Zhou Yi; Liu Zhenyi; Liu Yu; Duan Zaipeng; Qian Xinming

(State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081)

Flammability and Explosion Property of Gases in the One-Step Process of Propane Oxidation to Acrylic Acid

Zhou Yi; Liu Zhenyi; Liu Yu; Duan Zaipeng; Qian Xinming

(State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081)

In order to study the flammability and explosion property of gases during the propane oxidation to acrylic acid process, the explosion limits and the safety oxygen content of gases at the recycle gas compressor outlet, the reactor inlet, and the reactor outlet were theoretically calculated and experimentally tested. Finally, the inert limit was also determined. It showed that gases at the recycle gas compressor outlet and the reactor outlet were nonflammable based on three indicators: the explosion limits, the safety oxygen content and the inert limit. The C3H6and O2contents were higher at the reactor inlet, which made the mixed gases easily ignitable. However, the large amount of inert gases suppressed the possibility of explosion effectively. As a consequence, no explosion phenomenon would happen in all three locations. But gases at the reactor inlet are most dangerous, where more supervision on the concentration of gases and more strict control on the temperature and pressure should be implemented. Besides this, open flame, hot surfaces and other sources of ignition are prohibited in working spaces. The experimental results can be applied to similar process for oxidation of propane.

acrylic acid; propane oxidation; explosion limit; safety oxygen content

1 Introduction

Acrylic acid is an important organic chemical material, which is mainly used for the production of acrylate, and is also widely used in textile, adhesive, chemical fiber, papermaking, leather, building materials, plastics modification, synthetic rubber, radiation curing, water treatment and petroleum industries[1-2]. The main methods for production of acrylic acid include the ethylene cyanohydrin method, the acetylene carbonylation method, the ketene method, the acrylonitrile hydrolysis method, and the propylene oxidation method. The propylene oxidation process involves the one-step and two-step methods[3], and the latter was widely used in the acrylic acid manufacture facilities which are newly constructed or extended since the 1980s. Compared with propylene, the low price and rich supply of propane make it one of the hot topics on the development and utilization of low-carbon alkane. But alkane is more stable than olefin, and the activation process is usually realized under high temperature and critical oxidation conditions. Besides that, the low conversion rate of propane in one working process gives rise to the demand for a larger reactor and recycling equipment, which would increase the risk of flammability and explosion of gases[4]. So, it is worthwhile to study the key aspects concerning the flammability and explosion property of gases in the one-step process of propane oxidation to acrylic acid.

2 Theoretical Analysis

Taking the one-step process of propane oxidation to acrylic acid with 85% of tail gas being recycled as an example, the process flow diagram[5]is shown in Figure 1. Based on the analysis of gases at the recycle gas compressor outlet, and at the reactor inlet and outlet, such components as H2O, N2, O2, CO, CO2, C2H6, C2H4, PP (polypropylene), and C3+ hydrocarbons are identified along with a small amount of ACA (acetic acid) and AA (adipic acid). As PP in the working conditions is difficult to deal with, it is treated as C3H6together with C3+ hydrocarbons, and the insignificant amount of ACA and AA is ignored in the theoretical analysis. The adjusted gas composition and its concentration in the system are shown in Table 1, with the combustible gases consisting of CO, C2H4, C2H6and C3H6, the combustion-supporting gas composed of O2,andinert gases consisting of H2O, N2, and CO2.

Figure 1 Technical process of propane oxidation to acrylic acid in one step

Table 1 Component of mixed gases in three parts of the production process

When the gas concentration is within the range of explosion limit and the oxygen concentration is higher than the safety oxygen content in the system, the mixed gases would burn or explode after encountering sufficient ignition energy[6-7].

In order to make a better experimental plan, it is necessary to conduct theoretical estimate on the explosion limits and safety oxygen content in three key locations of the process flow scheme in advance.

2.1 Estimation of explosion limits and safety oxygen content at recycle gas compressor outlet

1) Explosion limits

Generally speaking, the Le Chatelier formula applies to flammable gases with similar activation energy, molecular heat of combustion and reaction rate. It is not precisely applicable to mixed flammable gases with CO. However, the theoretical analysis of this part is just used for reference on experiments and there are no other precise and convenient calculation methods. Therefore, according to Le Chatelier formula[8-9], the explosion limits of multicomponent combustible gas can be roughly calculated by:

in whichCmis the explosion limits of multi-component combustible gas, v%;V1,V2,…,Vnis the volume fraction of each component in the gas mixture, v%, and the sum is 100%;C1,C2, …,Cnis the explosion limit of each component in oxygen or air, vol%.

