Modification in energy and productivity of high-temperature variable pressure humidification–compression Open Access

Symbols

F

Feed

h

Enthalpy (kJ/kg)

i

Component i

j

Stage j

L

Liquid Phase

Mass Flow Rate (kg/h)

P

Pressure (bara)

T

Temperature (°C)

V

Vapor Phase

x

Liquid Mole Fraction

y

Vapor Mole Fraction

z

Feed Mole Fraction

Δ

Difference

Subscripts

fg

Liquid–Vapor Phase Change

i

Component i

in

Inlet

j

Stage j

min

Minimum

out

Outlet

Acronyms

CWCA

Close Water–Close Air Cycle

GOR

Gain Output Ratio

HC

Humidification Compression

HDH

Humidification Dehumidification

HETP

Height of Equivalent Theoretical Plate

OWCA

Open Water–Close Air Cycle

PR

Performance Ratio

RR

Recovery Ratio

SEEC

Specific Electrical Energy Consumption

SSC

Specific Steam Consumption

The increase in global population and improving of living standards in recent decades have increased freshwater demand, and new technologies have been introduced to meet this demand (Abdelmoez et al. 2013). It is very important to note that the produced fresh water must be healthy and cheap in order to be usable by the general public, therefore recent studies have focused on increasing water quality and decreasing the water production cost. There exist two main classifications for water sweetening, including membrane technologies and thermal technologies (Yuan et al. 2011).

The humidification dehumidification (HDH) desalination process has a simple mechanism for producing freshwater and is practical for remote areas where freshwater is scarce (Srithar & Rajaseenivasan 2018). The HDH system is a low-temperature process with lower energy quality in comparison with other thermal desalination methods (Parekh et al. 2004). The HDH system is categorized in three groups based on its circulation type (natural or forced circulation, close or open circulation and heating fluid) (Capocelli et al. 2018).

There exist many studies on HDH processes with the objective to increase freshwater production and improve system performance. The influence of natural and forced air circulation in the HDH process was investigated by applying a modified condenser and open water, close air (OWCA) cycles. Cellulose paper was used as a filler in the humidification section. The results revealed that the forced circulating air entering from the bottom of the humidifier has a greater impact on system performance. Also, modification in the condenser increased the condenser effectiveness up to 0.7 (Kabeel et al. 2014). A multi-stage HDH desalination apparatus was presented consisting of four closed water loops. The results indicated that with an increase in both air and water flow rates, the efficiency increases. The maximum performance ratio of the apparatus increased up to 2.6 at the maximum temperature of 85 °C (Wu et al. 2016). The effect of inlet air humidity into the humidifier on desalination performance was investigated. The results illustrated that an increase in inlet air humidity severely affects humidifier performance, while forming a condensation film on the walls (Kassim et al. 2011). A multi-effect HDH process was assessed theoretically and a mathematical model for different components of the system was provided. The results showed that applying the released heat from the condenser increases water productivity and GOR up to 72.6 kg/h and 2.4, respectively (Kang et al. 2014).

The mathematical modeling and simulation of the HDH system are a suitable method for studying the effect of important parameters on the system performance. An HDH system was modeled and analyzed where the saturated air and the collector effects were of concern. The proposed mathematical model consists of the humidification, dehumidification, and solar collector components. The results showed that an increase in heat absorption in the solar collector, an increase in the humidifier column height and a decrease in feed flow rate have a positive effect on the produced freshwater flow rate. Freshwater production error decreased by over 48% in comparison with the previous model (Campos et al. 2017). An HDH system using solar energy (applying heating and electricity energy) was designed and tested in different weather conditions. Energy and economic analyses for this HDH system were studied. The maximum energy efficiency for the system was 31.5%. The best time for the system running was 1–6 pm and the maximum freshwater production rate was 101.4 cubic metres per day (Deniz & Çınar 2016). In experimental and theoretical studies on an HDH desalination system, CWOA was run to clarify its efficiency. The system includes evaporator, heater, regenerator, two solution tanks, condensate collection tank, etc. A model was presented to achieve optimum operating conditions. The effect of performance parameters on the evaporation rate and specific steam consumption (SSC) were illustrated. The results indicated that SSC decreases with an increase in the heat recovery ratio. At different evaporation rates, the minimum volume of SSC was 0.34–0.40 kilograms (per kilogram of evaporated water) (Li et al. 2018). The effects of water heating, external reflectors, and weather condition on system efficiency and performance were studied analytically and experimentally to enhance water productivity. The results revealed that applying water heater and reflector simultaneously improves the daily water productivity in the system (Elminshawy et al. 2015).

