Energy, itself, has a cost, so there is the competing objective of meeting purity specifications with minimum use of energy. Traditional Control. The traditional approach to dual composition control of continuous distillation is simply to control the overhead temperature with reflux flow and control the bottom temperature with heat usually steam to the reboiler. These may or may not be configured as cascade loops with the temperature controller outputs directly controlling valves or providing a setpoint to the flow controllers.
Temperatures are selected as inferential measurements related to product composition, because usually on line analyzers are not available or are impractical. Level in the reflux accumulator and base of the column are typically controlled with the product flows. Again, there may be a cascade between level and flow or the level controllers may manipulate the control valves directly. This kind of strategy is simple to implement and works for most distillation processes.
Pressure can be controlled in different ways depending on whether there is non-condensable gas in the overhead that must be vented or not. Feed to the column is seldom available for control as part of the column controls as it is usually coming from somewhere else. The feed flow rate, temperature, and composition can all vary creating disturbances to the column. In the case of a side draw product, the side draw flow is often controlled manually or it may be manipulated to control a nearby temperature.
However, there are often better ways to configure the distillation controls and as a workhorse unit operation in the process industries, it is worthwhile to take a deeper look at how to control a column. Distillation is a very interactive process. Secchi 3. E-mail: natalia radixeng. E-mail: arge peq. The hydrodealkylation process of toluene HDA has been used as a case study in a large number of control studies.
However, in terms of industrial application, this process has become obsolete and is nowadays superseded by new technologies capable of processing heavy aromatic compounds, which increase the added value of the raw materials, such as the process of transalkylation and disproportionation of toluene TADP.
Distillation Control, Optimization, and Tuning : Fundamentals and Strategies
TADP also presents more complex feed and product streams and challenging operational characteristics both in the reactor and separator sections than in HDA. Plantwide control structures oriented to control and to the process were adapted and applied for the first time for this process. The results show that, even though both strategies are similar in terms of control performance, the optimization of economic factors must still be sought. The use of recycles and heat integration in the transformation processes is a consolidated solution to increase yields and to reduce operational costs.
These factors tend to increase the process complexity, demanding a control perspective not limited to the analysis of the individual units. Many authors point out that the need for a plantwide perspective on control arises mainly due to these changes in the way plants are designed. Indeed, these factors lead to more interactions and therefore the need for a perspective beyond individual units, as pointed out by Stephanopulos and earlier by Buckley These decisions are all taken during the basic design conception, unfortunately before the complex control studies that in general are not performed by process engineers.
Myers defined the aromatic complex as a combination of process units that can be used to convert petroleum naphtha and pyrolysis gasoline pygas into the basic petrochemical intermediates: benzene, toluene, and xylenes know as BTX. Benzene, toluene and xylenes are produced through the catalytic reforming of naphta, but the thermodynamic proportion obtained in this process is different from the market demand Toluene has the lower demand and several processes are used to convert toluene and to produce, preferentially, benzene and xylenes Serra et al.
Distillation Control, Optimization, and Tuning: Fundamentals and Strategies
This work focuses on the plantwide control of an aromatic complex, more precisely the Transalkylation and Disproportionation Unit TADP , whose function is the increase of xylenes and benzene production in the aromatic complex from surplus toluene. This process is considered to be a technological improvement of the HDA process because, besides benzene, the TADP process also produces xylenes and generates fewer by-products with low commercial value.
To the knowledge of the authors, this is the first study of plantwide control applied to the TADP process. According to Qiu et al. This is the mathematically-oriented approach or design of the control structure , i.
An alternative is the development of heuristics based on experience and understanding of the process and is referred to as the process-oriented approach Luyben, ; Luyben et al. The implementation of the methodology from Luyben et al. Morari stated that "in search of a control structure considered optimizing, the main objective is to incorporate the economic objectives to the process control objectives".
In other words, "the goal is to find a function c u , d of process variables, that, when held constant, leads the manipulated variables automatically to their optimal working values, and with it, to the optimal operating conditions [ They evaluated the effects of a loss function departure from optimum in the implementation of the reference value of the controlled variable.
The authors presented a design procedure based on a mathematically-oriented approach, but with some elements of the process-oriented approach. The procedure starts with a top-down analysis to select the controlled variables, based on ideas of self-optimization. At this stage, a rigorous steady-state model is needed and the operational objectives economic steady-states have to be defined.
The result consists of one or more alternative sets of controlled variables.
