GB2509721A

GB2509721A - Membrane distillation process

Info

Publication number: GB2509721A
Application number: GB1300406.4A
Authority: GB
Inventors: Nidal Hilal
Original assignee: Swansea University
Current assignee: Swansea University
Priority date: 2013-01-09
Filing date: 2013-01-09
Publication date: 2014-07-16
Also published as: GB201300406D0

Abstract

A method of treating liquid to remove contaminants using membrane distillation is claimed, wherein the liquid is fed from a source, through a membrane in communication with a heater to raise the temperature of the liquid to form a vapour. Subsequently, the vapour is collected by condensation on a surface. The liquid to be treated may be water, such as water which is used in oil or gas extraction or in a desalination process. Suitably, the distillation method is air gap membrane distillation whereby there is an air gap between the membrane and the surface. The method can involve altering one or more of the following parameters: feed temperature, feed flow rate and coolant temperature. Preferably, the feed liquid has a temperature in a range of 40 to 80 degrees centigrade and a flow rate of 0.5 to 1.9 l/min. In a preferred embodiment, the coolant temperature is 5 to 25 degrees centigrade.

Description

Water Treatment Process

Field of the Invention

The present invention relates to a water treatment process and in particular but not exclusively to a wäte±. treatment process using. distillation.

Background of the Invention

Produced water is Water from underground formations that is brought to the surface during oil or gas production. It is also referred to as a brine, saltwater, or formation water. Generally, produced water is composed of dispersed oil, dissolved organic compounds, production chemicals, heavy metals and natural radioactive minerals. Some of these are naturally occurring in the produced water while other are related to chemicals that have been added for well-control purposes. Produced water characteristics and physical properties vary considerably depending on the geographic location of the floW, the geological formation with which the produced water has * E been in contact for thousands of years and the type of hydrocarbon product being produced.

Moreover, additional water is often needed to maintain sufficient pressure in a reservoir for oil 0*S*S* * production and this adding of water increases the amount of produced water that is fonned * making the produced water the largest volume byproduct or waste stream associated with oil and gas production. For inland oil production facilities, more than 60% produced water is commonly 000 re-injected back into the wells.

*00000 * * Produced water can have different potential impacts on. environment depending on where it is discharged. Produced. water presents a potential environmental threat to surface, underground water and soil. The fact that the oil and gas industry requires huge quantities of fresh water is a major challenge, especially.in coastal and arid regions. Therefore it is desirable to be able to treat and reuse water that is being used in oil and gas production processes to provide a sustainable water resource for oil and gas industry. 1*

Different treatment technologies are proposed to treat wastewater such as physical, membrane separation, chemical and biological methods. The high cost treatment, toxic chemical usage and sppce requirements fqr installation are the main drawbacks of chemical and biological methods Conventional oily wastewater treatment methods such as gravity separation and skimming have several disadvantages such as low efficiency, high operating costs, corrosion, and recontamination problems. The use ofmembrane filtration processes offers many benefits over other conventional treatment methods, for example low susceptibility of permeate quality towards feed conditions, low energy cost,, and the low' usage of chemicaIsadditives.

Filtering of waste water has been looked at, for instance, microflltràtion (MF) has been used to treat oily water by two microfiltration membranes (0.2 and 0.8 pm pore sizes). In addition nanofiltration processes and low pressure reverse osmosis have been utilized to treat produced water but membrane filtration per se would have disadvantages in large scale treated due to the clogging ofmembranes with largethrough puts of water to be treated. *0et

* The current invention seeks to overcome the prdblems of the prior art by providing a niethod that * has been adapted to treat water that is a waste product from industrial processes in an efficient *. and economibal way while limiting environmental impact of the processes used. * .

Summary of the Invention

Actording to the present invention there is provided a method of treating liquid to remove contaminants using membranç distillation wherein the liquid is fed from a source, through a membrane in communication with a heater to raise the temperatureof the liquid to form a vapour followed by collection of said vapour by condensation on a surface.

Preferably the method involves air gap membrane distillation whereby there is an air gap between the membrane and the surface.

It is' envisaged that the method involves altering one Or more of the flowing parameters: feed temperature, feed flow rate and coolant temperature.

his envisaged that the method involves altering one or more of the flowing parameters: feed temperature, feed flow rate and coolant temperature.

Preferably the feçd temperature is in.a range of 40 to 80 degrees centigrade.

It is preferred that the feed flow.rate is 0.5 to 1.90 1/mm.

Preferably the coolant temperature is 5 to 25 degrees centigrade.

It is envisaged that the metho4 is used to fttat water.

Preferably water is used in oil or gas exfraction or in a desalination process.

