Fresh Feed Streams Research Articles

A novel “feed-backward” control structure for on-demand control of distillation column sequences

The typical conventional plantwide control structure sets the flow rate of the fresh feed entering the process. Sometimes marketing or safety requirements dictate that the flowrate of the product leaving the process must be flow controlled. This structure is called “on-demand.” Important examples of chemicals that must be supplied on an “as-needed” basis include hydrogen, oxygen, synthesis gas, toxic hydrogen cyanide and explosive nitrotoluene. Since the output flow is fixed in an on-demand control structure, the material balance loops controlling liquid levels and gas pressures must work their way back upstream through the process to eventually adjust the fresh feed streams. A novel on-demand control structure is developed in this paper that reduces peak dynamic disturbances throughout the process by immediately changing upstream flow rates in a “feed-backward” manner when the product flow rate is changed.

Process improvement for three-column extractive distillation by feed split

A three-column extractive distillation process may often be used when separating azeotropic components at a dilute feed condition. A feed-split method is innovatively proposed to save energy for this process. The method is to split the fresh feed stream into two streams, one for the pre-concentrator, and the other for the extractive distillation column. The applicability of this method is initially estimated. Four systems are used as main case studies including acetonitrile/water with ethylene glycol as the solvent, acetone/methanol with dimethyl sulfoxide as the solvent, n-propanol/water with N-methylpyrrolidone as the solvent and ethyl acetate/ethanol with furfural as the solvent. The results show that most three-column extractive distillation process can save energy by feed split. This method can be used alone or used for other energy-saving methods to save further energy.

Simplified plantwide control structure for the diethyl oxalate process

The diethyl oxalate process features two reaction sections with two of the fresh feeds (oxygen and ethanol) and a recycle stream containing nitric oxide fed into the first reactor, which produces ethyl nitrite and water. Water is removed in a distillation column and the ethyl nitrite is fed to the second reactor along with the third fresh feed (carbon monoxide). Diethyl oxalate and nitric oxide are produced in this reactor. The product diethyl oxalate is recovered in a distillation column, and the nitric oxide is recycled back to the first reactor. The intermediate components (nitric oxide and ethyl nitrite) and inert nitrogen circulate between the two reaction sections.A recent paper proposed a plantwide control structure for this complex process that requires four on-line composition controllers to handle disturbances in throughput and the compositions of the three fresh feed streams. The purpose of this paper is to demonstrate that a less complex control structure with only one composition measurement can achieve stable regulatory control.

Using a tracer technique to identify the extent of non-ideal flows in the continuous mixing of non-Newtonian fluids

The identification of non-ideal flows in a continuous-flow mixing of non-Newtonian fluids is a challenging task for various chemical industries: plastic manufacturing, water and wastewater treatment, and pulp and paper manufacturing. Non-ideal flows such as channelling, recirculation, and dead zones significantly affect the performance of continuous-flow mixing systems. Therefore, the main objective of this paper was to develop an identification protocol to measure non-ideal flows in the continuous-flow mixing system. The extent of non-ideal flows was quantified using a dynamic model that incorporated channelling, recirculation, and dead volume in the mixing vessel. To estimate the dynamic model parameters, the system was excited using a frequency-modulated random binary input by injecting the saline solution (as a tracer) into the fresh feed stream prior to being pumped into the mixing vessel. The injection of the tracer was controlled by a computer-controlled on-off solenoid valve. Using the trace technique, the extent of channelling and the effective mixed volume were successfully determined and used as mixing quality criteria. Such identification procedures can be applied at various areas of chemical engineering in order to improve the mixing quality.

A Microchemical System with Continuous Recovery and Recirculation of Catalyst‐Immobilized Magnetic Particles

A microchemical system for continuous flow catalytic reactions with magnetic catalyst is presented. The automatic separation of catalyst particles and recirculation by the microchemical system makes it possible to realize fully the advantages a catalytic microreactor can offer. It would be applicable to various catalytic reactions with aid of already reported magnetic catalyst or new magnetic catalyst.

Design and Control of the Monoisopropylamine Process

The monoisopropylamine (MIPA) process provides an interesting example of plantwide economic design optimization and plantwide control. The process consists of a tubular reactor and three distillation columns. There are both gas and liquid recycles. Fresh feed streams of isopropyl alcohol and ammonia are introduced into the process. The desired products are MIPA and water. An undesirable byproduct of di-isopropylamine (DIPA) is also produced and is recycled to extinction since its formation reaction is reversible. An excess of ammonia in the reactor inhibits the DIPA reaction, so ammonia is also recycled. The purpose of this paper is to present the details of the process for use by other workers as a test case for use in plantwide design and control studies. A heuristic economic optimum design and a control structure are developed. The process illustrates the classical design trade-off between reactor costs versus separation costs. In addition, the process has two recycles, which are both distillate products from different distillation columns, and this leads to another trade-off. Using more ammonia recycle produces less DIPA recycle. So the optimum design must balance the separation costs of the two recycles. Two of the distillation columns can be effectively controlled using a single temperature and a reflux-to-feed ratio control structure. The third column requires a dual control structure ratio (one temperature and one composition).

