Many materials are capable of organizing into multiple distinct solid phases, each exhibiting a unique set of material properties (e.g., mechanical, optical, electronic, catalytic, etc.). This material property diversity implies that a specific solid form or structure is typically preferred for a specific application. Thus, directing and controlling solid form during crystallization processes is a fundamental solid state engineering challenge. Here, for the first time, a general procedure is presented for designing continuous crystallizers that produce polymorphically pure crystal distributions of a preferred polymorph regardless of that polymorph's relative thermodynamic stability. The design rules were generated by developing and analyzing a multi-polymorph mixed suspension mixed product removal precipitator model, and they have been corroborated both by experimental data generated in our lab and by all of the applicable data in the published literature. These rules were developed to build understanding and aid in the process design of two sustainable energy technologies: carbon capture and utilization as structural materials (CCUSM) and methane pyrolysis (MP). Studying these processes also required the development of new models for calculating interphase mass transport in concentrated, electrolytic, reacting solutions and for describing the reactive transport that occurs in methane pyrolysis membrane reactors. The development and analysis of these models and a thermodynamic minimum energetic cost assessment of a wider set of sustainable energy technologies are also included in the dissertation.