: * Brief Description of the Figures * *

An embodiment of the invention will now be described by way of example only with reference *..*** to the accompanying figures in which: Figure 1 shows: a schematia diagram of apparatus. used in Air Gap Membrane Distillation (AGMD); * * Figure 2 shows: the typical concentration of components in water from the Arabian Gulf; Figure 3 shows: pore size effect at feed temperatures of 50 degrees centigrade, a coolant temperature of 10 degrees centigrade and a flow rate of 0.5 L/min; Figure 4 shows: the effect of flow rate on permeate flux (feed temp of 50 degrees centigrade and acondensing temperature of 10 degrees centigrade) Figure.5 shows: the effect of coolant temperature on permeate flux (feed temperature of 50 degrees centigrade and flow rate of 1.5 Llmin; Figure 6 shows: the effect of feed temperature on permeate flux (coolant temperature of 10 degrees centigrade and flowrate of 1.5 L/min; Figure 7 shows: the power consumption at different flow rates (feed,temperature of 50 degrees centigrade and condensing temperature oil 0 degrees centigrade) Figure 8 shows:, the power consumption at different condensing temperatures (feed temperature of 50 degrees centigrade and flow rate of 1.5 Lfmin); and Figure 9 shows: the power consUnipti.oi at different feed temperatures (ôpndensiñg temperature of 10 degrees centigrade and flow rate of 1.5 L/min); and **fl. * *

This separatiOn process is driven by the vapour pressure difference. existing between the porous *befi.

hydrophobic membrane surfaces. According to permeate collection and driving force generation, .4 * membrane distillation technology can be classified into four categories. (1) Direct Contact no Membrane Distillation (DCMD), where the hot.and cold fluids are in direct contact with the two * membrane side surface; (2) Air Gap membrane Distillation (AGMD), where a staguant air layer is introduced between the membrane. and the condensation surface; (3) Sweeping Gas Membrane Distillation (SGMl)), where an inört gas is used to sweep the vapour at the permeate membrane side to condense outside the membrane module; and (4) Vacuum Membrane Distillation (VMD), where vacuum is created in the permeate membrane side using a vacuum pump. In SGMD and VMD modes of operation, the condensation takes place outside the membrane module.

AGMD has many attractive features, such as low operating temperatures in comparison to thOse -encountered in conventional process; the solution (mainly water) is not necessarily heated up to the boiling point. Moreover, the hydrostatic pressure encountered. in AGMD is lower than that used in pressure-driven membrane processes like reverse osmosis (RO). In addition, itis suitable for desalination and removing volatile compounds from aqueous sol!ltions. Also, the heat lost by conduction in the AGMD is low compared to other types of MD.

There have been several studies to investigate the influence of high salt concentration on. the permeate flux.. The effect of high salt concentration, such as in NaC1 solutions, using DCMD, was reported by Martinez [8], who attributed the reduction of the permeate flux to the decrease in water activity. Furthermore, Air Gap Membrane Distillation was also implemented to treat high concentration of NaCI,, MgCI2, Na2CO3, and Na2'S04, The Permeate fluxes are measured for different feed concentrations and membrane pore sizes [9]. Also, Safavi and Mohammadi [10], who used VMD to treat highly saline water, found that the VMD performance improves with decreasing feed concentration, and that salt rejection is not affected by feed concentration. Yun [.111 reported that for a highly concentrated NaC solution there is variation in the permeate flux with time, and that it is difficult. to calculate the permeate flux using existing models.. It is postulated that the boundary layer solution at the membrane surface. reaches saturation, so its * properties become different from those of the bulk solution. * .

* : *.: In this work, an experimental study is employed to treat produced water. The penneate flux. and rejection factors for salts and organic compounds have been investigated in AGMD.; A comparative study involving three different pore sizes (p.2, 0.45 and I jim) is performed to * * examine the effect of pore size on the permeate flux and rejection factor. In this study, the energy * * ** consumption is also measured for the different pore sizes.

A Detailed Description of an Embodiment of the Invention Air Gap Membrane Distillation (AGMD) has been implemented to treat produced water. The permeate fluxes, rejection factor and energy consumption for three different membranes, TF200, TF450 and TF1000, with pore sizes of 0.2, 0.45 and 1 j.tm are measured at different operating parameters. The influence of membrane pore size is investigated for the produced water. Also, the effect of feed flow rate, coolant temperature and feed temperature on permeate flux is.

studied. The flux increases as the feed temperature and flow rate increase, and declines as the coolant temperatures increase. Moreover, the energy consumption was measured at different pore size and was found to be independent of membrane pore size.

The experimental tests were carried out using flat sheet membrane in an AGMD module, as shown in Figure 1. The membrane cell and piping system are thade of stainless steel. The membrane cell, which is here used in horizontal position, consists of three compartmçnts.. The top part was. occupied by the feed solution, the bottom was occupied by cooling water, and the middle was the penneation section, where the permeate liquid is collected. In addition, a hydrophobic porous polytetrafluoroethylene (PTFE) membrane is layered between the. feed and permeate compartments. The permeate vapour diffuses across the membrane potes to the air gap, and is then collected on the cooling plate. The energy consumption in each AGMD test was measured nsing the energy meter that registers the amount of electric energy consumed (in kWh) including all AGMD equipments such as heating and cooling systems as well as the feed circulation pump.