Explosion limits of combustible gases in oxygen or air are listed in Table 2. The combustible gases need to be reorganized by removing the inert and combustion-supporting components in the system, as shown in Table 3.

Table 2 Explosion limits of flammable gases (25 ℃, 1 atm)

Table 3 Volume fraction of combustible gases after reorganization

According to Le Chatelier formula, the lower explosion limit of combustible gases in air at the recycle gas compressor outletCtlis 3.72%.

And the upper explosion limit of combustible gases in airCtuis 16.65%.

2) Safety oxygen content

In general, there are two methods for estimating the safety oxygen content, namely: the stoichiometry method and the triangle method[10]. Based on the stoichiometry calculations of combustion reaction and the lower explosion limit, the minimum oxygen concentration is estimated assafety oxygen content by the stoichiometry method[11]. The explosive range of mixed gases including combustible gases, combustion-supporting gases, and inert gases can be displayed in triangle method, and the critical oxygen content is based on explosion limits of flammable gases in air or in oxygen[12].

As regards the stoichiometry method, according to Amagat law of volumes, the volume fraction of each component in the mixed gases is equal to its mole fraction. So the tail-gases can be approximately regarded as a single gas containing C, H, O and its condensed formula can be C2.1155H3.3628O0.4371. The theoretical minimum oxygen concentration is:

As regards the triangle method, the explosion limits of combustible gases in oxygen need to be calculated firstly according to Le Chatelier formula:

So if the explosion limitsX1,X2of combustible gases in air need to be drawn on the air line (axis) in the form of points and the explosion limitsX11andX22of combustible gases in oxygen—on the oxygen axis. Then the lineX1X11andX2X22will intersect at the point C. The safety oxygen content 12.05% can be found out after drawing a line parallel to the fuel-axis cross point C (Figure 2).

Figure 2 Oxygen content of flammable gases at recycle gas compressor outlet determined by triangle method

2.2 Estimation of explosion limits and safety oxygen content at reactor inlet and outlet

Similarly to section 2.1, the explosion limits of combustible gases in air at the reactor inlet are calculated as between 3.25% and 14.34% according to Le Chatelier formula, and the minimum oxygen content is 10.4% measured by the stoichiometry method while it is 12.37% obtained by the triangle method, as shown in Figure 3(a). The explosion limits of combustible gases in air at the reactor outlet are in the range of 3.72%—16.66%, and the minimum oxygen content is 10.19% obtained by the stoichiometry method while it is 12.10% determined by the triangle method, as shown in Figure 3(b).

Figure 3 Safety oxygen content of flammable gases measured by triangle method

In summary, the values of explosion limits and safety oxygen content of combustible components at three locations at 20 ℃ and 1 atm are shown in Table 4. The maximum pressure is 200 kPa and the maximum temperature is 375 ℃in working condition. The increase in initial temperature and pressure will increase the risk of flammable gases, which is tantamount to the increase of range of explosionlimits and decrease of the safety oxygen content[13-14]. The influence of initial pressure on gas explosion limits is not so apparent under a certain range of pressures[12], so there is little impact on explosion limits of mixed gases under a pressure of 2 atm. Besides, upon comparing the explosion limits of CO at the initial temperature of between 375 ℃and 25 ℃ (at a pressure of 1 atm), it can be found out that the lower explosion limit decreases less than 2% and the upper explosive limit increases less than 3%[12]. And C3H6shows a similar trend. It can be seen from Table 4 that the concentration of combustible gases is 19.70% at the recycle gas compressor outlet, which is higher than the upper explosion limit, and the oxygen concentration is 8.59% which is lower than the minimum oxygen content. So we may come to the conclusion that the mixed gases have no explosive risk by theoretical analysis. At the reactor inlet, the concentration of combustible gases is 19.25% which is higher than the upper explosion limit, but the oxygen content is 16.19%, which is lower than the minimum oxygen content. So explosion event may happen at the reactor inlet. Finally at the reactor outlet, the oxygen content is 17.19% that is lower than the minimum oxygen content, while the volume fraction of combustible gases is 16.48%, which is within the explosion range. So gas combustion or explosion is possible at the reactor outlet.

Table 4 Theoretical assessment of explosion limits and safety oxygen content (25 ℃, 1 atm)

3 Experimental

3.1 Experimental apparatus

A 20-L explosion experiment equipment for multiphase materials (owned by the State Key Laboratory of Explosion Science and Technology at the Beijing Institute of Technology) was used in this test, which was in compliance with the standard EN1839 (B) as the design principle. There are four parts in this system: explosive device, gas mixing system, ignition system and control system, as shown in Figure 4. Jet mixing technology is adopted for mixing of multi-component gases and multiphase liquefied gases.

