Column distillation purity levels

The column starts up on total reflux (no distillate is withdrawn) until the distillate composition reaches the desired purity level. The time at total reflux is called TE in the program. Then distillate is withdrawn at a fixed rate of 40 mol/h. 

Solution 6,2 We first consider the scenario where the feed is below the sidestream product. Thus, we use the distillate purity as the initial integration composition from the top of the column. By guessing ever increasing values for the reflux ratio in the topmost CS, it is possible to eventually find a reflux ratio that will allow this profile to enter the sidestream purity level. This is illustrated in Figure 6.24. Figure 6.24 shows that the first reflux that achieves a composition of 0.9 intermediate is extremely high at 65. The exact composition where this specification is met is X5 = [0.064, 0.036, 0.900]. 

In this chapter, we study distillation columns that have more than the normal two product streams. These more complex configurations provide savings in energy costs and capital investment in some systems. Sidestream columns are used in many ternary separations, and the examples in this chapter illustrate this application. However, a sidestream colmnn can also be used in a binary separation if different purity levels are desired. For example, two grades of propylene products are sometimes produced from a single column. The bottoms stream is propane, the sidestream is medimn-purity propylene, and the distillate is high-purity polymer-grade propylene. 

One of the most important applications of thermodynamics in our profession is in the design of distillation columns. Such columns are used to effect the sepa-rati[Pg.28]

The developments in this subsection have revealed that high-purity distillation columns exhibit a dynamic behavior with three time scales. Thus, according to the results in Sections 7.4 and 3.4, the design of a control system involves the synthesis of a tiered structure featuring three levels of control action. 

Once the large internal flow rates have been set via appropriate control laws, the index of the DAE system (7.21) is well defined, and a state-space realization (ODE representation) of the slow subsystem can be derived. This representation of the slow dynamics of the column can be used for the derivation of a model-based nonlinear controller to govern the input-output behavior of the column, namely to address the control of the product purity and of the overall material balance. To this end, the small distillate and bottoms flow rates as well as the setpoints of the level controllers are available as manipulated inputs. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

If the reactants A and B are lighter and heavier, respectively, than the product P, the flowsheet involves two distillation columns and two recycle streams (Figure 4.5). The control structure includes loops for reactor level and temperature, as well as for distillation columns top and bottom purity. The following dimensionless equations can be derived ... 

The control of the separation section is presented in Figure 10.11. Although the flowsheet seems complex, the control is rather simple. The separation must deliver recycle and product streams with the required purity acetic acid (from C-3), vinyl acetate (from C-5) and water (from C-6). Because the distillate streams are recycled within the separation section, their composition is less important. Therefore, columns C-3, C-5 and C-6 are operated at constant reflux, while boilup rates are used to control some temperatures in the lower sections of the column. For the absorption columns C-l and C-4, the flow rates of the absorbent (acetic acid) are kept constant The concentration of C02 in the recycle stream is controlled by changing the amount of gas sent to the C02 removal unit The additional level, temperature and pressure control loops are standard. 

The magnitudes of various flowrates also come into consideration. For example, temperature (or bottoms product purity) in a distillation column is typically controlled by manipulating steam flow to the reboiler (column boilup) and base level is controlled with bottoms product flowrate. However, in columns with a large boilup ratio and small bottoms flowrate, these loops should be reversed because boilup has a larger effect on base level than bottoms flow (Richardson rule). However, inverse response problems in some columns may occur when base level is controlled by heat input. High reflux ratios at the top of a column require similar analysis in selecting reflux or distillate to control overhead product purity. 

Figure 6.20 gives a control scheme for a single sidestream column. The flowrate of the sidestream can be manipulated, so we have an additional control degree of freedom. Three compositions can be controlled the impurity of B in the distillate (xDiS). the purity of B in the sidestream (xsS), and the impurity of B in the bottoms xBS). Note that we cannot control the two impurity levels (A and C) in the sidestream xsa and Xs,c because there are not enough degrees of freedom. 

