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Application of Palladium Catalyst in the Production of Hydrogen Peroxide by the Anthraquinone Process
Application of Palladium Catalyst in the Production of Hydrogen Peroxide by the Anthraquinone Process
(Shandong Tancheng , China)
Abstract: Based on the operating experience of palladium catalysts in domestic hydrogen-peroxide units, this paper describes the optimized control and key points for catalyst loading, activation, operation, and regeneration in the anthraquinone process. Measures for improving operating conditions—such as temperature, pressure, working-liquid flow rate, and circulating hydrogenated-liquid flow rate—are proposed. Regeneration methods using steam or organic solvents are also presented.
Key words: hydrogen peroxide; palladium catalyst; hydrogenation efficiency; degradation; regeneration; activity
- At present, the anthraquinone processes used industrially in China for hydrogen-peroxide production include the suspended-tank nickel-catalyst process, the fixed-bed palladium-catalyst process, and the fluidized-bed process. Among them, the fixed-bed palladium-catalyst process has become the mainstream technology, accounting for more than 95 % of total capacity, because of its low investment, high output, simple operation, and the advantages of the palladium catalyst itself—namely, low consumption, high activity, easy regeneration, and safe use.
- In the fixed-bed palladium-catalyst anthraquinone process, 2-ethylanthraquinone (EAQ) is dissolved in a mixed solvent of aromatic hydrocarbons and tris-(2-ethylhexyl) phosphate to form the working solution. In the hydrogenation reactor the solution reacts with hydrogen at specified temperature and pressure in the presence of a palladium catalyst. The hydrogenated solution is then oxidized with air, and the resulting oxidized liquor is extracted with deionized water and purified to give hydrogen peroxide. After treatment the working solution is recycled. The hydrogenation step is the core of the whole process, and its performance depends directly on the behavior of the palladium catalyst.
- Palladium catalyst is a costly key raw material in the anthraquinone process. During production it must be handled according to its characteristics so that it can operate safely and steadily; otherwise its efficiency will be impaired, leading to waste and losses in output and quality.
1. Palladium Catalyst Description
Palladium catalyst is prepared by loading palladium chloride onto an alumina support through steps that include support preparation, transformation, palladium loading, and post-treatment. The type, crystal form, particle size, bulk density, pore structure, strength of the support, and the loading method are the main control indices in catalyst manufacture, of which support quality is the most decisive factor influencing all product indices. The catalyst specifications are as follows:
(1) Catalysts for fixed beds are spherical, cylindrical, or trilobate, with various particle sizes. Smaller particles give larger contact area and higher hydrogenation efficiency, but also higher pressure drop and greater tendency to agglomerate.
(2) Palladium content is generally 0.2–0.3 %.
(3) Bulk density is 0.5–0.6 g mL⁻¹, which affects plant capacity.
(4) To avoid breakage during use, the crushing strength must exceed 40 N per particle.
(5) For hydrogen-peroxide production, hydrogenation occurs mainly on the catalyst surface; a large specific surface area increases hydrogenation efficiency. Smaller particles enlarge the external surface, and reactions also take place within the pores, so pore structure strongly influences hydrogenation rate. The support must therefore have sufficiently large pore volume. Excessive pore volume, however, lowers mechanical strength, leading to pulverization and palladium loss during operation; long residence time of working solution in wide pores causes over-hydrogenation and degradation of anthraquinone. Over-fine particles are easily affected by external factors, forming agglomerates or local blockages, channeling, localized over-hydrogenation, precipitation of hydroanthraquinone, and accelerated formation of degradation products.
All the above indices must be considered together so as to obtain a catalyst with good activity, selectivity, and service life.
2. Catalyst Loading and Activation
2.1 Catalyst Loading
The hydrogenation tower sections are first purged with hydrogen at a rate determined by the amount of catalyst. Palladium-catalyst activation is exothermic; the bed temperature rises 20–50 °C during activation and reaches 60–80 °C at the end (depending on ambient temperature). Activation is considered complete when the temperature in the catalyst layer begins to fall; this usually takes about 20 h. When activation is finished, hydrogen flow is stopped and nitrogen is introduced for 1–2 h until the hydrogen content in the exit gas is ≤ 2 %. The bed is then pressurized to 0.2–0.3 MPa and held for use. Some modern palladium catalysts possess high initial activity and need no pre-activation; they can be activated on stream, thereby preventing too-rapid release of activity.
Catalyst loading should be carried out in a dry environment to prevent moisture uptake. Particles must be handled gently; at the start of loading they are poured slowly through a canvas bag to the bottom of the reactor, and personnel entering the tower must not wear hard-soled shoes. Firm, uniform loading is essential. First, check the stainless-steel screens (degreased and passivated) for strength, aperture, and dimension to ensure they can support the catalyst and allow proper flow; prevent collapse, blockage, or loss of catalyst through gaps between screen and wall. Second, ensure uniform packing to avoid channeling. Spherical catalysts roll easily and pack densely; cylindrical or trilobate particles are irregular and must be leveled or tamped by hand if necessary. Do not exceed the recommended loading height; otherwise excessive bottom hydrogenation and hydroanthraquinone precipitation may occur. If necessary, load less initially and add more later, or use multiple beds in series.
