Environmental Engineering Reference
In-Depth Information
The incentive for a process analytical strategy, whether it is based upon
PAC or PAT, is to increase and stabilise production yields by minimising all
types of variations and keep the process within optimal operation conditions
at all times. The process analytical approach will reduce the need for
accurately determined analytical results from quality control laboratories,
but it will never eliminate the need for those analyses completely. In the
future, there will be a need for regular quality control and tests to run
advanced PAT monitoring process control systems on-line. The two
analytical strategies should, in fact, be viewed as complementary strategies.
10.3.5 Sampling issues in the context of AD process control
Fermentation processes at full scale are sensitive to sudden changes in
feedstock composition that cause significant variability in the process
conditions, i.e. organic overloading. Today, fermentation process control is
usually achieved through manual sample extraction with off-line analysis of
few key process parameters (Ahring et al., 1995; Nordberg et al., 2000).
The time constants involved in AD do not have a critically short time
response - from hours to days: the average hydraulic retention time of
feedstock in a semi-continuous digestion system is of the order of 10 to
25 days and much longer in crop digestion (up to 100 days). However,
organic overloading can often happen in a few hours and usually without
knowing it is happening. Therefore, there is a need for implementation of
inexpensive and robust on-line process monitoring technologies (Al Seadi,
2005; Hjort-Gregersen, 2005; Holm-Nielsen et al., 2007). In order to obtain
valuable results, correct sampling is needed, which has to be in accordance
with the theory of sampling (TOS). This can be expressed as a TOS toolbox
of unit operations to respect the principles of correct sampling under all
kinds of harsh conditions in nature; good examples are any kind of sampling
situations during fermentation processes of very heterogeneous bioslurry
systems.
10.3.6 Sampling unit operations
In order to perform correct sampling in practice, always respecting the
principles of the TOS, we only have to know about the following seven basic
sampling unit operations (for more details see Petersen and Esbensen
(2005)).
Structurally correct sampling - the only safeguard against sampling bias.
.
Heterogeneity characterisation of initial sampling material.
.
Homogenisation - mixing, blending.
.
Composite sampling - minimum of five composites per sample.
.
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