupted by the direct reaction of primaryor peroxide radicals with PTZ. Further PTZ consumption iscaused by the oxidation with dissolved oxygen or by the reac-tion with peroxides formed [8] (see Scheme 1).In the investigated system oxygen, PTZ and the monomer

were present at the beginning and during the reaction perox-ides were formed. This means that all the above radical andoxidation reactions are possible and influence the inhibitionperiod of acrylic acid.

2 Experimental

2.1 Measuring Devices

The measurements were made in a modified experimentalplant described in earlier works [4, 7]. Directly after the O2measurements the sampling of acrylic acid was conducted andPTZ concentration was analyzed via HPLC. Furthermore, acontinuous stirred tank reactor, described in [1], was used.To simulate the real conditions, all measurements were

made without the addition of radical starters. The differentoxygen partial pressures in the acrylic acid were directly ad-justed in the feed tank by gassing the liquid monomer with adefined oxygen/nitrogen mixture.

2.2 PTZ Analysis

For analyzing PTZ and the decomposition products a HPLCsystem (Hewlett-Packard, Model HP 1090 Series L LiquidChromatograph) was used. The UV-VIS detector (HP filterphotometric detector) works at 254 nm where PTZ has its ab-sorption maximum. As chromatographic column a modifiedRP 18 material with the model name NC-03 (250 3.0 mm)PRONTOSIL 1203-C18-AQ 3.0956 m was used. The columnwas tempered to 50 C. The mobile phase was a mixture of

75% (L L1) acetonitrile und 25% (L L1) water (volume flowrate 0.5 mLmin1). The injection volume of the auto injectorwas varied depending on the PTZ concentrations of the sam-ples.

2.3 Chemicals and Equipment Used

The investigations were carried out with pure acrylic acid fromBASF AG, Ludwigshafen, Germany. For the purification andelimination of the storage stabilizer the monomer was distilledand recrystallized three times. After the cleaning process PTZwas added to the unstabilized acrylic acid. The monomer waseither directly used or stored at 20 C to avoid side reactionssuch as the formation of diacrylic acid. The chemicals andequipment used are listed in Tab. 1 and Tab. 2.

Table 1. Equipment and producers.

Equipment Producer

O2 Sensor (electrochemical) Aero2-Mat 4125, Fa. Syland ScientificGmbH, Heppenheim, Germany

Pumps PTFE-Minidosierer BF 411,(0...60 mL/min), Fa. Telab,Dosiertechnik & HandelsgesellschaftGmbH, Duisburg, Germany

AD transducer Airflow Memory AM-2, Fa. AirflowLufttechnik GmbH, Rheinbach,Germany

Thermostats Julabo HC5, Fa. Julabo LabortechnikGmbH, Seelbach, Germany

Mass-Flow Controller MFC5850 TR, N2 from 05 L/min,Fa. Brooks Instrument B. V.,Veenendaal, The Netherlands

HPLC system HP 1090 Series L LiquidChromatograph

HPLC software Varian Star 5.3

Table 2. Chemicals used.

Chemical Purity Company

Acrylic acid purum>99.5%stabilized with200 ppm (g g1)MeHQ

BASF AG Ludwigshafen (D)

Acetic acid 99100% Riedle-de Haen AG, Seelze (D)

MeHQ >98% Fluka Chemie AG, Buchs (CH)

PTZ > 99% Acros, Neuss (D)

Nitrogen 99.999% (N2 5.0) Linde AG, Wiesbaden (D)

Acetonitrile 99.99% Fisher Chemicals (D)

Water bidistilled TUD, Darmstadt (D)


S

N

O

H

S

N

O O

H

+ ROOH + ROH

S

N

H

S

N

O

H

+ ROOH + ROH

Scheme 1.

932 H. Becker et al. Chem. Eng. Technol. 2006, 29, No. 8, 931936

3 Results and Discussion

3.1 HPLC Results

The investigated samples have a PTZ concentration between 0and 200 ppm (g g1). The main peak at a retention time of2.57 min is caused by the excess of acrylic acid and cannot beanalyzed quantitatively (see Fig. 1). The measurements showthat during the reaction of PTZ in acrylic acid five not identi-fied decomposition products (O1 PTZ, O2 PTZ, O3 PTZ,...)are formed.

