Abstract

Abstract of Biodiesel Making Process

Purification of crude biodiesel is mandatory for the fuel to meet the strict international standard specifications for biodiesel. Therefore, this paper carefully analyzed recently published literatures which deal with the purification of biodiesel. As such,dry washing technologies and the most recent membrane biodiesel purification process have been thoroughly examined. Although purification of biodiesel using dry washing process involving magnesol and ion exchange resins provides high-quality biodiesel fuel, considerable amount of spent absorbents is recorded, besides the skeletal knowledge on its operating process. Further, recent findings have shown that biodiesel purification using membrane technique could offer high-quality biodiesel fuel with less wastewater discharges. Thus, both researchers and industries are expected to benefit from the development of membrane technique in purifying crude biodiesel. As well biodiesel purification via membranes has been shown to be environmentally friendly. For these reasons, it is important to explore and exploit membrane technology to purify crude biodiesel.

Biodiesel dry washing process

Dry washing is another most regularly used technology in the dynamics of biodiesel purification process. This process is achieved via application of adsorbents such as Amberlite, purolite, cellulosics, Magnesol, Trisyl, activated carbon, activated fiber, and activated clay. Adsorbents comprise of basic and acidic adsorption sites and can easily attract polar substances, which include glycerol and methanol etc. To make the process more effective and efficient, a filter unit is provided as depicted in Fig. 1. The process is operated at 65°C and refining process is completed in 20–30min.

During dry washing, both glycerides and total glycerol are reasonably reduced to appreciable levels. The dry washing procedure is waterless, improves fuel quality, is simple to incorporate into existing plant, reduces washing time and zero waste- water, minimizes total surface area coverage of wash tank, and saves space . It was found by,that dry washing procedure for the refining of crude biodiesel decreases cost of production and lowers production time. The process offers quality biodiesel and because no water is added, the possibility of achieving water content below 500 ppm as specified by ASTM D6751 is significantly high. It is worthy to note that, in water washing process, fuel water content is mostly higher than 1000ppm, hence rendering water removal costly, complicated, and time-consuming . As well, samples of esters prepared from waste frying oil (WFO) were refined by means of rice husk ash (RHA) at varying concentrations of 1%, 2%, 3%, 4% and 5% (w/w), which were then compared with refining techniques; commercial adsorbent Magnesol® 1% (w/w) and conventional acid solution aqueous H3PO4). Rice husk ash with a concentration of 4%, presented excellent result in removing impurities in biodiesel as compared to other concentrations. The high adsorption capacity of impurities is as a result of high concentration of silica in its composition and the presence of meso and macropores. Hence, the rice husk ash, which is a by-product of rice processing, appears as a substitute material to refine crude biodiesel that ion exchange resin and magnesol powder are pioneered to replace water washing process.

Dry washing processes are easily adopted industrialized plants. The use of magnesol to refine biodiesel requires thorough mixing and needs 1.5–3 wt.% of biodiesel. Consequently, magnesol was used to refine biodiesel produced from soybean and grease, and it was found that both physical and chemical properties of the fuel met the required specification designed by ASTM D6751 and EN 14214. The polar compounds are attracted by magnesol, therefore effectively removing impurities such as Metals, alcohols, di-glycerides, glycerol and soap. Furthermore filtering of biodiesel mixture is easily achieved via cloth filter of size 5μm, with 1μm nominal filter used to perform final filtration process. Biodiesel is finally polished via filter (0.45μm or 0.55μm) prior to being applied as diesel engine fuel. In addition, application of magnesol to refine biodiesel was performed by, and the products obtained were found to offer properties similar to the values specified by ASTM D6751. Similarly, magnesol was employed by [38], to refine crude biodiesel with different concentrations of 0.25%, 0.50%, 0.75% and 1.00% at a temperature of 60°C, by means of a batch reactor. Thereactor of size 200ml immersed in a water bath was used for the purification process. Biodiesel samples were removed from the reacting vessel at an interval time of 10 and 20 min, but 30 min was the standard time for the washing process.

Thus the intermediate products were removed via a centrifuge. Separation of the final biodiesel product was performed using a Büchner funnel and water ejector. Consequently, biodiesel with free glycerol content of 0.03% and methanol content of 0.51% was recorded. It important to note that, when magnesol bag is opened, care must be taken to re-seal the bag as tightly as possible since magnesol is hygroscopic. Thus masks must be used to handle magnesol, in view of the fact that the powder is very fine[38].Nonetheless magnesol is limited because little is known about its performance and its catalytic efficiency.