To increase GOR and energy recovery, researchers examined the HDH system using heat pumps as heating and cooling sources (Rostamzadeh et al. 2018; Faegh & Shafii 2019). The HDH system with the heat pump was considered to assess its performance. The results showed that maximum GORs of 8.88 and 7.6 are obtained for air/water ratios of 0.6 and 1.3, respectively (Lawal et al. 2018). An HDH system coupled with a heat pump was investigated. System performance under different conditions was evaluated through mathematical models. Power consumption and water recovery were investigated for evaluating the system performance. The results indicated that optimal conditions (RR of 5.8% and SEEC of 335.4 kW·h/m³) can be achieved at T_sw,t = 70 °C, T_fw,t = 20 °C and mr_sw/fw = 1.05 (Dehghani et al. 2018). Another model was developed based on the governing equations to survey the combined system behavior for an HDH system coupled with a heat pump. The results showed that at the best operating conditions, water production rate and GOR are 82.1 kg/h and 5.1, respectively. According to the analytical analysis, the best value for the pressure ratio was 4, which corresponds to the highest GOR of 5.1 (He et al. 2018). The energy recovery in the HDH system through a heat pump, where the condenser acted as a heater in the HDH loop and the evaporator acted as a cooling system for humidification, was examined. The results illustrated the highest water production rate and GOR of 2.8 kg/h and 2.1, respectively (Shafi et al. 2018).

The previous HDH systems are usually designed for atmospheric pressure operations. To increase the air humidity of the outlet stream from the humidifier, the system needs to operate under atmospheric pressure. Variable pressure in the humidification and dehumidification unit is another option for increasing the GOR and system performance. A solar still at low pressures using a vacuum pump was investigated. For this purpose, a double slope solar still was fabricated and tested at different depths and different materials. The performance of the vacuum still was compared with conventional stills. The result showed that a sub-atmospheric pressure of 50 mmHg inside the still increases the production capacity of the still by 50.75% (Sriram et al. 2013). A variable pressure HDH desalination process was proposed, applying a compressor. A parametric study was performed to evaluate the different systems and main variables affecting system performance. The results indicated that the compressor-driven HDH system is of a higher performance than that of the available HDH systems (Narayan et al. 2011). The effect of dehumidification column pressure with bubble flow pattern subjected to different working conditions was investigated. The results indicated that an increase in dehumidifier pressure decreases the system performance, moreover, the system effectiveness decreases by 2% with an increase in humidifier pressure from 1 to 2 bar (Sharqawy & Liu 2015). Two variable-pressure HDH processes using solar heating were simulated. The results showed that under similar conditions, a decrease in humidifier pressure from 0.9 to 0.1 bar leads to an increase in freshwater productivity in both the processes. The maximum GOR for atmospheric dehumidification was 139.1% higher than that of the high-pressure dehumidification, while freshwater production was 5.7% lower (Rahimi-ahar et al. 2018). An HDH system with variable pressure to enhance system performance was studied. The effects of water/air ratio, the pressure of different sections and the system temperature on performance were investigated. The results illustrated the maximum GOR is obtained at 3.8 for a pressure ratio of 1.3 (Siddiqui et al. 2017).

According to the available studies, no investigation was found on improvement of HC desalination process based on energy consumption and water recovery criteria. In this article, in continuation of the previous studies (Ghalavand et al. 2014, 2018), performance evaluation of a new double-stage humidification–compression process where the variable pressure technology is applied at high temperature is discussed. Three arrangements of HC consisting of a single-stage process with a pressure of 1 bara in a humidifier (case A), a single-stage process with a pressure of 0.6 bara in a humidifier (case B) and a new double-stage process (1 bara pressure in the first and 0.6 bara in the second humidifiers) (case C) are considered. The simulated results are in good agreement with the experimental results. The three proposed processes are compared with each other based on water productivity and GOR at a constant value of compression ratio.

The available HC processes have been studied with constant pressure (atmospheric or vacuum) in the humidifier. In this newly proposed process, two different pressures are applied in a simultaneous manner in humidifiers of the HC process to reduce energy consumption and improve the process performance. This process is compared with two processes of atmospheric pressure (1 bara) and of vacuum pressure (0.6 bara) at a constant water flow rate (100 kg/h). The process initially operates at atmospheric pressure humidification and then the humidifier pressure is reduced to 0.6 bara in the second humidifier to improve the evaporation rate. Moreover, to increase the humidity ratio, the two different pressures for humidifiers mentioned above are applied. The main sections of the studied processes consist of humidification, dehumidification, compression and heat recovery.

Aspen HYSYS^® software (based on the equation mentioned in Appendix A) is applied to obtain the operating conditions and parameters in the three processes. The processes are simulated based on the following assumptions and specifications:

• All processes operate at steady-state conditions.

• The equipment is considered adiabatic.

• The only source of consumed energy is electricity.

