Refinery Energy Losses Due to Fouling in Heat Exchangers Figure 3: Validation on Four Units from Two Refineries 3 2 1 0 -1 -2 Est. -3 0 50 100 150 200


250 300 350 400 450 500 550 Time (days)


+ Ra-E01 ∇ Ra-E02 Rb-E01 x Rb-E02 Percentage errors on tube-side outlet temperature (simulations – measured).


Figure 4: A Four-pass, Two-shell Exchanger in the Hot End of a Pre-heat Train


A 16


14 12 10 8 6 4 2


0 B 12


10 8 6


2 4


0 0 50 100 150 Heat duty loss due to fouling Assessment of heat duty and energy loss due to fouling.


flowrates, the outlet temperatures can be predicted over a much longer period, and the quality of the predictions can be assessed against the actual measured data. The overlay plot of simulated and measured tube-side outlet temperature for one particular refinery unit (see Figure 2A) shows an excellent agreement up to the end of the run period (350 days). To ensure a wide applicability, the same procedure was repeated for three other industrial units belonging to two refineries operated by


200 Time (days) Cumulative energy loss 250 300 350


1,000 800 600


200 400


0 50 100 150 Heat duty fouled 200 Time (days) Heat duty clean 250 300 350


Of course, no single heat exchanger exists as a stand-alone unit in a PHT. Units are interconnected in complex network configurations, affecting each other’s behaviour. For example, the energy loss in one unit may be partially compensated by an increased driving force for heat transfer in a unit downstream. A simulation with a detailed mathematical model of the whole PHT is able to unveil such interactions as they evolve over time, show which exchangers are more affected by fouling and calculate instant and cumulative overall energy losses.


The above single exchanger model was defined as an object in a commercial process simulator (gPROMS, Process Systems Enterprise) that can be easily instantiated, replicated and flexibly interconnected with other units in a flowsheet. An Excel interface allows automatic importation of geometries for each heat exchanger from a standard spreadsheet or database. This gives flexibility and versatility to the model and makes setting up a network very easy. The overall PHT model allows capturing and assessing some complex interactions within the network which are highly dependent on the distinct fouling performance in the various exchangers, for example, the evolution over time of uncontrolled flow split on hydraulically unbalanced branches.18


Assessing the Impact of Fouling at Network Level One major benefit of a network simulation based on the above model is the possibility of assessing far more detailed cumulative, marginal and prospective costs related to fouling, including those due to extra furnace


fuel, CO2 emissions, electrical pumping power and loss in production occurring when a furnace firing limit is reached. As an illustration, Figure 5 shows the simulated extra costs in a 250,000 bbl per day refinery resulting from the decline in thermo-hydraulic efficiency due to fouling in the hot end of the PHT, starting from a clean situation, for a combination of different refinery margins (US$2 per bbl and US$10 per bbl) and furnace maximum heat duty (85 and 90MW). Utilities costs are the same as in Coletti and Macchietto,18


emissions are costed at US$30 per ton of CO2 and it is assumed that no cleaning is performed over Predictions


However, through the simulations it is possible to calculate at any time, for given process conditions, the heat exchanged in clean and fouled conditions (see Figure 4A), and thus the heat duty loss due to fouling as a difference between the two (see Figure 4B). After a few days of operations this loss, initially zero in clean conditions, becomes large with a maximum value in this case of 6MW after about 250 days (see Figure 4B). The area below the heat loss curve (below red line) represents the instant energy actually lost because of the thermal inefficiencies caused by fouling while its integral over time gives the cumulative energy loss for fouling in this heat exchanger (see Figure 4B, right axis).


major oil companies. Results indicate that for all units tested, the outlet temperatures are predicted over extended periods (four to 16 months) with an excellent accuracy of ±1% for the tube–side (see Figure 3) and ±2% for the shell–side (not reported here). A major advantage of this approach is that it can calculate the growth of deposit layers in various parts of an exchanger (see Figure 2B). From such a detailed picture, information about aggregate performance and efficiency of the exchanger over time can be calculated. For example, heat transfer losses at any particular time depend on (variable) inlet temperatures and flowrates, which makes it difficult to isolate the fouling effects.


22


HYDROCARBON WORLD – VOLUME 6 ISSUE 1


Error (%) Heat duty loss (MW) Heat duty (MW)


Energy loss (MJ)


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