Maleic Anhydride Production and Reactor Simulation

1.0 Introduction

Maleic anhydride is an organic compound known for its strong irritant properties when it comes into contact with the skin, eyes, and mucous membranes of the upper respiratory system. It has a melting point of 52.8°C and a boiling point of 202°C, commonly existing as a liquid or gas during production. Maleic anhydride plays a pivotal role as a multifunctional chemical intermediate, serving as a key feedstock for the production of unsaturated polyester resins, butanediol, gamma-butyrolactone, and tetrahydrofuran⁽¹⁾.

To achieve a high yield, which is preferred in commercial applications, a fixed-bed reactor is employed.

This type of reactor offers the advantages of ease of use, low maintenance, and approximately 65% conversion of butane.

Maleic anhydride contains a reactive double bond and readily reacts with water at 60°C to form maleic acid and at 100°C to form fumaric acid. Currently, around 70% of the demand for Maleic anhydride is met through the oxidation of n-butane using vanadium-phosphorus-oxide (VPO) as a catalyst. VPO not only facilitates the oxidation reaction but also eliminates the need for butane as a feedstock⁽¹⁾.

This report focuses on simulating a fixed-bed reactor using a catalyst composed of phosphorous vanadium mixed oxide promoted by metals.

2.0 Background for Reactor

The production of Maleic anhydride through the oxidation of butane primarily utilizes two types of reactors: Fixed bed reactors and fluidized bed reactors. Both fixed bed and fluidized bed reactors are commonly employed in industrial processes. In fixed bed reactors, the reaction occurs within multiple tubular reactors, each typically ranging from 3.5 to 6 meters in length and 21 to 25 millimeters in diameter.

The reaction in a fixed bed reactor is highly exothermic, leading to significant heat generation during the process. However, this excessive heat generation can result in a runaway reaction, especially during the partial oxidation of butane, ultimately limiting the yield to less than 65%. Consecutive reactions, along with issues like local catalyst overheating and poor heat transfer properties, contribute to this lower yield. On the other hand, total oxidation reactions release even more heat, leading to a loss of selectivity in the reactor. To prevent adverse inlet conditions and maintain no recycle stream, this process is carried out with a low feed n-butane concentration in air (< 1.8 m3/m3) within fixed beds⁽¹⁾.

Fluidized bed reactors, while offering better heat transfer properties, require a larger bed and consequently achieve lower yields (around 55%). Increased back-mixing is a primary factor leading to the lower yields in fluidized beds. Additionally, parallel reactions involving the combustion of n-butane and oxidative degradation of Maleic anhydride to acetic and acrylic acids further reduce selectivity, which is a significant concern as it affects overall yield⁽¹⁾.

Given the need for higher yields in these reactions, fixed bed reactors are often the preferred choice. Fluidized processes demand a considerable investment in expanding the bed size. The following reaction takes place during this process⁽²⁾:

Air + Superheated Vaporized Butane → Maleic Anhydride

During this process, air is compressed to modest pressure via a compressor and mixed with superheated vaporized butane. A static mixer is employed to ensure that the air-butan concentration remains below 1.7 mole%. Rupture disks are used for pressure control at both the inlet and exit reactor heads. Efficient removal of heat from the reactor is crucial to mitigate by-product formation, necessitating the use of a large multi-tubular heat exchanger. The reactor tubes are typically 3 to 6 meters long with an outer diameter of 25 to 30 millimeters, facilitating effective heat removal. Given the exothermic nature of the reaction, an external salt cooler is employed to dissipate the heat. The reactor operates at temperatures ranging from 390°C to 430°C with rapid salt circulation on the side. Under ideal conditions, these reactions can achieve an efficiency of up to 85% butane conversion, although typical conversions are around 65%. Waste heat from the plant can be utilized to drive an air compressor, generate electricity, or both.

3.0 Catalyst

In the process of converting butane to maleic anhydride, the method employed involves the air oxidation of benzene in the presence of specific heavy metal oxide catalysts. This approach is chosen primarily due to the inherent toxicity of butane fumes, and the catalyst serves the critical function of eliminating butane as a feedstock. Initially, the catalyst used for the conversion of butane to maleic anhydride predominantly consisted of vanadium and phosphorus. However, this catalyst had the drawback of yielding only 30% to 50% of maleic anhydride despite enabling the oxidation of butane. To enhance production yield, various activators, stabilizers, and promoters have been introduced.

