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Investigation of Non-conventional Batch Distillation Column

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Investigation of Non-conventional Batch Distillation Column

    Investigation of Non-conventional Batch

     Distillation Column Configurations

    Nem-hagyományos szakaszos desztilláló

    kolonnakonfigurációk vizsgálata

    László Hégely, Péter Láng

    BUTE Departement of Building and Process Engineering

    1521 Budapest, Műegyetem rkp. 3-5

Summary

    Conventional and new batch column configurations and their different operation modes were compared

    with the CC-DCOLUMN professional dynamic flow-sheet simulator. We simulated the separation of test

    mixtures (n-hexane-n-heptane and n-hexane-n-heptane-n-octane), and compared the recoveries of

    different methods under constant energy consumption. The open and six different closed operation modes

    of batch rectifier were studied. In case of negligible liquid hold-up, closed operation provided better

    recoveries. Batch rectifier and middle-vessel column were also compared. Middle-vessel column

    provided greater average recovery.

     These devices can be operated in closed I. Introduction mode (without product withdrawal) as well,

     which may reduce the energy consumption [6]. Distillation is the most widespread The aim of this work is to study the separation process in the chemical industry. The competitiveness of non-conventional components of a liquid mixture are separated on configurations and operation modes by rigorous the basis of differences in their volatilities. The simulation calculations. The recoveries obtained advantage of batch distillation over the with the different methods are compared under continuous one primarily in the separation of constant energy consumption.

    mixtures produced in small, changing quantities

    and compositions are well-known. In the II. The studied configurations and operation pharmaceutical industry, distilling industry, fine modes

    chemical production and solvent recovery

    distillation is mostly performed in batch mode. The different configurations are modelled The batch rectifier is the only widespread with the CC-DCOLUMN dynamic professional configuration in the industry operated in open flowsheet simulator. The following simplifying system (with continuous product withdrawal). assumptions are used:

    In order to reduce the energy requirement of this theoretical plates,

    operation researchers devoted great attention to constant liquid hold-up on the plates, newer configurations in recent years. Batch negligible vapour hold-up.

    stripper (first mentioned by [1]) can be Calculations are made for a binary mixture considered as an inverted batch rectifier, with n-hexane n-heptane (comparison of different the charge filled into an upper vessel. Middle-operation modes of batch rectifier), and for a vessel column [2,3] consists of two column ternary one n-hexane n-heptane n-octane

    sections connected through a middle vessel, and (comparison of batch rectifier and middle-vessel can produce three products simultaneously. The column). For both mixtures, the Soave-Redlich-generalisation of the middle-vessel column is Kwong (SRK) equation of state is applied for the multi-vessel column [4,5] which is built up calculation of vapour-liquid equilibria and from more column sections and intermediate enthalpies.

    vessels.

II. 1. Batch rectifier

    The open and different closed operation

    modes of batch rectifier are compared. The

    mixture to be separated contains 50 mol%

    hexane and 50 mol% heptane, its volume is 10 3dm. The prescribed purity of hexane is 99

    mol%. The column has a total condenser, and

    operates with a heat duty of 500 W.

    II. 1. 1. Open operation mode

    The model of the open operation mode

    (Figure 1) consists of a SCDS column, and two

    DYNAMIC VESSELs (reboiler, product tank).

    The reflux ratio is constant during the process.

    (The operation policy of constant distillate

    composition was also studied, but as it proved to

    be less efficient than this simpler policy, it is not Figure 2. Model of closed modes of a batch presented here.) The operation is stopped when rectifier (with level control, Modes 2-3). the hexane content of the accumulated product Six different closed modes are presented, decreases to 99 mol%. The duration obtained in which differ in the operation of the upper vessel, this way is prescribed for the closed operation that is in the method of varying the liquid flow modes. rate from the vessel. The closed modes have at

    least one additional degree of freedom

    compared to the open one.

    The (conventional) reflux ratio is always

    infinite during a closed operation, as there is no

    distillate withdrawal. However, if we take into

    consideration that the hold-up of the upper

    vessel may vary and we define the reflux ratio

    with the following equation

    LR V;L

    then R is only infinite, when the vapour and Figure 1. Model of the open mode of a batch liquid flow rates are equal.

    rectifier. Mode 1. Constant volumetric flow rate

    The constant value of the liquid flow rate II. 1. 2. Closed operation modes provides the additional degree of freedom. The models of the closed operation modes However, this specified value can only be (Figure 2) are very similar, the only difference ensured, when the flow rate of the condensate is the presence or lack of the control equipment. becomes sufficiently large. As the flow rate of One of the DYNAMIC VESSELs models the the condensate is almost constant after the initial upper vessel, in which the hexane-rich product part of the operation, the level in the upper is accumulated. The total condenser of the vessel increases almost linearly. In this mode no column is modelled with a HEAT controller is present. The advantage of this EXCHANGER module. The control equipment operation mode is the initial existence of the consists of a PIDC (a sensor and a PID reflux (contrary to Mode 2 a). However, R controller), a CVAL (control valve), and a always has a finite value. PUMP (creating pressure difference between the two sides of the valve) module.

