Reverse Osmosis with Energy Recovery

Introduction

This flowsheet represents a reverse osmosis (RO) with energy recovery device (ERD). The flowsheet is similar to the desalination block of the full-scale seawater RO treatment facility flowsheet, but includes a demonstration for optimization of the process. The user can select between pressure exchanger ERD, pump-as-turbine ERD, or no ERD.

Implementation

This flowsheet uses several different modeling features available in WaterTAP, including:

Costing packages:

• WaterTAP Costing Package

• Detailed Unit Model Costing

Property models:

• NaCl Property Package

Unit models:

• Reverse Osmosis (0D)

• Pressure Exchanger

• Product Block

• Separator

• Mixer

The demonstration file itself contains several core functions that are used to build, specify, initialize, solve, and (optionally) optimize the model, as well as helper functions that group these core functions together for convenience or to display results.

• Creating and instantiating the model using build():

  This function will create the core components and structure of the flowsheet including unit, property, and costing models. Different components will be added depending on the type of ERD used (dictated by the erd_type argument). An Arc is used to connect the different component flows to one another and default scaling factors are set.

• Specify the operating conditions with set_operating_conditions():

  Influent component flows and RO pump operating pressures are set with this function. The user can set via keyword arguments the volumetric flow rate (flow_vol), mass fraction of NaCl (salt_mass_conc), a target fractional mass-based water recovery water_recovery, and a fractional over pressure (over_pressure). Over pressure is the fractional increase in pressure over of the brine osmotic pressure to use as the an initial guess for the RO operating pressure as determined by the helper function calculate_operating_pressure. Initial values for relevant RO membrane parameters are provided in the Flowsheet Specifications section. After the RO unit is initialized, the area is unfixed and the mass-based recovery is fixed to the user provided value.

• Calculating the RO operating pressure with calculate_operating_pressure():

  This helper functions takes in the RO feed state block and will calculate the needed operating pressure to achieve the desired mass-based water recovery.

• Initialize entire system with initialize_system():

  After the model is built and specified, this function will initialize the feed, RO, and costing blocks. Depending on the erd_type different helper functions are used to initialize the relevant ERD-related unit models.

• Optionally optimize the system with optimize_set_up():

  The model is fully defined and can be solved with the provided solve() function after the user passes the model through initialize_system(). This function is provided as a demonstration of how the system could be optimized the minimize the levelized cost of water (LCOW). The optimization routine proceeds as follows:

  1. An Objectve is placed on the flowsheet set to minimize the LCOW expression in the costing package.

  2. Operating pressure for the RO booster pump is unfixed and realistic bounds are placed on the pressure.

  3. Similarly, the RO membrane area is unfixed and bounds are placed on the membrane area.

  4. To ensure the product water is still of acceptable quality, a Constraint is placed on the effluent concentration of the RO to be less than 500 mg/L.

  5. Additionally, there is a minimum water flux placed on the RO model to be over 2.8e-4 kg/m \(\text{}^2\)/s.

There are other helper functions, like display_system(), display_design(), and display_state(), that are used to print out the results of the model solve. The main() function is an example of building, specifying, initializing, and optimizing an RO-ERD system with WaterTAP.

Figure 1 presents the process flow diagram if erd_type == pressure_exchanger.

Figure 1: Process flow for RO with pressure exchanger ERD.

Figure 2 presents the process flow diagram if erd_type == pump_as_turbine.

Figure 2: Process flow for RO with pump-as-turbine ERD.

Figure 3 presents the process flow diagram with no ERD (i.e., erd_type == no_ERD).

Figure 3: Process flow for RO without ERD.

Degrees of Freedom

The degrees of freedom (DOF) for the flowsheet can change depending on model configuration options. For either pump_as_turbine or pressure_exchanger as erd_type, there are 15 DOF. Running the model with no_ERD results in 13 DOF.

• Influent conditions (component flows, temperature, pressure)

• RO membrane properties

• RO operating pressure

• Pump and ERD efficiencies

Passing any model build to the provided function set_operating_conditions() will result in a model with zero DOF.

Flowsheet Specifications

The influent and operating conditions for the different flowsheet configurations are presented in the following table, including the different build options for erd_type:

Description	Default Value	Units
Influent Conditions
Volumetric flow rate	1e-3	\(\text{m}^3/\text{s}\)
TDS mass fraction	0.035	\(\text{dimensionless}\)
Temperature	298	\(\text{K}\)
Pressure	101325	\(\text{Pa}\)
Desalination
RO water permeability coefficient	4.2e-12	\(\text{m/Pa/s}\)
RO salt permeability coefficient	3.5e-8	\(\text{m/s}\)
RO spacer porosity	0.85	\(\text{dimensionless}\)
RO channel height	1e-3	\(\text{m}\)
RO membrane width per stage	5	\(\text{m}\)
RO total membrane area per stage	50	\(\text{m}^2\)
RO permeate side pressure	101325	\(\text{Pa}\)
Pump 1 efficiency	0.8	\(\text{dimensionless}\)
Pump 1 operating pressure	70e5	\(\text{Pa}\)
if erd_type == "pressure_exchanger"
Pressure exchanger efficiency	0.95	\(\text{dimensionless}\)
Pump 2 efficiency	0.8	\(\text{dimensionless}\)
if erd_type == "pump_as_turbine"
Energy recovery device pump efficiency	0.95	\(\text{dimensionless}\)
Energy recovery device permeate side pressure	101325	\(\text{Pa}\)

Code Documentation

• watertap.examples.flowsheets.RO_with_energy_recovery