Biomedical Engineering Reference
In-Depth Information
form the basis of mathematically 'rigorous' equations used to
calculate yield, purity, throughput etc. and can thus enable material
or volumetric balancing.
• Facility E.g. related to provision of ancillary utilities
General plant-related items will have an impact upon overall pro-
cess costs as well as the capability to run a process on time and
within schedule e.g. reliable access to services such as steam,
glycol, chilled water etc.
• Times E.g. process durations and hold times
Inputs such as processing times can be set either directly or cal-
culated based upon other values e.g. flow rates, process volumes
etc. Models can be used to determine how best to arrange batches
within a campaign in order to ensure that all can be processed on
time. Hold times may also be critical if a product is sensitive or if
excessive downtime causes undesirable events such as degradation,
and such aspects should be included within a model. Using a
model, the impact of process changes upon manufacturing times
can be determined to see whether bottlenecks will be formed and
how to avoid them. Such scheduling considerations are related to
resource availability aspects, as discussed below.
• Resources E.g. disposable bags, chromatography resins, membranes, buffers
or labour
Typically, resource pools will exist within a facility and a model
will need to represent the quantity of these and their availability
(e.g. for labour, the impact of shift patterns will need to be
reflected). Resource availability will also impinge upon the like-
lihood of a process running into bottlenecks and may prompt the
acquisition of additional capital or consumable resources to prevent
these from impeding processing. Costs will also be associated with
the acquisition and usage of these resources, and so this needs to be
factored into a process simulation.
• Costs E.g. for capital, development or running costs, including overheads
In addition to those costs mentioned already (e.g. for resources or
overheads), there are other expenses which need to be included e.g.
capital expenditure, maintenance, taxes and also the cost of failed
batches (see below).
• Risk factors E.g. probabilities of batch failure
Risk factors and their impact upon annual product throughput and
process costs should also be incorporated, as this can give an
indication of not only the likely outcome on process or economic
grounds but also the probability that a process will succeed or fail
to meet its annual targets.
Related to the last point, although it is possible to model solely using fixed
values for the input parameters identified above (i.e. a deterministic analysis), it is
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