This invention relates to an ultrafiltration system and a process utilizing the system for concentrating latices.

More particularly, the ultrafiltration system and process of this invention relates to carrying out latices concentration, recovery of waste latex, and latex product isolation.

DESCRIPTION OF TH E PRIOR ART

Natural rubber latex as an industrial raw material is supplied as concentrated latex. It serves as the main raw material for natural rubber latex product manufacturing factories.

Known methods of concentrating latices are by way of centrifugation, creaming, and evaporation.

Centrifugation is widely used in Malaysia and accounts for some 95% of the total latex concentrate produced. Latex concentration by centrifugation involves the separation of preserved natural rubber field latex into two fractions. One fraction contains concentrated latex having high dry rubber content while the other having low dry rubber content. The latter fraction is also known as skim latex. Skim latex is generally recovered by coagulation with sulphuric acid. Once skim latex is recovered, there exists an effluent which consists of sulphuric acid-contaminated serum with a high biological oxygen demand value. This effluent is then discharged into effluent ponds (anaerobic ponds). Microbial breakdown takes place in these anaerobic ponds where sulphate ions originating from sulphuric acid is partially converted to poisonous hydrogen sulphide (H ₂ S) gas. This pungent smelling H ₂ S gas is detrimental to the environment.

Recent improvements to the above method include the development and utilization of membrane separation technology for the concentration of natural rubber field latex, in particular membrane ultrafiltration. Ultrafiltration is an environmentally friendly process and economically viable. It is possible to achieve 'zero discharge' with ultrafiltration as all the products from the concentration process have commercial value. Malaysian patent application PI 20071273 discloses an ultrafiltration process for natural rubber latex concentration. The apparatus used to carry out the process is one or more membrane modules having a plurality of tubular membranes. The process is carried out by pumping latex through the membrane modules. In this known process, an ultrafiltration system having tubular filtration membranes, separates natural rubber latex into two fractions i.e. permeate and retentate, by filtering the latex through the tubular filtration membranes. When concentrating natural rubber field latex, the resultants are concentrated natural rubber latex and a latex free natural rubber serum. The ultrafiltration system may also be used to concentrate skim latex (produced from centrifugation process) to produce latex- free serum and skim latex concentrate. One disadvantage to this system is that the membrane modules can only be used in a serial arrangement, which results in a system having a relatively large footprint. The system was also observed to take a substantially long time to stabilize i.e. 20 minutes before the concentration process can begin.

More importantly, however, is the issue of membrane fouling, the most serious problem when employing such prior ultrafiltration systems. A decrease in performance due to membrane fouling has hindered the widespread application of ultrafiltration to replace current centrifugation methods of concentrating latices mentioned previously. Membrane fouling has many adverse effects on such prior ultrafiltration systems such as membrane flux decline, significant requirement for increase in trans-membrane pressure, biodegration of membrane materials and ultimately system failure.

The decline in membrane permeate flux occurs as a result of concentration polarization, plugging of membrane pores, and adsorption of fouling material on the membrane surface or in the pore walls. Concentration polarization occurs when a concentration gradient of the retained components is formed on or near the membrane surface.

Studies have been conducted on methods for reducing the extent of concentration polarization and thereby increasing membrane permeation flux by applying ultrasound to the membranes during ultrafiltration. An article entitled "Evaluating the use of in-situ ultrasonication to reduce fouling during natural rubber skim latex (waste latex) recovery by ultrafiltration" by D. Veerasamy, A. Supurmaniam, and Z.M. Nor, presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5-9 November 2007 in Sydney, Australia discloses the optimum preservation level required for skim latex recovery by ultrafiltration with ultrasonication.

In this study, two membrane modules connected in series were used. The membrane modules contained one tubular membrane and were completely immersed in an ultrasonic bath. Ultrasound of 40 kHz by way of piezoelectric ultrasonic transducers was applied from beneath the tank of the ultrasonic bath. In this article, it was concluded that a composite preservation solution of 0.3% ammonia and 0.2% ammonium laurate was needed for carrying out ultrafiltration of natural rubber skim latex with continuous ultrasonication. Ammonia easily evaporates, which results in emission of ammonia fumes during ultrafiltration. Ammonia is harmful if inhaled in excessive quantities.

The main disadvantage of this system is that the ultrasonic transducers are placed beneath the tank only. As ultrasonic waves are concentrated at the bottom of the tank, the intensity of the ultrasonic waves are not uniformly distributed throughout the membrane area. The bottom portion of the tubular membranes receive a higher intensity of ultrasonic waves, as opposed to the top portion of the tubular membranes which is further away from the ultrasonic transducers. As the membrane modules are immersed in an ultrasonic bath, the number of membrane modules the system can have depends on the size of the ultrasonic bath used. As with the prior ultrafiltration system of PI 20071273, the membrane modules can only be used in a serial arrangement, which results in a system having a relatively large footprint.

Prior ultrafiltration systems have been unable to provide uniform distribution of the ultrasonic waves throughout the membranes for preventing premature membrane fouling as well as to provide for effective cleaning of the membranes.

During latices concentration there are two types of common foulants i.e. larger sized foulants and minute sized foulants. Smaller sized foulants would easily plug the membrane pores while larger sized foulants produce a gel layer formed on the membrane surface which will hinder permeation during further concentration runs.

Therefore, fouled membranes require cleaning. Current methods of ultrafiltration membrane cleaning, such as circulating cleaning solution through the membrane to remove fouling material, although effective in the short run, will eventually lead to a total fouling of the membrane where these membranes can no longer be cleaned by the cleaning solution. Frequent replacement of the membranes are then required. This incurs high operation and maintenance costs.

Thus, there is a need to develop an ultrafiltration system which enhances membrane permeation flux and prevents premature membrane fouling during ultrafiltration. Also there needs to be established a cleaning procedure to bring about good flux recovery after a concentration run is carried out to increase the economic life of the membranes.

This invention thus aims to alleviate some or all of the problems of the prior art.

SUMMARY OF THE INVENTION

In accordance with an aspect of the invention, there is provided an ultrafiltration system for the concentration of latices. The system comprises a first tank for storing the latex feedstock, a second tank for storing cleaning solution used in the cleaning of the system, a pump for pressurized circulation of the feedstock and cleaning solution through the system, and at least one vertically disposed ultrafiltration membrane module. Each module is provided with a plurality of vertically disposed tubular membranes for filtering the feedstock, and with a plurality of ultrasonic transducers to generate ultrasonic waves for enhancement of membrane permeation flux. The transducers are arranged in vertically spaced apart pairs so as to span the height of the module, with each pair of transducers arranged to also enable generation of ultrasonic waves across the width of the module. In use, the feedstock exits the first tank and is pumped through the membrane module for ultrafiltration, where lower molecular portions of the feedstock pass through the tubular membranes as permeate and higher molecular portions of the feedstock are retained as retentate. The resulting retentate is recycled back to the first tank and ultrafiltration is repeated until the desired latex concentration is reached. At the end of the ultrafiltration cycle, the cleaning solution exits the second tank and is pumped through the membrane module for cleaning of the membranes. The arrangement of the ultrasonic transducers enables the generation of ultrasonic waves across the width and along the height of the module for effective enhancement of membrane permeation flux during both the ultrafiltration and cleaning cycles.

