Engineering Data and Assumptions Guide

Three major areas of engineering assumptions are to be included. Sources of data, reaction assumptions, and unknown physical properties should be documented.

Sources of Data

The source of process data should be documented, whether it be from pilot plant data supplied by the process developer, an earlier conceptual design, or a complete "paper" process fabricated from physical principles. If pilot plant data were used, indicate the specific run numbers that were taken as a representative basis for scale-up. Methods used for data reduction and correlation should be specified.

Engineering Assumptions

Physical Properties

Many physical properties of intermediate streams have to be assumed in order to effect a energy and material balance and to size equipment. This may be due to a lack of full characterization from pilot plant data of mixed streams of new products. These assumptions may include heat capacities, phase equilibrium data, densities, thermal conductivities, viscosities, etc. All should be documented.

Reaction Kinetics

The reactor section is perhaps the unique feature of a process that distinguishes it from another. Because of this uniqueness, more detailed assumptions have to be reported concerning reaction conditions. Any vessel in which reactions occur (such as preheaters, methanators, etc.) are to be included. Among the assumptions that should be listed are:

• All input and output stream flow rates and compositions
• Temperature and pressure (temperature and pressure profiles, if applicable)
• Residence times for each phase
• Partial pressures of reactants in the gas phase
• Catalyst life, circulation rate, and make up rates (if catalysts are required)
• Percent conversion (define basis)
• Void volumes in packed beds
• Expanded bed densities in fluidized beds
• Recirculation rates in an ebullient bed
• Equilibrium temperature
• Space velocities
• Superficial velocities
• Compositions and flow rates of all bypassing, recycle or intermediate withdrawal streams
• Conversion efficiency (based on heating value, carbon utilization, etc.)
• Characterization of contaminants in the reactor effluent: particulates (quantity and size distribution), tars (in the case of gasification, both quantity and composition), etc.

Others

There are typically many other assumptions that have to be made in equipment sizing. This includes liquid/gas ratios in absorbers or distillation columns, design air cooler inlet temperatures, pressure drops, separation efficiencies (solid/liquid), regeneration efficiencies, etc. All such data should be reported. An estimate of the turndown ratio comparing the minimum throughput capacity to the design capacity for the overall plant and major processing steps should be presented.

Levels of Assumptions

For processes that are in the bench to pilot plant scale of development, a great number of processing and equipment performance assumptions will necessarily be used. Some assumptions are easily obtained from similar unit operations of commercial processes. Other assumptions must be regarded as speculative and subject to engineering judgments. All assumptions and design practices should be fully documented. This will allow an assessment of their impact on the overall economics.