Startup reboiler

An alternative technique that prevents excessive bottom temperatures and permits reboiler startup before vacuum is fully drawn is routing the column feed directly into the reboiler inlet (Fig. 12.4). This technique may generate substantial quantities of off-spec bottom product a provision for recycling bottom (and possibly also top) products into the feed tank may minimize product loss. 

Column Bottom Temperature. The bottom temperature is often controlled on the reboiler outlet line with a control valve in the heating medium line. The control point can also be on a bottom section tray. Care must be exercised in location of the temperature control point. It is recommended, especially for large columns, that a cascade arrangement be used. The recommended scheme has a complete flow recorder/controller (FRC) in the heating medium line including orifice and control valve. The set point of this FRC is manipulated by the temperature recorder/controller (TRC). This eliminates the TRC from manipulating the control valve directly (recall that temperature is the most difficult parameter to control). This makes for smoother control for normal operations. Also, it is handy for startup to be able to uncouple the TRC and run the reboiler on FRC for a period. 

It is easy to improperly design a steam trap. The design must work for two circumstances and often a designer will check only one of these. The circumstance often overlooked is as follows On startup or upset the steam control valve can open wide so that the steam chest (assume for this discussion that we are speaking of a reboiler) pressure rises to full steam line pressure. At a... 

Use a 750 MBtu/hr reboiler to allow for startup heat loads. Calculate Duties of Heat Exchangers... 

Following startup reboiler failed and gas released causing explosion to occur. 

Figures 23 and 24 show the liquid-phase compositions for, respectively, the reboiler and condenser as functions of time. After column startup, the concentration of methanol decreases continuously whereas the distillate mole fraction of methyl acetate reaches about 90%. A comparison of the rate-based simulation (with the Maxwell-Stefan diffusion equations) and experimental results for the liquid-phase composition at the column top and in the column reboiler demonstrates their satisfactory agreement (Figures 23 and 24). Figure 25 shows the simulation...

This operating condition may be approached in a number of ways. A continuous distillation column may be placed on total reflux operation by setting the external rates equal to zero. This type of operation is commonly used in the startup of either a continuous or a batch distillation column. After a given quantity of feed has been introduced into the column, it may be allowed to come to a steady state in which no streams enter or leave the column. As will be demonstrated below, the precise steady state which is reached depends upon the holdup on each plate as well as the holdups in the accumulator and the reboiler. 

Continuous distillation is a thermodynamically efficient method of producing large amounts of material of constant composition. However, when small amounts of material of varying product composition are required, batch distillation has several advantages. In batch distillation a charge of feed is heated in a reboiler, and after a short startup period, product can be withdrawn from the top of the equipment. When the distillation is finished, the heat is shut off and the material left in the reboiler is removed. Then a new batch can be started. Usually, the distillate is the desired product. 

For startup, a 2 or 3-inch diameter, gravity-flow by-pass is usually provided from the tower liquid space to a low point of the downcomer, at reboilers with high liquid draw-off nozzles. 

A dump line connecting the bottom of the column with the reboiler liquid inlet line should be provided (Fig. 4.7a to d). Without it, liquid will weep through the tray at startup (there is no vapor flow until the reboiler starts up), and no liquid will be available to commission the reboiler and initiate vapor generation. 

When a light mixture (e.g., top product, off-spec top product, or a feed stream void of the heavy components) is used for startup, an excessive bottom temperature is unlikely to occur. In this case, feed can be introduced and the reboiler started before vacuum is fully drawn. This practice is often used when startup time is critical. 

It is a good practice to desuperheat reboiler or stripping steam during startups in order to minimize hot-spot formation and, therefore, material degradation at the base of the column. 

If it is decided not to heat up the column prior to startup, it will behave as a mtgor heat sink when reboil is started. Reboil must be slowly applied until the coliunn heats up. Excessive reboil rates during this period may cause premature flooding and even damage to column intervals (105). As shown earlier (Pig. 12.4), it may be desirable to run the coliunn feed directly to the reboiler inlet sump. 