In an embodiment, the first tank may further comprise a temperature sensing means to detect temperature variation of the feedstock.

In a further embodiment, the first tank may further comprise a pH sensing means for reading the pH level of the feedstock. In yet a further embodiment, the first tank may further comprise a separating means for allowing only liquid portions of the retentate back into the first tank. In another embodiment, the system may further comprise a pressure indicator means to detect pressure variation of the feedstock entering the membrane module and pressure variation of the retentate leaving the membrane module. In another embodiment, each of the membrane modules may have from seven to thirty- seven cross-flow tubular membranes for filtering the feedstock.

In a further embodiment, each of the membrane modules may have seven tubular membranes for filtering the feedstock.

In yet another embodiment, the tubular membranes may have a pore size ranging from about 0.1 to about 0.2pm.

In an embodiment, the tubular membranes may be ceramic cross-flow tubular membranes.

In a further embodiment, the membrane module may be of a substantially cuboidal shape. In another embodiment, the membrane module may be provided with a compartment for holding the ultrasonic transducers on either side of the membrane module.

In an embodiment, ultrasonic transducers may further comprise a time regulator means to enable continuous or intermittent operation of the transducers.

In yet another embodiment, the ultrasonic transducers used may have a drive frequency of 25 kHz. Each membrane module may be provided with twenty four units of ultrasonic transducers having a drive frequency of 25 kHz. In a further another embodiment, the ultrasonic transducers used may have a drive frequency of 40 kHz. Each membrane module may be provided with thirty six units of ultrasonic transducers having a drive frequency of 40 kHz. In an embodiment, each membrane module may be provided with ultrasonic transducers having a drive frequency of 25 kHz and 40 kHz.

In another embodiment, the system may further comprise a pressure adjusting means for adjustment of the required pressure to drive the feedstock through each membrane module.

In an embodiment, the system may further comprise a plurality of valves for controlling the flow of the feedstock or cleaning solution entering each membrane module or, retentate or cleaning solution exiting each membrane module.

In a further embodiment, the membrane module may be operated in a serial arrangement or a parallel arrangement by opening or closing selected valves. In another embodiment, the system may further comprise a thermal cleaning device for cleaning of said tubular membranes. In use, the membranes are removed from the membrane modules and are subject to thermal cleaning with the device upon reaching membrane permeation flux of 50%. The system may be particularly suitable for recovering natural rubber skim latex, concentrating epoxidised natural rubber, and recovering waste nitrile latex.

In accordance with another aspect of the invention, there is provided an ultrafiltration process for concentration of latices utilizing the system of this invention. The process comprises the steps of:

i. pumping feedstock from the first tank to the membrane module to commence ultrafiltration;

ii. running step (i) continuously during which higher molecular portions of the feedstock is removed from the membrane module as retentate for recirculation to the first tank;

iii. running steps (i) and (ii) continuously during which lower molecular weight portions of the feedstock is removed from the membrane module as permeate; iv. running steps (i) to (iii) continuously until the required latex concentration is reached; and v. commencing the cleaning cycle by pumping cleaning solution from the second tank through the membrane module until the appearance of the cleaning solution leaving the membrane module is clear;

wherein the ultrasonic transducers are operated either continuously or intermittently throughout steps (i) to (v) for effective cleaning and enhancement of membrane permeation flux.

In an embodiment, for a first ultrafiltration cycle, prior to commencing step (i), a predetermined amount of deionized water is fed into the membrane modules.

In an embodiment, the tubular membranes may be operated at a pH range of 0 to 14.

In a further embodiment, the tubular membranes may be operated at a breaking pressure of not more than 9 MPa (90 bar).

In another embodiment, the tubular membranes may be operated at a running pressure of not more than 1 MPa (10 bar).

In yet another embodiment, the tubular membranes may be operated at a temperature below 400°C.

In another embodiment, the cleaning cycle may comprise the step of cleaning the tubular membranes with deionized water. In a further embodiment, the cleaning cycle may comprise the step of cleaning the tubular membranes with 1% NaOH solution.

In an embodiment, the cleaning cycle may comprise the step of cleaning said tubular membranes with a solution of 1% NaOH and 0.025% NaOCI.

In another embodiment, each cleaning cycle may be carried out for 30 minutes at a temperature of 50°C. In yet another embodiment, the cleaning cycle may be conducted at a transmembrane pressure of 50 kPa (0.5 bar).

In another embodiment, the process may further comprise the step of:

vi. removing fouled tubular membranes from the membrane module and placing the tubular membranes in the thermal cleaning device for thermal cleaning of the tubular membranes.

In another embodiment, the membrane modules may be heated at a temperature ranging from about 250°C to about 300°C.

The process may be particularly suitable for recovering natural rubber skim latex, for concentrating epoxidised natural rubber, and for recovering waste nitrite latex. The present invention seeks to overcome the problems of the prior art, and to provide an ultrafiltration system and process utilizing the system for the concentration of latices. Advantageously, the system and process of this invention enables enhancement of membrane permeation flux and provides for an effective cleaning of the ultrafiltration membranes.

The arrangement of the ultrasonic transducers enables the generation of ultrasonic waves across the width and along the height of the membrane modules, i.e. allows the ultrasonic waves to be distributed in uniform intensity over the entire membrane area of the tubular membranes. This greatly enhances membrane permeation flux and aids in preventing premature membrane fouling leading to an increase in the economic life of the membranes. The generation of ultrasonic waves across the width and along the height of the membrane modules can be applied for all type of membrane separation processes such as microfiltration, nanofiltration, reverse osmosis, electrolysis, and dialysis. The additional cost for an ultrafiltration system to have ultrasonic application is only 5% more than that of an ultrafiltration system without ultrasonic application. Therefore, there is only a minimal addition to operation costs, not enough to discourage use of ultrafiltration system having ultrasonic application. Water and chemical used in the cleaning of the membranes within the system can be recycled and can be used in more than one ultrafiltration cycle. This reduces the amount of water and chemicals needed for cleaning, which not only saves costs, but also makes the process environmentally friendly.

