Crude Terephthalic Acid Research Articles

A highly active and stable Pd–TiO2/CDC–SiC catalyst for hydrogenation of 4-carboxybenzaldehyde

Carbon has been widely used as a catalyst support and adsorbent in industry. However, it suffers from poor stability due to its limited mechanical strength, particularly under high pressures and temperatures. We report here a carbide derived carbon (CDC) layer on a porous SiC surface, which has the properties of high mechanical strength and is easy to shape. The CDC exhibits an amorphous structure and contains mainly mesopores with a BET surface area of 125 m2 g−1. The CDC–SiC composite yields a comparable performance to coconut activated carbon (AC) as a catalyst support in the probe reaction hydrogenation of 4-carboxybenzaldehyde. The further introduction of TiO2 nanoparticles enhances the activity and stability significantly because of the improved dispersion of Pd particles on CDC–SiC. The activity is 4 times higher than the Pd/AC catalyst. Pd–TiO2/CDC–SiC shows great promise as an alternative to the current AC supported Pd catalyst for the crude terephthalic acid hydropurification industry.

Recovery of Terephthalic Acid from Alkali Reduction Wastewater by Cooling Crystallization

AbstractA process for the recovery and purification of terephthalic acid (TA) from alkali reduction wastewater is reported. TA was first precipitated from alkali reduction wastewater by acidification with sulfuric acid, and then the produced crude TA was dissolved in dimethylacetamide (DMA) so that crude TA could be purified from the solution by cooling crystallization. The results indicated that acidification could reduce the chemical oxygen demand of the wastewater by 83 %, and the purity of TA by crystallization could reach 99.91 %. A correlation was proposed in describing the solubility of crude TA in DMA from 303.4 to 358.65 K, which gives a mean relative discrepancy of less than 1.14 %. The cooling rate of the mother liquor had a large influence on the crystal size distribution. At an average cooling rate of 1.18 K min–1, the particle size distribution of TA was narrow and the average size was about 100 μm. In a bench‐scale study, it was demonstrated that the crystallized product can be recycled as the raw material for polyethylene terephthalate production.

Cover Chem. Eng. Technol. 10/2011

AbstractTerephthalic acid recovered from alkali reduction wastewaterA process for the recovery and purification of terephthalic acid (TA) from alkali reduction wastewater was developed. The process consisted of two steps: TA was first precipitated from alkali reduction wastewater by acidification. After that, the crude TA produced was dissolved into dimethylacetylamide and purified by cooling crystallization. The results indicated that acidification could reduce the chemical oxygen demand of the wastewater by 83 %, and the purity of TA by crystallization could reach 99.9 %. Furthermore, at an average cooling rate of 1.18 K min–1, the particle size of TA was uniform and the average size was about 100 µm. The isolated terephthalic acid can be recycled as raw material for polyethylene terephthalate production.S.‐C. Wu, Z.‐M. Cheng*, S.‐D. Wang, X.‐L. ShanChem. Eng. Technol. 2011, 34 (10), 1614–1618.DOI: 10.1002/ceat.201100096

Isothermal Seeded Semicontinuous Reactive Crystallization of Crude Terephthalic Acid Crystals

In the reactive crystallization of terephthalic acid (TA) by liquid-phase oxidation of p-xylene (PX), four simultaneous physicochemical processes occurred, including the oxidation of PX to TA, the nucleation of TA from solution, the growth of TA crystals, and the incorporation of impurity 4-carboxybenzaldehyde (4-CBA). In this work, by using the experimental technique of seeded semicontinuous reactive crystallization, the growth of TA crystals and the incorporation of impurity 4-CBA were studied. The effects of supersaturation and temperatures on the crystallization kinetics were experimentally examined. A detailed semicontinuous reactor model including chemical reactions, TA growth, and 4-CBA incorporation was developed. The model was solved numerically to optimize the model parameters for crystallization kinetics. The effect of temperatures on the growth rate and the incorporation rate was studied and correlated. The correlated results agreed with the experimental results satisfactory. The obtained crystallization kinetics can be used as essential data to predict the growth of TA crystals and the incorporation of impurity 4-CBA.

