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n South African Journal of Industrial Engineering - Multi-objective optimisation in carbon monoxide gas management at Tronox KZN Sands

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Abstract

Carbon monoxide (CO) is a by-product of the ilmenite smelting process from which titania slag and pig iron are produced. Prior to this project, the CO at Tronox KZN Sands in South Africa was burnt to get rid of it, producing carbon dioxide (CO2). At this plant, unprocessed materials are pre-heated using methane gas from an external supplier. The price of methane gas has increased significantly; and so this research considers the possibility of recycling CO gas and using it as an energy source to reduce methane gas demand. It is not possible to eliminate the methane gas consumption completely due to the energy demand fluctuation, and sub-plants have been assigned either CO gas or methane gas over time. Switching the gas supply between CO and methane gas involves production downtime to purge supply lines. Minimising the loss of production time while maximising the use of CO arose as a multi-objective optimisation problem (MOP) with seven decision variables, and computer simulation was used to evaluate scenarios. We applied computer simulation and the multi-objective optimisation cross-entropy method (MOO CEM) to find good solutions while evaluating the minimum number of scenarios. The proposals in this paper, which are in the process of being implemented, could save the company operational expenditure while reducing the carbon footprint of the smelter.

Koolstofmonoksied (CO) is 'n newe-produk van die ilmenietsmeltproses waardeur titaniumslak en ru-yster geproduseer word. Voor die uitvoering van hierdie projek is die CO gas wat deur Tronox KZN Sands, Suid-Afrika produseer is, verbrand om daarvan ontslae te raak. Die verbranding het weer koolstofdioksied produseer. By hierdie aanleg word ongeprosesseerde materiale voorverhit met metaangas wat van 'n eksterne verskaffer aangekoop word. Die prys van metaangas het beduidend toegeneem, en die moontlikheid om CO te herbruik in die plek van metaangas is in hierdie studie ondersoek. Weens die variasie in vraag na metaangas, kan die gebruik daarvan nie volledig uitgeskakel word nie; dus moet of CO of metaangas aan sub-aanlegte oor tyd toegeken word. Sodra daar 'n oorskakeling tussen die twee gassoorte plaasvind, moet die toevoerlyne eers gespoel word; en dit veroorsaak verlies van produksietyd. 'n Multidoelwit-optimeringprobleem het dus ontstaan waarin verlies aan produksietyd minimeer word terwyl CO gasgebruik maksimeer word. Hierdie probleem bevat sewe besluitveranderlikes, en scenario's is met rekenaarsimulasie ge-evalueer. Rekenaarsimulasie en die kruis-entropie metode vir multidoelwit-optimering is toegepas om goeie oplossings te verkry terwyl die minimum aantal scenario's evalueer is. Die voorstelle, wat tans in die proses is om implementeer te word, sal die onderneming operasionele koste bespaar en die koolstofspoor verklein.

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/content/indeng/25/2/EJC165138
2014-08-01
2016-12-03
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