The vertical arrangement of the membrane modules within the system also allows for easy incorporation of additional membrane modules when needed, as well as to allow for convenient drainage of the concentrated latex before the cleaning cycle is carried out. The system and process of the present invention also produces concentrated latex that has superior film qualities when compared to the film properties of centrifuged latex concentrate.

The system and process of this invention provides for various other advantages which will be further elaborated in the following pages.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is illustrated, although not limited, by the following description of embodiments made with reference to the accompanying drawings in which:

Figure 1 is a schematic diagram of the ultrafiltration system according to an embodiment of this invention.

Figure 2 is a cross-sectional view of the membrane module having ultrasonic transducers used in the system of Figure 1.

Figure 3 is a cross-sectional view of a tubular membrane housed in the membrane module of Figure 2. Figure 4 shows a membrane module for use with the system of Figure 1, having thirty six units of ultrasonic transducers with a drive frequency of 40 kHz.

Figure 5 shows a membrane module for use with the system of Figure 1, having twenty four units of ultrasonic transducers with a drive frequency 25 kHz.

Figure 6 shows a thermal cleaning device for use with the embodiment of Figure 1.

Figure 7 is a graph showing membrane permeation flux during the concentration of natural rubber field latex with and without ultrasonic application as explained in Example 2.

Figure 8 is a graph showing the rate of increase of total solid content in concentrated natural rubber field latex with and without ultrasonic application as explained in Example 2.

Figure 9 is a graph showing membrane permeation flux during the concentration of epoxidised natural rubber (ENR25) latex as explained in Example 3.

Figure 10 is a schematic diagram of the cross section of a typical cross-flow filtration. Figure 11 is a graph showing membrane permeation flux during the concentration of epoxidised natural rubber (ENR50) latex with ultrasonic application (at frequencies of 40 kHz and 25 kHz) and without ultrasonic application as explained in Example 4.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention is an ultrafiltration system and process having ultrasonic transducers for enhancement of membrane flux and cleaning capabilities, developed to enable environmentally friendly latices concentration.

The feedstock

The system and process of this invention may be used for the concentration of various types of industrial liquids, for example natural rubber latex, epoxidised natural rubber latex (ENRL), nitrile butadiene rubber latex (NBRL), styrene butadiene rubber latex (SBRL) and polyvinyl chloride latex (PVCL).

The invention may be used for the concentration of natural rubber field latex (NRFL) and natural rubber skim latex (NRSL), where natural rubber serum and natural rubber skim serum, respectively, are produced as useful environmentally friendly by-products.

Concentration of natural rubber field latex by centrifugation typically requires the latex to be preserved before the centrifugation process can be carried out. Generally, natural rubber field latex is preserved with ammonia, but may also be preserved with various other preservatives in addition to ammonia. In the system and process of this invention, the natural rubber field latex can be concentrated with lower concentrations of preservation chemicals beforehand. This reduces costs as less preservation chemicals are required. The invention may also be used for recovery of waste nitrile latex, which is a scheduled waste of nitrile latex glove manufacturing. Generally, waste nitrile latex is disposed by way of incineration at high disposal costs. The system and process of this invention enables recovery of waste nitrile latex such that the concentrated nitrile latex may be reused in the processing of new batches of nitrile latex.

Epoxidised natural rubber (ENR) latex is a structurally modified natural rubber field latex. Structure modification is done to incorporate superior properties of synthetic rubber, such as resistance to oil and chemicals. Concentration of epoxidised natural rubber latex by centrifugation is not feasible. This is because the difference of specific gravity between epoxidised natural rubber latex and water is too small for any meaningful separation to occur during centrifugation. Epoxidised natural rubber latex can only be can only be concentrated by the process of this invention. The system

The ultrafiltration system of this invention mainly comprises a first tank 1 (feed tank) for storing the latex feedstock, a second tank 2 (washing tank) for storing cleaning solution used in the cleaning of the system, a pump 3 for pressurized circulation of the feedstock and cleaning solution throughout the system, and at least one vertically disposed ultrafiltration membrane module 4 provided with a plurality of ultrasonic transducers 9.

The feed tank 1 can be made of any suitable material such as stainless steel and can be of any suitable shape and configuration. In the embodiment of Figure 1, the tank is vertically disposed and is substantially of a cylindrical shape with a tapered bottom. The feed tank 1 can be of any suitable size depending on the processing capacity of the system. For example, the feed tank in Figure 1 has a capacity of 1250 L. If necessary, the size of this tank may be increased depending on the amount of feedstock to be processed. The feed tank 1 has an inlet located at the top of the tank for receiving feedstock as well as incompletely filtered latices (retentate) and an outlet located at the bottom of the tank for the egress of feedstock. The feed tank 1 may preferably be provided with a separation means 14 for allowing only liquid portions of the retentate back into the feed tank 1 for recirculation and further ultrafiltration. The separation means 14 may be a sieve having a suitable pore size, for example a 40 mm mesh sieve. To avoid high turbulence in the feed tank 1 during processing, the feed tank 1 may also additionally be provided with the appropriate conventional fittings such as baffles and the like. The feed tank 1 may have a temperature sensing means 6 connected to an electrical control panel 10 positioned near the tapered bottom of the tank such as a stainless steel thermocouple type K temperature probe. A pH sensing means 5 such as a panel mounted pH meter may also be provided for detecting the pH of the contents of the feed tank 1, and may be located adjacent the temperature sensing means 6. The washing tank 2 can be made of any suitable material such as stainless steel and can be of any suitable shape or configuration. The tank is vertically disposed and is substantially of a cylindrical shape with a tapered bottom. The washing tank 2 can be of any suitable size depending on the cleaning solution to be used in the cleaning cycle and the desired capacity of the system. For example, in the embodiment of Figure 1, the washing tank has a capacity of 300 L. If necessary, the size of this tank may be increased.

The feed pump 3 can be any type of pump of a suitable capacity for providing pressurized circulation of the contents of the feed tank 1 and washing tank 2 throughout the system and may be provided with industry-standard components such as an air regulator, air muffler and/or a non-stalling air valve. For example, the pump may be a compressed air powered double-diaphragm pump having a capacity of 10 hp.

