The fatty acids are predominantly oleic and linoleic Tall Oil Soap Recovery acids with small quantities of linolenic, stearic and palmitic acid. The rosin acids are monocarboxylic C. Douglas Foran diterpene acids having a general formula C20H30O2. Arizona Chemical Company, The predominant rosin acids are abietic and de- Savannah, GA 31402 hydroabietic acids, although numerous distinctive E-mail: [email protected] isomers occur. Both abietic and dehydroabietic acids contain three six-membered rings. Dehydroabietic Abstract acid contains one unsaturated aromatic ring, and thus This chapter describes what tall oil soap is and how differs from abietic acid, which contains three much should be recovered based upon the wood unsaturated rings. The neutral fraction, often called supply and season of the year. It further describes unsaponifiables, is a mixture of a variety of the benefits of good soap recovery including substances including phytosterols, fatty and wax improved evaporator and recovery boiler throughput alcohols, terpenes and hydrocarbons (3). and reduced effluent toxicity. Process variables that Table 4.3.1. Composition of crude tall oil (3). affect soap solubility are discussed and related to how pulping process changes affect where soap South- Northern Scandinavia separates. Soap removal during washing is discussed eastern USA & as well as those variables that most affect soap USA Canada skimming efficiency. Methods for improving soap Acid No. (1) 160 - 175 125-135 120-140 skimming efficiency and guidelines for soap Resin Acids % 35 - 45 25 - 35 20 - 30 handling equipment are also discussed. Factors Fatty Acids % 45 - 55 50 - 60 50 - 60 affecting soap liquor separation during storage and Unsaps % 7 - 10 12- 18 18 - 24 handling prior to acidulation are also discussed. Considerations affecting the recycle of tall oil plant 1. Acid Number - The number of mil-equivalents brine are presented. of potassium hydroxide required to neutralize one gram of tall oil (1) Introduction What is Tall Oil Soap? The extractives in black liquor are partially The alkaline pulping of softwoods in the kraft solubilized and, as a result black liquor is a colloidal process converts the resin acids and fatty acids in system. The extractives may be in several different wood to their sodium salts. Associated with the salts states in black liquor, including true solution, are neutral or unsaponifiable compounds. The salts micellar solution, liquid crystalline phases and, and unsaponifiables separate from the spent cooking eventually, neat soap. Most of these colloidal states liquor as black liquor soap. occur simultaneously and are in a sort of equilibrium with one another. The aggregative state of the soap “Black liquor soap naturally separates from black depends on the concentration of the dispersed soap as liquor at various points in the pulping process. The well as on the salt content and solids content of the soap separates as a lamellar liquid crystalline phase liquor. Since the weak black liquor is concentrated (1). It is this phase separated material that may be in evaporators prior to combustion in the recovery observed floating in pulp washer vats, at the surface furnace, the black liquor colloid is continually of filtrate and weak black liquor tanks, and in large undergoing stresses. The stresses result from a amounts at the foam tower and black liquor soap continual increase in solids content as the water is skimmer. The black liquor soap skimmings are evaporated from the black liquor. As the water is collected from various locations and are pooled removed, the salt content of the liquor increases, together in a central holding tank where the causing a natural separation of any organics present. skimmings are held prior to acidulation”(2). The fatty acid and rosin acid salts form micelles that Black liquor soap skimmings consist of a mixture of solubilize the neutral fraction of the extractives. fatty and rosin acid salts, (i.e., anionic surfactants), Micelle formation, which indicates the onset of phase fatty and rosin acid esters, and neutral components. separation, depends on a number of factors. These Black liquor soap skimmings, being a natural include salt content, fatty to rosin acid ratio and product, contain hundreds of different compounds. temperature. Generally, as the salt content increases, The predominance of a given constituent varies with the concentration of soap at which micelles form tree species, pulping chemistry and genetic factors. 