In a similar study by , magnesol® was efficiently used to eliminate numerous impurities in esters produced from used cooking oils. The magnesol® used was effective in removing methanol, glycerol and soaps formation. For the 0.50 – 1.00 wt.% of magnesol®, methanol and Soaps formation were reduced by around 98% and 67–92% respectively, The glycerol reductions were lower which range from 15% to 55%. Magnesol® with a concentration of 1.00 wt.% and 600 rpm provided the best results. The international standard, EN 14214 was met in terms of density, acid value, free glycerol, methanol, triglycerides and mono-glycerides contents. In the same way an investigation by employed adsorbent to refine crude biodiesel and the results obtained from magnesol 1% and silica 2% were 61 ppm of soap, 500 mg kg−1 of water, 0.17mg KOH g −1 for acid number, 0.03% of free glycerol and 0.22% of methanol. The results obtained for free glycerol were above the specified minimum by ASTM D6751 standard.

In another investigation, biodiesel was refined via activated carbon, activated clay, activated fibers, and acid clay. As well, glycerol was employed as a solvent to remove contaminants from biodiesel. Furthermore, biodiesel iseffectively purified with clay grain size that ranged between 0.1 mm and 1.5 mm. Better biodiesel purification process is achieved via clay with smaller grain size. However removal of spent adsorbent proves more cumbersome. When the size of clay grain is larger, purification process is inferior but separation after the treatment becomes simple. Usually, activated carbons are mostly employed as adsorbent for the removal of excess color in biodiesel. In another study, purified crude biodiesel using activated carbons was prepared from spent tea waste.

They used a chromatography column with internal diameter 1cm and a height of 15 cm which was packed with glass wool. The column was then filled with a bed of 2g of activated carbon. Subsequently, crude biodiesel samples were passed through the adsorbent bed of the column with a flow rate of 15drop/min. Biodiesel with high yields and physicochemical properties was obtained after refining via activated carbons besides, the limits specified by ASTM D6751 for biodiesel fuel were achieved. In addition, the spent activated carbons used for the biodiesel refining process were regenerated and reused. The results obtained showed compared to the other methods the activated carbons regenerated presented biodiesel with better fuel properties as well as higher yields. In addition, biodiesel silica refining was reported to offer easy and efficient procedure for removal of glycerides and FFA impurities in crude biodiesel . Hence, silica gel was experimented in the adsorption of glycerol and monoglycerides. The values obtained indicated efficiency and robustness of adsorption of glycerol and monoglycerides. Consequently, purification of biodiesel was carried out through a settling separator and a silica adsorption unit as an alternative to the currently used water washing technique.

In this sense, different adsorbents such as diatomaceous earth, impregnated activated carbon and bleaching using activated carbon were utilized to refine biodiesel produced from chicken oil containing high degrees of acidity. The process was carried out at an ambient temperature in a gas tight autoclave with stirring. The loading of the adsorbent was 0.3 – 5 wt.% and the contact time was 720 min. The experiments by means of silicagel as adsorbent were carried out in a vacuum (0.21 bar) with stirring at a temperature of 90°C for 90 min. The results showed that glycerol was almost completely removed and a significant reduction of glycerides was made by the adsorption process. Such process allowed the eradication of water washing steps, consequently minimizing wastewater effluents issues. Further, after methanol and water stripping, silica refining enabled the removal of glycerides and traces of FFA from biodiesel, hence fulfilling the EN 14214 limits for glycerides, FFA and methyl ester contents. Another suitable adsorbent used for the purification of biodiesel is Purolite (PD206), which is a dry polishing media purposely designed to eliminate impurities in biodiesel . Although purolite (PD206) is sold as ion exchange resin; the suppliers do not promote its revitalization since it is acting as adsorbents. The use of ion exchange resins for the refining of biodiesel samples was investigated by, at a time of 2h for removal of glycerol and methanol and established that ion exchange resin can proficiently decrease glycerol to a level of 0.01wt.% and significantly eliminate soap; however, methanol is not effectively removed. The results showed methanol content of 1.14%, and this value is more than the value provided by EN14214. Besides, little soap adsorption points to a restriction for feedstocks having higher content of soap formation, Ion exchange resins provide excellent performance in the eradication of water, glycerol, removal soap, salt and catalyst, and it is also a waterless process . But, its ability to remove methanol is low.

