Once the problem has been specified, the engineer must calculate the unknown variables. Often it is not necessary to solve the entire problem, since only limited answers are required. The first step is to do mass and energy balances around the entire column. For binary design problems, these balances can usually be solved without doing stage-by-stage calculations. Figure 3-8 shows the schematic of a distillation column. The specified variables for a typical design problem are circled. We will assume that the column is well insulated and can be considered adiabatic. All of the heat transfer takes place in the condenser and reboiler. Column pressure is assumed to be constant.