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Mechanical Engineering Department

Instituto Superior Técnico, UTL, Lisbon, Portugal

Dimensioning a water desalination plant by low temperature distillation

Levi Miguel Teixeira Pagaime levi.pagaime@gmail.com

October 2011

ABSTRACT

The most current technologies used in desalination have some limitations, such as water salinity and the availability of a nearby source of heat at high temperature. Therefore, the development of other techniques is needed. Distillation at low pressure has the advantage that it can be combined with heat sources of low temperature, such as thermal solar panels.

The aim of this dissertation was to quantify the main phenomena, pre-size the equipment and establish a first approximation of the energy consumption of a sea water desalination plant by distillation at low temperature, to see if it is cost-effective in relation to the existing processes.

For this, a computer program was designed, in order to calculate the energy consumption of the various system components and the investment needed to build the installation, according to various parameters in order to optimize it.

It was concluded that the technology in study, without resorting to heating the vaporizing water, is not financially competitive in comparison with the largest current technologies in the market. However, with the use of additional heating, with thermal solar panels, for example, this technology can have a competitive cost with other technologies.

KEYWORDS – Desalination, fresh water, low

temperature distillation, LTTD, C++ programming.

1 – INTRODUCTION

There are, nowadays, very efficient desalination processes (mainly reverse osmosis and distillation), but are rarely used for processing large quantities of water, because they consume too much energy, leading to a prohibitive water price. Reverse osmosis systems are very efficient to treat brackish waters, but the work pressures reach very

high values and the efficiency decreases as the salinity increases. Hence, the potential for the development of other techniques. The distillation at low pressure has the advantage of being able to be combined with low temperature heat sources such as solar panels. A variant of this system is used normally in ships, taking advantage of heat generated by the propulsion machinery.

The purpose of this dissertation was to quantify the main phenomena, pre-size the equipment and establish a first approximation of the energy consumption of a sea water desalination plant by distillation at low temperature, to know whether it is cost-effective in relation to existing processes.

To this end, it was conceived a program in C++ language, after the establishment of the formulas for the calculation of the energy balance. The objective of this program is the calculation of energy consumption of the various components of the system and, of the necessary investment to the construction of the installation, according to various parameters, in order to optimize it.

2 – DESCRIPTION OF THE SYSTEM UNDER STUDY

The system that we intend to study is a variant of the Low Temperature Thermal Desalination, or LTTD.

To be able to evaporate the water at ambient temperature we need a place where the pressure is sufficiently low for the saturation temperature to be lower than the environment. The evaporation pressure of water at room temperature is relatively low, but can be reached in a reservoir with a height of only 10 m, due to pressure at the top, about 2.0x10

3 Pa, in which the saturation temperature of

the water is about 18°C. In "ideal" conditions, it would be sufficient to fill one of these reservoirs with water to produce the distillation on top: in this case,

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a small pump would be enough to make the circulation of sea water in the facility. In reality, the process is complicated by the need to previously degasify the salt water and re-dissolve some air in the water that is returned to the ocean, by ecological imperatives. On the other hand, having received the sufficient steam flow (at low temperature and therefore low pressure), it is necessary to condense it.

This process can be carried out with a pressure increase and/or decrease of temperature: in any case, the equipment is not standard because, given the properties of steam under these conditions, the Reynolds numbers are very low and the steam volumetric flow rates are very large for a given liquid water volumetric flow rate. In the case we will study, two condensers will be used, which will be fed with water from the same area of the incoming water in main system, thus, at the same temperature Once condensed, it is easy to increase the pressure of water back to atmospheric pressure, because it corresponds to providing it with a lifting height of only 10 m.

The system can be represented with the following schematic:

Figure 1 – Schematic of the Desalination by Low Temperature Distillation System

The sea water enters through (1), towards the

deaerator, where a compressor (A) maintains the low pressure inside. Then, it goes to the vaporization tank where the pressure is lower than the saturation pressure of sea water in the range of 1.0x10

3 Pa to 2.0x10

3 Pa. A pump (B) sucks the

water into a sprinkler system that, while spraying the water, facilitates its evaporation. The salt water not evaporated (12) then goes to a regasification tank, driven by the pump (E), where some air is reintroduced, so that the marine ecological life does not resent with the water without dissolved oxygen. The air introduced in this regasification tank is pressurized by compressor (F), and a bomb (G), forces air circulation through the water, thus achieving its gasification.