Membrane separation process

Module configurations include among others tubular, hollow fiber, spiral wound and rotating devices and flat plate. Tubular modules are commonly applied where it is beneficial to have a turbulent flow regime, for instance, in the concentration of high solid content feeds. The membrane is cast inside of a porous support tube which is often housed in a perforated stainless steel pipe. Individual modules contain a cluster of tubes in series held in a stainless steel permeate shroud. The tubes are generally 1–6m in length and 10–mm in diameter. Tubular modules are cleaned without difficulty and a good deal of operating data exists for them. Their major disadvantages are the fairly low membrane surface area.

The separation of solution components via membrane is achieved by restricting thepassing of unwanted material via a semi-permeable barrier in a selective manner. The transportation using membrane is affected by diffusion of individual molecules, temperature or pressure gradient and concentration difference. In the refining of water, gas separations and protein separations, membrane technology has played a critical role, but industrial use of membrane technique has limited separations to relatively inert gases and aqueous solution. Therefore application of membrane technology in treating nonaqueous fluids is an area that is being newly explored. The membranes used for the pressure driven separation processes, are reverse osmosis (RO), ultrafiltration (UF), and microfiltration (MF). Further separation using membrane is mainly a size exclusion-based pressure -driven process, and theseparation of components is based on shapes and the sizes or weight of the particles.

During separation process, the performance of membrane is affected by interactions between membrane surface and components of the feed, velocity of flow, temperature, pressure, and membrane composition . Membranes are classified into organic and inorganic membranes. Inorganic membranes such as ceramic membranes have great potentials toward aiding purification processes. Inorganic membranes (Al2O3, TiO2, ZrO2, SiC) are presently given much attention for been superior to organic membranes in terms of increased resistance to fouling, long lifetime, narrower pore size distribution, mechanical, t hermal and chemical stability, resistance to microbial degradation, high flux, and high porosity . The organic membranes include poly sulfone, polyamide, poly carbonate and a number of additional highly developed polymers. Most of these synthetic polymers comprised of improved resistance to microbial degradation and chemical stability . It was commented by , that polyacrylonitrile (PAN) is asymmetric and porous membrane which joins high permeation rate and high selectivity, but polymeric membranes may swell up, which result in either instant swelling or long-term pore-size changes.

Therefore, application of polymeric membranes in solvent could lead to short life span . Biodiesel organic membrane separation process Membrane technology has been playing a vital role in the refining of various products. Consequently, hollow fiber membrane extraction(poly sulfone) was used to remove impurities in biodiesel. The hollow fiber membrane with the following dimensions: length: 1 m, diameter: 1 mm, was filled with distilled water and immersed into the reactor at a temperature of 20 °C. The unrefined biodiesel was transferred into the hollow fiber membrane with a low rate of 0.5 ml/min, and pressure of 0.1 MPa.

Subsequently the crude biodiesel was passed over heated sodium sulfate and then properly filtered. This procedure successfully decreased the loss of yield during the refining process and avoided formation of emulsion during the washing step. Further biodiesel purity of about 90% was obtained and the physicochemical properties met the ASTM standards. The results obtained showed that membrane extraction is very promising toward purifying biodiesel samples.It was found by, that membrane technique can be employed in refining crude biodiesel without involving water washing process.

The membrane systems could enhance treatment of effluents, and valuable products recovery, hence minimizing their atmospheric harmful effects and offering solutions to many ecological problems commonly faced during the refining of crude biodiesel. The polyacrylonitrile (PAN) membrane was tested at 25°C in order to eliminate glycerol, methanol, water, and soap. With the addition of little amount of water, removal of glycerol from biodiesel was considerably increased and glycerol content as low as 0.013 mass% was obtained, which is lower than the value (0.020 mass%) specified by ASTM D6751 standard. Removal of impurities from biodiesel via ceramic membrane process Recently, ceramic membrane technique is employed in separating and purifying bio diesel samples, particularly removal of glycerol from biodiesel. Biodiesel was produced by means of ethyl transesterification of degummed soybean oil and evaluated use of micro-and ultra filtration with ceramic membranes to remove glycerol from biodiesel.