Mode 2 Constant level

    The level of the upper vessel is controlled. Two variants are considered. In both cases, the additional degree of freedom is the set-point of the level controller.

    Mode 2a Upper vessel empty at the start

    The operation is started with empty upper vessel. The drawback of this mode is the initial lack of reflux, as the liquid must accumulate in the vessel. After this period, however, the reflux ratio is infinite.

    Mode 2b Upper vessel filled up at the start

    The charge is distributed between the upper vessel and the reboiler, and the initial liquid level is maintained by the controller. R is infinite during the whole operation, but the purity of the initial reflux is much lower than that of the condensate, and the dynamics of the upper vessel is slower at the beginning than by the other operation modes. Figure 3. The model of middle-vessel column.

    Mode 3 Constant liquid flow rate, then constant 3octane, with a volume of 20 dm. The level prescribed purities for hexane and octane were This mode can be considered as a 98 mol%, the quality requirement for heptane is combination of Modes 1 and 2a. In the first part the purity reached with the middle-vessel of the operation the liquid flow rate is specified, column. and after the level reaches its specified value The model of batch rectifier is basically the (which is the set-point of the controller), the one presented above, extended with a TIME level controller is activated. The two additional SWITCH (for separating the different cuts) and degrees of freedom are the liquid flow rate, and three DYNAMIC VESSELs (vessels for the liquid level in the vessel. Though the hexane-, heptane-rich products, and for the side-dynamics of the vessel is faster at the beginning, product). this decreases the duration of the period with The middle-vessel column (Figure 3) is built infinite reflux. up from two SCDS columns, connected through Mode 4 Composition control a DYNAMIC VESSSEL. The charge is filled Composition of the upper vessel is into the middle vessel. The reflux and the reboil controlled; the set-point is somewhat higher ratios can be changed by the aid of the two than the quality requirement, in order to ensure DIVIDERs. The TIME SWITCH and MIXER its fulfilment. The level in the vessel (and also R) modules are used for providing the total reflux begins to increase after an initial period, and at the beginning of the operation. The top and finally becomes almost constant. On the basis of bottom products are accumulated in the two the possible division of the charge, two variants DYNAMIC VESSELs. are proposed: at the beginning, the upper vessel is empty (Mode 4a) or filled up (Mode 4b). III. Results III. 1. Comparison of different operation II. 2. Middle-vessel column modes of the batch rectifier The performances of batch rectifier and Recoveries obtained with the open and middle-vessel column are compared. The charge different closed operation modes are compared is equimolar mixture of hexane, heptane and

for the same product quality (0,99 mole fraction 100,00%0 ml/plate99,00%hexane). In the case of closed operation modes, 50 ml/plate98,00%100 ml/plate97,00%the value of at least one operation parameter has 96,00%95,00%to be adjusted in order to maximise the recovery 94,00%while satisfying the quality requirement. 93,00%92,00%Additionally, the controllers in the closed 91,00%90,00%operation modes (except Mode 1) must be tuned. 89,00%88,00%The tuning is made by the Ziegler-Nichols OpenMode 1Mode 2aMode 2bMode 3Mode 4amethod: the controller’s proportional band is

    Figure 4. The recoveries at different levels of decreased until the critical gain is reached and

    hold-up (N=10, R=9) the controlled variable begins to oscillate. The

    gains of the PI controllers are calculated from The hold-up of the column is a very the critical gain and the duration of oscillation important parameter: in case of negligible hold-period. In some cases of Mode 4a, the quality of up, the closed operation modes (except Mode 1) control was not satisfactory with gains give better results than the open one. However, calculated by the Ziegler-Nichols method, and when the hold-up is greater, the open operation the proportional band had to be increased. mode provides better recoveries (Figure 4). Mode 4b does not differ significantly from The advantage of the closed operation Mode 4a. The difference between the set-point modes over the open one is also affected by the and the composition of the charge, and therefore reflux ratio and the number of plates, though to the error is so great initially, that the controller a smaller extent. The difference increases with very quickly empties the vessel. In this way, the increasing number of plates, and decreasing division of the charge is useless. reflux ratio.

