Product distribution directed modification of ZSM–5

Thesis (MSc (Chemistry))--North-West University, Potchefstroom Campus, 2012.Ethylene and propylene are important chemical feedstocks for the production of polyethylene and polypropylene. Ethylene and propylene can be produced by various methods including steam cracking of liquefied natural gas (LNG), naphta or light olefin fractions. The methanol to olefin (MTO) process provides an alternative means of producing ethylene and propylene, where ZSM-5 is frequently used as catalyst due to its hydrophobicity, strong acidity, molecular sieve properties and low tendency towards coking, which makes ZSM-5 one the most popular zeolite catalysts in the industry. The oil crisis 1973 and the second oil crisis in 1978 caused the development of a commercial MTO process. Mobil Research and Development Corporation built a fixed-bed pilot plant to demonstrate the feasibility of the MTO as well as methanol-to-gasoline (MTG) process. When the oil price dropped again during the 1980’s, further developments of commercial processes were stopped for the time being. However, investigations on a bench scale are still pursued, and applications for patents are still submitted. During this study ZSM-5 was synthesized with a hydrothermal method, which produced agglomerated polycrystalline grains with characteristic ZSM-5 morphology and a Si/Al ratio of approximately 40. The synthesis time, synthesis temperature and aging time were varied while keeping all the other synthesis parameters constant in order to determine their influence on crystallite size. The synthesis time was varied between 12-72 hours, synthesis temperature was varied between 130-170°C and aging time between 30-90 minutes. Using SEM to determine crystal size, it was found that a variation in the aging time produced the largest crystallites (average of 21.6μm ± 10.8μm) while also having the largest influence on crystallite size followed by synthesis temperature (average of 13.1μm ± 4.9μm) and finally synthesis time (average of 5.7μm ± 0.4μm). In all cases XRD and SEM confirmed the formation of ZSM-5. To evaluate the as-synthesized ZSM-5 and compare it to a commercial ZSM-5 catalyst, Catalyst A using the MTO process, ZSM-5 was synthesized for 72 hours at 170°C with an aging time of 60 minutes before synthesis. The as-synthesized as well as Catalyst A’s agglomerated polycrystalline grains were sieved into three size fractions: smaller than 75μm, 75-150μm and 150-300μm. All six ZSM-5 fractions of ZSM-5 were used as catalysts for the MTO process in a fixed bed reactor at 400°C, atmospheric pressure and a 20wt% methanol to water feed. At 3.5 hours time on stream (TOS), the intermediate 75-150μm fraction had the highest light olefin selectivity for both the as-synthesized as well as Catalyst A, followed by the 150-300μm fraction and finally the smaller than 75μm fraction with the lowest light olefin selectivity. From this results it is clear that the as-synthesised ZSM-5 did not perform as well as Catalyst A. While the intercrystalline voids of the agglomerated ZSM-5 form second-order pores where self-diffusion is enhanced, the increased diffusional barriers created by the intercrystalline boundaries reduce the diffusion rate, promoting secondary reactions at the strong Brönsted acid sites thereby reducing ethylene and propylene selectivity. Coking reduces access to the Brönsted acid sites and plays a more influencial role for smaller crystallite sizes. Accordingly, the smaller than 75μm fraction had the lowest light olefin selectivity, while the 150-300μm fraction was probably least influenced by coking. The increased pathways for products and reagents in the 150-300μm fraction resulted in more secondary reactions taking place within this catalyst than the 75-150μm fraction explaining the superior performance of the 75-150μm fraction. Since the grain size determines the ratio of the external to the internal surface areas as well as the amount of intercrystalline boundaries in the catalyst, it follows that the catalytic activity and polycrystalline grain size ratio should actually be tailored when optimising the product distribution of the ZSM-5 catalysed MTO process. The as-synthesized ZSM-5 didn’t perform very well when compared to Catalyst A and modification of the synthesis method is recommended.Master

Reproductive desires of men and women living with HIV: implications for family planning counselling

The reproductive desires of people living with HIV/AIDS (PLHIV) of low socioeconomic standing attending public health facilities in South Africa were studied. HIV-positive men, pregnant and non-pregnant women were recruited from two clinics at a large public hospital in Tshwane, South Africa. Individual interviews were used to explore the reproductive desires of HIV-positive participants. HIV counsellors’ perceptions of their clients’ reproductive desires were explored during focus group discussions. Parenthood proved to be an important factor to all participants in continuation of the family and establishing their gender identities, despite the possible risk of HIV transmission and community stigmatization. Different cultural procreation rules for men and women and stigmatizing attitudes towards PLHIV affected their reproductive decision making. Women had the dilemma of choosing which community expectations they wanted to fulfil. Community stigmatization towards PLHIV was visible in the negative attitudes of some HIV counsellors regarding HIV and procreation. Because the reproductive desires of PLHIV are currently not given high priority in HIV prevention and family planning in the public health sector in South Africa, the prevention of HIV transmission may be jeopardized. These results necessitate the integration of HIV and sexual and reproductive health counselling on a primary health care levelhttp://www.rbmonline.com/2016-09-30hb201