Component 92: Desalination - MSF-Stage

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Component 92: Desalination - MSF-Stage

Specifications

Line connections
1	Salt water pre-heating inlet
2	Salt water pre-heating outlet
3	Distillate inlet
4	Distillate outlet
5	Salt water inlet
6	Salt water outlet
7	Steam inlet
8	Steam outlet
9	Pressure specification

General User Input Values Characteristic Lines Physics Used Displays Example

General

Component 92 can model one stage of an MSF-Unit (Multi-Stage Flashing Desalination). The module is able to describe the evaporation of a brine released in the stage as well as of a distillate flashed in the MSF-Stage and the heat transfer from condensed steam to the primary medium (salt water pre-heating). In addition, the losses due to an increase in the flashing brine temperature with respect to the ideal equilibrium conditions (non-equilibrium allowance), pressure drop in demister and the steam losses due to vacuum system can be accounted for. Two calculation modes are possible. Either the steam pressure in the flashing tank P9 is specified and k*A is calculated, or the component computes the pressure P9 using k*A.

The computed mass flow of condensed steam is additionally sent to the line connection 9. Thus, this mass flow can be used as a parameter for the adaption polynomial.

Caution!!! There are obvious physical boundaries for specifying the steam pressure P9. The specified pressure P9 can not be higher than the saturation pressure for the enthalpy H5. On the other hand, if the specified pressure is too low, it can result in a negative temperature difference in condenser.

Pressure drop limitations in off-design (Extras --> Model Options--> Calculation -->Relative pressure-drop maximum) :
As the pressure drop rises quadratically with the mass flow, pressure drops that are significantly too high can quickly arise in the event of a transgression of the nominal mass flow. These will then cause phase transitions and convergence problems. For this reason, pressure drop limitations have been installed.

User Input Values

DP12N	Primary pressure loss (nominal)
TOL	Accuracy of the energy balance
DQLR	Heat loss by radiation to the surroundings (relative to the transferring flow)
FMODE	Flag for calculation mode 0: set as global 1: local off-design (i.e. always off-design mode, even if design is set globally) 2: Special local off-design (special case for compatibility with earlier Ebsilon-versions, not to be used in new models, because the results of the real off-design calculations are not consistent)
FSPEC	Flag for setting, which values will be specified and which values will be calculated Like in Parent Profile (Sub Profile option only) Expression 0: P9 given (Identification of kA) 1: P9 calculated from kA
FADAPT	Flag for using the adaptation polynomial / adaptation function Like in Parent Profile (Sub Profile option only) Expression =0: not used and not evaluated =1: Correction for kA [KA = KAN char line factor * polynomial] =2: Calculation of kA [KA = KAN polynomial] =1000: Not used, but ADAPT evaluated as RADAPT (Reduction of the computing time) = -1: Correction for kA [KA = KAN char line factor * function] = -2: Calculation of kA [KA = KAN function] = -1000: Not used, but EADAPT evaluated as RADAPT (Reduction of the computing time)
EADAPT	Adaptation function
FVALKA	Validation of kA Like in Parent Profile (Sub Profile option only) Expression =0: KAN used without validation =1: Pseudo measurement point identified by IPS used (can be validated) , e.g.. time. not available*
IPS	Index for pseudo measurement point
DTLOSS	Temperature loss owing to the thermal non-equilibrium
DPLOSS	Pressure loss in the demister
FTYPL8	Flag for specifying the mass flow of steam to the vacuum system Like in Parent Profile (Sub Profile option only) Expression 0: M8 as fraction of the total steam mass flow 1: M8 given from outside
M8MST	Steam fraction to the vacuum system (ignored, if FTYPL8=1)
DUM2	Spec value dummy 2
DUM3	Spec value dummy 3
KAN	kA (Heat transfer coefficient area, nominal)
M1N	Primary mass flow (nominal)
MSTN	Steam mass flow (nominal)
VSTN	Steam volume flow (nominal)
P9N	Stage pressure (nominal)
QN	Heat transferred at nominal load (nominal)

The identification value marked in blue is the reference value for the off-design mode. The actual off-design values refer to the values used in the equations.

Generally, all inputs that are visible are required. But, often default values are provided.

For more information on colour of the input fields and their descriptions see Edit Component\Specification values

For more information on design vs. off-design and nominal values see General\Accept Nominal values

Characteristic Lines

There are two characteristic lines, which describe the effect of the primary mass flow or that of the secondary mass flow on k*A. Multiplication of both the influencing factors results in the correction factor for k*A.

1^st Characteristic line FK1 = f (M1/M1N)
2^nd Characteristic line FK2 = f (MST/MSTN)

Total: (K*A)/(K*A)N = FK1 * FK2

Characteristic line 1, CKAM1: (k*A)-characteristic line: (k*A)1/(k*A)N = f (M1/M1N)

     X-axis    1        M1/M1N                    1^st point
                    2        M1/M1N                    2^nd point
                    .
                    N        M1/M1N                    last point

     Y-axis     1        (k*A)1/(k*A)N            1^st point
                    2        (k*A)1/(k*A)N             2^nd point
                    .
                    N        (k*A)1/(k*A)N            last point

Characteristic line 2, CKAMST: (k*A)-characteristic line: (k*A)2/(k*A)N = f (MST/MSTN)

     X-axis    1        MST/MSTN                 1^st point
                    2        MST/MSTN                 2^nd point
                    .
                    N        MST/MSTN                 last point

     Y-axis     1        (k*A)2/(k*A)N            1^st point
                    2        (k*A)2/(k*A)N             2^nd point
                    .
                    N        (k*A)2/(k*A)N              last point

Physics Used

Equations

Design

(Simulation flag:

GLOBAL = Design

and

FMODE = Design)

P2 = P1-DP12N

Off-design

(Simulation flag:

GLOBAL = Off-design

FMODE = Off-design)

F1 = (M1/M1N) ** 2
P2 = P1 - DP12N * F1

All cases

Simplification: The salt content of the brine is assumed to be constant within the stage. The value of the salt content WSALT is set as equal to the salt content at the salt water outlet (PIN 6).

Pressure balance:

FSPEC=0: P9 set from outside

FSPEC=1: P9 calculated from k*A (for k*A characteristic line)

P1, P5, P3, P7-set from outside

P6 = PM (Calculation of PM from equation (1))

P4 = P8 = P9 - DPDEM

Flashing:

A virtual pressure PM is necessary in order to compute the amount of steam MSTBRINE arising by flashing of brine under influence of thermal non-equilibrium conditions:

PM = PSAT [(TSAT(P9,WSALT) + DTLOSS),WSALT] (1)

Steam fraction in the brine:

XBRINE = (H5 - H’(PM,WSALT)) / (HSTBRINE - H’(PM,WSALT))

Steam enthalpy of the evaporated brine:

HSTBRINE = H(P9,TSAT(P9,WSALT)) (2)

Steam fraction in the distillate:

XDEST = (H3 - H’(P9-DPDEM)) / (H’’(P9-DPDEM) - H’(P9-DPDEM))

Calculation of the steam quantity and steam enthalpy arising during flashing:

Distillate:

IF(XDEST = 1) => MSTDEST = M3, HSTDEST = H’’(P9-DPDEM)

IF(XDEST = 0) => MSTDEST=0, HSTDEST=H3

IF(0 < XDEST < 1) => MSTDEST = M3 * XDEST, HSTDEST = H’’(P9-DPDEM)

Brine:

IF(XBRINE = 1) => MSTBRINE=M5, HSTBRINE out of equation (2).

IF(XBRINE = 0) => MSTBRINE=0, HSTBRINE=H5

IF(0 < XBRINE < 1) => MSTBRINE = M5*XBRINE, HSTBRINE out of equation (2).

Mass balance:

M1, M3, M5, M7 set from outside

M2 = M1

M4 = M3 + M7 + MSTBRINE - M8

M6 = M5 - MSTBRINE

FTYPL8=0: M8 set from outside

FTYPL8=1: M8=M8MST*MST

MST = M7 + MSTDEST + MSTBRINE

Salt content:

WSALT = WSALT6 = WSALT5 * M5/M6

WSALT2 = WSALT1

Energy balance:

H1, H3, H5, H7 given from outside

Q5 = H5 * M5

QDBRINE = MSTBRINE * HSTBRINE

H6 = (Q5 - QDBRINE) / M6

H4 = H'(P9 - DPDEM)

H8 = H’’(P9 - DPDEM)

Q3 = H3 * M3

Q4 = H4 * M4

Q6 = H6 * M6

Q7 = H7 * M7

Q8 = H8 * M8

DQ = (Q3 + Q5 + Q7 - Q4 - Q6 - Q8) * (1 - DQLR)

Q2 = Q1 + DQ

H2 = Q2 / M2

Heat transfer:

DTL = TSAT(P9 - DPDEM) - T1

DTU = TSAT(P9 - DPDEM) - T2

LMTD = (DTU - DTL) / (ln(DTU) - ln(DTL))

KA = DQ / LMTD

Component Displays

Display Option 1

Example

Click here >> Component_92 Demo << to load an example.