Ethoxylation Kinetics Experiments

3.1 Introduction

In Chapter 2 the mechanisms proposed in the literature for the base-catalysed ethoxylation of an alkylphenol were thoroughly reviewed. A classic SN 2 mechanism was considered to best describe the base-catalysed ring-opening reaction of ethylene oxide and which was assumed to be the rate-determining step. Following this derivation, Santacesaria et al. (1990) and Di Serio et al . (1995) had developed a mathematical model that described the kinetics of base-catalysed nonylphenol ethoxylation in a semi-batch stirred reactor. Their experimental approach for measuring the ethoxylation kinetics was also discussed in Chapter 2. In order to validate further the kinetic data published by Santacesariaet al. (1990) and Di Serio et al . (1995), a new set of kinetic experiments was conducted in this research project. The experiments were conducted by using the existing alkoxylation facilities in the Huntsman Corporation Australia Technical Centre, located at Ascot Vale, Victoria. This chapter describes the reagents, apparatus and experimental procedures used in the laboratory ethoxylation operation owned by Huntsman Corporation Australia (HCA). An overview of the experimental approach undertaken in the present study is also given in this chapter.

Because the experimental process involved handling of a hazardous, explosive chemical, laboratory safety measures were enforced through an autoclave operation training program and full time supervision from experienced HCA laboratory staff during the entire course of the experiments.

3.2 Materials

The following is a list of reagents used in this experimental study.

For ethoxylation experiments

•

Neodol 91: synthetic alcohol 91, consisted of C9 to C11 primary alcohol: 13 – 23% C9, 37 – 47%

C10, 33 – 43% C11, was supplied by Shell Chemical (Aust.) Pty Ltd

•

Nonylphenol: 4-nonylphenol (mixture of alkyl chain isomers), consisted of 2% maximum w/w di-nonylphenol, 10% maximum w/w o-nonylphenol and 90% minimum w/w p-di-nonylphenol, was supplied by Schenectady

•

Ethylene oxide (highest purity), HCA’s own manufacture

•

Nitrogen (high purity), supplied by BOC

•

Potassium hydroxide (caustic potash, liquid), 48.0-50.0% w/w KOH, supplied by Orica Chemnet

•

Acetic acid (industrial grade), 89-91 % acetic acid, supplied by Orica Chemnet

The kinetics of nonylphenol ethoxylation with the use of a basic catalyst was the main focus in this experimental study. Thus, nonylphenol was the sole hydrophobic substrate used in the kinetic experiments.

The use of neodol 91 will be explained in the later section.

For gas chromatography analysis

•

Dichloromethane (AR grade), supplied by Merck Australia

•

Bis(trimethylsilyl)trifluoroacetamide catalysed with 1% trimethylchorosilane, known as BSTFA+1% TMCS, supplied by Alltech Associates, Inc.

3.3 Apparatus

3.3.1 Two-litre autoclave

All laboratory experiments were conducted in a two-litre autoclave in the autoclave building at the Ascot Vale technical centre. The autoclave building was specifically designed to minimise the hazards associated with the use of toxic and highly flammable materials under conditions of high pressure and temperature. The building has two areas: a control room and six external reactor bays. The control room is always maintained above atmospheric pressure to prevent ingress of gases. Each autoclave is equipped with its own control unit and panel within the building so that an operator spends a minimal amount of time outside in the actual bay.

The control unit, shown in Figure 3. 1, has a main power switch and switches for the stirrer control and heater. It also has a heater dial to set the temperature for the heater and a graph for recording temperature changes within the vessel.

Figure 3. 1 The control unit (Courtesy of HCA).

The inside panel contains various control valves as shown in Figure 3. 2. The nitrogen pad valve feeds nitrogen to the top of the reactor only. The ethylene oxide/input valve feeds ethylene oxide or nitrogen from a storage cylinder through a dip-leg pipe into the bottom of the reactor. The cooling water valve controls the cooling water flow into the internal coil of the reactor when cooling is applied. The vacuum water tap supplies vacuum to the reactor from a water-driven vacuum pump. The reactor is placed under vacuum when both the vacuum valve and the cooling water valve are opened.

Figure 3. 2 The inside panel (Courtesy of HCA).

