Complex sentence: Today, the fundamental aspects of designing chemical plants are done by chemical engineers, although historically this was not always the case and many chemical plants were constructed in a haphazard way before the discipline of Chemical Engineering became established.
Split into simpler sentences: Today, the fundamental aspects of designing chemical plants are done by chemical engineers. Historically, this was not always the case and many chemical plants were constructed in a haphazard way before the discipline of chemical engineering became established.