Reboiler distribution

Kayihan, F., Optimum Distribution of Heat Load in Distillation Columns Using Intermediate Condensers and Reboilers, AfC/iS Symp. Ser., 192(76) 1, 1980. 

Kettle reboilers are generally assumed to require neghgible pressure drop. It is important to provide good longitudinal liqmd flow paths within the shell so that the hquid is uniformly distributed along the entire length of the tubes and excessive local vaporization and vapor binding are avoided. 

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. 

In the first experiment, Amberlyst-15, a strongly acidic cation exchange resin, was used as a catalyst to synthesize mesityl oxide, the precursor of MIBK, from acetone without hydrogenation. The effects of acetone feed rate, reboiler duty and reaction temperature on the mesityl oxide productivity and product distribution were investigated. Preliminary results of this experiment are outlined in Table 1. 

Table 1. The effects of reaction temperature, acetone feed rate and reboiler duty on the mesityl oxide productivity and product distribution for the first CD experiment...

Furnace coils heated a flowing combustible heat-transfer oil before this hot oil is distributed and routed through the shells of four heat exchangers (reboilers) (see Figure 5—1). One of the four heat exchangers required considerable repairs and was shut down. The system pump continued to circulate hot transfer oil at about 460° F (240° C) through the other three reboilers while repairs were underway on the faulty exchanger. [2]... 

The batch distillation column consisted of 3 internal plates, reboiler and a total condenser. The reboiler was charged with a fresh feed of 5 kmol with Benzene molefraction 0.6. The total column holdup was 4 % of the charge. Half the holdup was in the condenser and the rest was distributed over the plates. The vapour load to the condenser was 3 kmol/hr. The required product purities were x oi = 0.90 and x B2 = 0.15. The solution of Equations 8.1-8.4 therefore gives DJ = 3.0 kmol and B2 = 2 kmol. This problem is same as case 3 shown in Table 8.1. Three reflux ratio (control) intervals were used to achieve (Dl, x Di) and one control interval to achieve (B2, x B2). 

The column compositions are initialised to the composition of the mixed reboiler charge and a total of 2% of the fresh feed is used as column holdup. Half of the column holdup is assumed to be in the condenser and the rest is distributed equally over the plates. Piecewise constant reflux ratio was used, with 3 time intervals for the main-cut separation and 1 interval for the off-cut. 

This example is taken from Mujtaba (1989) and Mujtaba and Macchietto (1992) where the same ternary mixture as in example 1 was considered for the whole multiperiod operation which includes 2 main-cuts and 2 intermediate off-cuts. The column consists of 5 (NT) intermediate plates, a total condenser and a reboiler. The column is charged with the same amount and composition of the fresh feed as was the case in example 1. Column initialisation, holdup distribution and condenser vapour load are also same as those in example 1. 

In Policy 1, the feed is distributed equally among the reboiler, vessels and reflux drum. These holdups are kept constant and the column is run under total reflux. Only QR(t) and (/are optimised. 

Finally, the product obtained is separated from the distillation tower, such as gas product, light oil product and heavy oil product. Our target is light oil product and/or heavy oil product, which is generally obtained by control of reactor temperature and distillation system such as temperature gradient, reflux ratio and reboiler temperamre, etc. The distribution of the oil product must be decided by market circumstances. 

The results for the corresponding adiabatic column are also shown for comparison. The most important trays for distributed heating were the trays closest to the reboiler and condenser. 

The internal modifications to retrofit the distillation system consider the distillation column and its internals. In this level of retrofit, some changes are made in the internals or nozzles of the main column and the side strippers. Moreover, the reboilers and pump around loops are examined and adjusted as required. Therefore, more complex modifications are required at this level that lead to higher investments. The objective of these modifications is to find out the ideal distribution of stages for each section in the main column [4]. This ideal design is applied to the existing column by modilying its internal to the niinimrim extent to reduce the costs. 

Reasonable care should be used to disperse a side-entering gas (e.g., vapor/liquid return from a reboiler) across the tower cross section. For extremely critical cases, one or two crossflow trays may be used to improve distribution—but they cause increased pressure drop. 

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