Detailed Modelling and Optimisation of an Ethylene Plant Chemical

115 Pages·2013·3.44 MB·English

by  Francisco Borralho

Detailed Modelling and Optimisation of an Ethylene Plant

Francisco José Orantos Borralho

Thesis to obtain the Master of Science Degree in
Chemical Engineering

Examination Committee
Chairperson: Prof. Dr. Maria de Fátima Machado da Costa Farelo
Supervisors: Prof. Dr. Carla Isabel Costa Pinheiro
Dr. Maarten Nauta
Members of the Committee: Dr. Diogo Alexandre Cipriano Narciso
Dr. Vitor Manuel Vieira Lopes

October 2013
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An investment in knowledge pays the best interest.
Benjamin Franklin
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Acknowledgements
First of all, I would like to express my gratitude to Professor Carla Pinheiro, Professor Henrique
Matos and especially Professor Costas Pantelides for giving me the chance to work in Process Systems
Enterprise for seven months and learn a lot about gPROMS ModelBuilder.
I would also like to thank my housemates Pedro, Vasco and Tiago for easing my homesickness
and for playing an important role during the time we spent in London.
A word of appreciation goes also to all my friends from Instituto Superior Técnico, especially my
best friends Bernardo and Inês for all they’ve done for me in the past five years.
I’m also thankful to my PSE supervisor, Maarten Nauta, for being such a selfless and down to
earth guy and for helping me every time I needed help.
Also, huge thanks to my family, especially my parents and my brother for everything they’ve
taught me in twenty three years of life and for investing in my education.
Last, but not least, I would like to thank Mariana, for her patience, her belief in me and for
keeping up with me in the last seven months we’ve been apart.

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Abstract
This work comprises the detailed modelling and optimisation of an ethylene plant processing
fresh propane and recycle streams of ethane and propane.
Propane is converted in a steam cracker, operating at low pressure, but high temperature. Then,
the effluent is quickly chilled in one or more transfer line exchangers and then in a quenching tower.
After that, the cracked gas goes into a compression train, with intercoolers, to remove water, and is
dried in molecular sieves. Finally, the water-free cracked gas goes into the distillation column train, in
order to separate and recover valuable products, like ethylene and propylene.
This process is energy demanding: temperatures go as high as 900 ºC and as low as -121 oC
and the key of it is the choice and design of the right utilities.
Due to the cryogenic temperatures, at least one refrigerant has to be used in order to chill the
cracked gas and condense some of the distillate streams. In this case, two refrigeration cycles were
designed: an ethylene refrigeration cycle and a propylene refrigeration cycle. Ethylene refrigerant is
condensed using propylene refrigerant, whereas propylene refrigerant is condensed using cooling
water.
Finally, some key aspects of the designed flowsheet were subjected to an optimisation, and the
total annualized cost was reduced from 87.67 M$/yr to 50.55 M$/yr, corresponding to a reduction of
42.34% on the total annualized cost.

Keywords
Ethylene plant, steam cracking, olefins, alkenes, gPROMS, refrigeration
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Resumo
O presente trabalho consiste no modelo detalhado e otimização de uma fábrica de olefinas que
processa uma alimentação fresca de propano e reciclagens de etano e propano.
O propano é convertido num steam cracker, que opera a baixas pressões, mas altas
temperaturas. O efluente reacional é rapidamente arrefecido num ou mais transfer line exchangers e,
depois, numa torre de quenching. Posteriormente, o gás de cracking segue para uma unidade de
compressão, com arrefecimento intermédio, de forma a remover-se parte da água presente e depois é
secado em peneiros moleculares. Finalmente, o gás de cracking livre de água segue para a unidade
de colunas de destilação, de forma a recuperar-se produtos de alto valor, como o etileno ou propileno.
Este processo requer o uso de muita energia: as temperaturas vão desde – 121 ºC até
900 ºC e a chave do processo é a escolha e design das utilidades adequadas.
Devido às temperaturas criogénicas, pelo menos um refrigerante tem de ser usado para
arrefecer o gás de cracking e para condensar algumas correntes de destilado. No presente projeto, dois
ciclos de refrigeração foram modelados: um ciclo de refrigeração de etileno e um ciclo de refrigeração
de propileno, sendo que a condensação do etileno é conseguida usando propileno, enquanto que o
propileno é condensado com água de arrefecimento.
Finalmente, alguns aspetos-chave do flowsheet elaborado foram sujeitos a uma otimização,
onde o custo total anualizado foi reduzido de 87.67 M$/ano para 50.55 M$/ano, correspondendo a uma
redução de 42.34% no custo total anualizado.

Palavras-chave
Fábrica de olefinas, steam cracking, olefinas, alcenos, gPROMS, refrigeração
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