Page 313 - Design of Simple and Robust Process Plants

8.3 Automation of Operation 299
5. Start recycling the top and bottom streams to the feed vessel while feeding
continuously from the feed vessel and bringing the distillation column at its
operational conditions including both product qualities.
6. Reduce the recycle and gradually bring fresh feed to the required amount.
A breakdown of operation in its functional activities is: first, the system is filled;
second, the unit is brought at operational conditions, heat addition, pressure and
inventory control with flow and level loops active; third, qualities are in control; and
fourth, the unit is brought to full operation.
Alternative ways to start such a system include filling the reflux drum and bottom
section with material at or close to specification, in which case the column can be
brought more rapidly to its required operational conditions. Another option is to
recycle the product stream immediately to the feed vessel before the products are at
quality. Whichever route is used, the objective is the same ± to bring the unit opera-
tion to its operational conditions that enable smooth processing of freshly supplied
feed at any time, by achieving inventory control and simultaneous heat and mass
balance control (all control loops are now active).
The advantages are that the unit is available for processing, and the impact of
upset on successive unit operations is limited.

8.3.2.2 Start-up of an irreversible unit operation
Irreversible unit operation demands another approach in order to realize first-pass
prime operation, for which two options are available:
1. To determine if the unit operation can be made reversible by implementing
an inverse operation. For example, a crystallizer can be made reversible by
dissolving the crystals formed and recycling them back into the feed.
2. To accommodate the operational conditions of the unit in such a way that
when the feed starts it can perform its function. Start-up of a dryer is mainly
done by preheating the dryer beforehand to the required level; in this way a
dry product is obtained immediately after the feed is processed.

A start-up approach was generated for an irreversible reaction and studied by Ver-
wijs et al. (1996) for the process shown in a simplified form in Figure 8.4. This
study is summarized below to illustrate the possibilities for the start-up to meet
first-pass prime conditions. The reaction studied was an irreversible, noncatalytic
exothermic hydration reaction, and is described as
A+B ® C
Some consecutive reactions take place which were not relevant for this study.
The reaction is adiabatic and performed in a plug flow reactor, all in the liquid
phase. The reaction was first-order, with an Arrhenius-type rate equation. Reactant
A is in excess, and therefore after separation from the reactor effluent is recycled
back to the inlet of the reactor. The objective was to achieve a high level of conver-
sion for reactant B in order to avoid the formation of an undesirable reaction after
the reactor. This condition is particularly important during start-up as, during this