Alumina refinery process design typically starts with two fundamental elements: the detailed mass and energy balance and the flow availability. The detailed mass and energy balance is typically developed using process flow-sheet simulation software and represents the steady-state operation of the plant at normal operating conditions. The flow availability describes the ratio of plant production to the steadystate mass balance. These elements used together as the basis for equipment selection and design.
Flow availability takes into account a wide range of production loss factors including planned and unplanned maintenance, feed availability and composition changes, variability in operational performance and interaction between batch and continuous unit operations. The combined impact of these loss factors is complex and as a result flow availability is usually derived using a top-down approach such as comparison to an existing operation, a group of similar operations or to an anecdotal ‘industry leading’ benchmark. However, these top-down approaches are by their nature limited as they cannot factor in changes to equipment selection, flow-sheet or plant operating philosophy. As a result, scarce capital can be overspent, spent in the wrong areas, or even underspent, leading to production volume risk.
Operations simulation can be used to provide decision support using a bottom-up method. It provides a technique to quantify the combined impact of the loss factors described here and a tool to support process design and equipment selection by facilitating a range of sensitivity and what-if analyses. This paper presents a comparison of operations simulation with other techniques to estimate flow availability and a case study of how operations simulation was used to support the process design and equipment selection of an alumina refinery expansion project.