The main purpose of installing a C4 separation tower in the four-tower process for gas separation

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Upon research, it turns out that some refineries have alkylation units that require the use of C4 separation towers. The question then is: in a three-column gas separation process, after MTBE production, the C4 fractions can be sent directly to the alkylation unit, thereby eliminating the need for a C4 separation tower and saving energy. However, the downside is that the C4 fractions are not separated before being used in alkylation, resulting in a low concentration of isobutane; this leads to high separation costs during the alkylation process. This is probably why a four-column gas separation process includes a C4 distillation tower – by separating the C4 fractions first, the concentration of isobutane in the subsequent stages is higher, reducing separation costs, and overall energy consumption remains low. Do you understand? Thank you!