Tubular α-Al2O3/TiO2 membranes with average pore diameter of 0.2, 0.1 and 0.05μm and 20 kDa were used to perform the experiments. The experiments were performed using the following conditions: temperature of 50°C, trans membrane pressures (TMP) of 1.0, 2.0 and 3.0 bar, and mass concentrations of acidified water of 10%, 20% and 30%. The separation process was effectively carried out using mass concentration of 20% acidified water; however, a sharp reduction of flux over the filtration time was noticed which was attributed to the fouling of the retained phase on the membrane surface. At a mass concentration of 30% the aqueous phase containing glycerol was observed to permeate through the pores of the membrane. Further, addition of 10% of acidified water, promoted retention of aqueous phase containing glycerol which yields biodiesel with glycerol content below 0.02 wt.%, besides the fouling on the membrane was significantly reduced. The separation of glycerol through ultra filtration was advantageous since it eliminated the settling step and led to reduction of the amount of water required for washing.

The properties of biodiesel produced met the standards provided by ASTM D6751 for commercialization. In a similar study, It was noted that removal of free glycerol from crude biodiesel is a most critical factor.

This process was effectively performed using microfiltration with tubularAl2O3/TiO2 ceramic membranes, which involved average pore size of 0.2, 0.4, and 0.8 μm, and filtration area of 0.005m2. The samples of biodiesel were micro filtered at a temperature of 60°C, and trans membrane pressures of 1.0, 2.0, and 3.0 bar. For the feed solution with 5% ethanol, the lowest flux decline rate and 99.6% glycerol retentionwere recorded. The technique involving refining of biodiesel appears to be efficient, thus offering high quality biodiesel, as well as minimizing energy usage. In another study, biodiesel was refined using a ceramic membrane with a pore size of 0.02μm in order to achieve biodiesel fuel that meets both ASTM D6751 standard specifications. The membrane system used in the refining process was effectively developed and the process variables which include temperature, flow rate and transmembrane pressure were examined. The interaction between various process variables was well understood using central composite design (CCD) coupled with Response Surface Methodology (RSM). The best operating conditions were found to be temperature, 40°C, flow rate, 150L/min and trans membrane pressure, 2 bar.

These conditions provided permeateflux of 9.08 (kg/m2h), with corresponding potassium value of 0.297mg/L and free glycerol value of 0.007 wt.%; these values are quite lower than those specified by ASTM D6751 standard specifications. The membrane system also presented biodiesel with physical properties that are comparable to those of ASTM D6751 and EN 14214 standards. These showed that ceramic membrane with pore size 0.02 μm could provide high - quality biodiesel. In another similar study , membrane reactor (tube length: 1200mm, pore size: 0.05μm, surface area: 0.022m2, internal diameter: 6mm, and external diameter: 8 mm) was employed in producing biodiesel. The reactor is particularly useful for its the amount of soap present in the biodiesel. But, the proposed microfiltration process could not efficiently reduce the free glycerol content compared to water washing.

Also, the ultra filtration membrane of 30kDa could not reduce capability in retaining the unreacted triglyceride, which led to the achievement of high - quality biodiesel. The main advantage of the process is the achievement of free triglyceride esters. In another investigation , micro - and ultrafiltration techniques were proposed to be used in removing impurities from biodiesel. At different transmembrane pressures, the biodiesel mixture was filtrated in a dead -end process and using membranes of different pore sizes. Permeate fluxes obtained showed that greater pore sizes as well as greater transmembrane pressures enable greater fluxes. The properties of the produced biodiesel such as viscosity, density, and acid values met the international legislation for biodiesel quality. The process of water washing and membrane separation considerably reduced biodiesel glycerol content to the international legislation for free glycerol content. However, the ultra filtration membrane of 10 kDa provided biodiesel with glycerol content that is less than 0.02 wt.%. Addition of water in the raw biodiesel enhanced removal of glycerol via membrane filtration. nature of the membrane employed. Therefore membrane fouling is firstly controlled by careful choice of membrane type. Secondly, a good choice of module design offers appropriate hydrodynamic conditions for the particular application. Equally, process feed pretreatment is also essential. The degree of membrane fouling is dependent on the properties of the process feed and on the When membrane fouling occurred, the permeation rate can be substantially restored through back-flushing of the membrane.

