Unit Subroutines

A degrees-of-freedom analysis (Smith, 1963 Rudd and Watson, 1968 Myers and Seider, 1976) is incorporated in the development of each subroutine (or block, or model) that simulates a process unit. These subroutines solve sets of AEquaiions involving Nvanabies. where A Equations A variabies- Thus, there are Nd = A, ariabies Equations degrees of freedom, or input (decision) variables. Most subroutines are written for known values of the input stream variables, although HYSYS.Plant permits specification of a blend of input and output stream variables, or output stream variables entirely. 

Using temperature and pressure as the intensive variables, Eq. (4.4) becomes 

During execution of a unit subroutine, the stream vectors and equipment parameters are accessed, from a so-called B vector in ASPEN PLUS, and changes are recorded when new 

BLOCK FI IN=FEED OUT=VAP LIQ BLOCK FI FLASH2 PARAM TEMP=120 PRES=13.23 

TEMP PRES VFRAC DUTY ENTRN MAxrr TOL  

---

Early research in flowsheeting involved discovering automatically the better streams to use as guesses (tear streams) by selecting the order in which the unit subroutines should be called. Methods were also published for... 

Table 4.1 lists the unit subroutines (or blocks, or models) in each of the four simulators. Several of the subroutines are referred to in the sections that follow, with descriptions of many on the multimedia CD-ROM and detailed descriptions in the user manuals and Help screens. 

Figure 4.7 ASPEN PLUS unit subroutine—information transfer.

For each unit, the vector of parameters computed by a unit subroutine is saved for display and printing, and to initiate iterative computations for subsequent executions of the subroutine. 

Subroutine VLDTA2. VLDTA2 loads the binary vapor-liquid equilibrium data to be correlated. If the data are in units other than those used internally, the correct conversions are made here. This subroutine also reads the estimated standard deviations for the measured variables and the initial parameter estimates. All input data are printed for verification. 

The subroutines PARIN and PARCH are source routines written in American National Standard FORTRAN (FORTRAN IV), ANSI X3.9-1978, and should be compatible with most computer systems where input can be taken from logical unit 3. 

The use of the computer in the design of chemical processes requires a framework for depiction and computation completely different from that of traditional CAD/CAM appHcations. Eor this reason, most practitioners use computer-aided process design to designate those approaches that are used to model the performance of individual unit operations, to compute heat and material balances, and to perform thermodynamic and transport analyses. Typical process simulators have, at their core, techniques for the management of massive arrays of data, computational engines to solve sparse matrices, and unit-operation-specific computational subroutines. 

Program unit heading—Program name, function name, or subroutine name. 

Subprogram statements are those used to transfer control between program units—the main program, functions, and subroutines. A function call is performed by invoking the name of the function module in an assignment statement, such as... 

Program DGC04 solves the time-dependent problem. Subroutine EQUATIONS evaluates the coefficients of the unknown delys in the manner just outlined, and then subroutine GAUSS solves for the values of dely. Subroutine STEPPER steps forward in time by incrementing x and y. Subroutine SPECS sets the values of the parameters of the problem, converting units where necessary, and PRINTER writes the results to a file for plotting. 

Other parameters of the simulation are specified in subroutine SPECS. The quantity solcon is the solar constant, available here for tuning within observational limits of uncertainty. The quantity diffc is the heat transport coefficient, a freely tunable parameter. The quantity odhc is the depth in the ocean to which the seasonal temperature variation penetrates. In this annual average simulation, it simply controls how rapidly the temperature relaxes into a steady-state value. In the seasonal calculations carried out later in this chapter it controls the amplitude of the seasonal oscillation of temperature. The quantity hcrat is the amount by which ocean heat capacity is divided to get the much smaller effective heat capacity of the land. The quantity hcconst converts the heat exchange depth of the ocean into the appropriate units for calculations in terms of watts per square meter. The quantity secpy is the number of seconds in a year. 

Use the subroutine LONGD given in Table 18.2 to find the response of the closedloop system of Example 18.4 to a unit step load disturbance. Use values of... 

To gain the maximum benefit from the use of a flowsheet program, the operator/designer must be adequately trained. A suitable program will have 20-30 standard units available, numerous equation-solving procedures, control facilities and probably optimization facilities. The unit-equipment subroutine must adequately represent the process equipment, recycle streams need to be specified, and suitable solution convergence is required. For the effective use of CAD packages, it... 

The FCC simulator program was converted to subroutine form a few years ago and incorporated into a nonlinear programming model representing a complex of process units in the Toledo refinery. It is this subroutine version which has been linked with the LP preprocessor. 

WATEQ2 consists of a main program and 12 subroutines and is patterned similarly to WATEQF ( ). WATEQ2 (the main program) uses input data to set the bounds of all major arrays and calls most of the other procedures. INTABLE reads the thermodynamic data base and prints the thermodynamic data and other pertinent information, such as analytical expressions for effect of temperature on selected equilibrium constants. PREP reads the analytical data, converts concentrations to the required units, calculates temperature-dependent coefficients for the Debye-HKckel equation, and tests for charge balance of the input data. SET initializes values of individual species for the iterative mass action-mass balance calculations, and calculates the equilibrium constants as a function of the input temperature. MAJ EL calculates the activity coefficients and, on the first iteration only, does a partial speciation of the major anions, and performs mass action-mass balance calculations on Li, Cs, Rb, Ba, Sr and the major cations. TR EL performs these calculations on the minor cations, Mn, Cu, Zn, Cd, Pb, Ni, Ag, and As. SUMS performs the anion mass... 

---