| 1. |
- Ackermann, N.L., et al.
(författare)
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Local scour around circular piers under ice covers
- 2002
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Ingår i: Ice in the environment. - Dunedin : Dep. of Physic, Univ. of Otago. - 1877139521 ; , s. 149-155
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Konferensbidrag (refereegranskat)abstract
- This paper presents a laboratory investigation on the effect of ice cover on local scour around circular bridge piers. Experiments were performed in a 12-meter flume with recirculating sediment discharge. Both smooth and rough artificial covers were used. The bed material consisted of uniform ripple-forming sand. The tests were run for both clear water as well as live bed conditions. The results showed that for equivalent averaged flow velocities the existence of an ice cover could increase the local scour depth scour by 25 to 35 % from the free surface condition. The largest difference occurs at a live bed condition when the flow velocity, U, is in the rage of 1.5 to 2 times of the critical velocity for bed movement, U (sub c) . A rough cover gives slightly larger scour depth than a smooth cover. The movement of bed forms led to variations of scour depth with time.
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| 2. |
- Guo, X. W., et al.
(författare)
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Effects of SiO2/Al2O3, MgO modification and hydrothermal treatment on the catalytic activity of HZSM-5 zeolites in the methylation of 4-methylbiphenyl with methanol
- 2004
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Ingår i: Applied Catalysis A. - : Elsevier BV. - 0926-860X .- 1873-3875. ; 261:2, s. 183-189
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Tidskriftsartikel (refereegranskat)abstract
- The objective of this work is to study the effects Of SiO2/Al2O3 of HZSM-5, MgO modification and hydrothermal treatment on methylation of 4-methylbiphenyl (4-MBP) with methanol under fixed-bed down-flow conditions. The results show that, with an increase in the SiO2/Al2O3 of HZSM-5 (from 50 to 150, molar ratio), the selectivity to 4,4'-dimethylbiphenyl (4,4'-DMBP) increases from 25 to 79%. MgO modification also increases the selectivity to 4,4'-DMBP, but leads as well to low catalyst activity and rapid deactivation. Hydrothermal treatment not only increases the selectivity, but also improves the stability. When CBV1502 (SiO2/Al2O3 = 150) catalyst was hydrothermally treated at 500degreesC, the selectivity to 4,4'-DMBP increased to 85%. The selectivity to 4,4'-DMBP was further improved to about 90% by the increasing in the amount of mesitylene in the feed as solvent.
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| 3. |
- Guo, X. W., et al.
(författare)
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Methylation of 4-methylbiphenyl with methanol over metal oxide-modified HZSM-5 zeolite catalysts
- 2003
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Ingår i: Cuihuà xuébào. - 0253-9837 .- 1872-2067. ; 24:5, s. 333-337
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Tidskriftsartikel (refereegranskat)abstract
- A series of HZSM-5 zeolite catalysts modifed with metal oxides (MgO, CaO, SrO, BaO, ZnO, La2O3 and CeO2) were prepared by impregnation method and characterized by XRD, TPD and N-2-adsorption. Methylation of 4-methylbiphenyl (4-MBP) with methanol to 4,4'-dimethylbiphenyl (4,4'-DMBP) was carried out over the prepared catalysts under fixed-bed down-flow conditions. The results showed that the MgO-modified HZSM-5 can significantly improve the selectivity for target product 4,4'-DMBP (up to 80 % vs only 13 % over the parent HZSM-5). The effects of metal oxides on 4,4'-DMBP selectivity are arranged in the following order: MgO>SrOapproximate toZnOapproximate toCaOapproximate toLa(2)O(3)>BaO>CeO2. The effects of MgO contents, precursor-salt types, and loading methods were further investigated, and the results revealed that during 4-MBP conversion, proper MgO loading (5.6 %) can be more effective, and the ion-exchange methed is much better than the impregnation method. However, it is difficult to get higher metal oxide loading by using the ion-exchange method. The high selectivity over HZSM-5 modified with MgO largely results from the depression of 4,4-DMBP second reactions such as isomerization, dealkylation and alkylation, as demonstrated by the reaction of 4,4'-DMBP as reactant over MgO/ZSM-5 catalyst and HZSM-5 zeolite.
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| 4. |
- Guo, X. W., et al.
(författare)
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Shape-selective methylation of 4-methylbiphenyl to 4,4 '-dimethylbiphenyl over zeolite HZSM-5 modified with metal oxides of MgO, CaO, SrO, BaO, and ZnO
- 2003
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Ingår i: Catalysis Letters. - 1011-372X .- 1572-879X. ; 87:02-jan, s. 25-29
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Tidskriftsartikel (refereegranskat)abstract
- A series of ZSM-5 samples modified with metal oxides MO(M = Mg, Ca, Sr, Ba and Zn) were employed for the alkylation of 4-methylbiphenyl (4-MBP) with methanol to 4,4'-dimethylbiphenyl (4,4'-DMBP) under fixed-bed down-flow conditions. The methylation results showed that the use of basic metal oxides can effectively enhance the selectivity to the target product 4,4'-DMBP. MgO is the most effective modifier among the metal oxides used and it can improve selectivity to 4,4'-DMBP up to 80% as compared to only 13% over the parent zeolite HZSM-5. The modi. cation effectiveness of metal oxides on 4,4'-DMBP selectivity can be arranged in the order MgO> SrO similar to ZnO similar to CaO> BaO. The optimization of MgO modi. cation through the content, salt types and loading methods revealed that proper MgO loading (5.6 wt%) can be more effective, and the impregnation method is much better than ion exchange. The correlation of physicochemical properties (TPD, TGA, chemical analysis and chemical adsorption, etc.) of the modified HZSM-5 with the catalytic data showed that the high selectivity over ZSM-5 modified with MgO largely results from the effective suppression of 4,4'-DMBP secondary reactions such as isomerization, dealkylation and alkylation.
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