Figure 4 Experimental apparatus for explosion measurement of flammable gas (vapor)

Firstly, air tightness of the experimental system waschecked before experiments. Secondly, the gas mixture was prepared based on the Dalton Partial Pressure Law of Gases after evacuating the explosion chamber until a specified pressure was obtained. For the sake of convenience and accuracy in gas mixture preparation, trace amount of C2H4and C2H6was ignored in the experiment, while inert gases N2and CO2were mixed at first and C3H6, H2O, O2, and CO were added in turn according to the partial pressure law. The mixed gas was then ignited after it had been prepared according to the test schedule. Then the ignition phenomenon could be judged by the appearance of a flame, which could be monitored through the observation hole.

3.2 Experimental results of gases at recycle gas compressor outlet

3.2.1 Explosion limits

The theoretically estimated explosion limits of combustible gases at the recycle gas compressor outlet at 25 ℃and 1 atm was between 3.72% and 16.65%, which could serve as a reference for the experiment. The volume fraction of combustible gases CO and C3H6in the system was 8.61% and 11%, respectively. During the experiments, these two components maintained a constant volume ratio of 43.68%:56.32%. Whether there was critical points of blast could be identified by the trial and error method with the explosion limits determined eventually. Experimental results are shown in Tables 5 and Table 6, in which“1” represents an explosion or combustion phenomenon, while “0” represents the opposite.

Table 5 Experimental lower explosion limit of combustible gas in air

Table 6 Experimental upper explosion limit of combustible gas in air

It can be seen from Tables 5 and 6 that the experimentally determined data on the explosion limits of combustible gas in air at the recycle gas compressor outlet under working condition were in the range of 3.50%—15.10%. And the total volume of combustible gases in the system was 19.70% at this location, which was far above the upper limit of 15.10%. So the mixed gases would not pose any explosion or combustion hazards judging from the indicators of explosion limits.

3.2.2 Safety oxygen content

In general, CO2and N2are used as shielding gases in the experiment for testing the safety oxygen content. In this experiment N2+CO2and H2O were selected as inert gases in order to truly reflect the working conditions. The content of O2in the system can be changed by adjusting the concentration of inert components. Finally, the safety oxygen content at the recycle gas compressor outlet was 4.80% as specified by experiments. As listed in Table 4, the oxygen content in the system was 8.59%, which was far below the safety oxygen content. So no explosion phenomenon will happen at the recycle gas compressor outlet judging from the indicators of safety oxygen content.

3.2.3 Inert limit analysis

In order to determine the risk of mixed gases more accurately, the inert limit was also tested which was based on the explosion limits and safety oxygen content of flamma-ble gases at the recycle gas compressor outlet. According to the literature information[15-16], the range of explosion limits will be significantly narrowed with the increase of inert gas content, and the flammable gases will be beyond the flammability and explosion limits when the content of inert gases increases up to a certain value, which is called the inert limit. With the total volume fraction of inert gases comprising a constant of 10% in the system (containing 9.1% N2+CO2and 0.9% H2O), the explosion limits of this system can be determined to be in the range of 4%—12% step by step. Thus, the explosive area can be drawn in a triangular diagram to define the upper explosion limitX1and the lower explosion limitX2of combustible gases in air, as well as the upper explosion limitX11and the lower explosion limitX22when the inert gases content is 10% by volume, as shown in Figure 5. The inert point C corresponds to a concentration of 30.91%, which means that when the volume fraction of inert gases (N2+CO2, and H2O) is more than 30.91% in the gas mixture, no explosion phenomenon will happen no matter the flammable gas (CO and C3H6) and air would mix at whatever proportion. According to data listed in Table 1, the total volume of the inert gases in the system was 71.7%, which was much higher than the inert limit. So the gas composition was beyond the flammability limits at the recycle gas compressor outlet as evidenced by the inert limit indicators.

Figure 5 Explosive area of mixed gases at recycle gas compressor outlet

3.3 Experimental results of gases at reactor inlet

Compared with the temperature of gases at the recycle gas compressor outlet, the temperature of gases at the reactor inlet is apparently higher, coupled with a lower pressure, and a significantly larger mass flow rate and oxygen content. Experimental data on explosion limits of combustible gases at the reactor inlet were in the range of 2.80%—13.50% under the working condition, while the volume fraction of combustible gases was 19.25%, which was far above the upper explosion limit 13.50%. Therefore the gas composition was beyond the explosion limits. Upon employing the same method of section 3.2.2, the safety oxygen content of combustible gases at the reactor inlet is 15.60%. But the oxygen content in the system was 16.19% which was higher than the safety oxygen content. So combustion or explosion accidents might take place at the reactor inlet judging from the data on safety oxygen content.

