Handling delayed coker disturbances with APC
An advanced process control system provides automatic detection of disturbances during drum switch in a delayed coker to raise performance levels.
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A delayed coker unit (DCU) is an established candidate for the application of advanced process control (APC) with an attractive return on investment. The challenge in DCU-APC lies in handling the big disturbances that occur during drum switch-over and vapour heating events when the vapour enthalpy feed to the main fractionator is suddenly reduced. Timely actions need to be taken on several control loops to minimise the effects of sudden cooling of the column, which leads to large variations in coker gas oil quality as well as flow rates. The exact occurrences of these events are unmeasured and a conventional control system is not adequate to handle such big disturbances. The resulting economic losses due to quality give-away and off-spec product generation are substantial, not only in the DCU but also in downstream units where the disturbances are propagated. Hence continuous operator attention is required for managing these events.
Effective disturbance handling with APC requires DCS logic for unambiguous detection of the various events that lead to major disturbances in the downstream fractionation sections. Detected discrete events are then used for generating continuous disturbance functions, which, in turn, are utilised for multi-variable modelling as well as for predictive feed forward control, honouring multiple constraints. Since manual actions in the field are also involved in these events, the extent of disturbances varies in each coking cycle. This article also describes how intermediate variable and state observer (Kalman filter) concepts are utilised for robust control.
A DCU is one of the most profitable refinery units. The process involves thermal cracking for upgrading (converting) asphalt-like residue, typically from the vacuum distillation unit, into lighter distillates, coker gas oils and solid coke, which are further processed into marketable fuel products such as LPG, gasoline, diesel, fuel oil and petroleum coke.
Delayed coking is a semi-batch process where one or more pairs of coke drums are used for the thermal cracking and coking process.
Simultaneously in each pair of coke drums, one drum is online for the coking process while the other drum is offline undergoing decoking. Figure 1 is a simplified block diagram of a typical delayed coker with two pairs of coke drums. Vacuum residue (fresh feed) after preheating (by exchanging heat with run-down streams), is injected into the main fractionator bottom. The fractionator bottom is heated again in the two coker furnaces to a high cracking temperature (about 500°C), and hot, partially cracked feed flows from the coker furnace into the coke drums, where cracking continues. Cracked distillate vapour ascends in the coke drum and flows into the fractionator where it is separated into wet gas, unstabilised naphtha, light coker gasoil (LCGO), heavy coker gasoil (HCGO), and recycle oil. Coke is deposited in the drum. Two drums (one from each pair) are online at one time, accumulating coke until almost full. About every 24 hours, the filled coke drum is switched off for coke removal and the empty drum is connected. The drum that was just filled goes through a cycle of steaming out, cooling, opening, coke removal, closing, steaming, pressure testing, heating, and finally reconnecting to the furnace and fractionator.
The semi-batch process poses unique challenges for APC in DCUs, and the challenges are discussed in this article.
Disturbances due to drum switch events
The operations of two pairs of coke drums are staggered, and the coking-decoking cycles in each pair are scheduled in such a manner that drum switch-over happens every 12 hours (twice in every day). Vapour from the furnace at about 500°C is passed through a specially designed four-way valve to the bottom of the drum which is in operation. Vapour leaving the top (after cracking and coke deposition) is quenched on temperature control to 425°C, and fed to the main fractionator bottom for separation of the product streams. About six hours before the drum switch-over, a portion of (quenched) hot vapour from the operating (on-line) drum is diverted from flowing to the main fractionator towards the offline empty drum for gradual ‘vapour heating’. The hot vapour flows from the top vapour line of the empty drum and the condensate is removed from the bottom. The empty drum is gradually heated from about 150°C to about 300°C for six hours before eventual complete switch-over after every 24 hours. During drum switch-over, the flow of hot vapour to the top of the empty drum is stopped and is instead diverted to the bottom of the empty drum through a four-way valve gradually in three steps. In the first step, one-third of the vapour is diverted to the empty drum and two-thirds to the filled drum. In the second step, two-thirds of the vapour is sent to the empty drum and one-third to the filled drum. Eventually, the empty drum is taken on-line by diverting the entire (100%) vapour flow towards the empty drum and isolating the filled drum, and taking it off-line.
The load on the main fractionator thus varies widely due to drum switch events and feedstock quality fluctuations. The effects of vapour heating and drum switch events on the main fractionator temperature profile are shown in Figure 2, where the disturbances in the trend of HCGO draw temperature are shown for one complete coke drum coking cycle (24 hours). Two drum switches between the AB pair, from drums A to B and from B to A, occur at 24-hour intervals. Since the drum switches between two pairs of drums, AB and CD, are staggered (see Figure 3), two drum switches occur in a 24-hour period at 12-hour intervals. Each drum switch involves four major disturbance events (see Figure 2):
The hot vapour is diverted for the purpose of vapour heating (of the empty offline drum) about six hours before the switch-over between the drums of a pair (AB or CD). The sudden and substantial reduction of vapour flow from one of the coke drum pairs disturbs the main fractionator.
Once the drum is warmed up with vapour heating for about six hours, the vapour flow to the empty drum is stopped by closing the condensate drain (at bottom). Then, about one-third of the total hot vapour from the on-line drum is diverted to the bottom of the empty drum, by partly opening the four-way valve (at the bottom). As the direction of flow is changed, the lighter, uncondensed vapour hold-up inside the empty drum is flushed out, thereby suddenly increasing the vapour load on the main fractionator and increasing the temperature profile.
The drums are completely switched. Vapour from the furnace is completely diverted towards the empty drum and the filled drum is isolated, thereby taking the empty drum on-line. The temperature in the drum is around 400°C, which is much less than that required for cracking. The cracking reaction is abruptly quenched, causing a major disturbance to the main fractionator as both the heat and vapour mass flow are suddenly reduced.
As the empty drum heats up, the cracking reaction gradually resumes. The temperature and vapour flow to the main fractionator increase gradually over the next 3-4 hours.
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