At least one membrane module 4 is provided with the system of this invention. The module may be of any suitable shape and configuration. In the embodiment of Figures 1 and 2, four vertically disposed membrane modules is provided, each module being of a substantially cuboidal shape and having an inlet for receiving feedstock, a primary outlet for outflow of retentate (incompletely filtered latices) and a secondary outlet for outflow of permeate (lower molecular weight). The primary outlets of all four modules are operatively connected to a common retentate line that removes the retentate for recirculation back to the feed tank for further processing. The secondary outlets of all four modules are operatively connected to a common permeate line for removing permeates of the feedstock at predetermined intervals. Each membrane module 4 houses a plurality of cross-flow tubular membranes 13 of a suitable type. The membranes used should be able to withstand a pH range of 0 to 14, a breaking pressure of not more than 9 MPa (90 bar), a running pressure of not more than 1 MPa (10 bar) and temperatures of not more than 400°C. Ceramic tubular membranes may be used.

The number of tubular membranes provided per module and the total membrane area per module is dependant on the feedstock to be concentrated and the desired capacity of the system. The tubular membrane used may have any suitable number of channels and may be made of any suitable material. For example, in the embodiment of Figure 3, a ceramic tubular membrane is used. The ceramic membrane is made of Zr0 ₂ - Ti0 ₂ , has eight channels with each channel having a hydraulic diameter of 6 mm, a pore size of 0.14 microns, a length of 1178 mm and has a total membrane area of 0.2 m ² . When seven of these tubular membranes are provided per module, the total membrane area per module is 1.125 m ² and the total membrane area of the system is 4.5 m ² .

The tubular membranes used may have a pore size ranging from about 0.1 μητι to about 0.2 pm. The pore size is dependent on the type of feedstock to be concentrated. For example, for natural rubber field latex concentration, recovery of skim latex, and recovery of waste nitrile latex the tubular membrane pore size should be in the range of from about 0.1 pm to 0.2 pm. For epoxidised natural rubber latex concentration, the tubular membrane pore size should be in the range of from about 0.14 pm to 0.16 pm.

Each membrane module 4 is provided with a plurality of ultrasonic transducers 9 to generate ultrasonic waves for enhancement of membrane permeation flux and to enhance cleaning of the membranes during a cleaning cycle. The transducers are arranged in vertically spaced apart pairs so as to span the height of the module. Each pair of transducers are arranged to also enable generation of ultrasonic waves across the width of the module, for example by placing each transducer of a pair of transducers on either side of the module. The transducers are fixedly attached to each module, either directly on the module wall or housed in a separate holding compartment 11. In the embodiment of Figures 4 and 5, each membrane module is provided with a compartment 11 for holding the transducers on either side of the module. Any suitable type of ultrasonic transducers may be used in the system of this invention. For example, when the system is used for the concentration of natural field rubber latex, recovery of skim latex, concentration of epoxidised natural rubber, and recovery of waste nitrile latex, transducers having a drive frequency of 25 kHz or 40 kHz may be used. All of the membrane modules in the system may be provided with transducers of identical or different drive frequencies, for example, in a system having four modules, two of the modules may be fitted with transducers having a drive frequency of 40 kHz and the remaining two modules fitted with transducers having a drive frequency of 25 kHz. Alternatively, each of the membrane modules in the system may be provided with ultrasonic transducers of different frequencies. For example, each membrane module may be provided with transducers having a drive frequency of 25 kHz and 40 kHz. The number of ultrasonic transducers provided per module is dependent on the drive frequency of the transducers used. In the embodiment of Figure 4, each module is provided with thirty six units (eighteen pairs) of transducers having a drive frequency of 40 kHz. In the embodiment of Figure 5, each module is provided with twenty four units (twelve pairs) of transducers having a drive frequency of 25 kHz.

The configuration and arrangement of the ultrasonic transducers and the type of transducers used (drive so as to enable frequency) is chosen so as to enable propagation of the ultrasonic waves throughout each membrane module. A suitable generator may be provided with the system of this invention for generating power for the ultrasonic transducers. A suitable time regular means may also be provided for optional selection of continuous or intermittent operation of the transducers.

A suitable pressure meter for measuring the pressure of the feedstock entering the membrane module 4 and retentate exiting the membrane module 4 may be provided with this system. The pressure meter may have a pressure range of from 0 to 700 kPa (7 bar).

As is known in the art, the system of this invention may be further provided with suitable valves, pipes and fittings for operation and control of the system. Selected valves can be opened or closed for operation of the membrane modules either in a parallel or a serial arrangement. The valves can also be used for controlling the desired trans-membrane pressure for driving the feedstock through each membrane module 4.

A suitable main structure frame is provided to support all the apparatus of the system such as the feed tank 1, washing tank 2 and membrane modules 4. The main structure frame can be made of any suitable material such as SUS304 stainless steel. The system may have an electrical control panel 10 that may be mounted onto the main structure frame or separately provided. The electrical control panel 10 consolidates all the signals of the pH meter, temperature meter, and pressure meters. The system may further comprise a thermal cleaning device 12 such as a muffle furnace (Figure 6) for receiving and cleaning the tubular membranes. The thermal cleaning device 12 can be of any suitable shape or size. The size of the device is dependent on the desired number of tubular membranes to be cleaned at any one time. For example, in the embodiment of Figure 6, the length of the device is 1400 mm so as to be able to accommodate fourteen tubular membranes of 1020 mm length. The device is able to generate heat of up to about 500°C.

The process

The ultrafiltration process for concentration of latices utilizing the system of this invention, mainly comprises the following steps: i. pumping feedstock from the feed tank to the membrane module to commence ultrafiltration;

ii. running step (i) continuously during which higher molecular portions of the feedstock is removed from the membrane module as retentate for recirculation to the first tank;

iii. running steps (i) and (ii) continuously during which lower molecular weight portions of the feedstock is removed from the membrane module as permeate; iv. running steps (i) to (iii) until the required latex concentration is reached; and v. commencing the cleaning cycle by pumping cleaning solution from the washing tank through the membrane module until the appearance of the cleaning solution leaving the membrane module is clear.

Prior to commencing an ultrafiltration run, serial or parallel arrangement of the membrane module 4 is selected by opening and closing the appropriate valves.

In a first ultrafiltration cycle utilizing the system of the invention, deionized water is fed into the membrane modules 4 prior to commencing an ultrafiltration run. This is done to surround the tubular membrane with a medium for transmitting the ultrasonic waves. Preferably, deionized water is fed into the membrane module until it reaches the permeate line.

The pump is then turned on to commence ultrafiltration where feedstock from the feed tank 1 is circulated to the membrane modules 4. Trans-membrane pressure is set by adjusting the pressure adjustment means 8 after the system has stabilized. The system is usually stable after 10 minutes from the commencing of the circulation.

