Integrated product and process design has been approached by representing product specifications and process operations in a common space of mixture compositions. Interval reasoning can be a useful preprocessing step prior to detailed design in situations where there is uncertainty in feed composition, or it is sufficient to be within a given range of values for product properties. This paper presents new interval reasoning algorithms for abstract representations of unit operations such as reactors and separators. These methods enable the propagation of product specifications, given as ranges on properties and compositions, to ranges on feed conditions, and vice versa. Comparisons of different processes may also reveal relative preferences between systems.In this paper, we demonstrate these concepts and illustrate their application through a waste vitrification system example in which feed stream composition, product composition, viscosity and durability are all constrained. Two different vitrification systems are compared in terms of their ranges of acceptable feed compositions and the range of possible product mass. The determined ranges on variables provide a superset of system capabilities; points outside this space are proven infeasible, while points inside the space are possibly feasible.This paper demonstrates that interval propagation schemes that make specific use of the structure inherent in chemical engineering processes, such as conservation of mass, can be very effective in limiting the spurious inclusion of infeasible points. In addition, redundant equations are valuable in reducing spurious solutions-as has been shown elsewhere for other qualitative and semi-qualitative reasoning schemes.