* . : r Three flat sheet polytetrafluoroethylene (PTFE) microporous hydrophobic membranes were * * utilized. Those membranes are supplied by Sterlitech Corporation as laminated TF200, TF4SO and TFI 000 with thickness of I 75jtm and normal pore size of 0.2, .0.45 and 1.0 inn respectively.

* * The effective membrane area was 0,003688m * *.* Produced water from the oil field was treated by air gap membrane distillation. The produced *: : : :* watet was supplied by Saudi Arabian Oil Company (ARAMCO). Like any natural produced * : *: water, it contains organic, inorganic compounds and suspended particles. Removing sand. and suspended particles froth the, feed is important proce$s to prevent,aiiy pipe blockage, membrane fouling and to render the pump operation safer. For these reasons, microfiltration pretreatment has been implemented to the produced water. Figure 2 shows a table illustrating the produced water composition in the Arabian Gulf.

The effect of produced water flow rates as measured at 0.5, 1.0, 1.5 and.1.89 I/mn was analysed The desired flow rate is maintained by manipulating the pump speed. The produced water' is heated and pumped to the top part of membrane cell and maintained at the desired constant temperature by a thermostat (Autotune temperature controller) connected to the feed reservoir supplied by CAL Controls. Also, the cooling water temperature was kept constant at 10°C and circulated to the bottom cell compartment by refrigerated thennostatic bath (LTD6G) supplied by Grant Instruments. The inlet and outlet temperatures were continually monitored and measured.. The impact of the condensing temperature at 5, 10, 15, 20 and 25°C was also studied.

The levels of produced water temperatures studied are: 40, 50, 60, 70 and 80°C. The electrical conductivity (Jenway 3540) o.f produced water was monitored an4 recorded hourly. However, the electrical conductivity of permeate concentration was meastfrçd and recorded at the end of experiment in order to calculate the salt rejection factor. Moreover, the total orgaz)ic carbon analyzer (TOC) with autosampler (ASI-V) (Model TOC-VCPH, Shimadzu) has been used to analyze samples at the beginning and at. the end of each experiment. Each experiment was run for five hours and the penneat.e flux was. measured when steady state was teached (after approximately 45 mm). The electrical energy consumption was reported at the end of each experiment.

The effects of membrane pore size on.permèate. flux is shown in Figure 3. It is to be noted that the TF200 membrane has a pore size of 0.2 j.tm, the TF450 membrane has a pore size of 045 gin * and TFI 000 has pore size 1 gm, As. evident from this figure, the permeation flux of produced *.*..* S water at a feed flow rate of 0.5 1/mm and a temperature of 50°C approximately increases from 1.5 to 1.9 gIm2.s in going from TF200 to TF450. This corresponds to an enhancement of 26% in * the flux of produced water corresponding to an increase in pore size of 125%. Also, the permeation flux of produced water increases from 1.5 to 2.2 g/m2.s in going from TF200 to TF1000, which corresponds to an enhancement of 46 % in the. flux of produced water. This is a S...

* : direct result of the enhanced mass transfer in, the pores, which is controlled by Knudsen / ordinary diffusion mechanism that results in increased permeability and. therefore a higher flux.

However, the permeate conductivity from TF200 membrane was lower than that from TF450 and TFI 000. Furthermore, the salt and organic rejection for TF200 and TF450 are more stable than TF1000. For example, the salt rejection for TF200 found to lie between 99.99 -99.98 % and between 98.6-98.1% for organic rejection. On the other hand, the salt rejection for TFI000 varied between 97.8-97.1% and 96.1 -95.5% for organic rejection.

The influence of. the feed flow rate on permeate flux under the conditions of constant feed temperature and constant coolant temperature for TF200,TF4SO and TFI 000 membranes are shown in Figure 4. For all membranes studied under the constant conditions stated above, increasing the feed flow rate lçads to an increase in the permeate flux. For example, the penneatc flux for TF200 increases from around 1.5 (0,5 1/min) to 2.3 g/m2.s (1.89 1/mm), whereas the permeate flux increases from aroun4 1.9 (0.5 11mm) to 2.7 g/th2.s (1.89 lhnin) for TF450. Also; the permeate flux for TFI000 increases from around 2.2 to 2.9 g/m2.s when the feed flow rate increases from 0.5 to 1.89 1/mm, respectively. This can be. explained by the temperature and concentration polarization phenomena. Increasing the mass transfer coefficient will reduce the difference between the bulk and membrane surface concentration and also reduce the temperature difference between the feed bulk and membrane surface due to the increase in.