In an embodiment of the invention where the membrane modules 4 are used in serial arrangement, feedstock is pumped from a first membrane module to the last membrane module in sequence. In an embodiment of the invention where the membrane modules are used in parallel arrangement, the feedstock is pumped through more than one membrane module simultaneously. During ultrafiltration, lower molecular portions of the feedstock pass through the tubular membrane as permeate and higher molecular portions of the feedstock are retained in the tubular membrane as retentate. For both serial and parallel arrangement of the membrane modules 4, permeate is removed from the membrane modules 4 through the permeate line, while the retentate is removed through the retentate line. Retentate is recirculated via the retentate line to the feed tank 1 for further ultrafiltration, until the desired latex concentration is reached. During the concentration process, the pH and temperature of the feedstock is monitored. For example, the temperature of the feedstock should not exceed 55°C when the feedstock is natural rubber field latex. Pressure of feedstock entering the membrane module 4 and retentate exiting the membrane module 4 should also be monitored periodically.

The ultrafiltration cycle continues until the required concentration is reached. The required concentration of the product is calculated using the initial concentration of the feed and the volume of permeate produced during an ultrafiltration run.

Upon reaching the required concentration, the ultrafiltration process is stopped. Retentate is isolated in the feed tank 1 by opening and closing the appropriate valves. The resulting retentate is the fully concentrated latex which is removed from the system by draining it from the feed tank 1. The cleaning cycle is then commenced. Cleaning solution from the washing tank 2 is pumped through the membrane module 4 until the appearance of the cleaning solution leaving the membrane module is clear. Throughout the ultrafiltration and cleaning cycles, the ultrasonic transducers are turned on either continuously or intermittently.

The preferred cleaning cycles of the process of this invention are set out below. Cycle 1

Clean water is circulated from the washing tank 2 to the membrane modules 4. The circulation of clean water is carried out until the milky appearance of the rinsed water disappears. During this cleaning cycle, no trans-membrane pressure is set. After the milky appearance of the rinsed water disappears, a final rinsing is done. This final rinsing is done by circulating deionized water having a temperature of 50°C and at a transmembrane pressure of 50 kPa (0.5 bar).

Cycle 2

After cycle 1 is completed, a 1% NaOH solution having a temperature of 50°C is circulated from the washing tank to the membrane modules 4. This cleaning cycle is carried out for thirty minutes at a trans-membrane pressure of 50 kPa (0.5 bar). The system is then packed with the 1% NaoH solution and left idle overnight. The next day, the 1% NaoH is discharged and deionized water is circulated through the system. Cycle 3

After cycle 2 is completed, a 1% NaOH and 0.025% NaOCI solution having a temperature of 50°C is circulated from the washing tank to the membrane module 4. This cleaning cycle is carried out for thirty minutes. The system is then packed with the 1% NaOH and 0.025% NaOCI solution and left idle overnight. The next day, the 1% NaOH and 0.025% NaOCI is discharged and deionized water is circulated through the system.

After repeated use and cleaning of the system, e.g. repeated ultrafiltration and cleaning runs, the membrane flux of the tubular membranes is expected to deteriorate due to accumulated foulants consisting of rubber particles and scales of latex proteins plugging the membrane pores.

When the membrane flux of the tubular membranes deteriorates below 50%, a thermal cleaning cycle is carried out. The tubular membranes are removed from the membrane module 4 and are subsequently placed in the thermal cleaning device 12. After placement of the tubular membranes in the device, the device 12 is turned on to generate heat at a preferred temperature of about 250°C to about 300°C. The membranes are heated in the device 12 for a predetermined amount of time. This thermal cleaning cycle further cleans the fouled membranes by ashing out rubber particles and scales of latex proteins that are plugging the membrane pores. The thermally cleaned tubular membranes are then ready for reuse in new ultrafiltration runs.

EXAMPLE

The following Examples illustrate the various aspects, methods and steps of the system and process of this invention. These Examples do not limit the invention, the scope of which is set out in the appended claims.

Example 1 : The effect of different cleaning solutions on membrane permeation flux during the cleaning cycle A study was carried out with the system and process of this invention to determine the effects of different cleaning solutions on membrane permeation flux. In this study, two membrane modules connected in parallel, each module having fourteen tubular membranes were used, with each membrane module having ultrasonic transducers with a drive frequency of 40 kHz.

Material and method

Cycle 1 of membrane cleaning is carried out by rinsing the tubular membranes with clean water with the ultrasonic transducers turned on, without trans-membrane pressure to remove remnants of latex from the system, until milky appearance of rinsed water disappears. A final rinse is done with deionized water (DIW) at a temperature of 50°C with the ultrasonic transducers turned on and a trans-membrane pressure (TMP) of 50 kPa (0.5 bar). Cycle 2 of membrane cleaning is carried out after cycle 1 has been completed by circulating the system with 1% NaOH solution at a temperature of 50°C with the ultrasonic transducers turned on at a trans-membrane pressure of 50 kPa (0.5 bar) for thirty minutes and packing the system with the cleaning solution overnight and rinsing with deionized water the next day.

Cycle 3 of membrane cleaning is carried out after cycle 2 has been completed by circulating the system with 1% NaOH and 0.025% NaOCI solution at a temperature of 50°C with the ultrasonic turned on for thirty minutes and packing the system with the cleaning solution overnight and rinsing with deionized water the next day. Conclusion for Example 1

Membrane flux recovery is calculated as shown in Equation (1).

. _a permeate of deionized water after cleaning cycle

Membrane flux recovery = ^£ — (1) permeate of deionized water when the membrane was unused

1. After cycle 1, the average membrane flux recovery of two membrane modules was recorded at about 55% to 65%. 2. After cycle 2, the average membrane flux recovery of two membrane modules was recorded at about 80% to 85%.

3. After cycle 3, the average membrane flux recovery of two membrane modules was recorded at about 90% to 92%.

Therefore, implementing all 3 cycles of membrane cleaning proved to be most effective in attaining higher membrane flux recovery.

Example 2 : A comparison between natural rubber field latex concentrated using ultrafiltration with ultrasonic application and natural rubber field latex concentrated using ultrafiltration without ultrasonic application

Natural rubber field latex concentrated using ultrafiltration with and without ultrasonic application are compared to show the extent of flux enhancement obtained using ultrasonic transducers.

Material and method

To investigate the extent of flux enhancement induced by ultrasonic transducers, two 100 L batches of natural rubber field latex were concentrated using ultrafiltration on different days. Two membrane modules with a total membrane area of 2.125 m ² were used for the ultrafiltration process. The ultrafiltration process was carried out for nine hours at a transmembrane pressure of 50 kPa (0.5 bar). The first batch was concentrated with continuous ultrasonic application at a frequency of 25 kHz, and the second batch was concentrated without ultrasonic application.