The outside bay where the autoclave is housed is shown in Figure 3. 3. The main features of the autoclave system are shown in the schematic illustrated in Figure 3. 4. The vessel is of 316 stainless steel construction and capable of sustaining pressures up to 38MPa, however the maximum working pressure is set at 600kPa and the normal operating pressure is 400kPa. The vessel also has several bursting discs which are designed to release pressure when the disc’s pressure rating is exceeded. Internally, the reactor is equipped with an agitation system, which is a packless magnetic driven stirrer motor with variable speed control. The agitator has a working speed between 540 and 1800rpm. The impeller is a six-blade disc turbine mounted on the shaft. The shaft has an aspiration hole near the bottom end. The aspiration hole allows the gas to be drawn through the hollowed shaft then sparged into the bulk liquid. In combination with the effect of agitation, the interfacial contact between dispersed gas and bulk liquid is enhanced.

the reactor include a thermowell, which holds a thermocouple, and a dip-leg pipe, which extends through to the bottom of the vessel. The dip-leg pipe provides the path that delivers ethylene oxide or nitrogen in direct contact with liquid mixture upon entry. The top end of the dip-leg pipe is connected to a flexible hose and an ethylene oxide/input line, linking the vessel to a storage cylinder of ethylene oxide. The cylinder is made of 304 stainless steel and fitted with manifolds to facilitate ethylene oxide transfer as shown in Figure 3. 5. The cylinder is normally pressurised by nitrogen to 700kPa but not beyond 900kPa and placed onto a digital balance. The nitrogen bypass valve on the feed cylinder manifold, when opened, allows nitrogen to be passed through the flexible ethylene oxide addition line.

Figure 3. 3 Two-litre autoclave (Courtesy of HCA).

Figure 3. 4 Autoclave schematic.

Figure 3. 5 Feed cylinder manifold (Courtesy of HCA).

The reactor body is wrapped with an electrical heating jacket that provides the heating to the reactor contents. The vessel is also equipped with a pressure gauge that measures the pressure of the vessel itself. The minimum and the maximum working volume for the vessel are 80ml and 1.6 litres respectively.

Table 3. 1 summarises the specifications of the vessel used in the present study.

Table 3. 1 Summary of the specifications of the vessel used in this study.

Vessel Specifications 2-litre

Autoclave

Materials of construction 316SS

Minimum volume to stir, ml 80

Maximum operating volume, ml 1600

Maximum working pressure, kPa 600

Normal operating pressure, kPa 400

Bursting disc settings at 22°C, kPa 1000; 37,900 Stirring speed working range, RPM 540-1800

Heating External

Cooling Internal coil

Minimum volume to coil, ml 130

Minimum volume to thermowell, ml 130

3.3.2 Gas chromatograph

The homologue distribution of low molecular weight ethoxylates of an alcohol or an alkylphenol was quantified by a Hewlett Packard 5890 Series II gas chromatograph (GC), equipped with single flame ionisation detector (FID). The GC column used was a DB-1 capillary column with 0.250mm internal diameter, 15m in length and 0.1

µ

m film thickness (J&W Scientific Inc.). The column oven temperature was programmed from 35 to 65°C at a rate of 15°C/min (no hold), then ramped to 340°C at 4°C/min and held for 30 min. Injector and detector temperatures were set at 38 and 350°C, respectively. Helium was used as both carrier and make-up gases with the flow rates of 1.5 ml/min and 19.3ml/min, respectively. The detector hydrogen flow was 33ml/min and air flow was 300ml/min. GC signals were processed using a Hewlett Packard ChemStation Series II software.

The GC method was applicable to alcohol ethoxylates or alkylphenol ethoxylates between molecular weights of approximately 100 to 900.

3.4 Ethoxylation experiment procedures

A typical ethoxylation run consists of a sequence of operational procedures. They are vessel pressure test, raw materials charge, dehydration, ethylene oxide addition and product neutralisation. Each of these procedures requires absolute precautions to ensure safety.

3.4.1 Vessel pressure test and raw material charging

It is important to take precautions against ethylene oxide explosion hazard before ethylene oxide is admitted into the vessel for a reaction. For this purpose, safety precautions including an initial pressure test and elimination of oxygen were conducted.

First, it was important to ensure that the vessel was cool, dry and clean. This was tested by the introduction of nitrogen, approximately to 200kPa, to the sealed vessel. The nitrogen was introduced via the nitrogen pad valve. The sampling tube was then cautiously opened to expel nitrogen. As the nitrogen was expelled, any physical sign of moisture was visually and manually inspected. As discussed in Section 2.4.3.4, water also reacts with ethylene oxide, resulting in a competitive side-reaction. If no sign of moisture was evident, the vessel was confirmed dry and clean, otherwise, further drying was necessary.

Next, the vessel was tested for pressure leaks. It was essential that the vessel was cool and isolated.