Partition Coefficient of 4-Carboxybenzaldehyde between Crude Terephthalic Acid Crystals and the Corresponding Aqueous Acetic Acid Solution at (423.2 to 453.0) K

By using a specially contrived experimental technique, the partition equilibrium of 4-carboxybenzaldehyde (4-CBA) between crude terephthalic acid crystals and the corresponding aqueous acetic acid solution was achieved. The equilibrium mole fractions of 4-CBA in both the liquid and solid phases were measured over a temperature range from (423.2 to 453.0) K, and the mass fractions of acetic acid in the solvent mixtures ranged from 0.800 to 0.995. The partition coefficients, defined as the ratio of the equilibrium mole fraction of 4-CBA in the solid phase to the equilibrium mole fraction of 4-CBA in the liquid phase, were experimentally determined. The experimental results show that: (1) within the temperature range of the measurements, the partition coefficient shows a decreasing trend as the temperature increases at a constant solvent composition, and (2) within the solvent composition range of the measurements, the partition coefficient increases with increasing water content in the solvents at a constan...

Aging of Crude Terephthalic Acid Crystals at High Temperatures

By using a specially contrived experimental technique, crude terephthalic acid (TPA) crystals have been aged in their own saturated 90 mass % acetic acid + 10 mass % solution at 467, 477, 487, 497, and 507 K. The time evolution of crystal size distribution, as well as contents of the contaminant 4-carboxybenzaldehyde (4-CBA) in both solid and liquid phases, has been experimentally measured. Experimental results reveal the following: (1) The effect of stirring rate on the aging of crude TPA crystals is negligible under the conditions of this study. (2) The crystal size distribution is observed to broaden with time, accompanied by a decrease in number and an increase in averaged crystal size during aging. The aging rate increases with the increasing of temperature. (3) Under the conditions of this study, substantial purification occurs during aging. The rate of purification increases as the temperature increases. The Ostwald ripening mechanism is proposed to model the effect of aging on crystal size distri...

Metal dispersion and distribution in Pd-based PTA catalysts

Abstract The influence of the pH of the impregnating H 2 PdCl 4 solution was studied in the preparation of Pd catalysts on granular active carbon. All catalysts were tested in the purification of crude terephthalic acid (4-CBA hydrogenation), in conditions strictly similar to those of industrial operation. The pH of the impregnating H 2 PdCl 4 solution was found to strongly influence both Pd surface area and activity (best results with pH 1.5–2.0), while has a relatively small influence on Pd distribution. A linear relationship between catalytic activity for 4-CBA hydrogenation and Pd surface area was confirmed.

PdRu/Carbon Composite 촉매를 이용한 테레프탈산의 수소화 정제

CTA(crude terephthalic acid)의 수소화 정제 반응이 고온의 회분식반응기에서 PdRu/CCM(carbon-carbonaceous composite material) 촉매 상에서 수행되었다. 반응 시간에 따른 In(4-CBA; 4-carboxybenzaldehyde)의 의존도가 선형성을 보임에서 수소화 정제는 1차 속도론을 따름을 알 수 있었다. 촉매량에 따른 반응 속도의 선형성에서 조사된 반응 조건은 정제 반응을 잘 대변할 수 있음을 알 수 있었다. 반응 전환율이 증가하면(4-CBA가 감소하면) 고체 및 액체의 p-toluic acid(p-tol)의 농도는 증가하였으나 벤조산(BA)의 농도는 크게 변하지 않았다. 4-CBA 농도가 대략 0.15% 이하일 때에는 PTA의 AT(alkalitransmittance)는 4-CBA농도에 반비례하며 이는 4-CBA의 수소화에 따라 발색물질도 제거 됨을 보여 준다. 4-CBA농도가 약 0.2% 이상이면 AT는 일정하였는데 이는 4-CBA 자체는 발색 효과를 가지지 않음을 보여준다. (0.3%Pd-0.2%Ru)/CCM 촉매는 0.5%Pd/C 상업 촉매에 비해 초기 활성은 낮으나, 상업 공장 반응기에서 사용한 후에는 오히려 큰 잔존 활성을 보였고 PdRu/CCM 촉매는 루테늄 함량이 약 <TEX>$0.2{\sim}0.35%$</TEX> 일 때 활성의 상승효과를 보였다. PdRu/CCM 촉매는 0.5%Pd/C 상업 촉매를 대체할 가능성이 높음을 알 수 있다. The hydropurification reaction of CTA (crude terephthalic acid) was carried out with hydrogen over PdRu/CCM (carbon-carbonaceous composite material) catalyst in a batch reactor at high temperature. The first order kinetics of hydropurification is confirmed with the linear dependence of ln(4-CBA; 4-carboxybenzaldehyde) with reaction time. The reaction condition studied is thought to represent the hydropurification well because of the linear dependence of catalytic activity on the catalyst weight. The p-toluic acid (p-tol) in solid and liquid increases with the conversion of reaction or the decrease of 4-CBA. However, the benzoic acid (BA) concentration does not depend much on the conversion. The AT (alkali transmittance) does not depend on the 4-CBA when the concentration is higher than about 0.2% which shows the 4-CBA, in itself, does not cause the coloring effect. The AT of PTA depends inversely with the concentration of 4-CBA when the 4-CBA is less than about 0.15%. This may show the coloring materials are removed in parallel with the hydrogenation of 4-CBA. The (0.3%Pd-0.2%Ru)/CCM shows larger residual catalytic activity than a commercial catalyst, 0.5%Pd/C, after using in a commercial reactor even though the former has smaller fresh activity than the latter. The palladium and ruthenium in PdRu/CCM show the synergetic effect in activity when the ruthenium concentration is about <TEX>$0.2{\sim}0.35$</TEX> wt%. It may be supposed that the PdRu/CCM catalyst can be a promising candidate to replace the commercial Pd/C catalyst.