Evaluation of a two-temperature control structure for a two-reactant/two-product type of reactive distillation column

Several different control structures have been proposed for reactive distillation columns. The appropriate control structure depends on the flowsheet and on the type of reactions occurring in the column. If two reactants are involved and if it is desirable to operate the process without any excess of reactant, it is necessary to manage the fresh feed streams so that the stoichiometry is exactly balanced. A composition analyzer that measures an internal composition in the column is often required. However, if two products are produced, it is possible to avoid the use of an analyzer by using two temperatures in the column to adjust the two feed streams. This type of structure was proposed by Roat et al. [Roat, S., Downs. J., Vogel, E., Doss, J., 1986. Integration of rigorous dynamic modeling and control system synthesis for distillation columns. In: Chemical Process Control—CPC III. Elsevier, Amsterdam.] for the ideal reaction A + B ↔ C + D in one of the earliest papers dealing with reactive distillation control. The purpose of this paper is to explore the effectiveness of this two-temperature control structure for various column designs (number of reactive stages) to quantify the impact of design on controllability. We also discuss the issues of the selection of the trays whose temperatures are to be controlled and the tuning of the two interacting temperature controllers. Disturbances in production rate and fresh feed compositions are made to examine the rangeability of this control structure. Both an ideal reaction system and the methyl acetate system are studied. One of the main conclusions is that the locations of the temperature control trays should be made such that the two temperature controllers both have direct action (an increase in temperature increases feed), which requires negative openloop process gains for both loops.

Comparison of Two Types of Two-Temperature Control Structures for Reactive Distillation Columns

The purpose of this paper is to compare the effectiveness of two different types of two-temperature control structures for reactive distillation columns. The first was a structure proposed by Roat et al. (Roat, S.; Downs, J.; Vogel, E.; Doss, J. Integration of rigorous dynamic modeling and control system synthesis for distillation columns. Chemical Process Control; CPC III; Elsevier: Amsterdam, The Netherlands, 1986) almost 2 decades ago in which two tray temperatures in the column are controlled by manipulating two fresh feed streams. The second was recently proposed by Yu and co-workers (Yu, C. C. Private communication, 2004) in which two tray temperatures are controlled by manipulating one of the fresh feed streams and vapor boilup. The second fresh feed stream is flow controlled. We discuss the issues of the selection of the fresh feed stream to be manipulated and the importance of the tuning procedure for the two interacting temperature controllers. Both an ideal reaction system and the methyl acetate system are studied. The effectiveness and robustness of these control structures are compared in the face of disturbances in the production rate and fresh feed compositions. One of the main conclusions is that the selection of the manipulated fresh feed stream in the second structure has an important role in the stability of the system. Sequential tuning of the interacting temperature controllers is sometimes necessary.

Control of Isopropyl Alcohol Dehydration System: Case with Single Temperature Break in Azeotropic Column

Semi-conductor manufacturers in Taiwan often produce large amount of waste solution containing isopropyl alcohol (IPA)and water (H2O).In order to purify IPA, a simple pre-purification column is used first to purify the waste solution into the azeotropic composition of IPA and H2O.Then, an entrainer is added to further separate into high purity IPA product and waste water. In this work, control of an industrial scale complete heterogeneous azeotropic distillation column system of isopropyl alcohol (IPA) + water(H2O)with cyclohexane (CyH)as an entrainer is investigated. The complete system includes a heterogeneous azeotropic column, an additional recycle column, a decanter, and a recycle stream from the recycle column back to the azeotropic column. In early study of this system, the IPA product purity at the bottom of the azeotropic column is specified to be ultra pure with over 99.999 mol% IPA. The temperature profile of the azeotropic column for this ultra pure separation is characterized by two temperature breaks inside the column. In this paper, the IPA product purity is relaxed to specify at 99.9 mol%. With this purity, the temperature profile and the route of the composition profile for the azeotropic column is completely different than the previous case. The suitable overall control strategy of this complete heterogeneous azeotropic distillation column system has been studied extensively in this paper. From process analysis and dynamic simulation, the overall control strategy is developed which requires reverse-paired two temperature loops in the first column and one temperature loop in the second column. The overall control strategy is tested with composition and flow rate variations in the fresh feed stream. The specifications of the IPA product and waste water stream with both purities at 99.9 mol% can be met despite various disturbance changes.