Results and discussion

Figure 7 compares the flux values against processing time with and without ultrasonic application. The flux profiles and other readings obtained are shown in Table 1 and Table 2. Total solid contents (TSC) can also be indirectly calculated from the volume of permeate collected. Table 3 gives the total solid content values of the feed, calculated from the volume of permeate collected at hourly intervals. The initial and the final total solid content were accurately calculated by in-house accredited laboratory.

From Figure 7, it can be inferred that ultrasonic application during ultrafiltration had suppressed the concentration polarizations (CP) from the onset of the concentration process. A constant flux was maintained for the first four hours of concentration. Subsequently there was a drop in flux by 0.26 Liters/M ² /Hr and 0.54 Liters/M ² /Hr during the fifth and sixth hour when total solid content reached more than 50%. This is shown in Table 3 and Figure 8. From the sixth hour onwards, when the total solid content was more than 56%, ultrasonic application could not arrest the concentration polarizations completely which might have allowed gel layer to form on the membrane surface and caused the flux drop to 0.7 Liters/M ² /Hr.

concentration with ultrasonic application

concentration without ultrasonic application Processing time 0 1 2 3 4 5 6 7 8 9 (hours)

Total solid content 31 36 42 47 52 56 60 63 64 65 with ultrasonic

application (%)

Total solid content 31 33 35 37 39 41 42.5 44 45.5 46 without ultrasonic

application (%)

Table 3: Hourly increase of tota solid content values wit 1 and without ultrasonic application Without ultrasonic application, the concentration polarizations caused the sudden drop of flux by 0.55 Liters/M ² /Hr after one hour of processing. The drop in flux continued, all the way until the ninth hour of concentration. After concentration was concluded, the total solid content attained was only 46% for ultrafiltration without ultrasonic application compared to 65% for ultrafiltration with ultrasonic application. With ultrasonic application the required total solid content of 50% was attained after three and a half hours of concentration compared to more than nine hours without ultrasonic application.

Using Darcy's law on resistance-in-series model from Equation (2), filtration resistances are sub-classified into five types and each one impact the filtration process at different stages. where J _v is flux through the membrane (cm/s), ΔΡ is trans-membrane pressure (Pa), μ is dynamic viscosity (Pa.s or gscm ^-1 ). The resistances are as given below (all resistances are in cm ^-1 ): r _m is membrane hydraulic resistance, r _c is concentration polarization resistance, r _g is gel layer resistance, r _ai is weak adsorption resistance and r _a2 is strong adsorption resistance. r _m which remains constant, is the intrinsic membrane resistance, but r _c increases with an increase of total solid content. For this example, the concentration polarization, gel layer, and weak adsorption resistances may be considered to be reversible by clean water and NaOH extraction, while strong adsorption resistance is not. Ultrasonic application during ultrafiltration creates micro-streaming which disrupts the formation of concentration polarization. Subsequently gel layer formation would be delayed. Concentration polarizations would eventually occur even with ultrasonic application, but only at a higher total solid content value. The target total solid content is 60%. Full scale concentration polarizations may occur at a total solid content more than 50% and by the time it reaches 56% to 60% gel layer would have formed on the membrane surface and would start reducing the flux considerably. However, the targeted total solid content would already have been reached when flux declined. Since the concentration polarizations and gel layer formed at a much later stage, r _a i (weak adsorption resistance) would be higher than the r _a2 . Ultrasonic application at higher temperatures would be able to flush out most of the r _al leaving behind a small parentage of r _a2 left in the membrane.

Conclusion for Example 2

1. Time taken to reach a total solid content of 50% with ultrasonic application was three and a half hours while without ultrasonic application it took almost ten hours. Therefore, flux enhancement and percentage reduction in concentration time to achieve a total solid content of 50% due to ultrasonic application can be calculated as follows:

. , mean flux with ultrasonic 3.2

1. Flux enhancement = ■— = 2.3

mean flux without ultrasonic 1.4

.. . . . . . . . reduction in processing time with ultrasonic . _nn a. % reduction in processing time =— _: — x 100 time taken to process without ultrasonic

= ^{10 ~ 3"5} x 100 = 65%

10

)

2. Concentration of natural rubber field latex using an ultrafiltration system with ultrasonic application is environmentally friendly, produces two raw materials and paves way for zero waste scenarios latex concentrate factories.

3. Latex and film properties of latex concentrated using ultrafiltration are superior to that of latex concentrated using centrifugation as shown in Table 4 below:

concentrated using centrifugation a. The properties of films prepared indicate Tensile and EB are slightly higher for latices concentrated using ultrafiltration compared to centrifugation.

b. For the modulus values at one hundred, three hundred, and five hundred a similar trend was observed.

c. The film prepared from latex concentrated using ultrafiltration has a higher tear resistance and is stiffer compared to latex concentrated using centrifugation. Higher tear resistance is an advantage but a stiffer glove is not comfortable to the user.

d. Overall film properties indicate film from latex concentrated using ultrafiltration is somewhat higher in all the parameters tested. In most cases it is superior compared to latex concentrated using centrifugation except for its stiffness.

4. The throughput of an ultrafiltration system could match that of centrifugation, by scaling accordingly (such increasing its membrane area and pump capacity etc.).

Processing latex by ultrafiltration results in discharging reduced quantities of effluent, reduced water consumption, and reduced effluent treatment costs. Options are also available to reuse the clean water produced from the process and also recover membrane cleaning solutions for reuse purposes. Example 3 : Ekoprena 25 (ENR25) latex concentration using ultrafiltration with ultrasonic application at a frequency of 40 kHz

Epoxidised natural rubber (ENR) latex is a structurally modified natural rubber latex. Modification is carried out to incorporate superior properties of synthetic rubber, such as resistance to oil and chemicals. Natural rubber latex concentrate undergoes structural modification through chemical reaction/ Upon reaching a total solid content of 32% to 34% it is subsequently processed into epoxidised natural rubber blocks for the manufacture of green tires. Epoxidised natural rubber can be used as raw material for manufacturing niche latex products. For manufacturing latex dipped goods, epoxidised natural rubber latex has to be concentrated from its original total solid content value of 31% to 50%. The concentration of epoxidised natural rubber latex can only be done by membrane separation process. The concentration of epoxidised natural rubber latex by centrifugation is not feasible, because the difference of specific gravity between epoxidised natural rubber latex and water is too small for any meaningful separation to occur.

Epoxidised natural rubber latex had to be adequately preserved before concentration process is carried out. Although remnants of preservation chemicals from its production stage is still in the latex, additional preservation chemicals still needs to be added to prevent destabilization of the latex when ultrasound is applied to enhance flux. Even with the additional preservation, _, the _. behaviour of epoxidised natural rubber latex during ultrafiltration is quite unpredictable at times. If the preservation chemicals used are not suitable, the feed could become destabilized resulting in the coalition of destabilized latex particles which could eventually cause blockages of the piping and plugging of membrane pores.