• The thermal efficiencies of heat exchangers are assumed to be 100%.

• The areas of mass- and heat-transfer in the column are assumed to be equal.

• The polytropic efficiencies of compressors are considered to be 75%.

• The compression ratio is considered equal to 1.67 for all processes.

• The equilibrium efficiency for the humidifier is constant.

• The Peng–Robinson equation of state is applied for property estimation and vapor–liquid equilibrium calculations.

• The humidifier is considered with three theoretical stages.

• HETP for the packing section is considered equal to 500 mm.

• Temperature approaches for the heat exchanger are considered between 7.5 and 15 °C.

• Water to air flow-rate ratio is considered equal to 2.

• Feed water temperature is 25 °C.

For a better comparison, all independent operating conditions for the three processes are the same as according to Table 1. The effect of temperature difference (minimum approach temperature ΔT_min) is studied for all processes. For this purpose, four temperature differences of 7.5, 10, 12.5, and 15 °C for the heat exchanger are investigated.

A single-stage HC process with atmospheric pressure humidification process (case A) is drawn in Figure 1. In this process (based on ΔT = 7.5 °C), seawater enters at 25 °C temperature, 1 bara pressure and 100 kg/h flow rate, next, is heated in the heat exchanger by the released heat from the outlet humid air of a compressor up to 99.6 °C and then enters the humidifier from the top. The air enters the humidifier at 32.5 °C temperature and contacts the seawater in the humidifier, and is humidified and leaves the humidifier with a temperature and flow rate of 96.9 °C and 91.6 kg/h, respectively. This humid air first enters into a centrifuge compressor to be compressed up to 1.67 bara, where its temperature reaches 149.6 °C. Then the compressed air is cooled to 32.5 °C in the heat exchanger and is fed into a flash drum. In this drum, freshwater and air are separated and the air enters into the humidifier column after pressure regulation. Water productivity for this process is 41.2 kg/h.

A single-stage HC process with a humidifier pressure at 0.6 bara (case B) is drawn in Figure 2. Similar to the conventional HC process, the air is humidified and the freshwater and air are separated from each other after compression and cooling. To improve the evaporation rate in the humidifier, the humidification pressure is reduced to 0.6 bara and a pump is added to the process for discharging the brine from the humidifier. Water productivity for this process is 28.7 kg/h.

To investigate the effect of humidifier pressure on increasing air humidity ratio, a double-stage variable pressure HC process is designed. In this newly proposed process, seawater enters the heat exchanger with mass flow rate of 100 kg/h and is preheated by the discharged air from the compressor. This heated water is divided into two equal parts. The first part of the heated water (50 kg/h) enters into the first humidifier at 99.5 °C (first humidifier pressure is 1 bara). In the second humidifier with a pressure of 0.6 bara, the second part of the heated water (50 kg/h) contacts the outlet humid air from the first humidifier. The sub-atmospheric pressure in the second humidifier reduces the water's boiling point, thus, the driving force for humidification is increased. The humid air (81.3 °C) leaves the second humidifier from the top and enters the compressor (pressure ratio = 1.67). The discharged hot air from the compressor (131.5 °C) enters the heat exchanger to become cooled at 32.5 °C. After cooling, the condensate phase is formed and is extracted from the gas phase with a flow rate of 52.4 kg/h as fresh water. Then the separated saturated air enters the first humidifier. Figure 3 shows the scheme for this process.

The qualitative performance curves for these three processes are drawn in Figure 4, where the air humidity and temperature trends through each process are evident.

Before applying the simulation results, to validate their values, the results of the humidifier column are compared with an HC desalination system which is shown in Figure 5.

In the experimental rig, the process is similar to the conventional HC. The saline water is pumped from the storage tank and fed into the humidifier. The air enters into the bottom of the humidifier after heating in the solar heater. The humid air is discharged from the top of the humidifier and enters the compressor. After increasing the air pressure and temperature in the compressor, the air enters the heat exchanger to condense the humidity. Then, the air is separated and returned to the solar heater.

The height of the filled part of the humidifier is 1.5 metres, which corresponds to the three theoretical stages (since the HETP for Raschig Rings is approximately 50 cm). The equations applied in calculating the equilibrium stages are given in Appendix A. The applied packing specifications in the experimental rig are tabulated in Table 2.

The inlet water and air temperatures and the inlet air humidity inside the components are considered as the input parameters, and the values from the software including outlet air and water temperatures and outlet air humidity from the humidifier are considered as the output and are compared with the experimental data. The simulation is run for different operating conditions based on available experimental data for the humidifier. Variations of the operating conditions on air humidity and temperature in the humidifier are assessed and shown in the validation plots. The simulation values and experimental data for the outlet water temperature and outlet air humidity from the humidifier are shown in Figure 6. The well-matched simulation and experimental results with the lowest absolute error percentage validate the simulation. The average absolute error for the outlet air and water temperatures and the outlet air humidity from the humidifier is 7.0%. After validation of the humidifier simulation, parametric study for proposed cases is performed to compare their performance with each other.

