Split Flow Reflux

The name SFR (Split Flow Reflux) derives from the additional reflux. Due to this economical development of the GSP-process, the propane yield can be boosted to the range between 97 to 99% at constant power consumption. 

Accumulators are not separators. In one application, an acciunulator placed after a total condenser provides reflux to a fractionator and prevents column fluctuations in flow rate from affecting downstream equipment. In this application the accumulator is called a reflux drum. A reflux drum is shown in Figure 6.3. Liquid from a condenser accumulates in the drum before being split into reflux and product streams. At the top of the drum is a vent to exhaust noncondensable gases that may enter the distillation column. The liquid flows out of the drum into a pump. To prevent gases from entering the pump, the drum is designed with a vortex breaker at the exit line. 

When carrying out a batch distillation on a binary mixture it is very useful to have a quick and user-friendly analysis of the overheads. It is even better if the analysis can be performed on the plant without the need to take a sample to the laboratory and the process operator has a continuous record of the progress of the batch. A record of this sort can be provided by an on-line specific gravity (i.e. density corrected for temperature) meter. Many binary pairs such as mixtures of methanol, acetone and THF with water have large density differences between the organic distillate and water. This property can be used to control the split between reflux and product flow rates. 

In a sidestream column, changing the flow rates of liquid streams above or at a product withdrawal location affects the composition of that product and all other products below it. Changing the flow rate of the sidestream has little effect on the products above it. Therefore, sidestream composition can be controlled by vapor boilup, reflux flow rate, liquid split, or sidestream flow rate. As discussed later, we want to control a composition near the top of the prefractionator, and the logical manipulated variable to achieve this control is the liquid split. If reflux is used for distillate control, we are left with using either sidestream flow rate or vapor boilup for the control of the sidestream composition. 

Here, with the otherwise same basic process, a split flow of the compressed sales gas is recycled, completely condensed in the heat exchanger (7), subcooled and finally fed to column (4) as reflux. This reflux is significantly leaner in ethane compared to the reflux used in the GSP-process. Thus, better retention of the ethane at the top of column (4) is possible owing to which efhane yields between 95 and 99% are economically achievable. 

The fix for the erratic reflux drum pressure problem was to provide for separate pressure control of the fractionator column and the reflux drum. A new pressure control valve was installed upstream of the condenser and the old condenser outlet control valve was removed. A hot gas bypass, designed for 20% vapor flow, was installed around the pressure control valve and condenser. A control valve was installed in the hot gas bypass line. The column pressure was then maintained by throttling the new control valve upstream of the condenser. The reflux drum pressure w as controlled by the hot gas bypass control valve and the psv saver working in split range. The new system is shown in the figure below. 

The split-fraction coefficients are not very dependent on the feed composition, providing the reflux flow-rate is adjusted so that the ratio of reflux to feed flow is held constant Vela (1961), Hachmuth (1952). 

Inside the column a liquid stream flows downward and a vapor stream rises. At each point in the column some of the liquid vaporizes and some of the vapor condenses. The vapor leaving the top of the column, which contains 97 mole% benzene, is completely condensed and split into two equal fractions one is taken off as the overhead product stream, and the other (the reflux) is recycled to the top of the column. The overhead product stream contains 89.2% of the benzene fed to the column. The liquid leaving the bottom of the column is fed to a partial reboiler in which 45% of it is vaporized. The vapor generated in the reboiler (the boilnp) is recycled to become the rising vapor stream in the column, and the residual reboiler liquid is taken off as the bottom product stream. The compositions of the streams leaving the reboiler are governed by the relation... 

In this entire process, only one equation could not be solved analytically, Eq. (6.4). In order to solve the problem, you needed the Ai-values of the chemicals (or vapor pressures), the flow rates of the feed stream, the thermal condition of the feed stream, the desired split for the light and heavy keys, and either the number of stages or the reflux ratio. The equations then And the splits of all the other components, the minimum number of stages, the minimum reflux rate, and the actual number of stages needed to achieve the desired split. These last three items are very useful when using a more rigorous method of calculation, as shown below. 

One possible set of specifications would include the flow rates of three of the products and the column reflux ratio. The way the components are split among the products, the identity of the key components in each section are determined by the product rates, and the locations of the feed and products. In this example the products are specified at flow rates that correspond to the component flow rates in the feed. Thus, the distillate is specified at 12 kmol/h, the upper side draw at 48 kmol/h, and the lower side draw at 25 kmol/h. By overall material balance, the resulting bottoms flow rate is 15 kmol/h. 

A stream containing benzene, toluene, and biphenyl is to be separated in a distillation column to produce purified benzene in the distillate. The separation will take place in an existing column with a total condenser, a partial reboiler, and several optional feed locations. The feed stream is of fixed flow rate, composition, and thermal conditions. The entire feed may be introduced at any one of the available feed trays, but may not be split and introduced at more than one feed tray. The condenser pressure is controlled by an inert gas flowing in and out of the reflux drum. Using column modules representation, determine the degrees of freedom for this operation, and recommend a set of specifications to define the column performance. 