The steam is sucked from the vaporization tank into a first condenser, through the fan (C1), which compresses slightly, increasing the pressure and, consequently, the saturation temperature. A fraction of the steam will condense in contact with the condenser tubes, because the circulating water (sea water) will be cooler than the steam. The steam that will not be able to condense in the first condenser, is slightly compressed (C2) to a second condenser, where the rest of the process of condensation will be done. At the end of this second condenser, there is only a tiny amount of water vapour with air that was not possible to extract in the deaerator, being compressed to the atmosphere using a compressor (C3).

The fresh water is then extracted with two pumps (H1) and (H2), which will give it the power needed to counter the low pressure present inside the condensers.

Figure 2 – Thermodynamic cycle associated to the

process An improvement in cycle may be the addition

of energy at low temperature using thermal solar panels. This will cause the vaporization temperature to be higher, and then so powerful fans are not needed, reducing the electricity consumption. They are used for heating the water that is put into circulation in the tank of vaporization, which goes from the point (5) to (6). The water is then sprayed through several holes to facilitate its vaporization, due to the decrease in the size of the droplets, which increases its surface/volume ratio. The consequent cycle can be represented by the following figure:

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Figure 3 – Thermodynamic cycle with Thermal Solar Panels

3 – DEVELOPMENT OF THE COMPUTER PROGRAM

The chosen language was C++. Although the student did not know in depth, it was the most widely used programming language for many years, so there are many sources of code accessible for fast learning and habituation. The compiler chosen to program development was the Microsoft compiler, the Visual Studio 2010. The chosen language for the program’s interface is English, so the program can be used by a much wider range of users than just those who dominate the Portuguese language.

3.1 – General structure of the program

The program was organized into a structure of primary and secondary functions. The primary functions are called from the main menu, which are: the data insertion, the energy consumption of the system, the cost of the installation and the extras. The main menu is represented in the following image:

Figure 4 – Main Menu The data insertion is made to a text file that is

located in a folder attached to the program, to be able to run the program without having to constantly put the necessary data for the calculations. So we can make changes to data that is not asked during

the normal running of the program, such as pipe diameters and rugosity, components prices and environmental characteristics of the place where the plant will be built, such as average solar irradiance. In order to know what each number means, there is another file (starting_data_help.txt), which says what each line means.

What is asked from the user in the data insertion into the program is: the sea water temperature; the required mass flow rate of desalinated water; the expected project lifetime; and even the use or not of alternative energy source, including thermal solar panels, photovoltaic panels and wind turbines. In addition, all other values in the file are reset to their original values.

After the data insertion, some parameters are optimized to reduce the total cost of the project throughout its life, such as: pipe diameters, number of tubes in condensers, among others.

In the energy consumption function, the program initially checks the data file, for its extraction. If it does not find the file, it displays an error message, alerts the user to the data insertion and returns to the main menu. When it detects the file correctly, it displays on the monitor the more relevant starting data, such as input water temperatures, flow rates, ambient temperature and vaporization temperature. Then it pauses, for the user to confirm the data, waiting for him to press any key.

Figure 5 – Starting data example

Successively it presents the results of the power required at each pump and compressor, the height that the water is in the degasification tank and the vaporization tank, the pressure in the same tank, the amount of energy exchanged with the environment around it and the outlet water temperature at that point. Continuously, it shows results for the condensers, such as the required cold water flows, its outlet temperatures and length of inner tubes, in order to know the size of condensers necessary in the plant.

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Figure 6 – Example of the description of the system power consumption

If it has been chosen the use of auxiliary

equipment based in alternative energies, the program shows the results for the auxiliary selected in the data insertion. In the case of the thermal solar panels, it displays the necessary area, and the power saved with this option. With the photovoltaic panels, it only shows the panel area needed, while with the turbine, it shows the rotor diameter necessary for the production of the required power. Note that the calculation of these technologies is interconnected, having the program into account the use of one or several different systems.

