Cycling reboilers

Most glycol dehydration processes are continuous. That is, gas and glycol flow continuously through a vessel (the contactor" or absorber ) where they come in contact and the glycol absorbs the water. The glycol flows from the contactor to a reboiler (sometimes called "reconcentrator or regenerator where the water is removed or stripped from the glycol and is then pumped back to the contactor to complete the cycle. 

The process engineer identifies heat exchange equipment in a process by the operation or function it serves at a particular location in the flow cycle. For example, the bottom vaporizer on a product finishing distillation column is usually termed Finishing Column ReboUer E-16, or Reboiler E-16 the overhead vapor condenser on this column is termed Condenser E-17 etc. The usual operations involved in developing a process flowsheet are described in Table 10-11, or Chapter 1, Volume 1. 

Reboiler Boils liquid by heating medium in a recirculation cycle. Feed may flow by... 

Heavy cycle oil, heavy naphtha, and other circulating side pumparound reflux streams are used to remove heat from the fractionator. They supply reboil heat to the gas plant and generate steam. The amount of heat removed at any pumparound point is set to distribute vapor and liquid loads evenly throughout the column and to provide the necessary internal reflux. 

The debutanizer separates the feed into two products. The overhead product contains a mixture of C3 s and C4 s. The bottoms product is the stabilized gasoline. Heat for separating these products comes from an external reboiler. The heating source is usually the main fractionator heavy cycle oil or slurry. Steam can also be used. 

Varying the steam-to-condensate interface level to control the reboiler duty will promote steam leaks in the channel head-to-shell flanged closure. This is caused by the thermal cycling and stresses that result from constantly varying the level of condensate in the channel head. However, when low-pressure steam (<60 psig) is used, this becomes a minor problem, which may be safely ignored. 

FIGURE 11.19 A simple fractional distillation column used in a chemistry laboratory. The vapors from a boiling mixture of liquids rise inside the column, where they condense on contact with the cool column walls, drip back, and are reboiled by contact with more hot vapor. Numerous boil/condense cycles occur before vapors finally pass out the top of the column, reach the water-cooled condenser, and drip into the receiver. 

Dynamic optimisation of this type of periodic operation was first attempted and reported in the literature by Mayur et al. (1970), who considered the initial charge to the reboiler as a fresh feed stock mixed with the recycled off-cut material from the previous distillation task. Each batch cycle is then operated in two distillation tasks. During the Task 1, a quantity of overhead distillate meeting the light product specification is collected. The residue is further distilled off in Task 2 until it meets the bottom product specification. The overhead during Task 2 meets neither specifications (but the composition is usually kept close to the that of the initial charge for thermodynamic reasons) and is recycled as part of the charge for the next batch. As the batch cycle is repeated a quasi-steady state mode of operation is attained which is characterised by the identical amount and composition of the recycle (from the previous batch) and the off-cut (from the current batch). Luyben (1988) indicates that the quasi-steady state mode is achieved after three or four such cycles. 

In all the case studies presented here it is assumed that the amount of fresh feed to be processed in the long production campaign is fixed for every batch cycle, but the reboiler is oversized to some extent. The optimal amount of recycle is obtained within this bound so that maximum benefit can be achieved out of a given column. 

The cracked gas stream is cooled and purified in the primary fractionator (3) and quench water tower (5). Waste heat is recovered by a circulating oil cycle, generating dilution steam (4) and by a water cycle (5) to provide heat to reboilers and process heaters. The cracked gas from the quench tower is compressed (6) in a 4- or 5-stage compressor and dried in gas and liquid adsorbers (8). C02 and H2S are removed in a caustic-wash system located before the final compressor stage. 

The best operational policy Table 7) is when the total amount of the charge is fed into reboiler a (Ub / U h = 1 ).The duration of the cycle is nearly equal for the two configu-... 

Four fractions of pseudo-compounds are obtained by solution analysis of crude oil residue or its cracked product. During solution analysis of the cracking product from thermal treatment of vacuum residue or mixtures of vacuum residue and plastics (such mixtures were used in our investigation), a first step of solution analysis is soxhlet extraction. In the soxhlet extractor, the liquid/solid product is extracted with fresh warm solvent (THF) that does not contain the extract. This can increase the extraction rate, as the sample is contacting fresh warm solvent. The sample is placed inside a cellulose thimble and placed in the extractor. The extractor is connected to a flask containing the extraction solvent, and a condenser is connected above the extractor. The solvent is boiled, and the extractor has a bypass arm that the vapor passes through to reach the condenser, where it condenses and drips into the sample in the thimble. Once the solvent reaches the top of the siphon arm, the solvent and extract are siphoned back into the lower flask. The solvent reboils, and the cycle is repeated until the sample is completely extracted, and the extract is in the lower flask. 

The above value is not bad. In reality much more exergy is destroyed because of irreversibilities, particularly due to the cooling. For example, if we take into account only the hot utility for separation (feed preheating and reboiler) the total heat is g, = 138 + 1147 = 1285 kW. This could be extracted virtually from surroundings and converted by means of a Carnot cycle working between 298 and 406.15 K, for which the following work can been obtained ... 

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