3.7-1 decreases; i.e., the soap is less soluble. The soap for the mills from various wood procurement separation of the soap is dependent on many regions is as follows: variables such as wood species, dissolved solids content of the liquor, temperature and residual Table 4.3.4. Typical tall oil yields (3) effective alkalinity. (3,4,5) Region Tall Oil Tall Oil kg /1000 kg OD lb./ODT Wood How Much Tall Oil Soap Is Present and Wood Recoverable? Piedmont 24 48 The quantity of tall oil soap recovered by a given Coastal 26 52 mill varies according to wood species, season of the Canada 8.5 17 year, and wood storage practices. The quantity of tall Southwestern 31.5 63 oil available in several southeastern wood species has West of Cascades 7.5 15 been determined by solvent extraction (6). This Finland 19.5 39 method tends to overstate the true availability but is Sweden 25 50 valid for assessing the different wood species by the same method. Tall oil precursors in the wood vary seasonally. The seasonal variation in tall oil soap recovery is very Table 4.3.2. Tall oil availability by species (6) pronounced in southeastern U.S.mills. This is due to the warmer weather that promotes biological Pine Species Tall Oil Tall Oil degradation during chip storage. Figure 4.3.1 shows kg /1000 kg OD lb./ODT the seasonal variation of tall oil soap availability Wood using data from two southeastern U.S. mills in the Longleaf 43.5 87 coastal plain.. Slash 41.5 83 Soap Recovery Loblolly 35.5 71 52 26 Shortleaf 34.5 69 Spruce 31 62 51 25.5 50 25 Tall oil soap recovery is usually somewhat lower 49 24.5 than what is available in the wood due to wood 48 24 storage practices process losses and differences in operating practices. 47 23.5 kg CTO/ODtkg Pine Wood Lb. CTO/ODT Pine Wood 46 23 Table 4.3.3. Tall Oil Recovery (7,8,9) 45 22.5 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Bleached Bleached Linerboard Canada S.E. US S.E. US Average kg CTO kg CTO kg CTO /ADT /ADT /ADT Figure 4.3.1 Soap recovery seasonal variation curve CTO in Wood 27 61 59 The type and duration of wood storage also affect tall CTO lost on Pulp 4 6 9 oil soap recovery. Tall oil soap loss from wood CTO lost to occurs very rapidly during the first two months of Sewers 2 1 2 storage (10). Figure 4.3.2 shows that tall oil losses CTO lost to can be 50 -65% after two months of storage. This Recovery loss is highly dependent upon the manner in which Boilers 9 8 5 the wood is stored. The tall oil soap loss is lower if Recoverable the wood is stored as roundwood rather than when CTO 13 46 43 stored as chips. Crude tall oil soap recovery is regularly reported by the Pine Chemicals Association for its member companies. Most members are located in the southeastern U.S. The annual recovery of tall oil 3.7-2 Impact of Storage Time on Impact of the Soap Content in Liquor Tall Oil Loss on the Evaporator Heat Transfer 100% Coefficient Loss 80% 26 24 60% 22 20 40% 18 Tall Oil Loss Tall 16 20% 14 0% 12 0 2 4 6 8 10121416182022242628 10 Storage Time (weeks) 00.511.522.5 Hours 15%for Lower 1st Effect U Soap Evaporator Feed (% CTO of Liquor Solids) Outside Chips Roundwood Chips Pilot Plant Mill Figure 4.3.2 Impact of storage time on tall oil loss (10) Figure 4.3.3 Impact of soap content in liquor on evaporator heat transfer efficiency expressed as the time for a 15% reduction in the heat transfer Why Should the Tall Oil Soap Be coefficient (14). Removed? Tall oil soap should be recovered to: Impact of Black Liquor Soap Content on • Improve evaporator operation, Pilot Evaporator Overall Heat Transfer • Reduce effluent toxicity, 110% • Improve recovery boiler operation, 100% • Reduce loading on the recausticizers, 90% • Reduce accidents due to slips and falls 80% • It is a valuable byproduct of the alkaline pulping 70% operation.. 60% Improve Evaporator Operation - In 1975, a study 50% (11) was done to define the nature and extent of 0 3 6 9 12 15 18 21 24 27 30 33 scaling problems in the alkaline pulping industry. Hours Running Heat Transfer Coefficient (of Initial Maximum) Initial (of Coefficient Transfer Heat Over 75% of the mills responding reported 4.64% Soap Content 1.3% Soap Content 0.6% Soap Content encountering one or more types of scales. The most commonly encountered scales were calcium scales Figure 4.3.4 Impact of soap content in liquor on and soluble Na CO -Na SO scales. Others (12,13) 2 3 2 4 evaporator heat transfer coefficient loss (14). have shown that soap is a significant scale component, particularly in soluble carbonate-sulfate Influence of Skimmer Outllet Residual scales in evaporators and concentrators. Mill and on Evaporator Lost Time and Evaporation Rate pilot plant studies (14) were done to clarify the 140 340 760 importance of soap in evaporator scale formation and 120 335 740 330 quantify its effect on scaling rates.