The McCabe-Thiele diagram is most useful when the operating line is straight. This requires that the energy balance is automatically satisfied and liquid flow rate/vapor flow rate = constant. In order for energy balances to be automatically satisfied, we must assume that
1. The heat of absorption is negligible.
2. Operation is isothermal.
These two assumptions will guarantee satisfaction of the energy balances. When the gas and liquid streams are both fairly dilute, the assumptions will probably be satisfied.
If the solute mole fraction in the feed yB,N+1 is very low, then transferring most or even all of the solute to the liquid will have very little effect on the overall vapor flow rate V or ...