Therefore this concentration process was carried out to optimize the use of preservation chemicals and to set standard operating procedures for concentrating epoxidised natural rubber latex using ultrafiltration.

Material and Method

Two sets of membrane modules (with membrane area of 2.125 m ² ) regenerated by thermal cleaning at 300°C and having obtained membrane flux recovery of 96% to 97%, were used to carry out epoxidised natural rubbe (ENR25) latex concentration. The initial total solid content of the feed after the addition of preservation chemicals was 36%. 100 L of this feed was concentrated with a low trans-membrane pressure of 50 kPa (0.5 bar), with the ultrasonic application at a 40 kHz frequency. The flux and temperature profiles of this concentration process are as shown in Table 5. Figure 9 shows flux against processing time.

able 5: Flux > and temperature profiles of ENR25 concentration, using ultrasonic application at 40 kHz frequency

Results and discussions

The initial total solid content of 36% is relatively high. As can been seen from Figure 9, there was a gradual increase in flux for about forty five minutes. This was partly due to the ability of ultrasonic waves to destroy any immediate build-up of concentration polarization. Also, time taken by the system to stabilize and to get the full impact of the trans-membrane pressure also causes increase in flux. As the feed concentration increases due to permeation of the aqueous phase, the rate at which the concentration polarization being built is faster than the rate at which it was destroyed by the micro streaming of ultrasonic waves. Permeation has been curtailed considerably causing a reduction in flux as the concentration increased. After the third hour of processing, the permeation still continued but at a reduced rate. This caused a slow increase in feedstock concentration. Nevertheless, the presence of ultrasound managed to minimize the impact of concentration polarizations and gel layer resistances due to the increase in feedstock concentration. This can be seen by a reduced drop in the flux from the third hour onwards. Subsequently, a drop in flux remained almost constant after at three and a half hours of processing time. The application of ultrasound has enabled a constant flux even when the feed has reached high concentration.

This is because there could be a dynamic equilibrium between, feed temperature increase, reduction in feed viscosity, and the ability of ultrasonic waves to mitigate or diffuse any increase in concentration polarizations and gel layer formed resistances until a critical total solid content is reached.

Conclusion for EExample 3

1. The application of ultrasound enhances flux at any initial concentration and is capable of diffusing concentration polarizations and gel layer resistances even at high feed concentration.

2. The epoxidised natural rubber latex concentrate at a total solid content of 50% can be obtained by ultrafiltration enhanced by ultrasound within three and a half hours of processing time.

3. * Increase in feed temperature due to prolonged ultrasonic application reduces feed viscosity, which in turn enhances flux. The parameters are shown in Figure 10. sd„ ² P _T

J = " ^T (3) 4. Ultrafiltration enhanced by ultrasound is the only option available to produce EN latex concentrate as an industrial raw material to manufacture value added niche latex products. 5. Ideal conditions required to produce epoxidised natural rubber latex concentrate with a total solid content of 50% and a processing time less than three and a half hours are as follows:

i. Optimum feed preservation (feed is preserved with 0.3% ammonia and 0.2% ammonium laurate)

ii. Trans-membrane pressure at 50 kPa (0.5 bar)

iii. Maximum feed temperature of not more than 55°C

iv. Continuous ultrasonic application at a frequency of 40 kHz

Example 4 : Enhancing of membrane permeation flux during the concentration of Ekoprena 50 (ENR50) using ultrafiltration with ultrasonic application at frequencies of 25 kHz and 40 kHz

The following investigation is aimed at comparing the effectiveness of 25 kHz and 40 kHz frequencies of ultrasonic application used for enhancing flux during the concentration of epoxidised natural rubber (ENR50) latex. ENR50 latex is prone to destabilization.

Material and method

3 samples of 65 L of well-preserved Ekoprena 50 (ENR50) latex, with an initial total solid content of 31% were concentrated on different days and were enhanced by ultrasonic application at 40 and 25 kHz and without ultrasound (control) respectively (details of experiments shown in Table 7). For each sample, the concentration process was carried out for six hours, using thermally cleaned ceramic membrane to remove irreversible foulants. This enables the three membrane modules to have almost equal membrane flux recoveries neutralizing any differences in membrane conditions from previous concentration processes. At every half an hour interval, permeates were collected for two minutes and subsequently the flux values were calculated. Using concentration process without ultrasonic application as a control, the percentage increase in flux due to ultrasound at 25 kHz and 40 kHz frequencies for each time intervals were calculated (Table 8). An average percentage of flux enhancement value was obtained for each frequency (Table 9).

Table 6: Experiments on epoxidised natura rub er atex concentration EN 50 wit and without ultrasonic application

Table 7: Flux profiles with ultrasonic application of 40 kHz, 25 kHz and without ultrasonic application

Table 8: Flux enhancement profiles with 40 kHz and 25 kHz frequencies

Results and discussion

Table 7 shows the flux profiles obtained for the concentration processes carried out using 65 L of feedstock for each concentration process of epoxidised natural rubber latex using ultrasonic application with a 40 kHz frequency, 25 kHz frequency and with no ultrasonic application (control). Table 8 shows the percentage of flux enhancement at every thirty minute intervals as well as the average flux enhancements of 26% and 31% which were obtained for the concentration processes using ultrasonic frequencies of 25 kHz and 40 kHz respectively. At lower feed concentration, 40 kHz frequency enhanced membrane filtration process more effectively compared to 25 kHz. At higher frequencies, energy absorption is higher and thus greater acoustic streaming occurs which in turn increases the flow rates compared to the lower frequencies for the same power intensity. This mechanism caused the bulk water movement toward and away from the membrane cake layer, with velocity gradients near the cake layer that scoured latex particles from the surface. Once the feed concentration increased, gel layers which had formed membrane surface reduced the flux.

(25 kHz and 40 kHz) and without ultrasound

From Figure 11, it can be seen that the flux values during the initial three hours of processing time using ultrasound of 40 kHz were higher and this would have enabled in attaining a higher total solid content value at a faster rate, compared to using 25 kHz. However during the subsequent three hours of processing time until the end of the process, the flux values were somewhat lower with ultrasonic application of 40 kHz compared to 25 kHz. This phenomenon could be due to the higher total solid content value attained using 40 kHz compared to 25 kHz. This enabled the concentration polarizations and gel layer resistance to have set in. Ultrasound could not prevent their occurrences completely as the concentration increased. Lower frequency seemed to be more efficient in mitigating foulants of a larger size such as the gel layer resistance.

