Principles of Steady-State Flowsheet Simulation

Steady-State Simulation of Chemical Process

Principles of Batch Flowsheet Simulation

Process and simulation flowsheetDescribe an existing process or hypothetical process in

sufficient detail to convey the essential featuresSimulation is the tool chemical engineers use to interpret

process flowsheets, to locate malfunctions and to predict the performance of processes In order of increasing complexity, involve: material

balances, material and energy balances and equipment sizingMost basic chemicals processes involve conventional

process equipments: heat exchangers, pumps, distillation columns, absorbers and so onDesign team need to prepare one or more equation solving

algorithms for each process unit to solve the material and energy balances and to compute equipment sizes.

1. Process flowsheet

Collection of icons to represent process units and arcs (flowrates, temperature, pressure, enthalpy, etc..) to represent the flow materials to and from the units

2. Simulation flowsheet

Collection of simulation units to represent computer programs that simulate the process units and arcs to represent the flow of information among the simulation units

To convert from a process flowsheet to a simulation flowsheet, one replaces the process units with appropriate simulation unit

Each of the simulators has an extensive list of subroutines (blocks or models) to model and solve the process unit equations

In most simulators, new subroutines may be programmed by a user and inserted into the library

The steady-state models in simulators do not solve time-dependent equations. They simulate the steady-state operation of process units and estimate size and cost of process units

When using process simulators, it is important to recognise that, some exceptions, most stream are comprised of chemical species that distribute within one or more solution phases that are assumed to be in phase equilibrium

Process Flowsheet

Simulation Flowsheet

1. Degree of Freedom (DoF)

DoF analysis is incorporated in the development of each subroutine that simulates a process unit

The subroutines solve sets of NEquation involving NVariables where NEquation < Nvariables

ND = Nvariables - Nequation

2. Bidirectional information flow

Material and energy balances for the process units are solved given specifications for the inlet streams and equipment parameters, along with selected variables of the outlet streams

The unknown variables to be computed are usually those of the outlet stream (flowrates and compositions)

Whenever stream variable is altered – adjacent process units are recomputed. This causes the information to flow in parallel to the material stream when a unit downstream is recomputed, or opposite to the material stream when unit upstream is recomputed

Unit Subroutines (Cont.)

3. Control blocks – design specifications All simulators provide a facility for iterative adjustment of the

variables and parameters that are permitted to be specified so as to achieve the desired specifications

Guesses are made (manipulated variables) – simulation calculation are performed – control subroutine compares the calculated value with desired specification (set points) – errors detected – control subroutine prepares a new guesses using numerical method

The procedure is analogous to that performed by feedback controllers – it is common to refer to these convergence as feedback control subroutines

4. Calculation order In process simulators, unit are computed one in a time –

calculation order is automatically computed to be consistent with flow of information in simulation flowsheet

Variables of process feed streams are specified and information flow parallel to the material flows (calculation proceed from unit to unit)

1. Simpler distribution – fractional conversion (extents of reaction) are known, split reaction are specified, and no purge streams, flowrates of the species in the recycle streams can be calculated directly (without iteration)

2. Reaction operation – involve reversible reactions (competing reaction), split fraction of species leaving separators are complex function of operating conditions and purge streams exist, iterative calculation are necessary

3. Simulation flowsheets usually contain information recycle loop –cycles for which too few stream variables are known to permit equation for each unit to be solve independently

4. Tear stream guesses – one solution technique in recycle loop –information passes from unit to unit until new value of variables of tear stream are computed, new values are used to repeat calculation until convergence tolerances are satisfied

5. In process simulator, recycle convergence unit are inserted into tear stream – convergence unit use convergence subroutines to compare newly computed variables with guessed value and to compute new guess value when two stream are not identical

heater Mixer Reactor DISTL

Conv’ unit

(Recycle)

Newly computed

variables Guess Value

1. Involves separation of mixture consisting of HCl, benzene and monochlorobenzene (MCB), effluent from reactor to produce MCB by chlorination of benzene

