Because the tower was reboiler-limited but was not flooding, the column pressure was reduced during this period. The overhead condensing capacity did not limit operations at the lower pressure because the cooling water temperatures were cooler than the summer design values.

The reduced column pressure mitigated the reboiler limitation, and allowed unit charge rates to be increased somewhat, to the level shown in Table 1.

Even at the lower column pressure, the DIB limited unit operation and the reboiler steam-supply valves remained wide open. Additionally, during the summer, higher cooling water temperatures would eliminate the option to decrease column pressure.

With the help of unit personnel, the authors gathered and analyzed the unit operating data. The data revealed two major areas of concern:

• The 250 psig steam supply to the bottom reboiler had about 70° F. of superheat.
• More important, the bottom-reboiler inlet temperature was 10° F. greater than the alkylate product temperature.

The temperature difference between the reboiler inlet and alkylate product suggested that a significant amount of bottom tray liquid was bypassing the reboiler inlet and going directly to the product side of the bottom baffle.

Review of the tower internals showed that the designers attempted to ensure that liquid from the bottom tray would feed the reboiler preferentially. In operation, however, this was not always occurring.

Further analysis of the operating data revealed reboiler heat-transfer coefficients that were significantly less than design. Coefficients in this range indicated fouling.

Reboiler Review

Thermosyphon reboilers are either once-through or recirculated. The outlet from a once-through thermosyphon returns to the column so that none of its liquid effluent enters the reboiler inlet sump. This is usually accomplished by returning the effluent to the column below the inlet draw location.

The inlet temperature to a once-through reboiler is less than that of a recirculated reboiler in the same service. The lower inlet temperature increases the temperature driving force, as defined by log mean temperature difference (LMTD), and therefore yields a greater duty for the same area.

A once-through reboiler functions as about one theoretical stage in the tower. Recirculated thermosyphons return their outlet to the column so that part of their liquid effluent returns to the reboiler inlet, where it is mixed with light feed from the tray above. Recirculating a thermosyphon has the advantage of lowering the percent vapor of the reboiler outlet.

Thermosyphon reboilers typically are limited to an outlet vapor content of 30-50 wt % to reduce the potential for liquid slugging and to provide a liquid wash for the tube surface. Increasing the concentration of reboiler effluent liquid in the reboiler feed reduces the outlet vapor fraction.

Recirculation also can help ensure a consistent liquid height feeding the thermosyphon. Having a constant liquid head on the reboiler inlet generally results in more stable reboiler operation.

Recirculating a thermosyphon, however, increases equipment size because of the higher flow rates and lower temperature driving forces. The lower driving forces result from the mixed feed entering the reboiler at a higher temperature than if it were fed from only the tray above.

A recirculated thermosyphon equates to some fraction of a theoretical stage. Recirculated thermosyphons are often designed such that, although there is recirculation from the exchanger outlet to its inlet, all of the light liquid from the tray above is preferentially fed to the reboiler. This minimizes the reboiler inlet temperature, thereby maximizing the temperature driving force and the exchanger duty for a given heat-transfer area.

Preferentially feeding the liquid to the reboiler typically is accomplished by installing appropriate internal baffles to direct the light liquid flow to the reboiler inlet.

For the case reviewed here, the side reboilers were not designed for preferential flow. The bottom reboiler was of preferential design, but the internals did not guarantee that the bottom tray liquid would be preferentially fed to the reboiler.

As discussed previously, the initial operating data indicated that the DIB reboiler inlet temperature was higher than that of the DIB bottoms product (alkylate). This surprising observation was checked and rechecked.

The lower product temperature indicated that a large portion of the light liquid feed to the reboiler was bypassing the reboiler inlet and leaving the column directly with the alkylate. The primary effect of this would be to increase the inlet temperature of the bottom reboiler, thereby lowering the temperature driving force and capacity of the exchanger.

The authors developed a process simulation to match the operating data.

In most tower simulations, it is not critical that the reboiler be modeled exactly. Usually, the reboiler duty is simply added to the bottom stage.

In this case, however, to accurately model the existing tower and match the plant data, the process simulation had to include bypass of the reboiler inlet and the appropriate reboiler recirculation rate.

The simulation matched the plant data by sending 50% of the light liquid from the bottom tray directly to the alkylate product. This means half the normal reboiler feed did not enter the exchanger.

The simulation verified that the observed plant temperatures and the theorized bypassing were valid.

The bypassing had several detrimental effects on reboiler performance. It reduced the separation efficiency of the reboiler to much less than one theoretical stage. More important, with half of the bottom tray liquid being sent to product, the reboiler temperatures had to be greatly increased to maintain the alkylate RVP specification.

Essentially, the bottom tray material almost had to meet the RVP specification, and the reboiler outlet had to exceed the specification.

The higher reboiler temperatures reduced the reboiler LMTD and, hence, available duty by almost 40%. The effect was dramatic because of the large volatility difference between n-butane and C8 alkylate.

The lower concentration of butane in the reboiler feed also reduced the heat-transfer coefficient.

The DIB side reboiler also is recirculated, and the design does not attempt to feed preferentially. The exchanger operating conditions, however, are such that the negative effect on exchanger capacity is not as great as it is for the bottom reboiler.

For this reboiler, the narrower differences between the adjacent tray temperatures and compositions lead to a smaller LMTD penalty for bypassing the side reboiler. Calculations suggest that only 5% of the exchanger design capacity is lost because of the nonpreferential feed arrangement.

Superheated Steam

The steam supply to the DIB bottom reboiler had 70° F of superheat. Unfortunately, hotter is not always better for heat transfer.

Saturated steam condensing on tube surfaces has a very high heat-transfer coefficient. Superheated steam, however, must be cooled to its saturation temperature before it can begin condensing. Much of this cooling occurs through dry-wall desuperheating, for which the heat-transfer coefficient can be 50% or less of the condensing steam heat-transfer coefficient.

Rigorous modeling of this exchanger indicated that the lower heat-transfer coefficients for desuperheating reduced the overall reboiler capacity by about 10%.

Superheated steam also can lead to a mistaken impression about the available temperature driving force.

Figure 2 illustrates typical heating and cooling curves for a steam reboiler. Assuming saturated steam, Equation (2) defines the standard LMTD for points on Figure 2.