3.2 PTZ Consumption

Die measurements of PTZ consumption were made in acrylicacid (AA) and acetic acid (HAc). Acetic acid was used as a po-lymerization inert reference solvent. In both solvents the con-sumption kinetics under air (21 vol.-% O2) and under nitro-gen (0 vol.-% O2) atmosphere was investigated. The resultsshow a linear PTZ decrease as a function of retention time (seeFig. 2). Under air atmosphere the PTZ consumption rates inacrylic acid and acetic acid are higher than under nitrogen at-mosphere. Between 40 and 100 C these consumption rates ofthe linear consumption curves are represented graphically inFig. 3.With the linear Arrhenius plots (see Fig. 4) the PTZ con-

sumption rates could be extrapolated to temperatures beyondthe measuring range. Under air atmosphere the activationenergy of PTZ decomposition is lower in acrylic acid(67 kJmol1) than in acetic acid (87 kJmol1). One possibleexplanation could be that in acrylic acid many radicals areformed by instable copolymers and peroxides which react withPTZ and consume the stabilizer.


Figure 2. Decrease of PTZ concentration as a function of reten-tion time (RT) at different temperatures: PTZ in acrylic acid withair (a), PTZ in acetic acid with air (b).

Figure 1. Chromatogram of PTZ in acrylicacid und air atmosphere (90 C) at the be-ginning and after 52.5 h (75% acetonitrile/25% H2O, 254 nm, 0.5 mL/min, T(HPLC) =50 C). Peak 1: O5-PTZ(3.972), peak 2:O4-PTZ(4.508), peak 3: O3-PTZ(4.704), peak4: O2-PTZ(4.893), peak 5: O1-PTZ(5.102),peak 6: PTZ(5.259).

Chem. Eng. Technol. 2006, 29, No. 8, 931936 Acrylic monomers 933

3.3 Discrimination of PTZ Consumption in ThreePartial Reactions

In acrylic acid PTZ is consumed at least by three decomposi-tion reactions [9]: reaction with the formed radicals (PTZ R), oxidation with dissolved oxygen (PTZ Ox), thermal decomposition at higher temperatures (PTZ T).With the following assumptions it is possible to calculate

the radical formation rates in acrylic acid with the PTZ con-sumption measured in acrylic acid and acetic acid: In the presence of O2 all three decomposition reactions takeplace.

In acetic acid PTZ consumption by radical reactions is negli-gible.

In N2 atmosphere no oxidative decomposition is possible. The solvents acetic acid and acrylic acid are comparable intheir pH value, polarity and O2 solubility.With the differences in PTZ consumption rates in acrylic

and acetic acid under the influence of the two atmospheres(air and nitrogen) the amount of the different PTZ decompo-sition reactions could be estimated. The results show that with

increasing temperature the amount of oxidative PTZ con-sumption also increases (27% at 60 C 54% at 90 C). Inthe temperature range between 90 and 100 C the oxidationrepresents the greatest part, whereas at temperatures below90 C the radical reactions represent the greatest part (68% at60 C 38% at 90 C) of PTZ consumption. The thermal de-composition of PTZ is of secondary importance and only attemperatures 90 C is the part of PTZ consumption greaterthan 10% (see Fig. 5).On the assumption that for the consumption of one PTZ

molecule one radical is necessary the radical formation rates inacrylic acid under air/N2 atmosphere can be calculated fromthe inhibitor consumption rates. Hereby, it is noticeable thatthe radical formation rates correspond between 50 and 60 Cunder nitrogen and air atmosphere. Above 70 C a sharp slopeof the radical formation rate in air is detectable. By constrast,the radical formation rate under N2 atmosphere is approxi-mately constant (see Fig. 6). One probable explanation is thestart of the decomposition of peroxides formed by the oxygenand monomer during the inhibition period. Hereby, the addi-tional radical sources could increase the radical formationrates at higher temperatures ( 70 C).


Figure 3. PTZ consumption rates in acrylic acid (a) and aceticacid (b) as a function of temperature under air () and N2 at-mosphere (): (a) EA = 67 kJmol

1 (), EA = 12 kJmol1 ();

(b) EA = 87 kJmol1 (), EA = 90 kJmol

1 ().

Figure 4. Arrhenius diagrams of PTZ consumption rates in acryl-ic acid (a) and acetic acid (b) under air () and N2 atmosphere().


