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The objective of this report is to design a process for the production of terpenes from softwood. This is in principle a very general objective which will be elaborated further. The most important step in the beginning of a design is to define the design requirements. The required capacity is 3,2 tons dry substance (ds) per hour. Additionally, the softwood feed ds is predetermined at 50 %wt (e.g. the softwood feed contains 50 wt% of water).
In this report, terpenes are the only products of interest. They are often produced in the form of turpentine in the pulp and papermaking industry, which will be explained in Chapter 2. This chapter will be devoted to defining wood, terpenes and turpentine, and will discuss the most important turpentine producing process; The kraft (or sulfate) pulping process. The chapter is concluded by summarizing important design features and requirements that form the basis of the design of the terpene production process. Questions that are relevant for the terpene production process are:
What is a suitable feed (e.g. type of softwood with a high terpene content)?
Which processing steps are required for the extraction of the terpenes?
What are the parameters for these processing steps (e.g. temperature, pressure etc.)
Chapter 3 will start with the material balances which form the basis of the process design and will determine the quantities of raw materials required (which is softwood in this case). Balances over individual process units set the process stream flows and compositions (Sinnott. 2005).
Background, Wood, Turpentine and Terpenes
Wood displays a universal composition in terms of its major constituents; cellulose (40-50%), lignin (18-35%), hemicelluloses and polyphenols, but has specific species-related components which can be polymeric, like poly-isoprene (natural rubber), and suberin, or small molecules, like terpenes, steroids etc. (Belgacem and Gandini. 2008).
Fig. 1 shows a schematic diagram of wood. Sapwood conducts moisture, minerals, oxygen, and nitrogen. As the tree grows in diameter, the sapwood cells cease their conductive function and form the inactive heartwood. Mineral deposits, gums and resins in the heartwood gives a darker color than the sapwood (Wang et al. 2004).
Figure 1: Schematic diagram of wood (Wang et al. 2004)
Cellulose dominates the wood composition, although its proportion compared to the other main components can vary significantly among different species (Pettersen. 1984). General chemical analysis on wood distinguishes between hardwoods (angiosperms) and softwoods (gymnosperms), whereas softwoods are richer in lignins, and hardwoods are richer in hemicelluloses. Typical chemical compositions of some wood species are shown in Table 1. Overall, wood has an elemental composition of about 50% carbon, 6% hydrogen, 44% oxygen, and trace amounts of several metal ions (Sjostrom. 1993).
Table : Chemical Composition of Different Hardwood Species (Sjostrom. 1993)
Scots Pine (Pinus sylvestris)
Spruce (Picea glauca)
Eculyptus (Eucalyptus camaldulensis)
Silver Birch (Betula verrucosa)
- Other polysaccharides
Softwoods consist mainly of long (3 to 5 mm) cells called tracheids which are about 20 to
80 x 10-6 m. Hardwoods consist mainly of two kinds of cells; wood fibers and vessel elements.
Wood fibers are elongated cells which are similar to tracheids except they are smaller, only 0.7 to 3 mm long and less than 20 x 10-6 m in diameter, and they do not serve for fluid transport in the living tree. The vessel elements do serve for fluid transport in the living tree, and they can have a wide range of sizes.
Figure : Typical morphology of a wood fibre (Belgacem and Gandini. 2008)
Fig. 2 shows the role of the three basic components respectively as the matrix (lignin), the reinforcing elements (cellulose fibres) and the interfacial compatibilizer (hemicelluloses) of a wood fiber. The middle lamella (0.5-2 Î¼m) is mainly composed of lignin (70 per cent), associated with small amounts of hemicelluloses, pectins and cellulose. The primary wall, often hard to distinguish from the middle lamella, is very thin (30-100 nm) and is composed of lignins (50 per cent), pectins and hemicelluloses. The secondary wall is the main part of the vegetal fibers. It's essential component is cellulose and it bears three layers, namely, the external, S1 (100-200 nm), the central S2 (the thickest layer of 0.5-8 Î¼m) and the internal or tertiary layer, S3 (70-100 nm) situated close to the lumen (Belgacem and Gandini. 2008).
