Tuesday, January 15, 2008

Visual techniques in QbD: the Process Scheme

The QbD initiative is leading to increased use of tools like Ishikawa / fishbone diagrams in pharma, to brainstorm and visualize the impacts of process parameters on product quality attributes. See for example the ICH Guidance document at http://www.ich.org/LOB/media/MEDIA4349.pdf.

Our customers have found another visualization tool, the 'process scheme', very powerful; this was developed originally in the Zeneca Agrochemicals Process Studies Group, to facilitate sharing of information between chemists and engineers at the unit operation level. Zeneca observed the different ways in which many chemists and engineers approach and discuss synthesis steps and that they lacked a common language and visual representation that would facilitate subsequent process development and scale-up. The process scheme is at first unfamiliar to both (few chemical structures, if any; very little equipment detail) but is built using a simple procedure that each can understand and leads to a compact representation of their current process understanding.

You can find examples anywhere that DynoChem is presented, see our case studies page for some quick links. A process scheme for a hydrogenation reaction is shown here, but all operations, including work-up and isolation, lend themselves to this approach.

The process scheme summarizes the phases (gas, liquid, etc.) and rate processes (reaction, phase transfer, hydrogen supply, heat removal) necessary for the operation to proceed; not all of these are always appreciated prior to the discussion. The scheme indicates certain process parameters likely to affect critical quality attributes (like impurity levels) and provides a rational basis for a systematic program to quantify their impact and how those parameters will vary with scale. Later, as more development data become available, the process scheme may be revised to reflect the current state of process understanding.

The procedure for creating the process scheme is:
  1. Assemble a small group that is familiar with the project, ideally including both the process development chemist and a chemical engineer. An analytical chemist can often provide invaluable additional input.
  2. Internal phases: draw each of the phases which you think are present:
    - For the continuous (normally liquid) phase, use a large rectangle
    - For solid phases, use a triangle at the lower left edge of the liquid phase rectangle
    - For dispersed gas phases, use a rectangle above the liquid, indicating the headspace
    - For sparged gas phases (gas introduced via a dip-leg), use a single bubble near the top of the liquid phase rectangle
    - For a dispersed liquid phase, use a single droplet at the edge of the liquid phase rectangle
  3. Internal rate processes:
    - Wr
    ite down (or propose) a working reaction scheme for the major reactions taking place in each phase
    - Write down the major components in each phase, on each phase
    - Represent any transport processes including mass or heat transfer between phases using a pair of single arrows, one pointing in each direction. Next to each transfer, mark the type of rate process (e.g. for mass transfer mark kLa, for heat transfer mark UA)
  4. External rate processes:
    - Using arrows that cross the model boundary, mark on the picture any process streams (i.e. flows) entering or leaving the system for each phase. These arrows should enter or leave the appropriate phase(s). For fed batch systems, show the feed tank as a separate phase connected to the main liquid phase by a flow. Beside each arrow, list the major components in each stream
    - Represent any vessel heating or cooling via a loop entering and leaving the liquid phase rectangle; mark with a pair of single arrows and UA.

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