Carbon-supported palladium-ruthenium catalyst for hydropurification of terephthalic acid

The ruthenium-incorporated catalyst, (0.3% Pd–0.2% Ru)/CCM (carbon carbonaceous composite material) catalyst, has been studied as a candidate to replace an existing 0.5% Pd/C catalyst for hydropurification of crude terephthalic acid; it gives an excellent performance and is a promising catalyst. It has been confirmed that the (0.3% Pd–0.2% Ru)/CCM catalyst has higher stability in actual commercial condition and aging in lab condition compared with a commercial catalyst. The increased stability may be explained as the effect of ruthenium to prevent the sintering of supported metal particles. The (0.3% Pd–0.2% Ru)/CCM catalyst shows equal or less amount of side reaction of benzene ring hydrogenation. The dust from carbon support can also be decreased from the (0.3% Pd–0.2% Ru)/CCM catalyst due to the increased strength of the CCM. The (0.3% Pd–0.2% Ru)/CCM catalyst may be very competitive to the existing catalyst due to the inexpensive ruthenium and the expected long life of the catalyst.

Fluorescence characterisation and analysis of 4-carboxybenzaldehyde, a colour precursor to the manufacture of terephthalic acid

The fluorescence properties of 4-carboxybenzaldehyde (4-CBA), a main oxidation product in the manufacture of terephthalic acid, have been investigated using different polar and acidic solvents. It is shown that these properties are very dependent upon the nature of the solvent, with concentrated sulphuric acid causing the most intense fluorescence. An unexpected blue shift was observed in the excitation spectra for solutions of 4-CBA and the models, dimethylterephthalate (DMT) and acetophenone, in sulphuric acid, with decreasing concentration and decreasing acid strength, indicating the existence of a ground state aggregate of the sandwich type. A method for determining the levels of 4-CBA in crude terephthalic acid (CTA) has been developed using the condensation reaction between 4-CBA and o-phenylenediamine (o-phd).

Spectroscopic analysis of organic contaminants in terephthalic acid: colour implications in poly(ethylene terephthalate) manufacture

Colour measurements and doping experiments on polyethylene terephthalate (PET) show that the compounds in crude terephthalic acid which cause the yellow colour of derived PET polymer are carboxylic acid derivatives of anthraquinone and fluorenone. Fluorene derivatives are found to contribute to the PET polymer yellowness. On the other-hand, PET polymer luminance, was not affected by the small variations in impurity levels investigated in this work. 4-carboxybenzaldehyde (4-CBA) the major impurity in manufactured crude terephthalic acid (CTA), dominates the fluorescence spectrum. The fluorescence emission spectrum of 4-CBA is strongly influenced by the polarity and protic nature of the solvent. Associated dimers and aggregates of 4-CBA are primarily formed via the aldehyde and carboxylic acid groups causing marked red shifts in excitation and absorption spectra. Complexation also occurs between the 4-CBA and PTA groups.