A reflux arrangement is now added at the lower end of the column. The extract is sent to a solvent removal unit, and the solvent-free extract is split into an extract product and an extract reflux which is sent back to the bottom of the column. Without the solvent, the extract reflux is now composed of the raffinate component and the solute (components R and E), so that this reflux is actually on the raffinate side of the equilibrium curve (Section 11.2 and Figure 11.2) flowing countercurrent to the extract phase. The extract phase is thus interacting with a raffinate phase which is richer in the solute than the feed. As a result, the extract enrichment with the solute is greater than it would be if the extract were interacting directly with the external feed in the absence of the extractor section below the feed. A higher-purity extract product can therefore be expected. 

Compute the minimum reflux ratio (Lr/D), the distillate rate D, and the flow rate of dk for each split key component for the following mixture... 

While the net flows patterns for each structure are now clearer, it still remains difficult to comprehend the effects of changing the reflux in one CS in the rest of the column. The reflux ratio in a specific CS is an important parameter in finding feasible structures and therefore it is necessary to fully understand these effects. As shown in previous chapters, the reflux in a specific CS is a limitless parameter valid anywhere from negative to positive infinity. Notice howev that in the side-stripper configuration, for example, that the liquid stream is split into two parts. 

Initially, the total feed is split equally between the two columns. This is achieved in the Splitter labeled Tl on the flowsheet shown in Figure 5.33. Two Design SpecA ary are set up in each column to adjust distillate flow rate and reflux ratio to attain the 99.9 mol% product purities of all foiu streams. The optimum feed tray location is determined by finding the feed stage that minimizes reboiler heat input. In column Cl, it is Stage 19. In column C2, it is Stage 18. 

A distillation column that is designed to produce both a liquid distillate stream and a vapor distillate stream from the reflux drum has an additional design degree of freedom. This is usually specified to be the reflux-drum temperature. Under these conditions, the split between the flow rates of the vapor and liquid distillates is fixed. The condenser heat duty is also fixed. Specifying a reasonable design minimum temperature differential temperature (at the either the cold or the hot end of the condenser), and a reasonable overall heat-transfer coefficient fixes the heat-transfer area. This also fixes the required flow rate of... 

In addition, there are four degrees of freedom that are adjustable during design and are also adjustable during operation of the column reflux flow rate (/ ), vapor boilup (V), sidestream flow rate (5), and the liquid split ratio (jSl = i-p/i-R)- The variable Lp is the liquid flow rate fed to the prefractionator side of the wall, and Lp is the total liquid leaving the bottom tray in the rectifying section. Of course, the rest of the liquid coming from the bottom of the rectification section is fed to the sidestream side of the column. Distillate and bottoms flow rates are used to maintain liquid levels in the reflux drum and column base, respectively. 

These three controlled variables require three manipulated variables. There are four available reflux flow rate, sidestream flow rate, vapor boilup, and liquid split. Vapor boilup has an immediate and strong effect on all compositions throughout the system, and therefore, in theory, could be used to control any of the three product compositions or a composition in the prefractionator. Reflux affects all compositions, but the only composition that it affects quickly is the distillate composition. Its effect on products lower in the column can take considerable time because of the liquid hydraulic lags. Therefore, it seems logical to control distillate composition with reflux. Reflux-drum level is then controlled by manipulating distillate. This choice is for situations in which the reflux ratio is not high. If the reflux ratio is greater than about 3, conventional distillation control wisdom says to control composition with distillate and control the reflux-drum level with reflux. 

Liquid from the column base is split Splitter S2 in Fig. 13.3) between the bottoms and a circulating stream that flows through a HeatX model used for the reboiler. The bottoms flow rate is set equal to that foimd in the base case (720.7 kmol/h). Note that the feed and bottoms flow rates are set to same values as the base case. Therefore, the distillate flow rate must also be the same. Using the same reflux flow rate should yield exactly the same tray and product compositions, which is indeed true (99.9 mol% purities of both product streams). 

This chapter presented a concept drawing of a distillation process that included flow rate sensors, liquid level sensors, temperature sensors, and process control valves. The process streams were identified and labeled. The standard definition of reflux ratio was presented, and the concepts of separation power and material balance split were introduced. 

The separation power base for distillation control can be set either by the ratio of reflux/feed or by the ratio of boilup/feed. If the separation power base is set by the ratio of boilup/feed, then either the reflux or the distillate flow rate can be manipulated to control the distillate/feed material balance split. If the separation power base is set by the ratio of reflux/feed, then the steam, that is, boilup, flow rate can be manipulated to control the distillate/feed material balance split. 

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