Distillation Control, Optimization, and Tuning: Fundamentals and Strategies - CRC Press Book
This top-down analysis is followed by a bottom-up analysis, starting with the regulatory control layer. After this stage, the setpoints of the regulatory layer and some unused manipulated variables are the remaining degrees of freedom, which can be used to control the primary controlled variables. This control layer is called the supervisory layer. Two main approaches are possible for this layer: single-loops decentralized controllers with feedforward connections, or multivariable control.
According to the authors, appropriately designed multivariable controllers will have better performance, but this must be negotiated against the cost of obtaining and maintaining the models used in the controllers. In the sequence, an optimization layer is applied with the purpose to identify active constraints and compute optimal set-points c s for the controlled variables. Finally, nonlinear dynamic simulations should be performed to validate the proposed control structure.
As pointed out by Qiu et al. Luyben et al. The strategies of Luyben et al. They were chosen after a literature review since they are up-to-date and representative of typical process- and mathematically-oriented plantwide control approaches. The technologies for transferring the methyl group convert toluene to benzene and xylenes at the same time, while the hydrodealkylation scheme mainly produces benzene. Due to the growing demand for xylenes, the process of transfer of the methyl group is preferred. The main products of the disproportionation of toluene are benzene and xylenes.
There are also some side reactions including disproportionation of xylenes producing A10 aromatic with 10 carbons and dealkylation of alkylbenzenes producing light gases. The hydrogen is recycled from the first separator through a recycle compressor. Figure 2 presents the flowsheet of the assumed process.
Some changes in the generic TADP process were implemented aiming a closer approximation to the concept of a petrochemical complex with the inclusion of input streams from adjacent units. The main changes were: the inclusion of a feed stream of a mixture of benzene and toluene bentol , from a hypothetical reprocessing complex; the xylene and heavy aromat- ics columns were removed from the study aiming an approximation with the HDA process for comparison purposes; a fresh stream of toluene was added in the feed of the benzene column, since toluene is not a product of the complex and therefore this stream can present variations in compositions that are absorbed in the separation columns.
Sidestreams were included in the stabilizer column and in the benzene column based on industrial plant design information of real units Klafke, The kinetic model is presented in the Appendix and was proposed by Ouguan et al. The design data were based on information of industrial units Klafke, and are summarized in Table 1 and Table 2.
The streams data are also available in Klafke The number of steady-state degrees of freedom determines the number of steady-state controlled variables that can be chosen.
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In complex processes, it is useful to sum the number of degrees of freedom of the individual units, as given in Table 3. From these rules, the degrees of freedom are calculated according to Table 4. This analysis can be verified by a balance of the 22 manipulated variables considered in this process see Table 5. However, 7 of the original degrees of freedom are flowrates used to stabilize liquid levels with no steady-state effect. For this process 49 variables were considered as controlled variables. This selection is presented in Table 6. With 15 degrees of freedom and 49 candidates for controlled variables, an analysis of all possible structures is impractical.
To avoid this combinatorial explosion, the active constraints are first determined and then an optimization analysis can be applied to define the remaining set. In this case study there are three valuable products of the distillation columns: the composition of benzene in the sidestreams of the stabilizer and benzene columns and the composition of xylene in the bottom of the toluene column. As these are the main products of the unit, their specification in these streams must be considered as active constraints.
The composition of toluene in the recycle stream to the unit is also a pre-defined variable, since an increased content of xylenes in the recycle stream demands higher energy consumption, and may also cause an undesired concentration of ethylbenzene in the process. Thus, this composition is also considered to be an active constraint. Just as considered for xylene, it is not interesting to recirculate back the benzene to the reaction section; therefore, in the bottom of the benzene column, benzene composition must be controlled.
The bottom stream of the stabilizer column will be manipulated in order to control the concentration of benzene in the bottom. These constraints are not degrees of freedom since their values should be set. It is considered that all by-products purge, distillate vapor of stabilizer and benzene column are sold as fuel. The constant setpoint strategy is simple, but will not be optimal, as a result of disturbances. The effect of these factors or, more specifically the loss depends on the choice of the controlled variables, and the goal is to find a set of controlled variables in which the loss is acceptable.
The objective function described in Equation 1 was set up with its constraints. Concentration variables with no active constraints were pre-selected, because they have a significant impact on the objective function. The decision variables were the concentrations mol fractions from Table 6. Variables defined as optimal from the standpoint of self-optimizing control were those with less deviation from their optimal value, which implies that the constant setpoint policy has the least impact on the profit function. Table 8 presents the results. From the described methodology, the selected variables should have been: i the separator liquid outlet toluene mol fraction and ii the mol fraction of xylene in the reactor outlet.
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