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John McCulloch - Process Control Trouble-shooting

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Many problems overlap the interface between the process and the control system. Addressing the problem from either discipline on its own is doomed to failure. I have a thorough knowledge of Process Engineering as well as of Control Engineering and have found this to be a valuable asset in solving process and control problems.

Often, trying to solve the problem with control alone leads to very complicated control systems; trying to solve the problem with process design changes alone leads to expensive additional plant; taking a multi-disciplinary approach leads to reduction in both capital cost and complexity.

Let me give some examples:

  1. Distillation Column:

    The distillation column was designed with a split base and a weir. This was to provide enough depth of liquid to keep the reboiler flooded. Unfortunately, the column was required to remain on control during upsets where the feed flow could reduce to zero. Under these conditions, the requirement was for the column inventory to be locked in until feed could be restored. Because of the weir, there was no means of preventing some overflow and this meant that inventory was lost and so the column went off specification.

    The process solution was to have a pump-around to return material from the base to the upstream side of the weir. The control solution was to have an alternative control scheme which was selected by a switch when feed was lost. The multi-disciplinary solution was to have no weir and to raise the column base to provide sufficient head to operate the reboiler. This was opposed on grounds of cost - the column would be taller. On estimating the modified column, it was found that the shell diameter and wall thickness could both be reduced because of the increased down-comer area in the base. So, although the column was taller, the overall cost was significantly lower.

  2. Effluent pH Control System:

    The system consisted of a holding tank and an in-line mixer to blend concentrated acid with the caustic waste, (mostly sodium carbonate), to achieve an environmentally acceptable pH. Downstream of the inline mixer was another vessel and a pump-back system that diverted the outflow if the pH went outside the environmental constraints. The feed could have very variable pH and flow-rate. pH control, of course, is highly non-linear, and carbonates have a double kink in the response curve. Starting and stopping the pump-back also kicked the pH control system and could cause cyclic behaviour. Experience with similar units showed that it was a very difficult control situation and that the pump-back was running frequently. The process solution was to have a secondary pH control system; tests showed that this made no significant improvement. The control solution was to use a complex feed-forward and secondary feed-back control system, which also did not fully address the problem.

    The multi-disciplinary approach was to have a single holding tank with a pump-back loop around it with a high but steady flow-rate, (about ten times the throughput), and to inject the acid into this loop. The pump-back went into the tank with a jet mixer so that the tank contents were of uniform composition. The net effect was that the tank contents were at a much higher, steady pH so that the pH controller was operating in a more linear region of the response curve. There was no need for a second tank, nor for automatic start and stop of the pump. A bleed from the pump-back took the effluent away, and this had a stop valve in case of pH excursions which then only occured when the primary pH sensor became fouled.

I have worked in both conceptual and detailed process design situations as a member of a team to optimise the process and control system design.

This page last updated on 2013-08-06 JGM