From this experiment, it was inferred that 40 kHz of ultrasound enhanced a higher initial -fIux-compared-to-25 -kHz— This- could have-enabled-in ^" achievin ^" g ^_ the ^" targ^tHI ^_ totar ^" ^lid content of 50% and a higher final total solid content value at a shorter processing time as can be seen in Table 10. Nevertheless, at a higher feed concentration, 25 kHz was marginally more efficient in mitigating gel layer resistance and this can be seen from Figure 11, where flux were slightly higher during the final stages of the concentration process, compared to 40 kHz. During the entire duration of the three experiments, the preserved ENR50 feed remained stable, as there were no excessive masses of coagulum found on the retentate sieve.

Conclusion for Example 4

1. Both 25 kHz and 40 kHz frequencies of ultrasound enhanced . flux during the concentration of epoxidised natural rubber (ENR50) latex. 2. 40 kHz frequency of ultrasound enhanced overall flux by 31% compared to without ultrasound and attained a final total solid content value of 56.4% during the six hours of processing and was found to be more efficient in preventing fouling by smaller sized particles.

3. 25 kHz frequency of ultrasonic enhanced overall flux by 26% compared to without ultrasound and attained a final total solid content of 51.0 % during the six hours of processing and was found to more efficient in preventing fouling by larger sized particles, such as gel layer.

Example 5 : Thermal cleaning of membranes in a thermal cleaning device

After prolonged use of the ceramic membrane for concentrating natural rubber latex and subsequent chemical cleaning, the membrane flux recovery would deteriorate to below 50% after many cycles of chemical cleaning caused by irreversible fouling. Study showed that the permanent fouling of the membrane is caused by accumulation of latex particles plugging the membrane pores and could no longer be removed by chemical and physical cleaning (ultrasonic application). The accumulated latex particles plugging of the membrane could easily be ashed at temperatures around 300°C. To reduce the use of chemicals and to exploit the versatility of ceramic membrane which could withstand temperatures more than 300°C, thermal cleaning of fouled ceramic membrane was investigated. Material and method

Experiments on thermal cleaning of ceramic membranes were carried out using an in- house designed and fabricated locally muffle furnace, as shown in Figure 6.

Ceramic membranes are usually made from thermal and hydrothermal stable material derived from or in combination of γ-alumina, titania and zirconia. At temperatures lower than 900°C (alumina), 600°C (zirconia) and 450°C (titania), these membranes and are stable for phasing and structural transformation. Ceramic membranes which were made from zirconia-titania (Zr0 ₂ - Ti0 ₂ ) in combinations were used for this study. This specially designed muffle furnace which measures a length of 1400 mm and could accommodate a total of 14 membranes of 1020 mm length and 20mm of external diameter, arranged systematically. It is also incorporated with a slow heating-cooling facility and can attain a highest temperature of 600°C. It was used to identify an ideal temperature to carry out the thermal cleaning of fouled ceramic membrane. The totally fouled membranes from latex concentration process which is known to be plugged by latex particles and latex proteins and could easily be ashed at temperatures below 300°C.

Fouled membranes were placed in the muffle and were heated at temperatures 100, 150, 200, 250, 300 and 350 °C each temperature separately for different membranes and were allowed to cool down slowly, as per procedure. The cooled membranes were observed carefully all along the entire 1020 mm length of the surface using a magnifying glass. No structural damages found. The membrane flux recoveries obtained were as shown in Table 2 (average of three readings) respectively.

This membrane cleaning method was only employed for membranes which were completely fouled and could not be rehabilitated by chemical and physical cleanings. Heating of the tubular membranes is done for 20 minutes and they are then allowed to cool slowly. Thermal cleaning does not require chemical usage and uses no water except for conducting water flux tests. Heating at lower temperatures of 100°C and 150°C, the flux recoveries obtained are as shown in Table 9.

heating temperatures

Conclusion for Example 5

It can deduced from the results of Example 5, that an ideal temperature for thermal cleaning should be around 250 - 300°C. General Conclusion

It can be concluded that the invention provides for an environmentally friendly process for concentrating latices. The system and process of this invention does not produce any harmful chemicals such as those produced when natural rubber field latex is concentrated using centrifugation, for example.

Concentration of natural field rubber latex is greatly enhanced by ultrasonic application. This can be seen in Example 1 where a targeted total solid content of 50% is reached within three and a half hours in an ultrafiltration system with ultrasonic application. In an ultrafiltration system without ultrasonic application, the targeted total solid content of 50% took almost ten hours to reach. This is because ultrasonic waves mitigate or diffuse any increase in concentration polarization and gel layer resistances during ultrafiltration and premature membrane fouling was prevented. Epoxidised natural rubber latex can be only be concentrated using membrane separation. From the results of the tests carried out, it was concluded that concentration of epoxidised natural rubber latex (ENR25) using ultrafiltration is enhanced when a frequency of 40 kHz is applied. The ultrasonic waves destroyed build-up of concentration polarization and gel layer resistances formed on the tubular membranes during the ultrafiltration process.

In the concentration of epoxidised natural rubber latex (ENR50) using ultrafiltration, it was found that a frequency of 40 kHz was more effective in enhancing ultrafiltration during the first three hours of ultrafiltration. After the first three hours, a frequency of 25 kHz was more effective. This is because a frequency of 40 kHz is effective in preventing fouling by smaller sized particles which are formed during the initial stages of ultrafiltration and a frequency of 25 kHz was more effective in preventing fouling by larger sized particles such as gel layer which is formed after the later stages of ultrafiltration. Cleaning of tubular membranes during the cleaning cycle is also enhanced with ultrasonic application. Further, the cleaning chemicals used and how many cleaning cycles are carried out play a part in the recovery of membrane permeation flux. Three cycles of cleaning firstly with deionized water, secondly with a 1% NaOH solution, and lastly with a 1% NaOH and 0.025% NaOCI solution proved to give a higher recovery of membrane permeation flux.

It can also be concluded that a thermal cleaning device such as a muffle furnace can further improve membrane permeation flux. Tubular membranes are placed in the muffle furnace to undergo ashing. The ashing process removes rubber particles and scales of latex proteins that are plugging membrane pores, and subsequently restores membrane permeation flux. The ideal temperature for thermal cleaning should be around 250°C - 300°C.

As will be readily apparent to those skilled in the art, the present invention may easily be produced in other specific forms without departing from its scope or essential characteristics. The present embodiments are, therefore, to be considered as merely illustrative and not restrictive, the scope of the invention being indicated by the claims rather than the foregoing description, and all changes which come within therefore intended to be embraced therein.