The next function, available on the main menu, deals with the financial part of the installation. It starts at the same point of the energy calculation function, verifying the data file, through the same steps. Then, it makes all necessary calculations and shows in terms of raw materials, the quantity needed and their respective cost. If alternative energy collection components have been selected, it shows, their initial investment, followed by the time required for the recovery of that investment, called payback. The main equipment investment is then shown, being discriminated in plumbing costs, cost of vaporization tank, condensers and even of pumps and compressors. Being completed with the total cost of the installation and, very importantly, the relative cost of the energy, in euro per tonne of distilled water.

Figure 7 – Example of the description of the plant cost

The last function accessible to the user is the

extras, which makes available the water properties that were used in the program. This serves to make quick and primary calculations that the user needs, without having to do tedious queries to tables, which may not be readily available, even if such calculations are not connected with the installation directly. It's just an add-on to the main program.

3.2 – Main Equations

The used equations are quite diverse and adapted to each specific component. Here we'll show the characteristic equations of the main elements of the system, which are, pumps and compressors. The power at the pumps is given by the following equation:

(1)

Where is the water mass flow rate to be pumped, is the height to be given to the water,

is the gravitational acceleration, and is the pump efficiency. The equation can also be in relation to the pressure loss in the pipes:

(2)

In which, is the pressure loss and is the water density.

The specific energy needed in the compressors is given by the following equation:

(3)

Being, the specific heat of the fluid to be

compressed, is the temperature before the

compression and the temperature after, which is found through the subsequent equation:

(4)

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Where, is the heat capacity ratio of the fluid

to be compressed, is the pressure before the compression and the pressure after.

4 – TECHNICAL STUDY

4.1 – Energy Consumption Calculation

The presented values correspond to a plant sized to produce 1 litre per second (using in most calculations the equivalent mass of 1 kg/s) of distilled water. The ratio of quantity of distilled water to the quantity of water required to circulate is 1/100, i.e. only 1% of water brought from the sea is distilled in the system, while the rest returned to the sea. The sea water temperature used is 15°C. The ambient temperature is considered to be 25°C.

Components Energy (J/l)

Compressor A 1223

Pump B 111

Compressor C1 51457

Compressor C2 12553

Compressor C3 146

Pump D1 904

Pump D2 367

Pump E 283

Compressor F 186

Compressor G 130

Pump H1 36

Pump H2 36

TOTAL 67432

Table 1 – Specific energy needed in the pumps and compressors

The total of energy consumption is 67.4 kJ/l,

or 18.8 kWh/m3.

4.2 – Possible cycle improvements

The cycle can be improved with small additions of energy at low temperature using thermal solar panels. This will cause the vaporization temperature to be higher, so there is no need for fans to compress the steam if the pressure increase is not required, which will reduce electricity consumption. The panels are used for heating the water that is put into circulation in the vaporization tank, which is sprayed through several holes to facilitate its vaporization, due to the decrease in the size of the droplets, which increases its surface/volume ratio. The optimization of the parameters concluded that the best temperature for the heated water is 24°C. To increase the evaporating water temperature to the best one, the needed panel area is 536 m

2.

Components Energy (J/l)

Compressor A 1223

Pump B 112

Compressor C1 0

Compressor C2 0

Compressor C3 136

Pump D1 2400

Pump D2 2400

Pump E 283

Compressor F 186

Compressor G 130

Pump H1 36

Pump H2 36

TOTAL 6880

Table 2 – Specific energy in the pumps and compressors, with Thermal Solar Panels

The total energy consumption is 6.9 kJ/l, or

1.9 kWh/m3.

The salt water that leaves the system, at a temperature of about 5°C lower than the input temperature, can be directed to the condensers, thus lowering the saturation temperature required, so reducing the power necessary to the fans in its input (C1) and (C2).