    The effects of three operational parameters The closed operation modes can be placed in a on the recovery are studied. The reflux ratio (R) relatively fix order according to the recoveries. and the number of theoretical plates (N) have Mode 1 proved to be always worse than the only slight effects, while the influence of the open mode, and generally worse than the other plate hold-up is very significant. Some of the closed modes. The order of the other closed recoveries reached with the different operation operation modes (with decreasing recoveries): modes under different operation parameters are 2b, 4a, 3, 2a. Therefore Mode 2b gives the best presented in Table 1. recovery with the exception of two cases, in

     which it is preceded by Mode 4a. The operation

    N 8 10 12 of closed modes is illustrated with Figs. 5 and 6. Hold- up 0 0 50 100 0 3 III. 2. Comparison of the batch rectifier and (cm/p.) middle-vessel column Open 92.2% 93.3% 95.2% 94.4% 93.6% First, the near-optimal operation parameters Mode 1 88.7% 92.0% 91.7% 89.8% 92.5%

    of the middle-vessel column are determined Mode

    2a 93.0% 97.1% 92.8% 88.4% 97.9% with the well-known simplex optimising Mode algorithm. The independent variables are the 2b 93.9% 97.7% 93.3% 88.9% 98.5% volumetric liquid flow rate from the middle Mode 3 93.3% 97.1% 92.8% 88.4% 98.0% vessel (W), the reflux (R) and the reboil (S) Mode ratios, the objective function is the sum of the 4a 94.1% 97.4% 93.3% 89.0% 98.2%

    deviations from product quality requirements (instead of the sum of recoveries). The Table 1. The effect of N and the hold-up on the prescribed heptane purity (98%) is not reachable recoveries. if the other two products are of acceptable

     purity. Table 2 contains the coordinates of the

     13/h) 40 Purities W (dm0,8R 9 Hexane 0.9813 0,6S 12 Heptane 0.7969 Mode 1Time (min) 110.5 Octane 0.9938 Mode 2a0,4Mode 2bObj. function 0.1982 Mode 30,2Mode 4aTable 2. The coordinates and results of the 001234567initial point. Time (h)

     Figure 5. Evolution of product compositions

    3(closed operation modes). W (dm/h) 42.88 Purities Recoveries

     R 13.81 Hexane 0.9830 0.9541 0,16S 13.25 Heptane 0.9309 0.7367 0,14Time (min) 157 Octane 0.9802 0.9000 0,12Obj. func. 0.052 0,1

    ;0,08Q (MJ/h) Mode 18.721 Mode 2aLevel (m)0,06Mode 2bTable 3. Operating parameters and results of the 0,04Mode 3Mode 4aMVC. 0,02

    0Comparing the results of the two 01234567Time (h)configurations, it can be seen that the hexane Figure 6. Evolution of the level in the upper and octane recoveries are higher in the middle-vessel (closed operation modes). vessel column, while the recovery of heptane is

    higher in the batch rectifier. The largest point, from which the initial simplex was

    difference occurs in the case of octane. These generated with a step size of 2, in the direction

    results can be explained by the different of increasing values. The results of this initial

    composition of the hold-ups: that of the middle- run are also presented.

    vessel column contains mostly heptane, while Due to convergence problems, the method of batch rectifier’s one is mostly octane. The calculation was modified several times. First, average recoveries (0.8636 for MVC, 0.8164 for the reflux and the reboil ratios were specified to BR) show more definitely the better be equal, then W was replaced with 2W. Finally, performance of middle-vessel column. the original method with three independent variables was used later, but with 4W instead of W. The characteristic data of the final point are Purities Recoveries R 5.51 ;Qgiven in Table 3. ( is an average heat duty.) Hexane 0.982 0.8996

     Heptane 0.930 0.8357 Time 157 (min) Octane 0.981 0.7138

    Table 4. Results for the BR.

    provided better recoveries. Modes 2b (level control with initially filled up vessel) and 4a (composition control with initially empty vessel) proved to be the best closed modes. The decrease of reflux ratio and increase of number of theoretical stages increased the advantage of closed modes. For higher hold-up, the open operation mode gave the highest recovery. Batch rectifier and middle-vessel column were also compared for a ternary mixture. Middle-vessel column

    Middle-vessel column provided greater 1

    recoveries for hexane and octane, but smaller 0,8

    for heptane. The average recovery was higher 0,6

    for this non-conventional configuration. In the 0,4middle-vessel column, we are able to Hexane0,2Heptanesimultaneously recover all three components, Octane0and no intermediate fractions are needed 020406080100120140160180Time (min)contrary to batch rectifier. However, its

    Figure 7. The evolution of compositions in the operation is more complicated.

    MVC and the BR.

    Acknowledgement In the case of the batch rectifier, the hexane

    This work was supported by the Hungarian and octane recoveries are further decreased with

    Scientific Research Foundation (OTKA, project the material lost in the intermediate fractions.

    No.: T-049184) Another advantage of the middle-vessel

     column is that only one operation step is needed,

    References as all three components can be recovered

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    negligible liquid hold-up, closed operation

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