3.4 Correlation between O2 and PTZ Consumption

The parallel measurements of the O2 and PTZ consumption inthe continuous stirred tank reactor should yield a more preciseoverview of the inhibition processes. If PTZ were oxidation-re-sistant and effective enough to immediately trap all primaryradicals formed, no oxygen would be consumed.In reality, oxygen is consumed by the oxidation of PTZ and

by the reaction of O2 with the formed radicals. The consump-tion rates could be calculated using the concentrations of O2 andPTZ in the stationary operation state at different retentiontimes. The ratio of these total consumption rates is kges (O2AA)/kges (PTZAA) and increases slightly with increasing tempera-tures from 1.7 to 2.1 (see Fig. 7).For the direct comparison of the concurrent reactions of O2

and PTZ with primary radicals the oxidative part must be cal-culated from the total consumption rates of O2 and PTZ. Esti-mation is possible, assuming that for the oxidation of 1 molPTZ 1 mol O2 is necessary and the percentage of the partial re-actions in Fig. 7. With this correction the ratio of the partialconsumption rates kR (O2AA)/kR (PTZAA) increases from 2.2 at

2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 5. Rate constants of PTZ partial reactions (a) and theirpercentage on total PTZ consumption (b) in acrylic acid with air(for details, see text).

Figure 6. Radical formation rates (PTZR) in acrylic acid underair () and N2 atmosphere ().

Figure 7. PTZ () and O2 () consumption rates (a) and thecorresponding ratio of O2/PTZ (b) in the continuous stirred tankreactor (VR = 5079 mL, RT = 820 h, T = 6090 C).

Chem. Eng. Technol. 2006, 29, No. 8, 931936 Acrylic monomers 935

60 C to 4.0 at 90 C (see Fig. 8). This indicates that the capa-bility of PTZ to directly react with primary radicals decreasesat higher temperatures and will be overcompensated by the re-action of oxygen with primary radicals. It seems that at highertemperatures the formation of peroxide and copolymer radi-cals again plays a role before these radicals react with PTZ.

4 Conclusions

The investigation of PTZ consumption kinetics in acrylic acidshows that under air atmosphere oxygen is consumed. This O2consumption could be explained by the oxidation of PTZ andby the reaction of oxygen with primary radicals during the in-hibition period. This is why only one part of the formed pri-mary radicals is directly intercepted by PTZ whereas the otherpart first reacts with oxygen. It is only after this has taken placethat the formed peroxide radicals can react with PTZ. There-fore, under process conditions, the PTZ and oxygen concentra-tions should be constantly measured to guarantee that bothstabilizers are present in adequate concentrations.At temperatures 80 C PTZ oxidation increases, and this

must be taken into consideration under production condi-tions. In plant units with higher temperatures (130 C) a high-er PTZ concentration is necessary to inhibit polymerization.With the extrapolation of the inhibitor consumptions in acryl-ic acid an early warning system is possible which calculates theend of the inhibition period as a function of PTZ concentra-tion. This extrapolation only yields relatively good results inthe temperature range between 80 and 100 C, at temperaturesbelow 80 C it fails (see Fig. 9).With the results of this work it is possible to adjust an opti-

mal PTZ concentration to avoid polymerization of acrylic acidin the different reprocessing steps. For example, the PTZ con-sumption rates in a rectification column could be calculated.With some measuring points in a rectification column optimi-zation of the PTZ concentration could be achieved. This re-duces raw material costs and breakdowns in productionplants.

Acknowledgement

Financial support by the BASF AG Ludwigshafen, Germany, isgratefully acknowledged.

References

[1] S. Schulze, H. Vogel, Chem. Eng. Technol. 1998, 21, 829.[2] Markets & Economics, Acrylic Acid Chemical Week, Jan.

2001, 3.[3] W. Kurze, F. Raschig, Ullmanns Enzyklopdie der Technischen

Chemie, Antioxidantien, Vol. 8, VCH, Weinheim 1975, 19.

[4] H. Becker, H. Vogel, Chem. Eng. Technol. 2002, 25, 547.[5] P. Gladyshev, D. K. Kitaeva, V. A. Popov, E. I. Penkov, Proc.

of the Acad. Sci. USSR 215, 1974, 354.[6] J. J. Kurland, J. Polym. Sci. Polym. 1980, 18, 1139.[7] H. Becker, H. Vogel, Chem. Eng. Technol. 2004, 27 (10), 1122.

DOI: 10.1002/ceat.200302114[8] W. Kurze, F. Raschig, Ullmanns Enzyklopdie der Technischen

Chemie, Antioxidantien, Vol. 8, VCH, Weinheim 1975, 19.[9] H. Roseboom, J. H. Perrin, J. Pharm. Sci. 1977, 66, 1392.


Figure 8. Corrected ratio of O2/PTZ in continuous stirred tank re-actor (VR = 5079 mL, RT = 820 h, T = 6090 C).

Figure 9. Calculation of inhibition period (IP) by extrapolation ofPTZ consumption rates (white, gray) in comparison to mea-sured inhibition period (black).


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