If the zone of installation of the plant was isolated and without access to the electricity grid, photovoltaic panels could be used, thus the plant would more costly, to the extent that the photovoltaic energy cost is quite high, due to the panels with current technology having a much reduced efficiency, which is around ±12%. Wind turbines would also be a good addition, if the zone where the plant was to be placed had good quantity and frequency of wind, which is the truth in oceanic coasts, in most cases.

4.3 – Financial analysis

This analysis quantifies the cost of the necessary material for the entire system, starting in piping, passing by the vaporization tank and condensers and ending in the water pumps and compressors. It was considered a price of 860$/tonne (€600) for carbon steel and 3800$/tonne (2600€) for stainless steel, having regard to the July 2011 listing in international markets. The thickness of the tubes was taken from ANSI B36.10 tables, where the thickness chosen was for standard efforts.

The plant investment cost is, in total 156.1x10

3 €, noting that this price is highly

speculative and only takes into account the necessary equipment, not counting the cost of assembly, of land and required permits to operate.

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Components Investment

(x103 €)

Piping 35,41

Vaporization Tank

7,84

Condenser 1 26,84

Condenser 2 17,93

Pumps and Compressors

68,10

TOTAL 156,12

Table 3 – Investments in the Facility

Given an annual interest rate of 5%, with regular monthly payments, the total investment cost, in current values, is 222.22x10

3 €.

In relation to running expenditure, the energy cost varies with different parameters, among which, the distilled water flow needed, the sea water temperature and the ambient temperature. Having regard an electricity price of 0.13 €/kWh, as the calculated consumption was 18.75 kWh/m

3, the

reference cost is 2.44 €/m3.

The total energy cost throughout the duration of the plant, 15 years, is 1.15x10

6 €. This makes the

cost of the whole project to be 1.38x106 €, for a total

relative cost of 2.91 €/m3.

The most important parameters were optimized in the calculation of the cost of the plant, so this cost is as low as possible for this type of installation.

To contemplate possible cycle improvements with the addition of energy at low temperature, a brief analysis was made on the use of thermal solar panels, to see the change in energy and investment costs. The result of the cost of the plant with thermal solar panels is 1.21 €/m

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It shows a large drop in consumption, which is in 1.91 kWh/m

3, a drop of almost 90%, but as the

panels require a large investment, the final relative cost descends only 58%.

5 – CONCLUSIONS

5.1 – Comparison between technologies

Here we will look at the differences between the technology covered in this study, the Low Temperature Thermal Desalination (LTTD), and the two most used in the market today, Reverse Osmosis (RO) and Multi-Stage Flash distillation (MSF).

In the following table we see some important values, in terms of energy consumption for each of the technologies. This comparison is carried out for the same fresh water flow from each, 31,822 m

3/day, or 367 kg/s. The consumption of the LTTD

is retrieved using the program made in the project,

with all parameters equal to the theoretical study done. The energy price considered is 0.13 €/kWh.

RO RO ER

MSF LTTD LTTD TSP

Heat Consumption,

MJ/m3

0 0 290 0 0

Electricity Consumption,

kWh/m3

4,2 3,5 3,6 18,8 1,9

Electricity Cost, €/m

3

0,55 0,46 0,47 2,44 0,25

Investment Cost, €/m

3

0,70 0,64 0,93 0,47 0,96

Total Cost, €/m

3

1,25 1,10 1,40 2,91 1,21

Table 4 – Comparison of cost between several technologies

Being that RO ER is reverse osmosis with

energy recovery, and LTTD TSP is low temperature thermal desalination using thermal solar panels.

It is confirmed that the LTTD technology, without the use of thermal solar panels, is not financially competitive in comparison with the biggest current technologies in the market. It costs 106% more than the MSF and 165% more than the most efficient version of RO technology.

With the use of thermal solar panels, the LTTD technology can reduce its cost by more than half. It can thus have a competitive price with the other technologies, being comparable to reverse osmosis without energy recovery.

5.2 – Final thoughts

We come to the conclusion that the desalination technology studied in this dissertation, is theoretically competitive in certain situations, and makes room for future studies on the same, to complete its commercial development. It can, in the future, be a good option to complement the existing technologies, filling gaps in the market that these technologies are still unable to fill.

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