2.2 Catalyst Activation
Fresh or regenerated palladium catalyst is normally activated to achieve full performance. First, displace air in the catalyst bed with nitrogen of ≥ 99 % purity until the oxygen content in the exit gas is ≤ 2 %. Stop nitrogen flow and introduce hydrogen at a rate that keeps the bed temperature rise below 50 °C. When the bed temperature begins to fall, activation is complete. Replace hydrogen with nitrogen for 1–2 h until the hydrogen content in the exit gas is ≤ 2 %. Pressurize to 0.2–0.3 MPa and hold for use.
3. Factors Influencing Catalyst Performance
3.1 Temperature
At the start of operation hydrogenation temperature is low; efficiency increases markedly with temperature. After an upper limit is reached the effect becomes less pronounced, while side reactions and anthraquinone degradation increase. Therefore, temperature should not exceed 75 °C. Temperature fluctuations should be minimized to prevent excessive efficiency, hydroanthraquinone precipitation, and agglomeration.
3.2 Pressure
Higher pressure accelerates hydrogenation and increases efficiency; the effect is evident in the 0.2–0.3 MPa range. During start-up, pressure can be adjusted to control efficiency; in normal operation efficiency is regulated by preheat temperature rather than by increasing pressure.
3.3 Liquid Spray Density in the Hydrogenator
The working-liquid flow rate strongly influences hydrogenation. Within the design spray-density range, higher flow gives faster surface renewal, better flow pattern, lower risk of channeling or agglomeration, and higher capacity, but hydrogenation efficiency per pass decreases. Increased recycle of hydrogenated liquor enhances spray density, consumes residual oxygen, and improves temperature uniformity. Insufficient total flow causes channeling, localized over-hydrogenation, degradation, hydroanthroquinone precipitation, and agglomeration. The flow rate must therefore be set according to catalyst capacity, tower design, and operating temperature.
3.4 Working-Liquid pH and Hydrogen-Peroxide Content
Slightly alkaline working liquor improves catalyst activity and selectivity; liquor treated in the alkali dryer and clay bed meets this requirement. Deviations reduce efficiency and selectivity. Excess hydrogen peroxide releases oxygen in the bed, decreases selectivity, and weakens the support.
3.5 Hydrogen Tail-Gas Venting
Accumulation of inert gases in hydrogen lowers its partial pressure and reaction rate. Analyze tail-gas purity and increase venting to keep hydrogen content ≥ 75 %.
3.6 Feedstock Quality
Feed quality markedly affects hydrogenation efficiency. Impurities such as CO, H₂S, Cl₂ in hydrogen rapidly poison the catalyst, sometimes irreversibly. Heavy aromatics, tris-(2-ethylhexyl) phosphate, and EAQ must also meet specifications, especially sulfur content.
3.7 Working-Liquid Composition
A moderate increase in the content of hydroanthraquinone (H₄EAQ) raises total anthraquinone solubility and hydrogenation efficiency, lowers the extent of hydrogenation, and slows degradation. However, when H₄EAQ exceeds ~80% of total anthraquinone, overall solubility falls and oxidation becomes slow; EAQ must then be replenished to keep the ratio stable. Typically H₄EAQ is maintained at about 50% of total anthraquinone. Intensifying hydrogenation to raise capacity often conflicts with improving selectivity and reducing degradation; both must be balanced for stable operation.
4. Catalyst Regeneration
With time on stream, catalyst activity declines. When hydrogenation efficiency or product concentration can no longer meet requirements, regeneration is needed. Common methods are steam and organic-solvent regeneration.
4.1 Steam Method
Loss of activity is usually caused by coverage of active sites or pores by impurities (hydroanthraquinone, alumina fines, alkali, degradation products). Removing these restores activity. Pass large amounts of saturated steam uniformly through the bed to sweep out the impurities until the condensate is clear. Then dry the catalyst with hot nitrogen; the nitrogen can be recycled after condensation and compression. Regenerated catalyst is activated with hydrogen and put back into service. This method is simple, pollution-free, and low in labor intensity.
4.2 Organic-Solvent Method
When the catalyst bed is seriously broken or agglomerated and steam regeneration is ineffective, the catalyst must be unloaded into a special vessel, washed with large amounts of water while stirring to recover working liquor until the wash water is clean, then soaked and stirred with an organic solvent (heavy aromatic hydrocarbon) until the catalyst appears shiny black. After solvent removal, the catalyst is dried and activated as above. This method gives good regeneration and maintains activity for a long time, but some catalyst loss occurs.
Dalian Chuangge Technology Co., Ltd
Richard Han
Doris Zhou
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