Ingenieria

Simulation of the steady-state and dynamic behaviour of multiple effect evaporation plants
Part2 :dynamicbehaviour
Ulf Bolmstedt
A computerprogram for simulation of the dynamic behaviour of general multiple effect evaporation plants has been developed. The computer program, called D YNEFF, consists of a number of dynamic blocks comprising the necessary differential and algebraic equations forevaporator vessels, flash tanks, heat exchangers, etc., and an administrative part governing the input/output procedures, the solution procedures, etc. D YNEFF is used in combination with a previously presented computer program, called INDUNS, for calculation of all the necessary initial values. INDUN5 performs a design or evaluation calculation of the desired multiple effect evaporation plant,and transfers the steady-state data as well as the flow sheet data to D YNEFF. Using the program combination, simulation of the dynamic behaviour of complete multiple effect evaporation plants is easily carried out. The main objective of the work is to increase the possibilities of achieving better knowledge and understanding of the dynamic behaviour of muHiple effect evaporation plants. It shouldalso be possible, to use the program combination to predict the dynamic behaviour of plants under construction as well as running plants, and thereby facilitate the determination of proper controller settings, suitable equipment sizes, etc.
Steady-state calculations of multiple effect evaporation plants for design purposes use the solution of a system of heat and mass balances. The solutionprocedure is very tedious and so is given computer treatment. A number of computer programs for d~sign calculations of multiple effect evaporation plants is presented in the literature, the first one by Marek 16. The main purpose of a steady-state design calculation is to determine the proper heating areas of evaporators and heat exchanges, as well as the proper vapour and liquid schemes. However, manyother important design parameters cannot be determined with the aid of steady-state calculations. Such parameters are liquid holdups of evaporators and storage tanks, control schemes and controller settings: these parameters may be determined from dynamic calculations, and it is clear that a combination of steady-state and dynamic calculations would facilitate a proper design of multiple effectevaporation plants. Considering this, two computer programs have been developed at the Department of Chemical Engineering at the Lund Institute of Technology. The first program, called INDUNS (Bolmstedt4~; Bolmstcdt and Gudmundson 6) performs steady-state design and evaluation calculations for any multiple effect evaporation plant. INDUNS uses a special module, called the unit cell, as well as aconnection matrix for the description of plant flowsheets. The unit cell comprises all equipment normally used in evaporation plants, for instance evaporator vessel, flash tanks, and heat exchangers. The program presented in this report, DYNEFF, uses INDUNS for the computation of all the necessary steady-state initial values, and then performs calculation of the desired dynamic simulation. Although anumber of investigations concerned with the dynamic behaviour of evaporators has been presented in the literature in the years 1960 to 1973, no program for simulation of the dynamic behaviour of general multiple effect evaporation plants has yet been reported. Johnson14 studied a single effect evaporator used in the concentration of a urea-water mixture. Some empirical transfer-function modelswere developed in order to obtain a proper plant design, as well as a proper control scheme. Narsimhan 18studied theoretically the start-up of a triple effect plant. Only the concentration dynamics was considered, and since some quite rough assumptions were made, very simple algebraic equations could be derived. The first more comprehensive model analysis was presented by Andersen et al. I. A...