The catalyst is prepared in aqueous solvents using inorganic acids and metal oxides. Once the aqueous solution becomes clear, and a significant reduction of vanadium has occurred, a substantial quantity of water-hydrogen chloride is removed from the catalyst solution, resulting in a thick syrup. This syrup is then diluted with methanol and other suitable alcohols. The catalyst preparation involves the following reactions⁽⁵⁾:

1. Mixing vanadium pentoxide (approximately 91 grams and 0.5 moles), hydrochloric acid (1.51 grams, 38% concentration), and molybdenum trioxide (4.4 grams).

2. Refluxing the mixture for 2 hours at 108ºC, observing a color change from red-brown to blue-green and ultimately blue.

3. Adding 1.28 moles of 85% H3PO4 and initiating distillation. After removal of H2O-HCl, stopping the distillation, and leaving a viscous blue syrup in the flask.

4. Dissolving half of the syrup in 400ml of methanol.

5. Refluxing for 5.5 hours with 150ml of O-xylene using a round-bottom flask with a reflux condenser and stirring bar.

6. Performing distillation until a blue syrup with a layer of O-xylene is obtained.

7. Drying the syrup in a vacuum oven to remove the brown crust covering the blue catalyst.

This catalyst exhibits significantly higher activity than conventional catalysts and can eliminate 100% of the vanadium feedstock. It comprises a phosphorous vanadium mixed oxide promoted by various metals. The atomic ratios include vanadium to phosphorous at 0.5:1 to 1.25:1, co-metal to vanadium at 0.75:1 to 1:1, molybdenum to vanadium at 0.001:1 to 0.2:1, and phosphorous to vanadium at 1:1 to 1.5:1. The co-metal can be added alongside vanadium or introduced separately into the solution. Suitable molybdenum compounds include molybdenum oxide and various soluble molybdenum salts⁽⁵⁾.

Additionally, the catalyst facilitates the oxidation of butane to maleic anhydride by enabling contact with oxygen. This oxidation occurs when n-butane contacts the catalyst at low oxygen concentrations. While air is a suitable source of oxygen, synthetic oxygen and diluent gas mixtures can also be employed⁽⁶⁾.

For the gaseous feed stream to the oxidation reactors, a concentration of 0.8% to 1.5% n-butane is required to achieve the desired yield of maleic anhydride. Concentrations below 1% are not economically favored. The flow rate of the gaseous stream through the reactor typically ranges from 1000 cc to 2400 cc of feed per cc of catalyst per hour, with a residence time of less than one second⁽⁵⁾.

4.0 Results and Design Considerations

A simulation of a fixed bed reactor for the production of maleic anhydride from butane, utilizing the catalyst "Phosphorous vanadium mixed oxide promoted by metals.",  aims to achieve an 80% conversion rate in Aspen HYSYS⁽⁵⁾. The table below presents the results obtained from the simulation.

According to US Patent US4416802A, the conversion rate achieved using VPO at 400°C is approximately 80%. The inlet mole flow rate of oxygen is 2.35 times that of butane⁽⁵⁾.

Table 1: Stream Flows and Properties Around Fixed Bed Reactor

Bed	Fixed bed Reactor
Inlet Temp (℃)	400
Outlet Temp (℃)	400
Feed Flow of butane (kmol/h)	20

To determine the reactor's diameter with a 5 psi pressure drop across the entire reactor, Excel is used with the Solver tool, applying the Ergun equation. Based on the experimental gas hourly space velocity (GHSV) of 1200⁽⁵⁾, the volume of the catalyst bed is calculated to be 1.5 m³. The characteristics of the fixed reactor in the simulation are summarized in Table 2 below.

Table 2: Design Properties of Fixed Bed Reactor

Diameter (m)	Cross-sectional Area (m²)	Bed Length (m)	Total Length (m)	Aspect Ratio
0.7657	0.4602	3.3312	3.6360	4.7484

Within the Aspen HYSYS simulation, the fixed bed reactor is represented as a vertical vessel storage tank under pressure. The table below provides information on the capital cost of constructing the reactor and the estimated operating cost.

Table 3: Capital Cost of Fixed Bed Reactor from Aspen HYSYS Simulation

Diameter (m)	Length (m)	Capital Cost (USD)
0.7657	3.6360	1,706,460

Waste Heat Considerations:

To manage the heat generated during the reaction, several strategies can be implemented. First, the heat produced by the reaction can be utilized to heat the incoming airflow. This can be achieved by installing a heat exchanger around the reactor and passing the air through it before it enters the reactor. This approach helps raise the temperature of the inlet airflow, reducing the energy required by the heater and thereby saving power.

