|
Soap Industry Introduction The industry manufactures soap by saponification of fatty matter with caustic soda. Fatty matter + Caustic soda �� Soap + glycerol Manufacturing process The process involves boiling of fatty matter with a slight excess of strong caustic soda. Brine (NaCl) washing is employed to remove glycerol and colour, and separate three layers namely neat soap, niger and lye. The lye is an effluent in this saponification process and the industry discharges about 4 tonnes of this spent lye per day to the surface drains. In addition a small amount of floor wash water including colouring materials and oil is also discharged to the surface drain. Effluent generation The lye is a main effluent source in this industry and it mainly consists of unreacted fatty matter, caustic soda, sodium chloride and glycerol. According to the above effluent analysis, it is shown that the effluent is highly polluted and that it should not be discharged into the surface drains. The effluent should be treated to satisfy the tolerance levels specified for discharge into inland surface waters. Treatment could be done either to recover glycerol and sodium chloride from spent lye or by increasing the glycerol content of the spent lye to an economical level. Waste Minimization Options available There are several methods available in the literature for recovery of glycerol from the spent lye. In each case pre treatment of lye is necessary before it is subjected to evaporative concentration. The process is as follows: Step A The lye should be allowed to cool and solid soap should be skimmed off or filtered. This soap can be re-circulated in the process. Initially the lye is boiled with fatty acid to remove free caustic and soda ash. It is desirable to have two or more spent lye tanks with a total storage capacity of about one week's production to allow for cooling and skimming while the operation is carried out smoothly. If the lye is found to be lower than the desired quality, then preliminary chemical treatment with calcium chloride and lime is necessary to remove a portion of the impurities present. Step B Addition of an acid and a coagulant followed by filtration removes the bulk of impurities. Alum (Al2(SO4)3� 14 H2O) or Ferric chloride (FeCl3� 6H2O) is used as a coagulant and muriate acid (HCl) can be used for the neutralization. The quantity of coagulant required can be determined by adding it until colourless filtrate is formed. pH should be maintained between 4.5 - 5.0. The lye sample received requires alum about 1% (by weight) for the precipitation of the bulk of impurities and to produce a colourless filtrate. After adding appropriate amounts of alum and muriate acid the spent lye should be mixed thoroughly in a container by providing mechanical or air agitation. Then this slurry is filtered through a filter press using a suitable filter cloth. The filter cake should be washed thoroughly and wash water can be recycled several times until its glycerol concentration reaches to optimum washing level. The dried filter cake can be disposed safely or utilization can also be considered. Step C The pH of the filtrate from the above treatment should be raised up to 8.5 to 9.0 by adding caustic soda (NaOH) to remove excess Aluminium present. The pH range is very important in this treatment, otherwise the excess alkalinity will cause glycerol polymerization during evaporation. The clear liquid from this operation can be either filtered or decanted. The first filter press should not be used for the filtration of this solution. This will prevent the reaction of fatty acid present in the filter cake with caustic soda. Step D The dilute glycerol liquors after being purified as described above are concentrated to crude glycerin by evaporation. This is usually carried out in conventional, natural circulation evaporators under reduced pressure using steam. Spent lye which deposit salt crystals on concentration require a salting out type evaporator. The standard, vertical short tube evaporator specially adapted for salting out liquors is the most commonly used unit for spent lye. Otherwise longer tubes tends to salt up more readily. At the bottom of the evaporator a salt box is provided for disposing of salt which forms, as the spent lye is concentrated. The slurry collects in the slurry box and is blown periodically as required with air and steam to salt handling equipment for separating the glycerol liquor from the salt. In an older type salt box, the salt is filtered on an internal screen, washed, steam dried and finally scraped out manually through a door in the side of the salt box. Standard material of construction for spent lye evaporators are cast iron for the body and copper tubes and tube sheets for the calendria. Serious losses can take place by the entrainment of glycerol liquor in the vapor passing out of the evaporator. These losses can be minimized by a sufficiently large and high vapor space or by a effective entrainment separator. When two evaporators are operated as a double effect the total rate of throughput is almost the same as for the single effect. But the quantity of steam and condensing water required is reduced almost by half. In normal operation, glycerol liquors are concentrated in two steps. In these steps they are brought to a half crude state and discharged from the evaporator to a holding tank. The half crude containing about 40% glycerol is stored until it reaches a sufficient quantity to return to the same evaporator and to produce to crude in one step, since the evaporation rate is greatly retarded in the later stages as the liquor becomes more concentrated. A good overall rate of production is possible with a two step concentration. In the double effect evaporation, the two evaporators may be operated as single effects for concentrating the half crude to finish crude. However, for more economic utilization of equipment it is preferable to provide a smaller single finishing evaporator. Two step concentration of glycerol liquors also facilitates the subsequent processing of the crystallized salt. The major portion of the salt is always obtained during the half crude state. At this stage the crystals obtained are larger and more easily washed and drained. The first effect evaporator operates under a vacuum of about 10 - 12" Hg and the second at about 26" Hg. When the salt boxes are full, the salt slurry is discharged to the salt handling plant where the adhering liquor is separated and returned to the process. The boiling temperature of the second effect evaporator is about 60 to 650 C for half crude glycerin. The crude finishing evaporator also operates under vacuum of 26 - 27" Hg and boiling temperature of about 850 C. Salt slurry is also removed from the finishing evaporator as and when required. Crude glycerin containing about 80% glycerol can be obtained at this stage. Using centrifuging or other filtration method can separate salt from the slurry. The dried salt can be reused for the process.
If the industry is unable to find out the method for utilization of crude glycerol, or cannot install a glycerin recovery plant, it is necessary to have an alternate disposal method for this effluent. By increasing the glycerol content of the spent lye up to economical level, it is possible to sell to some other glycerin manufacturing industry as a raw material. There are several systems described in literature for the manufacture of soap by saponification of fatty matter using caustic soda. The spent lye discharged from the three soap manufacturing methods analyzed is as follows: Table 1: Analysis of spent lye from soap manufacturing methods
Note: The lye ratio is weight of spent lye per unit weight of 63% TFA soap. Therefore it is necessary to consider the improvement of glycerol content in the spent lye without affecting the present quality of the soap by developing the existing technology. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
DANA MITRA LINGKUNGAN {Friends of the Environment Fund} Pusat Niaga Duta Mas Fatmawati, Blok B1/12 Jl. RS. Fatmawati 39, Jakarta 12150 - INDONESIA Telp. : (62-21) 724 8884, 724 8885 | Fax. : (62-21) 724 8883 Email : [EMAIL PROTECTED] | URL# http://www.dml.or.id Konperensi ke 4 APRCP @ http://aprcp.dml.or.id FORLINK @ http://forlink.dml.or.id Bursa Limbah @ http://w2p.dml.or.id Forum KMB Indonesia @ http://forumkmb.dml.or.id Join Milis PB, kirim email ke mailto:[EMAIL PROTECTED] ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ -- ~~~~~~ PRODUKSI BERSIH (PB) MAILING LIST ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Posting = [EMAIL PROTECTED] Berhenti = Kirim Email kosong ke mailto:[EMAIL PROTECTED] Berlangganan = Kirim Email kosong ke mailto:[EMAIL PROTECTED] Administrator = mailto:[EMAIL PROTECTED] Arsip = http://www.mail-archive.com/[email protected]/ FORLINK @ http://www.forlink.dml.or.id Environmental News @ http://forlink.dml.or.id/e-news/ Forum KMB Indonesia @ http://www.forumkmb.dml.or.id Bursa Limbah Indonesia @ http://www.w2p.dml.or.id ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ |
