Tutotrial: Thermal crystallizer (TCr) unit

TCr is a thermal based process that is used to crystallize salt. In this work, TCr aims to recover high quality water from brine solutions and crystallize salts.

In desalsim package, the TCr unit is used to model the operation of a Thermal crystallizer technology. Upon simulation, it will generate the influent/effluent mass flows and their concentrations, the cooling water, and the energy requirements. The TCr function consists of three classes: thermal_calc, conc_cal, calculate_energy. In this tutorial, we will focus on how to use the classes and their methods. Table 1 gives an overview of the main inputs and outputs for each process unit of Thermal crystallizer.

Table 1. Overview of the main inputs and outputs of Thermal crystallizer.

Process	Input	Output
Thermal crystallizer	Feed flow rate [m³/h]	Flow rate of water [kg/h]
	Ion concentration [g/L]	Flow rate of NaCl [kg/h]
	Feed temperature [°C]	Cooling water flow rate [m³/h]
	Steam temperature [°C]	Electrical [kWhel] and thermal [kWhth] requirements

The mathematical description of Thermal Crystallizer technology is given in Mathematical description, see Section A.3.

1. Getting started

1.1. Import class

import desalsim

Then import the class:

from desalsim.thermal_cryst_f import thermal_calc
from desalsim.thermal_cryst_f import conc_cal
from desalsim.thermal_cryst_f import calculate_energy

Additionally, function for calculating density (density_calc.py) need to be imported or constants (comparison.py) where user can add constant values like MW, prices etc, need to be imported.

from desalsim.density_calc import density_calc
from desalsim import constants
from desalsim import scaleup

1.2. Define feed characteristics

You can initialize the feed solution by setting the flow rate, specifying the focus components and their concentration.

# Feed concentration
components = ['Na', 'Cl', 'K', 'Mg', 'Ca', 'SO4']
Cf_tcr_in = [80.42, 116.69, 2.29, 0.01, 0.04, 0.54]
# Total salt concentration (g/L)
Cf_s = sum(Cf_tcr_in)

# CaSO4 concentration in feed solution (g/L)
Cf_caso4 = Cf_tcr_in[4] * MW_Caso4 / MW_Ca

# Feed flow rate
Qf = 1000 # kg/h

# input conditions
T_in = 40 # °C

Note

Note that if you want to add more components, you need to update the components list and include the concentration of the new component in the Cf_tcr_in.

You can calculate the density of the feed solution:

d_sol = density_calc(T_in, Cf_s) / 1000 # density of feed

1.3. Set operating assumptions

You need to set operating assumptions related to temperatures such as the temperature in the last effect, the intake/outake cooling water temperature. Additionally, the moisture in the salt stream needs to be set.

# Assumptions:
T_in = 40 # (°C)
T_cw_f = 25 # intake cooling water temperature (°C)
T_cw_o = 40 # out cooling water temperature (°C)
T_op = 60 # Operating temperature (°C)

salt_mois = 20 # % moisture in salt stream (units: %)

Thermodynamic data

The latent heat of motive steam and vapor is calculated based on steam tables.

# Heat Transfer Constant
UA = 45990
LHV_v = 2199.7 # 2107.92 # kJ/kg (gathered from steam table)
LHV_s = 2357.69 # kJ/kg (gathered from steam table)

Finally, you need to set assumptions related to pumping like pressure drop (dp) and pump efficiency (npump).

dp = 0.1 # pressure drop (units: bar)
dp_slurry = 3.5 # pressure drop for slurry streams (units: bar)
dp_f = 3.5 # pressure drop for feed (units: bar)
dp_w = 1 # pressure drop for water streams (units: bar)
dp_cw = 2 # pressure drop for cooling water (units: bar)

npump = 0.8 # pump efficiency (units: -)

1.4. Set constants

You need to set constant parameters like the specific heat capacity of water.

Cp_f = 3.14 # Feed specific heat capacity (units: KJ * kg * °C)
CP_cw = 4.187 # Water specific heat capacity (units: KJ * kg * °C)

After setting all the required inputs, then you can create the functions’ objectives.

2. Use thermal_calc class

thermal_calc is a class used to represent Mass Balance for Thermal Crystallizer Unit. In particular, it calculates the evaporated mass and mass of recovered solid, the cooling water requirements and the energy requirements. thermal_calc takes as input the operating temperature (T_op), the inlet flow rate (Qf), the total ion concentration of solution (Cf_s), the concentration of CaSO4 in the solution (Cf_caso4), the inlet feed temperature (T_in), the list of concentration of ions in the solution (Cf_in), the salt moisture in slurry stream (salt_mois),

the latent heat of vapor (LHV_v), the latent heat of steam (LHV_s), and the temperature of intake/outake cooling water (T_cw_o, T_cw_f).

2.1. Overview

The following attributes are available within the thermal_calc class:

• T_op: Operating temperature (°C)

• Qf: Inlet flow rate (kg/hr)

• Cf_s: Total ion concentration of solution (g/L)

• Cf_caso4: Concentration of CaSO4 in the solution (g/L)

• T_in: Inlet feed temperature (°C)

• d_sol: Inlet feed density (kg/L)

• Cf_in: List of concentration of ions in the solution (g/L)

• nacl_sat_wt: Weight percent of NaCl at saturation

• nacl_sol: NaCl in the solution (kg/h)

• salt_solids: Salt solids (kg/h)

• caso4_sol: CaSO4 in the solution (kg/h)

• solid_mass: Solid mass (kg/h)

• ev_mass: Evaporated mass (kg/h)

• BPE: Boiling point elevation (°C)

• heat_req: Heat required (kJ/h)

• T_s: Steam temperature (°C)

• steam_mass: Steam mass (kg/h)

• cw_mass: Cooling water mass (kg/h)

The thermal_calc class provides the following two methods:

# This method calculates the mass balance for thermal crystallization
mass_bal_cryst()

# This method calculates the heat balance for thermal crystallization
heat_bal_cryst()

They are called upon initialization of the class instance.

2.2. Create thermal_calc objects

thermal_calc takes as input the operating temperature (T_op), the inlet flow rate (Qf), the total ion concentration of solution (Cf_s), the concentration of CaSO4 in the solution (Cf_caso4), the inlet feed temperature (T_in), the list of concentration of ions in the solution (Cf_in), the salt moisture in slurry stream (salt_mois), the latent heat of vapor (LHV_v), the latent heat of steam (LHV_s), and the temperature of intake/outake cooling water (T_cw_o, T_cw_f).

# Create an instance of the thermal_calc class
th_cryst_dat = thermal_calc(T_op, Qf, Cf_s, Cf_caso4, T_in, Cf_tcr_in, salt_mois, LHV_v, LHV_s, T_cw_o, T_cw_f)

2.3. Use mass_bal_cryst method

The method calculates the mass balance for thermal crystallization. In particular, it calculates the recovered salt flow rate (solid_mass) and the evaporation mass flow rate (ev_mass).

th_cryst_dat.mass_bal_cryst()

It doesn’t take additional inputs.

2.3.1. Assigned the results to output parameters

After the mass calculation, you can assigned the results to output parameters:

# Recovered salt flow rate (kg/h)
M_Nacl=th_cryst_dat.solid_mass

# Evaporation mass flow rate (water recovery), (kg/h)
Q_evap_mass=th_cryst_dat.ev_mass

2.4. Use heat_bal_cryst method

The method calculates the energy balance for thermal crystallization. In particular, it calculates the steam_mass (steam_mass), and the total distillate flow rate (cw_mass) and required heat (heat_req).

th_cryst_dat.heat_bal_cryst()

It doesn’t take additional inputs.

2.4.1. Assigned the results to output parameters

After the mass calculation, you can assigned the results to output parameters:

#Cooling water requirements (kg/h)
Qcw_tcr=th_cryst_dat.cw_mass

#Calculate energy consumption
heat_req=th_cryst_dat.heat_req

2.5. Print results

You can print results from mass and energy calculations.

print("Mass flowrate of recovered salt is "+str(round(M_Nacl,2))+"kg/hr")
print("Mass flowrate of recovered water is "+str(round(Q_evap_mass,2))+"kg/hr")

#Cooling water requirements (kg/h)
Qcw_tcr=th_cryst_dat.cw_mass

Mass flowrate of recovered salt is 0.22kg/hr Mass flowrate of recovered water is 999.78kg/hr

Mass flowrate of required cooling water is 35016.66kg/hr

3. Use conc_cal class

conc_cal is a class used to calculate the concentration of different ions in a solution. For this calculation, you need to use the solution flow rate (Qf), the mass of solid stream (solid_mass), the name of ion and its concentration in the stream (C1, Cc1, C2, Cc2, C3, Cc3, C4, Cc4, C5, Cc5, C6, Cc6).