Three HC processes with different pressures in the humidifier are investigated. For better comparison, independent operating conditions for all are considered the same. The effect of changes in minimum temperature approach (ΔT_min) on the performance of these proposed processes is investigated. The ΔT_min values of 7.5, 10, 12.5 and 15 °C are considered and the simulation results are tabulated in Table 3 for all processes. It is noted that the cost of electricity is considered based on utility price in South Pars Industrial Zone located at Asaluyeh, Iran.

The preliminary results indicate that case C (double-stage process) is more efficient than the other two due to its maximum values of GOR and fresh water productivity, and minimum power to desalinated water ratio (specific energy consumption) in all ΔT_min. In thermal desalination processes, the minimum temperature approach between the hot and cold side is one of the important parameters in energy saving. A decrease in ΔT_min leads to a decrease in the energy consumption and an increase in the fixed capital cost. In HC processes due to the existing closed air loop, stream conditions change as the ΔT_min changes. An increase in the ΔT_min leads to an increase in inlet air temperature into the humidifier and increasing the humidity content in the outlet air stream at the top of the humidifier subsequently. An increase in the humidity content in the air increases both the air enthalpy and the exchanged energy in the heat exchanger.

As observed in Table 3, at a constant ΔT_min, the changing of the humidifier pressure from 0.6 to 1 bara and eventually applying the double-stage HC process increases the power consumption, while producing higher desalinated water improves the GOR, specific energy consumption and water recovery percentage. This result clarifies the importance of the ΔT_min effect on process performance. It is observed that an increase in ΔT_min leads to an increase in the GOR for the three processes. The only energy source applied in this study is electricity, considered for GOR calculation. As observed in Figure 7 the effect of a change in ΔT_min in case A is more intense than in the two other processes and at all ΔT_min values, the process performance for double-stage method is higher.

The desalination rate and GOR for these three cases at ΔT_min = 7.5 °C are bar-charted in Figure 8, where, as observed, the desalination rate and GOR for case C are higher than for the other cases.

According to the performance curve for case C in Figure 4, it is revealed that, in the double-stage process, air humidity increases in two stages. At the first time, the air humidity increases in the atmospheric humidifier and reaches near-saturation condition. Then the humidification pressure is decreased in the second humidifier to increase the potential of evaporation. In this condition, the air humidity increases for the second time. Outlet humid air from the second humidifier (with high humidity content) is dehumidified and more water volume is gained compared with case A and case B.

The main criteria for evaluation of a desalination process are produced water quality, water productivity and consumed energy per unit of produced water. In the thermal desalination processes, since the produced water is gained based on an evaporation mechanism, the product has high quality. Therefore, the main comparative parameters for the thermal desalination processes are GOR and water productivity. The desalinated water rate (kg/h), GOR and the cost of produced water (US$ /m³) of this study are compared with the available HDH processes in Table 4, where, as observed the double-stage variable pressure HC process (case C) outperforms its counterparts.

The obtained results show that the high-temperature HC process has higher GOR and water productivity. High temperatures in saline water increase equipment corrosion, consequently, in constructing this equipment, it is necessary to apply special materials like Duplex or Super-Duplex steel. Also, it is proposed to add food-grade antiscalant in the feed to avoid fouling in the equipment.

Improving desalination processes to increase water quality and decrease water production cost has been a focus of researchers recently. In general, reducing the pressure in the humidifier and increasing the pressure in the dehumidifier (at constant temperature and constant mass flow ratio) improve HDH process efficiency. In this study, three cases of HC (humidification–compression) processes with different humidifier pressures are investigated. To validate the simulation procedure, a humidifier column is simulated and the results are compared with the available experimental data. The results indicate that the simulated values and the experimental data are in good agreement. The effect of changes in minimum temperature difference (ΔT_min) on performance of these proposed processes is investigated. For this purpose, four values are considered for ΔT_min (7.5, 10, 12.5 and 15 °C) and the simulation is run with Aspen HYSYS^® software. The obtained results indicate that an increase in ΔT_min in the three cases increases the volume of produced desalinated water and GOR. Moreover, subject to similar conditions, the volume of desalinated water (66.08 kg/h) and GOR (17.19) for this newly double-stage HC process with different pressures (1 and 0.6 bar) is higher than that of the other two methods.

The simulation was performed in the computer laboratory of the University of Isfahan. The authors are grateful to the University of Isfahan for assistance in this article.

All relevant data are included in the paper or its Supplementary Information.