Reboiler heating upon startup should proceed in slow, incremental steps to avoid excessive thermal stresses. The thermal stress problem is most severe when reboiler tubes are roll-welded to the tubesheet (358). 

A total reflux startup is best performed with a mixture whose composition resembles the feed composition. Alternatively, total reflux operation can be carried out using one or more of the feed components. The column, reboiler, condenser, and reflux system are stabilized at... 

Failure to thermosiphon. At low heat fluxes, reboiler liquid flow is sometimes difficult to initiate. If the flow is not started, the reboiler may vaporize some of the liquid, leaving a residue too heavy to lift or too heavy to boil. The problem is common at startups, especially when... 

Installing a valved dump line connecting the column bottom outlet line with the reboiler inlet line (Fig. 15.4a). This technique is only needed when the column reboiler sump is separated from the column bottom sump by a baffle or when the reboiler liquid comes from a trapout pan. The valve remains shut during normal operation, but is opened during startup to lower the level and inspire thermosiphon action during startup. One case where this technique was successfully used has been described (237) the author has had several similar experiences. 

Set the highest liquid level to match the expected liquid level in the reboiler under the most severe turndown conditions required. Keep in mind that these may occur under startup conditions, when the reboiler is clean and the heat transfer coefficient is high. 

An erqrlofiion and fire were caused a butane release during a startup following an interruption. After bolts in the reboiler pump suction screen housing were loosened, heavy dqxnits trapped in the plugged suction isolation valve broke loose, reteasing 150-psi butane to atmosphere. 

A once-throu thermosiphon reboiler could not be started up because tray weeping at startup starved the reboiler of liquid. A valved dump line cotmect-ing rdxriler liquid and bottom sump was added and solved problem. 

A dump (or startup) line connecting col-lunn bottom sump and rdwiler liquid line should not be overlooked for once-throu reboilers. 

In tray columns the first mechanism is dominant. This can lead to a large number of different steady state solutions for a given set of operating conditions. If N is the number of steady states (typically an odd number). Then (N + l)/2 of these steady states are stable. This can lead to complex multi-stable dynamic behavior during column startup and set-point or load changes. These phenomena were observed for vanishing as well as for finite intra-particle mass transfer resistance. An example with a total number of six trays (two reactive and two non-reactive trays plus reboiler and condenser) is shown in Fig. 10.16 for the well-known MTBE process. In contrast to the previous section, the column is now operated in the kinetic... 

Solvent contaminated] carryover from upstream equipment example oil from compressor brines, corrosion inhibitors, sand, [corrosion products, see Section 1.3] /oxygen leaks into storage tank/inadequate corrosion control, example low pH causing corrosion/degradation via overheating, ex hot spots in reboiler tubes or fire tubes/ineffective filters/ineffective cleaning before startup/for amine absorbers corrosion products/FeS/chemicals used to treat well. 

Intermediate condensers are useful since the coolant can be at a higher tenperature. See Problem 4.C6. Intermediate reboilers and condensers are fairly common because they allow for more optimum use of energy resources, and they can help balance column diameters (see Section 10.4). During normal operation, they should cause no problems however, startup maybe difficult (Sloley. 1996). 

Oantrol of the condensate valve is sometimes achieved by a steam-flow measurement, sometimes by a condensate-flow measurement (more accurate and cheaper), and sometimes by another process variable. Response is more sluggish than when steam flow is throttled. No trap is required. The theory and design equations are presented in Chapter 15. For troubleshooting, performance tests, startup, and, in some cases, overrides, a liquid-level transmitter should be installed on the reboiler shell side. 

When the column base is too small for smooth level control, an external surge tank must be used. One way of connecting the two is to overflow from the column base into the bottom product receiver as shown in Figure 4.17. A recycle line back to the column base (or to a feed tank) facilitates startup and can be used to protect the column base—and therefore also the reboiler— from excessively low level. The bottom product receiver, as a minimum, should have enough holdup to contain the entire column contents. 

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