Reynolds number and, consequently, the improve of the transport coefficients.

The influence of coolant temperature was examined by varying the coolant fluid temperature between 5 to 25 °C at constant hot,feed temperature and flow rate. The permeate flüA declined * when the coolant temperature increased. For instance, the peineate flux of TF200 dropped off from 2.4 at a coolant temperature of 5°C to 1.4 g/m2.s at a.eoOlant temperature Of 25°C. In * addition, the permeate flux of TF450 dropped off from 2.8 at a coolant temperature of 5°C to 1.7 * * Wm2.s at a coolant temperature of 25°C This result can be attributed to the fact that decreasing *: * the coolant temperature leads to an increase in the vapour pressure difference acrossthe membrane4 thus an enhancement. of the flux. These results areS clearly displayed. in..Figure S. * * * The influence of feed Temperature has been investigated over a wide temperature range, i.e., 40 °C. As can be seen by the results presented in Figure 6, the permeate. flux is greatly affected by feed solution temperature. The effect observed looks as an exponential increase in AGMD flux with feed temperature. This can be explained by the exponential increase of the vapour piessure with temperature (e.g., Antoine equation) which implies exponential increase in the driving force; Increasing the temperature gradient across the membrane will affect the. di fThsion coefficient positively, which leads to increased vapour flux. Moreover, temperature polarization decreases. with increasing feed temperature.

It has been observed in this work that the energy consumption is almost independent Of membrane pore size. The range of energy consumption was between about 2.5.. 3.0 kWh.

Figures 7 to. 9 show clearly this behavior for the different membranes at different operating conditions. In terms of rejection factor, the salt and the organic rejection factors are not affected by the operating conditions.

The operating parameters on the performance of three different membranes was analysed and in particular,. the permeate flux, rejection factor and energy consumption for TF200, TF4SO and TFI000 membranes, with pore sizes of 0.2, 045 and I tm, respectively, were investigated. The permeate flux is found to be direcly proportional to the feçd temperature and the fóed flow rate.

However, it is inversely proportional to the coolant temperature. With rising feed temperatures, an exponential increase in the permeate flux was observed, while surprisingly the energy consutnption:is almost independent of membrane pore size.

* r *j The present invention has the advantage that it could provide a viable source of new water for beneficial use combined with economical and environmentally friendly methods of disposal of **...* producedwater are vital in orderto prevent serious environmental damage.

*... In particular the invention iS applicable to the oil and gas industries and water management in those industries, for example the process can be Used in the treatmentof: * . 1. Injection of produced water into the same formation from which the oil is produted or handled to another formation.

2. The produced water to meet the discharge regulations and then discharge it to the environment.

3. The produced water to meet the quality required for use in oil and gas fields operations.

4. The produced water to meet the quality required for beneficial uses such as drinking water and irrigation.

It is to be understood that the above embodiments have been provided only by way of exemplification of this invention, such as those detailed below, and that. further modifications and improvements thereto, as wouki be apparent to persons skilled in the relevant art, are deemed to fall within the broad scope and ambit of the present invention described. Furthermore where individual embodiments are dIscussed, the invention is intended tp cover combinations of those embodiments as well. The systems shown and described are not limited to the precise details and. conditions disclosed. Method steps provided may not be limited to the order in which they are listed but may be ordered any way as to carry out the inventive process without dejmrting from the scope of the invention. Furthermore, other substitutions, modifications, changes and ssions may be madc in. the design, operating conditions and arrangements of the exemplary embodiments without departing from the scope of the invention as express.edin the appended claims. *

**.... * . * * 0* *... * * *.o.. * *

Claims

Claims 1. A method of treating liquid to remOve contaminants using membrane distillation wherein the liquid is fed from a source, through a membrane in communication with a heater to raise the temperature of the liquid to fbrm a vapour followed by collection of said vapour by condensation on a surface.
2. A method accot ding. to claim 1., wherein the distillation method is air gap. membrane distillation whereby there, is air gap between the membrane ant the surface.
3. A method according to claim I or daim 2 wherein, the method involves altering one or more of the flowing parameters: feed temperature, fecd flow rate and coolant tcmperature. *.e. 0**

* * .
4. A method according to claim 3, wherein the feed temperature is in a range of 40 to 80 degrees centigrade. *.* * *
5. A method according to claim 3 wherein the feed flow rate. is 0.5 to 1.90 I/mm. * ***
6. A method according to claim 3 wherein the ooolant temperature is 5 to 25 degrees centigrade.

****** * *
7. A method according to any preceding claim used to treat water.
8. A method according to any preceding claim whetein the water is used in oil or gas extraction.?. A method of treating liquid to remove contaminants substantially described herein with reference to and as illustrated in the accompanying Figures