small time, the change in the amount of liquid in the still is and the amount of vapour withdrawn is . The following differential mass balance equation may be written as: Total material balance: = ----- (i) Component A balance: ----- (ii) ----- (iii) Equation (i) means that the total amount of vapour generated must be equal to the decrease in the total amount of liquid. Similarly, equation (ii) means that loss in the number of moles of A from the still because of vaporization is the same as the amount of A in the small amount of vapour generated. Putting in Equation (iii) and rearranging, = ------(iv) If distillation starts with F moles of feed of concentration and continues till the amount of liquid reduces to W moles (composition =xw), the above equation can be integrated to give = = ------(v) Equation (v) is the basic equation of batch distillation and is called as the Rayleigh equation.Rayleigh equation is used for calculation of data in the batch distillation column. Aspen Plus software History During the 1970s, the research have develop a novel technology at the Massachusetts Institute of Technology (MIT) with United States Department of Energy funding. The undertaking known as the Advanced System for Process Engineering (ASPEN) Project, was originally intended to design nonlinear simulation software that could aid in development of synthetic fuels. In 1981, AspenTech, a publicly traded company was founded to commercialize the simulation software package. Aspen Tech went public in October 1994 and has acquired 19 industry-leading companies as a part of its mission to offer complete, integrated solution to the process industries. As the complexity of a plant integrated with the several process unit increase, solving a large equation set becomes a challenge. In this situation, we usually use the process flowsheets simulator. Type of Aspen simulator package The sophisticated Aspen Software tool can simulate large process with a high degree of accuracy. It has a model library that includes mixers, splitters, as phase separator, heat exchanger, distillation columns, and reactor pressure changers manipulators, etc. By interconnecting several unit operations, we are able to develop a process flow diagram (PFD) for a complete plant. To solve the model structure of either a single unit of a chemical plant, required Fortran code are built-in in the Aspen simulator. Aspen simulator has been developed for the simulation of wide variety of processes such as chemical