Furthermore, the efficient utilization of waste heat from the maleic anhydride production process focuses on economically harnessing the by-product steam. This steam can be used to power compressors, generate electricity, or both. Alternatively, for maximum efficiency, an energy-intensive process such as a plant can be strategically located near the maleic anhydride plant. In such cases, the waste heat from maleic anhydride production can be transferred to the nearby plant for heating purposes⁽¹⁾.

Additionally, heat recovery systems play a crucial role in maleic anhydride plants, as seen in commercial processes. The unreacted butane can be recovered to produce additional heat within the reactor. Moreover, by-products such as carbon monoxide, which cannot be directly released into the air, can undergo deconstruction in a thermal oxidizer or modified boiler. This process allows the heat generated during the reaction to be used for carbon monoxide treatment as well⁽¹⁾.

5.0 Conclusion

The predominant choice for commercial production of maleic anhydride is the fixed bed reactor. In this report, we utilized Aspen HYSYS to simulate the conversion reactor, mimicking the fixed bed reactor's behavior with the catalyst vanadium-phosphorus-oxygen under constant 80% butane conversion and a steady stream temperature.

Based on the simulation results, a vertical pressure storage vessel with a 1.5 m³ catalyst bed was determined, with a specific diameter and height. The simulation also provided cost estimates: the construction cost for this reactor is USD 1,706,460, with an annual operating cost of USD 950,791.

Furthermore, considering the highly exothermic nature of the reaction in the reactor, effective waste heat management is essential. One approach is to utilize this excess heat to elevate the temperature of the incoming air by incorporating a heat exchanger around the reactor. Additionally, the waste heat can be harnessed as a power source for compressors or electricity generation. It is also crucial to address the issue of carbon monoxide produced during the reaction, as it poses health risks. The heat generated by the reaction can be employed to process and mitigate this toxic byproduct.

6.0 References

1. Felthouse, Timothy R., et al. 'Maleic anhydride, maleic acid, and fumaric acid.' Kirk‐Othmer Encyclopedia of Chemical Technology (2000).
2. Partopour, Behnam, and Anthony G. Dixon. 'n‐butane partial oxidation in a fixed bed: A resolved particle computational fluid dynamics simulation.' The Canadian Journal of Chemical Engineering 96.9 (2018): 1946-1956.
3. Aspen HYSYS Simulation of Maleic Anhydride Production from n-Butane via Partial Oxidation.
4. Sharma, Ramesh K., David L. Cresswell, and Esmond J. Newson. 'Kinetics and fixed‐bed reactor modeling of butane oxidation to maleic anhydride.' AIChE journal 37.1 (1991): 39-47.
5. Catalysts for the production of maleic anhydride by the oxidation of butane, US4416802A.
6. Improved vapor phase catalytic oxidation of butane to maleic anhydride, US4668802A.

7.0 Appendix

7.1 Sample Calculation

1. Gas Hourly Space Velocity (GHSV) Calculation:

GHSV = 1200 h^-1

Catalyst bed volume = volumetric flow rate in STP / GHSV = 1840 m³/h / 1200 h^-1 = 1.5 m³ = 54.1 ft³

2. Bed Diameter and Length Calculation:

Diameter of bed = 0.77 m

Cross-sectional area = Π * r² = 0.46 m²

Bed length = catalyst bed volume / cross-sectional area = 1.5 m³ / 0.46 m² = 3.33 m

Total bed length = Bed length + bottom support length = 3.33 m + 0.3 m = 3.63 m

Vessel aspect ratio = Bed length / diameter = 3.63 m / 0.77 m = 4.75

3. Ergun Equation for Pressure Drop:

∆𝑃 / 𝐿 = 150(1 − ϵ)² 𝐷𝑝 / (2 * ϵ³ * μ * 𝑢𝑜) + 1.75(1 − ϵ) 𝐷𝑝 / (ϵ³ * 𝜌 * 𝑢𝑜²)

Where:

∆𝑃: Pressure drop across the bed

L: Length of the bed = 3.33 m

𝐷𝑝: Equivalent spherical diameter of the packing = 0.0048 m

ρ: Density of fluid = 0.62 kg/m³

μ: Dynamic viscosity of the fluid = 0.00002083 kg m/s

𝑢𝑜: Superficial velocity = volumetric flow rate / cross-sectional area = 1.19 m³/s / 0.46 m² = 2.59 m/s

ϵ: Void fraction of the bed = 0.45

Using the Ergun equation:

∆𝑃 / 3.33 = 150(1 − 0.45)² * 0.0048 / (2 * 0.45³ * 0.00002083 * 2.59) + 1.75(1 − 0.45) * 0.0048 / (0.45³ * 0.62 * 2.59²)

∆𝑃 = 34482.8 Pa = 5 psi