Then the vessel was pressurised with nitrogen to approximately 400kPa and allowed to stand for a period of 15 minutes. Any pressure drop during this period of time was observed. If a pressure drop was less than 10kPa, the vessel was confirmed pressure tight and safe to use. If the vessel failed the pressure test, it was unsafe to operate and any leaks, however small, had to be rectified prior to raw material charging.

Leaks at joints or valves could be detected with a spray of a soap solution.

Once the vessel was confirmed pressure tight, the charging of the raw materials began. The liquid raw materials, including the initiator and aqueous potassium hydroxide (50% w/w), were drawn into the vessel via the sampling line under the vessel evacuation through a fully opened vacuum valve. Once charged with raw materials, the vessel was evacuated and nitrogen purged three times to eliminate any air that may have entered the vessel with the raw materials. Exposure of ethylene oxide to oxygen creates an explosion danger. The purged nitrogen was delivered via the feed cylinder manifold with the nitrogen bypass valve opened. The heating of the raw materials commenced after the oxygen elimination.

3.4.2 Dehydration and sampling procedures

In order to prevent competitive polyoxyethylation of water during the polyoxyethylation of an alkylphenol, significant effort must be made to remove residual water, which produces polyethylene glycol with ethylene oxide. The water was initially introduced to the reactor system from the addition of aqueous potassium hydroxide. More water was formed during the activation step (see Equation 2. 12). As

previously mentioned in Section 2.4.3.4 in Chapter 2, the activation step was an equilibrium reaction. The removal of the water would facilitate the high conversion to the potassium salt.

The water was removed by sparging nitrogen under vacuum during heating of the raw materials mixture. The nitrogen was introduced via the nitrogen bypass valve on the feed cylinder. The heating normally started at ambient temperature after the vessel, filled with the raw materials, was purged to eliminate oxygen. The heating continued and the temperature rose. When the temperature approached 120

°

C, the heating was adjusted so that it maintained at the intensity that stabilised the temperature at this value. The mixture of the raw materials was then left to dehydrate with the vessel placed under vacuum.

Nitrogen bleed was applied to facilitate dehydration process, but was kept at a very low flow (the vacuum gauge registered <10kPa). This minimised any raw material to be carried over into the catch pot (see Figure 3. 4). Stirring could also be applied occasionally. The dehydration continued for approximately one hour under constant monitoring to prevent reactor contents carryover. At the end of the one-hour drying step, the reactor vessel was cooled to approximately 80

°

C for a sampling routine. One waste beaker and a sample jar were prepared and both pre-weighed. The vessel, with the stirrer off, was pressurised to approximately 150kPa of nitrogen. First, the pre-weighed beaker was placed under the sampling tube and approximately 20ml of waste was collected. Then, a further approximately 10ml of reactor contents was collected into the sampling jar for water analysis. Both waste beaker and sampling jar were weighed again and the weights recorded. The contents of the sampling jar was tested for its free water by the Karl Fisher titration method (ASTM E203). The Karl Fisher method determines the free water and water of hydration in most solid or liquid organic and inorganic compounds with the use of the Karl Fischer reagent and electrometric end-point detection.

Hall and Agrawal (1990) investigated the effect of water content on the ethoxylation rate and found that a 32-fold increase of water content in fatty alcohol resulted in a 22% increase in the reaction rate.

They also suggested that the effect of residual water on the reaction rates became insignificant when the water content in a hydrophobic substrate was kept sufficiently low, for example, at 0.04% by weight. Van Os (1998) suggested the water content to be lowered to a level less than 0.1% by weight (Section 2.4.3.4).

In our experimental work, it was decided that the water content in the raw materials was removed to a level below 0.05% by weight. When the Karl Fisher analysis showed a water content below this set point in the sample taken at the end of one hour dehydration, it indicated that the dehydration was satisfactorily achieved. Otherwise, further dehydration was necessary until the water content in a sample met the requirement. Once a testing sample showed a satisfactory water level, the vacuum valve was closed and the remaining contents allowed to heat or cool to reaction temperature.

3.4.3

3.4.3 Ethylene oxide additionEthylene oxide addition

Once the temperature stabilised at the desired set temperature for an initiation reaction, ethoxylation was Once the temperature stabilised at the desired set temperature for an initiation reaction, ethoxylation was set to commence. For the kinetic study, ethoxylation was to take place at a fixed temperature and set to commence. For the kinetic study, ethoxylation was to take place at a fixed temperature and pressure so that a reaction rate could be determined.

pressure so that a reaction rate could be determined.