2. To obtain nearly pure HCl, benzene and MCB can be absorbed in absorber

3. Benzene and MCB have significantly different boiling points, they can be separated by distillation

Monochlorobenzene Separation Process

Feed is partially vaporised in preheater, H1, separated into two phases in flash vessel, F1. The vapour from F1 sent to absorber, A1, where most of HCl vapour passes through, but benzene is largely absorbed using recycled MCB as absorbent. Liquid effluent from F1 and A1 are combined, treated to remove remaining HCl with significant losses of benzene and MCB, distilled in D1 to separate benzene from MCB. Distillate rate is set equal to benzene flowratein feed in D1, reflux ratio is adjusted to obtain indicate MCB impurity in distillate. Bottoms are cooled to 120oF in heat exchanger, H2, after one third of the bottom is removed as MCB product, with remain two third recycled to absorber. Fraction recycle is specified during distribution of chemicals in process synthesis, with temperature of recycle, in an attempt to absorb benzene without sizable amount of HCl. Temperature of stream S02 is specified to generate adequate amount of vapour, 3 equilibrium stages are judged to be sufficient for the absorber, and number of stages and reflux ratio are estimated for distillation column.

1. The simulator used: ASPEN PLUS

2. Unknown temperatures and flowrates determined by satisfying the mass and energy balances

3. MCB separation process is simulated in steady state

4. By using simulator, it also calculate equipment size and estimate capital cost

5. Then, profitability will be performed

6. Process controllers are added and their response to various disturbances are computed using HYSYS in dynamic mode

1. Task-integration step of process synthesis – equipment items are selected, key decision are made regarding whether they operate in continuous, batch or semicontinuous modes

2. When throughput is small (on laboratory scale) continuous operation is difficult and impractical to maintain, it is simpler and more profitable to complete a batch in hours, days or weeks

3. Larger throughput – multiple products are produced, with variably sized orders received regularly, batch processes offer ease of switching from production of one product to another – flexibility –which is more difficult to achieve in continuous operation

4. Batch process simulator – BATCH PLUS; SUPERPRO DESIGNER and etc

5. Carried out mass and energy balances, prepare an operating schedule in form of Gantt chart for process

6. After the equipment and operating cost is estimated, profitability measure are computed, batch operating parameters and procedures can be varied to increase the profitability of the design

1. Steady-state simulation of continuous processes –convenient to convert from a process flowsheet to simulation flowsheet

2. Simulation flowsheet – arcs represent stream that convey the batches from equipment item to equipment item

3. Each arc bears stream name and represent transfer of information associated with each stream – mass of each species per batch, temperature, pressure, density and other physical properties

4. Icon represent models for each of the equipment items5. Simulation of batch processes involve sequence of

process operations, which specified by designer 6. These operations are defined as recipe or campaign for

each equipment item, involve charging chemical into vessel, processing the chemical removing chemical from vessel and cleaning vessel

1. Refer to textbook pg 140 table 5.2 (old version – pg 147 table 4.2)

2. For each equipment, engineer must specified the details of its operation –specification of charging, processing, emptying and cleaning

1. SUPERPRO DESIGNER Flowsheet is defined by engineer either batch or continuousBatch mode – stream results are reported on per-batch

basis, even for stream associated with continuous process in batch flowsheetEach equipment designated as operating in

batch/semicontinuous or continuous modeScheduling information must be included for all items

designated as operating as batch/semicontinuousSemicontinuous units operate continuously while utilised

but shut down between usesEquipment items designated as continuous are assumed to

operate at all time and excluded from operation scheduleSUPERPRO in continuous mode – stream are reported per-

hour basis, scheduling information not required, no overall batch time is calculated

2. BATCH PLUS Stream values are reported on per-batch basisContinuous operations can be inserted A feed is loaded, vessel is filled to its surge volume, and

effluent stream immediately begins to transfer product downstream

3. SUPERPRO and BATCH Caution must be exercised when introducing continuous

operation into batch processes – no warning are provided when continuous process unit is running dry

When feed to continuous unit runs dry, simulator assumes that this unit is shut down and restarted when feed returns –this operation is infeasible for many units such as distillation column and chemical reactors

Advantage of adding continuous operation arises when process bottleneck is transferred to continuous unit. Continuous unit is always in operation, batch cycling is avoided