Eollowing are two examples (16.1 and 16.2) of a distillation column that demonstrate the effect of applying different pairing strategies. In both examples the control loops for the column pressure and the liquid levels in the condenser accumulator and the column bottom are determined independently based on practical considerations. Thus, the column pressure is controlled by various techniques that may involve the condenser coolant rate, and the liquid levels are controlled by the product flow rates. What remains to be decided is how to pair the distillate and bottoms compositions with the reflux rate and the reboiler heat duty. The same distillation column is used in both examples, having a total condenser and a reboiler, one feed and two products. The column is designed to separate a benzene-toluene mixture into benzene and toluene products with specified purities. 

Figure 15.64 shows a distillation column that reaches an upper limit on the reboiler duty. When the remote setpoint for the steam flow rate to the reboiler is consistently greater than the measured steam flow, an override controller switches to using the column feed rate as a manipulated variable to keep the bottom product purity on specification. When the column feed rate is adjusted back to its normal level and the control valve on the steam to the reboiler is no longer saturated (i.e., fully open), the control configuration is changed so that the reboiler duty is manipulated to control the bottom product purity. 

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

Let s consider a simple one-feed two-product distillation column. When examined as stand-alone item with a fixed feed, its control is typically a 5x5 multivariable problem. Among the five controlled variables three are for the basic inventory pressure, level in reflux drum and reboiler. The two remaining are for quality eontrol purity of distillate and bottom products. The five manipulated variables are distillate flow D, bottom product B, boilup V or reboiler duty Qr, condenser duty and reflux flow L. The combination of controlled and manipulated variables may lead to several control structures. Here we present some typical situations useful for dynamic simulation. 

The second column supplies benzene of high purity. One-point composition control is more robust in a recycle system. In inferential mode the heat input in reboiler controls a sensitive temperature in the column. If the sampling point is placed in the stripping section, this control loop ensures good composition control of the bottom product. The reflux is set in ratio with the feed flow rate. For moderate disturbances, this allows good purity of distillate simultaneously with high recovery. The levels in the flash drum and in the base are hold by manipulating distillate rate and bottom product, respectively. 

The quality of the intermediate DCE must fulfil strict purity specifications. Low impurity levels imply high energetic consumption, but higher impurity amounts are not desired for operation. The intermediate DCE is conditioned mainly in the distillation column S2. In the bottom product the concentration of the two bad impurities E and E must not exceed an upper limit, of 100 and 600 ppm, respectively, while the concentration of the good impurity E must be kept around optimal value of 2000 ppm. Because these impurities are implied in all three reaction systems through recycles that cross in the separation system, their inventory is a plantwide control problem. The problem is constraint by technological and environmental constraints, as mentioned. 

As sketched in Fig. 6.7, control structure scheme A has reactor level controlled by column feed. Column base level is held by bottoms. Reflux drum level is held by distillate recycle back to the reactor. Reflux flow rate is flow controlled. Distillate composition is not controlled since the recycle is an internal stream within the process. Bottoms product purity is controlled by manipulating heat input. Note that this scheme violates the rule for liquid recycles since the streams in the recycle loop F and D) are both on level control. 

In order to keep the reboilers down to a reasonable size, the column has to be heated either with steam at a pressure of not less than 3.5 to 5 bar or wiA waste heat at a temperature level above 140-150°C. Unlike the prerun column, the pressurized column is a genuine distillation column as the overhead product has to meet the purity requirements of US Grade AA methanol. The reflux rate, the number of trays and the heat input can be varied within certain limits, and the most favourable design of the column and its economical operation have to be established by optimizing calculations. A column with the above-mentioned number of trays reaches its operating optimum with a reflux ratio of approximately 3.0 and a heat input of about 2.0 GJ per ton of total methanol produced. As the overhead product ftom the pressurized column is used to heat the atmospheric column, either of the two coliunns has to be used to distill some 50 % of the total methanol produced, except for slight differences in the reflux ratio. 

Since the RR is 1.07, reflux-drum level can be controlled by either distillate or reflux. We select distillate flow to control level to gain the advantage of proportional-only control smoothing out the disturbances to a downstream process. The reflux is manipulated to control the temperature on Stage 3 at 339.5 K, which maintains the purity of the DME product. Figure 10.10 gives the temperature and composition profiles in the column. Note that the BuOH vapor composition yBuon is smaller than the BuOH liquid composition BuOH on all trays below the feed tray. This illustrates why the sidestream is removed as a vapor instead of a liquid. 

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