3.1. Oveview

The following attributes are available within the conc_cal class:

• Qf: Flow rate (units: m³/h)

• solid_mass: Mass of solid stream (units: kg)

• C1, C2, C3, C4, C5, C6: Names of six different ions

• Cc1, Cc2, Cc3, Cc4, Cc5, Cc6: Concentrations of six different ions (units: ppm)

• d_sol: Density of the solution at 40°C (units: kg/m³)

The conc_cal class provides the following method:

conc_cal()

This class consists of three methods that calculate the concentration of different salts in the solution (conc_salt_comp) and the the concentration of different salts in the outlet stream (salt_conc).

3.2. Create conc_cal objectives and call functions

# Calculation of the concentration of different ions in the solution
th_cryst_dat_2=conc_cal(Qf, M_Nacl , 'Na',Cf_tcr_in[0], 'cl',Cf_tcr_in[1],'k', Cf_tcr_in[2], 'mg', Cf_tcr_in[3], 'ca', Cf_tcr_in[4], 'so4', Cf_tcr_in[5], T_in)
   th_cryst_dat_2.molarity()
   th_cryst_dat_2.conc_salt_comp()
   th_cryst_dat_2.salt_conc()

3.2. Assigned the results to output parameters

You can assigned the results to output parameters:

#Salt stream concentration (g/L)
Csalt_out=[th_cryst_dat_2.CNa,th_cryst_dat_2.CCl, th_cryst_dat_2.CK, th_cryst_dat_2.CMg, th_cryst_dat_2.CCa, th_cryst_dat_2.CSO4]

4. Use calculate_energy class

calculate_energy is a class used to represent the calculation of energy consumption and the specific energy consumption for Thermal Crystallizer Unit. For this calculation, the flow rate of the feed solution (Qf), the distillate flow rate (Q_evap_mass), flow rate of cooling water (Qcw), the mass of recovered NaCl (M_Nacl), the required heat (heat_req) calculated in thermal_calc, density of the feed solution (d_sol) are used. In addition, the calculate_energy takes as input the expected pressure drop in cooling water stream, slurry stream and water stream (dp_cw, dp_slurry, dp_w) and the pump efficiency (npump).

4.1. Oveview

The following attributes are available within the calculate_energy class:

• Qf: Flow rate of the feed solution (m³/h).

• Q_evap_mass: Distillate flow rate (m³/h).

• Qcw: Flow rate of cooling water (m³/h).

• d_sol: Density of the feed solution (kg/m³).

• npump: Pump efficiency.

The calculate_energy class provides the following method:

calculate_energy_consumption()

This method returns the the Specific energy consumption for electrical energy (SEC_el), Specific energy consumption for electrical energy per unit mass of evaporated water in kWh/kg (SEC_el_prod), Specific energy consumption for electrical energy per unit mass of NaCl produced in kWh/kg (SEC_el_prod2)

4.2. Create nf_energy objectives, calculate Energy Consumption and assign the results to output parameters

The following objective is created for energy consumption. Assumptions for pressure drop and pump efficiency need to be made.

E_el_th_Cr, E_th_th_Cr, SEC_el_f, SEC_el_NaCl, SEC_el_w = calculate_energy(Qf, Q_evap_mass, Qcw_tcr, M_Nacl, heat_req, d_sol, dp_f, dp_w, dp_slurry, dp_cw, npump)

4.3. Print results

You can print results from energy calculations. The specific energy consumption is also calculated so you can validate easier the results.

print(f"SEC_el_prod is {SEC_el_w} KWh/m3 product")
print(f"SEC_el_prod2 is {SEC_el_NaCl} KWh/kg of NaCl product")
print("SEC_th_prod is "+str( round((E_th_th_Cr/(Qf/1000)),2))+" KWh/m3 intake")

SEC_el_prod is 5.02 KWh/m3 product

SEC_el_prod2 is 23.06 KWh/kg of NaCl product

SEC_th_prod is 628.34 KWh/m3 intake