|Future growth within the chemical process industries depends on various factors such as raw material and energy availability, sustainability etc. A systematic process synthesis and design framework integrated with molecular design is needed to synthesize processes that perform this efficiently. Hence, this dissertation describes the development of a novel hybrid method for Computer Aided Flowsheet Design (CAFD) and its effective integration with molecular design. The interactions among process synthesis, process design and molecular design is through a common set of properties that are employed to analyze the processes as well as external agents involved in the process. Knowledge of these specific properties is needed to establish the feasibility of a unit operation in a process and the corresponding conditions of operation. The same information is needed for design of a component as an appropriate external agent. This forms the very basis of the proposed hybrid methodology for flowsheet synthesis/design integrated with molecular design. Both the Computer Aided Flowsheet Design (CAFD) and Computer Aided Molecular Design (CAMD) frameworks developed are group contribution (GC) based approaches. CAFD makes use of functional process groups, characterized by the type of unit operation/process and their corresponding driving force, to generate and represent flowsheets; process group contribution based property models to predict flowsheet properties from a-priori regressed contributions of process groups; a notation system (called SFILES) for storing the flowsheet structural information; and a synthesis method to generate and identify the feasible flowsheets. The identified candidate flowsheets are ranked based on flowsheet properties (like energy consumption, amount (mass) of external agents used and/or cost/profit) representing flowsheet performance in a quantitative sense. Once the promising flowsheet structures are identified, the flowsheet design parameters that describe the process will be estimated. The reverse simulation method is used to calculate the design variables of the unit operations involved in the process. This also gives a good estimate of the important design parameters. Some alternatives may involve unit operations that require an external agent. Conventional agents may not always meet the property constraints set by the reverse simulation design problem of such operations. Novel agents can be identified by solving a product design problem satisfying the property constraints. This is done by integrating the flowsheet design problem with a molecular design problem. Depending on the type of unit operation in the process where an external agent is required, the CAMD problem is formulated accordingly and the effect of the solution alternatives from the CAMD problem on the process is evaluated by the process models. CAMD includes building blocks (atoms and functional groups) to generate and represent molecules; group contribution based property models to predict target properties; a standard molecular structure notation system to store and visualize the molecular structure information; and a synthesis method to generate and screen molecules that match the target (design) properties. Once a set of near optimal flowsheet alternatives have been identified, rigorous simulation is used to verify the predicted performance and select the best flowsheet. The framework also aims at maintaining a good accuracy of solutions and large application range. A completely automated tool to perform the above tasks is also developed.