Further experimental analysis of the inert limit was conducted to evaluate the flammability risk of mixed gases. When the total volume fraction of inert gases was 10% in the system (containing 8.44% N2+CO2and 1.56% H2O), the explosion limit was in the range of 3.50%—10.0%. It can be seen from the explosive area of mixed gases in the triangular diagram (as shown in Figure 6(a)), an inert point C corresponding to a concentration of 26.39% is located, which indicates that when the volume fraction of inert gases (N2+CO2, and H2O) is more than 26.39% in the gas mixture, no explosion phenomenon will happen no matter at whatever proportion the combustible gases (CO and C3H6) would mix with air. It can be seen from the data listed in Table 1 that the total volume fraction of inert gases in the system is 64.56% which is much higher than the inert limit. So no combustion and explosion phenomenon will happen at the reactor inlet judging from the inert limit data.

3.4 Experimental results of gases at reactor outlet

C3H6content decreased distinctly when it passed through the reactor, and its temperature increased sequentially with its pressure decreasing slightly. Now the volume ratio of CO and C3H6is 43.75%: 56.25%, and the measured explosion limits of flammable gases are in the range of3%—15%. Judging from the data on explosion limits, the total volume fraction of combustible components at the reactor outlet was 16.48% according to Table 4, which was higher than the upper explosion limit of 15%, leading to the conclusion that mixed gases had no blasting risk. The experimentally measured safety oxygen content of combustible gases at the reactor outlet was 12.30% and the oxygen content in the system was 7.19% which was lower than the data obtained by theoretical calculations and experiments. So no combustion or explosion phenomenon will happen at the reactor outlet judging from the data on safety oxygen content.

Figure 6 Explosive area of mixed gases

Although it is recognized that the mixed gases were nonflammable based on the data on explosion limits and safety oxygen content, experiments on inert limit were also conducted. When the total volume fraction of inert gases was 10% in the system (7.44% N2+CO2and 2.56% H2O), the explosion limits of this flammable gas mixture ranged from 3.8% to 11.2%. It can be seen from the explosive area of mixed gases in the triangular diagram (as shown in Figure 6(b)) that the inert point C corresponds to a concentration of 25.64%. According to the data depicted in Table 1, the total volume fraction of inert gases at the reactor outlet is equal to 73.33% which is much higher than the inert limit data. So no combustion or explosion phenomenon will happen at the reactor outlet.

4 Conclusions and Suggestions

1) The theoretical volume fraction of combustible gases at the recycle gas compressor outlet is 19.70% under working condition which is higher than the upper explosion limit, and the oxygen content is 8.59% which is lower than the safety oxygen content, so the mixed gases do not have any risk of blasting. At the reactor inlet, the total volume fraction of combustible gases is 19.25% which is beyond explosion limits under the same condition, but the oxygen content is 16.19% which is higher than the safety oxygen content, so combustion and explosion accidents might occur at the reactor inlet by theoretical analysis. Finally at the reactor outlet, the oxygen content is 7.19%, which is lower than the safety oxygen content, but the total volume fraction of combustible gases is 16.48% which is within the range of explosion limits, so combustion or explosion phenomena might happen at the reactor outlet according to theoretical calculations.

2) There is no risk for gas blasting at the recycle gas compressor outlet judging from the data on explosion limits and safety oxygen content, and furthermore, the excessive amount of inert gases can suppress the explosion effectively. It is not so safe at the reactor inlet, because the oxygen concentration is higher than the safetyoxygen content notwithstanding the total volume of combustible gases is beyond the range of explosion limits. But the amount of inert gases equating to 64.56% in the system can suppress the risk of explosion effectively. With regards to the gases at the reactor outlet, it is safe as evidenced experimentally by the explosion limits and the safety oxygen content, as well as the inert limit.

3) It is also found out that the gas mixture at the rector inlet is most dangerous as it contains more C3H6and O2. Thus, the on-line analysis of gas composition, monitoring and control of pressure, temperature and gas concentration in critical working locations need to be strengthened. In addition, open flames, hot surfaces and other sources of ignition are strictly prohibited in working space.

Acknowledgement:The project is financially supported by the National Science and Technology Support Program of China (2012BAK13B01).

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Recieved date: 2012-07-05; Accepted date: 2012-10-30.

Liu Zhenyi, E-mail: zhenyiliu@bit. edu.cn.