Plantwide Control of Continuous Multiproduct Processes: Three-Product Process

A previous paper (Kapilakarn, K.; Luyben, W. L. Ind. Eng. Chem. Res. 2003, 42, 1890−1904) explored the design and plantwide control of a continuous process in which two reversible reactions produced two desired products M and D. Because chemical equilibrium established the amounts of the products leaving the reactor, the use of product recycle was required to operate over a range of product ratios M/(M + D). This paper extends the previous work to consider the more complex system in which there are three reactions producing components M, D, and T. Desired product ratios can span the entire ternary distribution space (M/D/T), depending on market conditions. Both steady-state design and dynamic control are explored for this three-product process, which features one reactor, four distillation columns, and two recycle streams. Several conventional control structures are studied in which the flow rates of the fresh feed streams are fixed or manipulated by level or composition/temperature controllers and the pr...

Extensions of the Simultaneous Design of Gas-Phase Adiabatic Tubular Reactor Systems with Gas Recycle

Luyben studied the simultaneous design of a simple gas-phase tubular reactor system with a single feed stream and independent reactor preheating and cooling for a reversible reaction. Reyes and Luyben studied an irreversible reaction system with a reactor feed preheating system (feed-effluent heat exchanger and furnace) but assumed equimolal concentrations of reactants in the recycle gas. A simple separation was assumed in both of these studies. Reyes and Luyben recently explored systems with more realistic separation systems (a distillation column) for gas-phase tubular reactors with liquid recycle and with a dual recycle system. This paper extends this work for two cases: (1) the reversible reaction system is explored with a realistic feed preheating system and with two fresh feed streams, and (2) the irreversible reaction system is studied for nonequimolal reactant concentrations. The exothermic gas-phase reaction A + B ⇔ C occurs in an adiabatic tubular reactor. A gas recycle returns unconverted reactants from the separation section consisting of a simple separator in which product C is removed in the liquid phase and reactants A and B are recycled in the gas phase back to the reactor inlet. Optimum steady-state economic designs are shown to lead to poor dynamic responses. Slight modifications of the plant design lead to a much more easily controlled plant. In the reversible case, the additional furnace heat input leads to a better dynamic performance. In the irreversible case, a higher reactor inlet temperature improves dynamics.

Alternative designs for a catalytic converter operating under autothermal conditions

This paper focuses on the study of two different autothermal designs of a catalytic combustor, based on natural gas oxidation for heat generation. The feed is preheated by means of two different alternatives: (a) indirect heat exchange between the products and feed streams (Design I), (b) direct heat exchange by recycling a fraction of the hot gases leaving the catalytic combustor (Design II). Steady-state and dynamic results have demonstrated that autothermal catalytic units, after an appropriate start-up procedure, can operate with high methane conversion, without exceeding the maximum allowable temperature and minimizing harmful emissions such as NO x (to preheat the reactants, the flame of conventional units is replaced by a heat feedback of a fraction of the total heat generated by the catalytic combustor). Due to the dilution originated by the recycle stream (which admits the use of higher methane molar fractions in the fresh feed stream) Design II appears as the most convenient alternative because it can provide significantly higher heat fluxes than Design I for the same process constraints. Moreover, Design II can provide an exit stream at the highest thermal level (for a given maximum allowable temperature) feasible for an autothermal converter.

Effect of Feed Impurity on the Design and Control of a Ternary Two-Recycle Process

This paper studies both the steady-state design aspects and the dynamic controllability aspects of processes in which a fresh feed stream contains significant impurities. The ternary process with t...

Dynamics and control of recycle systems. 4. Ternary systems with one or two recycle streams

This paper is the fourth in a series of papers that explore the challenging problems associated with the dynamics and control of recycle systems. The reactions considered in previous papers were fairly simple. Only first-order reactions were considered, so there was only one fresh feed stream. In this paper, second-order kinetics are considered with two fresh-feed makeup streams. Two cases are considered: (1) instantaneous and complete one-pass conversion of one of the two components in the reactor so there is an excess of only one component that must be recycled and (2) incomplete conversion per pass so there are two recycle streams. It is shown that the generic liquid-recycle rule proposed by Luyben applies in both of these cases: snowballing is prevented by fixing the flow rate somewhere in the recycle system. An additional generic rule is proposed: fresh feed makeup of any component cannot be fixed unless the component undergoes complete single-pass conversion. In the complete one-pass conversion case, throughput can be set by fixing the flow rate of the limiting reactant. The makeup of the other reactant should be set by level control in the reflux drum of the distillation column. In the incomplete conversion case, two workablemore » schemes were found: (1) Both recycle flow rates are fixed and both fresh-feed makeups are brought in on level control. Throughput is controlled by changing either the reactor temperature or the recycle flow rates. (2) One fresh-feed makeup controls reactor level and the other controls the composition in the reactor. Throughput is controlled by setting reactor temperature or reactor effluent flow rate.« less