In our previous blog posting, we discussed Potential Design Implications of Certain Objectives for a Pilot Plant. In that discussion, we highlighted some of the most common market development, technology development and risk mitigation goals and explained how these goals can affect pilot plant design. This blog will focus on some important considerations for pilot plants built for the purpose of technical validation and scale-up.
Technical validation and scale-up have been the primary motivators for chemical engineers when building a pilot plant. The specific issues that engineers want investigated depend on the nature of technology, how much is already understood about the process and the safety and technical risks involved. The following discussion presents a few issues that are commonly encountered during design of pilot plants for validation and scale-up of reactors, distillations systems and recycle streams.
Reactor Validation and Scale-Up
Shell and tube reaction systems are often used to control exothermic reactions. When designing a pilot plant for technical validation or scale-up, the use of a single (or few) tubes with the same dimensions as the expected commercial scale reactor offers several benefits. First of all, it allows a fairly direct comparison to commercial designs. It also allows accurate study of temperature effects and changes along the tube as well as heat generation, transfer and dissipation within catalyst beds. Heat dissipation within the bundle on the shell side will still need further modeling for a commercial design, but data collected on a single tube can be used as the design basis.
When piloting agitated systems, such as batch or continuously stirred-tank reactors (CSTR), the reaction vessels may require close study of mixing characteristics. This is especially important for viscous fluids, multi-phase systems and mass transfer limited systems. Various agitator and baffle conditions or configurations may need to be tested to find the best solution. If you intend to collect data to validate a computational fluid dynamics (CFD) model for scale-up to a commercial design, you will need to accurately collect data to accomplish this goal. The pilot plant design must allow for installation of the necessary instrumentation for data acquisition.
Small scale equipment must be designed to minimize ambient heat loss. Heat lost to the environment can hide exotherms or impact operating temperatures. A process that appears to be adiabatic may in fact have considerable heat transfer through the wall, tubing, supports, and hot surfaces. Inaccuracies of measurement due to ambient heat loss can lead to a flawed thermodynamic model. Such models when scaled-up can have disastrous consequences.
Distillation Validation and Scale-Up
Distillation is not commonly scaled-up directly from pilot plant data. Distillation columns are normally sized by applying process modeling results to sizing correlations. Tray or packing vendors provide correlations specific to their product, which can be used in conjunction with a process model for accurate sizing of the full-scale equipment. Models are, however, only as good as the data we put into them. The choice of thermodynamic and hydraulic input parameters can have a significant impact on the output of the model. A reliable model must reflect reality. It is not uncommon for distillation models to diverge from reality. This can be due to components that are not accounted for in the model, the nature of the materials, or unexpected side reactions taking place in the column. Trace components can be difficult to model accurately, but they can be critical to quality. Therefore, It is important that thermodynamic and hydraulic models be validated against reality. MATRIC recommends using at least a two inch glass Oldershaw column for testing to provide representative data for validating thermodynamics. Anything smaller is subject to too many other variables such as ambient heat loss, wall effects, etc. MATRIC uses several techniques to help minimize ambient heat losses and improve data quality, and MATRIC personnel have successfully scaled-up processes with 40,000 to one scale ratios based on this method.
Closing Recycle Streams and Accounting for Components
Closing recycle streams is important in understanding the build-up and removal of components from the system. If a process has recycles, it is critical to understand where components could accumulate and how they are getting out of the system. Known components may be modeled, but models can be based on wrong assumptions. There can be unexpected components that are generated in the system or come in with raw materials and can build up if they do not have a way out. It can be important to know the level these components can build up to, where they accumulate and how they impact the equipment performance or product quality. For example, intermediate boiling components can build to high concentrations in sections of distillation columns, even if their generation rate is very small. Accumulation of these components can impact equipment performance, generate conditions for unanticipated reactions, or generate unexpected corrosion in accumulation areas. Operating with closed recycles and observing this can lead to designs that will allow for removal of these materials. It is important to ensure that the method of removal doesn’t impact the process economics, product quality or product performance. Discovery of component build up can send you back to the drawing board, but it eliminates possible surprises during the operation of a commercial plant.
In summary, there can be many objectives for building a pilot plant. These include technical process development and validation, scale-up, business or market development and risk mitigation. The sponsor’s objectives must be clearly understood before going into the design of a pilot plant. This understanding helps make the best use of available resources in the shortest possible time. Once the objectives are understood, the design team must ensure that these objectives are implemented in the design. Multiple objectives can add complexity to the decision making during design. Project sponsors should be consulted about tradeoffs that need to be made. Pilot plants, when conceived, designed, built and commissioned properly, greatly increase the chances of commercial success.
Click here to read Part 1: Possible Drivers and Goals for Building and Operating Pilot Plants.
Click here to read Part 2: The Importance of Understanding Owner’s Project Objectives for a Pilot Plant.
Click here to read Part 3: Potential Design Implications of Certain Objectives for a Pilot Plant.
If you have any questions about MATRIC’s pilot plant capabilities, please contact Rob Nunley.