Nevertheless, this is rarely totally effective; thus, chemical cleaning is eventually required. On start-up of a process, a reduction in membrane permeation rate to 30 – 10% of the pure water permeation rate after a few minutes of operation is common for ultra filtration. Such a quick decrease may be even more intense for microfiltration. This is often followed by a more steady decrease throughout processing. More recent approaches to the control of membrane fouling include the use of more hydrodynamic control affected by pulsated feed flows or non - planar membrane and the application of further perturbations at the membrane surface such as continuous or pulsated electric fields .

Membrane cleaning process

During membrane cleaning process, components are removed physically, chemically or hydraulically. A module is temporarily out of order, when the cleaning process is performed. As a result, dead- end management is a discontinuous process. The length of time that a module performs filtration is called filtration time and the length of time that a module is cleaned is called cleaning time. In practice one always tries to apply the lowest possible cleaning time and make filtration time last as long as possible. In addition, ceramic membranes are ideal for in- place chemical cleaning at high temperatures, while using hydrogen peroxide, chlorine, caustic, steam sterilization and/or ozone and strong inorganic acids. These membranes can also be back- pulsed, which is essentially a permeate flow reversal technique to decrease fouling and to increase filtration efficiency. Back - pulsing is an in situ technique for the cleaning of membrane by periodically reversing the permeate flow by applying pressure to the filtrate side. In this manner, permeate liquid is forced back through the membrane to the feed side. This permeate flow reversal dislodges deposited foulants, which are then carried out of the membrane module by the tangential flow of retentate, or which may re - deposit on the membrane surface later on . The cleaning process is more efficient when using trans membrane pressure of 0.45 bar.

It was suggested that an efficient way to clean and recover permeate flux of a ceramic membrane that is submitted to crude soybean oil ultrafiltrat ion, is by using only hexane. In this way it can help to the understanding of the behavior of the cleaning process of degumming of crude oil using ceramic membranes . Successes and failures of biodiesel refining techniques The Refining of crude biodiesel is first and foremost done to ensure that high -purity biodiesel fuel that can be conveniently used on diesel engines is achieved . The processes developed for the purification of crude biodiesel have considerably played a vital role in ensuring that high - quality is achieved. However these processes are limited; for instance, water washing presents huge amount of wastewater discharges as well as high energy consumption. Owing to the problems associated with water washing, dry washing was introduced which is also faced with problems such as lack of spent adsorbent regeneration and little knowledge of its chemistry. The difficulties commonly encountered by the two prominent traditional washing processes have called for the exploration and exploitation of biodiesel refining via membrane systems. In this context, the membrane reactor seems an appropriate candidate to be used producing biodiesel, due to its capacity to keep unreacted triglycerides. Besides, development of separative membrane to refine crude biodiesel has provided more impetus in the dynamics of biodiesel refining processes. Although membrane process has provided biodiesel with good physicochemical properties more efforts are required to determine the pattern of membrane fouling during biodiesel membrane refining process.

It is worth mentioning that the key factor to industrial production and application of biodiesel is its level of refining; thus, the need to continue developing structures that will assist in achieving highly refined biodiesel is paramount. Achieving high-quality biodiesel could offer benefits such as decrease in fuel injector blockages, better quality exhaust emissions and better lubricant properties, reduction in elastomeric seal failures, reduced corrosion owing to absence of soap, catalyst, and glycerol, and reduced engine oil degradation thus offering high engine performance.

Conclusions and recommendations

Although, dry washing using ion exchange resins and magnesol leads to refined biodiesel products, the process does not provide product that meets the required methanol value by EN14214. Besides, the process generates spent adsorbent that is not regenerated. Consequently, development of membrane process is essential to explore its intrinsic characteristic of operating under reasonable conditions. Membrane process was found to produce biodiesel with 0.007 wt.% glycerol content, and this value is lower than the one provided by EN14214 and ASTM D6751. As well the process generates almost zero wastewater and was demonstrated to be requiring a lesser amount of energy. advantages provided by membranes have made the process to be more environmentally benign in comparison with other purification process such as dry washing. In addition, considerable removal of glycerol and unreacted triglycerides has put membrane technique far above a number of dry washing technique.