Fired heaters balancing

Design studies had shown that to recover the high heat content in the reactor effluent stream efficiently, it is desirable to split the effluent streams from each reactor into two parts, with the major part going to preheat the recycle gas to that reactor. A small fuel-fired heater is provided in each reactor train to trim the temperature of the recycle gas to that reactor. The excess hot effluent streams from all the reactors are combined and used to preheat and vaporize the methanol feed to the dehydration reactor. Although the process design is nearly in heat balance, a discrete quantity of... 

The first methanol was vaporised and reacted to produce hydrocarbons in the first conversion reactor as anticipated. However, the hi(J> prepane production due to high initial catalyst activity was not simulated. The actual unit heat balance was therefore not as predicted with the result that the process fired heater duty was significantly higher than expected. The startup heater which was in service to provide additional methanol superheat upstream of the DME reactor during startup was not taken off-line towards the end of startup as predicted. 

Based on the tuned models, equipment performance could be evaluated and compared with design. The models could then be readily adapted to simulate an additional process fired heater (to unload the methanol superheater) and to evaluate it s inpact on the overall MTG unit performance. Multiple case studies were performed to determine optimum heater size with regard to maximising heat recovery and providing unit heat balance flexibility. The additional heat balance flexibility allows the ZSM-5 catalyst outlet temperature to be reduced hence maximising gasoline yield. 

Coil flow paths should be balanced within 5% accuracy once per shift to obtain equal outlet temperatures. On large fired heaters, this may be as often as every 2 hours, or continuously with control systems. 

For the MP header, steam input comes from the process-fired heater F-5001 where the radiation section is used for process feed heating, while the convection section for MP steam generation. MP steam is also made from the extraction of CT-1401 turbine, the HP letdown, and two WHBs. On the other hand, MP steam is used as column reboiling and stripping as well as in the steam turbines and MP letdown. CT-1601 turbines run a compressor and boiler PTs include four small turbines used for running a boiler water circulation pump and three AD fans. Since MP letdown is not measured, it is unknown and to be determined together with the MP imbalance. The MP imbalance includes metering error, steam losses due to steam traps, leaks, and so on. Table 16.10 shows the balance for the MP header. 

The lesson learned from this painfrd event is that the plant fuel gas balance needs to accurately model the amount of fuel gas produced in individual processes as well as the amount consumed in individual fired heaters and boilers. The balance model should account for variations in feed rates and process conditions as well as weather changes. The variation could be very significant and thus modeling of variation in fuel gas production is a major challenge in building the plant fuel balances. 

A furnace or fired heater can be classified as natural, induced, forced, or balanced draft. The pressure inside a warm furnace is typically lower because of buoyancy differences in the cooler outside air. A natural-draft furnace can operate using this approach however, when fans are used to push or pull the air through the furnace, greater heat transfer rates can be achieved. A natural-draft fired heater is severely limited in contrast to these systems. 

A fired heater or furnace is used to heat large quantities of hydrocarbons for industrial use in a distillation system or reactor. Fired heaters are characterized by three basic designs cabin, cylindrical, and box. The basic components of a furnace include shell, refractory lining, burners, radiant tubes, convective tubes, damper, stack, and firebox. Air and fuel are proportionally balanced as temperatures in the furnace are held constant. Figure 7-12 shows the two standard symbols used for a fired heater or furnace and a boiler. 

A common oversight is not taking account of the fact that process dynamics vary with feed rate. Consider our example of a fired heater. If it is in a nonvaporising service we can write the heat balance... 

Fired heater pass balancing is often installed to equalise pass temperatures in order to improve efficiency. Chapter 10 shows that the fuel saving is negligible and that, in some cases, the balancing may accelerate coking. However there may be much larger benefits available from the potential to debottleneck the heater. 

Efficient heater operation requires that excess air entering the convection section be minimized, whieh is indicated by a very small negative pressure at the convection section inlet. To achieve this, it should have a well balanced draft pressure profile between the firebox and stack. The hot gas pushes so that the pressure is always greatest at the firewall while the stack draft pulls. When this draft is eorreetly balanced, the pressure at the bridge waU should be around 0.1-0.2 WG (water gauge). Too mueh draft allows cold air leakage into the fired box resulting in wasted fuel. 

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