When the temperature was stabilised at the set value for the initiation in ethoxylating the remaining When the temperature was stabilised at the set value for the initiation in ethoxylating the remaining hydrophobic substrate, the heating or cooling was discontinued. The nitrogen bypass valve was shut and hydrophobic substrate, the heating or cooling was discontinued. The nitrogen bypass valve was shut and the liquid isolation valve was opened to allow ethylene oxide into the ethylene oxide/input line. The digital the liquid isolation valve was opened to allow ethylene oxide into the ethylene oxide/input line. The digital balance was switched on and manipulated to obtain zero tare so that any weight loss from the cylinder balance was switched on and manipulated to obtain zero tare so that any weight loss from the cylinder could be monitored. Ethylene oxide was added into the autoclave by cracking open the ethylene could be monitored. Ethylene oxide was added into the autoclave by cracking open the ethylene oxide/input valve on the inside panel as shown in Figure 3. 2. Initial additions of ethylene oxide were oxide/input valve on the inside panel as shown in Figure 3. 2. Initial additions of ethylene oxide were stepwise, by bringing the reactor pressure from vacuum to the experimental set point. At each of the initial stepwise, by bringing the reactor pressure from vacuum to the experimental set point. At each of the initial additions, the pressure increase was kept below 50kPa; the temperature was maintained by controlling the additions, the pressure increase was kept below 50kPa; the temperature was maintained by controlling the exotherm via the internal cooling coil. The stepwise additions prevented the accumulation of a large exotherm via the internal cooling coil. The stepwise additions prevented the accumulation of a large amount of unreacted ethylene oxide in the system, which could cause possible hazards if the reaction got amount of unreacted ethylene oxide in the system, which could cause possible hazards if the reaction got out of control. When the reactor pressure reached the experimental operational level, the ethylene out of control. When the reactor pressure reached the experimental operational level, the ethylene oxide/input valve was closed. The pressure was allowed to drop and the temperature was maintained oxide/input valve was closed. The pressure was allowed to drop and the temperature was maintained constant via cooling. When a pressure drop was observed to be 15-25kPa, further ethylene oxide was constant via cooling. When a pressure drop was observed to be 15-25kPa, further ethylene oxide was added in those amounts that sufficiently compensated for the pressure drop. Time of each injection was added in those amounts that sufficiently compensated for the pressure drop. Time of each injection was recorded, as well as the weight of ethylene oxide added, which was read from the digital balance panel as recorded, as well as the weight of ethylene oxide added, which was read from the digital balance panel as the loss of weight from the ethylene oxide storage cylinder. Temperature, pressure and ethylene oxide the loss of weight from the ethylene oxide storage cylinder. Temperature, pressure and ethylene oxide injections were all manually manipulated via the inside panel and the control unit. Temperature variations injections were all manually manipulated via the inside panel and the control unit. Temperature variations were controlled to within

were controlled to within

±±

2 2

°°

C from the preset value.C from the preset value.

When the desired amount of ethylene oxide had been added, the liquid isolation valve on the feed When the desired amount of ethylene oxide had been added, the liquid isolation valve on the feed cylinder was shut and the nitrogen bypass valve was opened. However, it was estimated that up to 10g of cylinder was shut and the nitrogen bypass valve was opened. However, it was estimated that up to 10g of ethylene oxide remained in the flexible hose and the lines leading to the vessel. This remaining amount of ethylene oxide remained in the flexible hose and the lines leading to the vessel. This remaining amount of ethylene oxide in the line had to be purged by nitrogen via the ethylene oxide/input valve. Repeats of ethylene oxide in the line had to be purged by nitrogen via the ethylene oxide/input valve. Repeats of nitrogen purge were necessary to ensure all possible ethylene oxide in the lines was flushed. The ethylene nitrogen purge were necessary to ensure all possible ethylene oxide in the lines was flushed. The ethylene oxide was then allowed to react fully under the additional nitrogen pressure (to 400kPa) from the purging oxide was then allowed to react fully under the additional nitrogen pressure (to 400kPa) from the purging procedure whilst the reaction temperature was maintained. Constant pressure and temperature signalled procedure whilst the reaction temperature was maintained. Constant pressure and temperature signalled the end of reaction and the product was ready to be neutralised (see below).

the end of reaction and the product was ready to be neutralised (see below).

3.4.4

3.4.4 Nitrogen solubility in ethylene oxideNitrogen solubility in ethylene oxide

On selected kinetic runs, a study of nitrogen solubility in ethylene oxide was performed prior to the

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