Heat exchangers fouling control

The way in which the outlet temperatures of a heat exchanger are controlled could have a profound effect on the extent of the fouling. In general one of the outlet temperatures, usually the principal process stream is required to be fixed within a narrow range. The inlet temperature of this stream for some reason may be variable. The usual way of controlling the principal process fluid temperature is to restrict or increase the flow of the other fluid. The technique is illustrated in Fig. 13.2). 

It would appear that most installations of sonic technology to control heat exchanger fouling, fall into the category of retrofit applications. The use of the... 

Temperature control requiring additional heat input is normally controlled by regulating the flow rate of steam to the process heat exchanger. A desuperheater should be installed to prevent steam quality variation from causing heat exchanger fouling due to temperature spikes at constant flow. 

Cooling system failure could occur due to failure of pumps or controls supplying cooling media to the reactor vessel jacket, coils, or overhead reflux condensers. Piping to or from the condensers could become plugged or any of the heat exchange surfaces could become excessively fouled. 

Garrett-Price, B. A. (1985) Fouling of Heat Exchangers characteristics, costs, prevention control and removal (Noyes). 

Adaptive controllers can be usefully applied because most processes are nonlinear (Section 7.16) and common controller design criteria (Section 7.12) are based on linear models. Due to process non-linearities, the controller parameters required to give the desired response of the controlled variable change as the process steady state alters. Furthermore, the characteristics of many processes vary with time, e.g. due to catalyst decay, fouling of heat exchangers, etc. This leads to a deterioration in the performance of controllers designed upon a linear basis. 

Special attention should be paid to controlling the risk of fouling of the cooling waterside of plate heat exchangers, perhaps through control of total calcium content, or use of sidestream filters, or high quality polymeric dispersants. 

An often-used method for the limitation of the heat release rate is an interlock of the feed with the temperature of the reaction mass. This method consists of halting the feed when the temperature reaches a predefined limit. This feed control strategy keeps the reactor temperature under control even in the case of poor dynamic behavior of the reactor temperature control system, should the heat exchange coefficient be lowered (e.g. fouling crusts) or feed rate too high. 

Figure 4.11 present the complete flowsheet together with the control structure. The reaction takes place in an adiabatic tubular reactor. To avoid fouling, the temperature of the reactor-outlet stream is reduced by quenching. A feed-effluent heat exchanger (FEHE) recovers part of the reaction heat. For control purposes, a furnace is included in the loop. The heat-integrated reaction system is stabilized... 

Step 5. The azeotropic distillation column does not produce the final salable vinyl acetate product. Its primary role is to recover and recycle unreacted acetic acid and to remove from the process all of the vinyl acetate and water produced. So we want little acetic acid in the overhead because this represents a yield loss. Also, the bottoms stream should contain no vinyl acetate since it polymerizes and fouls the heat-exchange equipment at the elevated temperatures of the column base and the vaporizer. Hence we have two control objectives base vinyl acetate and top acetic acid compositions. And we have two manipula-... 

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