Terpenes & Turpentine
Table 3: Major groups of terpene compounds
according to the number of isoprenic units
First of all, this section will start by explaining what terpenes are. Terpenes refer to a family of naturally occurring compounds which share isoprene (2-methyl-1,4-butadiene) as a common carbon skeleton building block. This structural relationship was identified by Wallach in 1887 and led to the 'isoprene rule'. Based on this generic rule, terpenes can be classified according to the number of isoprene units (Table 3).
Figure : Chemical structures of the most common turpentine monoterpene components. (Belgacem and Gandini. 2008).
Turpentine  is the common term given to the volatile fraction isolated from pine resin which consists mainly of terpenes (Definition of turpentine in CAS RN 8006-64-2). The nomenclature for turpentine is confusing as turpentine is a name applied to numerous semi-fluid oleoresins obtained from coniferous trees as well (Definition of turpentine in CAS RN 9005-90-7).
From the context of most literature considered, the term "gum" refers to the oleoresin (or resin) from pine trees.
Distillation of the oleoresin results in the volatile turpentine oil and rosin. Rosin is the brittle, transparent, glossy, faintly aromatic solid that remains once all the turpentine oil has been extracted. Oleoresin obtained from pine trees consists of 75 to 90 percent resin and 10 to 25 percent oil. When distilled, it yields turpentine. Turpentine is a mixture of terpenes and essential oils, which vary in percentage based on geographic location, tree species, and the distillation process. In this report, the turpentine definition given by CAS RN 8006-64-2 will be used. Turpentine is often classified by its means of production (i.e., steam-distilled, destructively distilled, sulfate-distilled, or sulfite-distilled turpentine). There is considerable difference between the types of turpentine in production levels and current use patterns.
Gum spirits of turpentine: Spirits of turpentine obtained by distillation of oleoresin (gum) from living trees.
Steam-Distilled Wood Turpentine: Spirits of turpentine obtained by steam distillation from the oleo resinous component of wood.
Destructively Distilled Turpentine: Spirits of turpentine prepared from the distillate obtained in the destructive distillation (carbonization) of wood.
Sulfate Wood Turpentine: Spirits of turpentine prepared from condensates that are recovered in the sulfate process of cooking wood pulp.
The turpentine chemical composition is strongly dependent on the tree species and age, geographic location and the overall procedure used to isolate it. In turpentine produced in the United States, the major constituents are the volatile terpene hydrocarbons; Î±-pinene (75 - 85%), Î²-pinene (up to 3%), camphene (4 - 15%), limonene (dipentene, 5-15%), 3-carene, and terpinolene (percentages not provided) (Chinn. 1989), see Table 4 and Figure 3. Constituents that are derived from turpentine for use in other products are listed in Table 5.
Table 4: Constituents (in weight percent) and density of sulfate turpentine's originating in different countries (Gscheidmeier and Fleig. 1996)
Spec. Gravity (g/mL at 20 Â°C)
Table 5: U.S. Companies that produce products derived from turpentine (Haneke. 2002)
Aldrich Chemical Company, Inc
Bush Boake Allen Inc
Florida Chemical Company, Inc
Florida Chemical Company, Inc
Millennium Specialty Chemicals Inc
According to Gscheidmeier and Fleig (1996) there are five types of turpentines (turpentine oil, steam-distilled turpentine, destructively distilled turpentine, sulfate-distilled turpentine and sulfite-distilled turpentine), categorized by their starting material and production method. Since the last two decades, the production of sulfate turpentine dominates the total turpentine production (approximately 70%) and the rest almost exclusively as gum turpentine (which is obtained from tapping living trees). This is explained by the high labor costs and/or the requirement of specialized equipment associated with the production of other turpentine types.
Annual worldwide production of turpentine has been estimated at 330,000 tons and 250,000 tons for 1995 and 1998, respectively (Coppen and Hone. 1995, Plocek. 1998). 55% Percent of the worldwide pulping factories use the sulfate process, generating sulfate turpentine as a byproduct (Gscheidmeier and Fleig. 1996).
Production of crude sulfate turpentine is performed at the large paper producers that use the kraft wood pulping process. When kraft pulping is carried out continuously, as much as 74% of the original turpentine present in the wood can be extracted. Due to the presence of the sulfur compounds, the product has a dark color and a foul odor (Chinn. 1989). The crude sulfate turpentine is sold to other companies for further distillation and purification to obtain constituent products. In the United States, several companies derive terpenes from sulfate turpentine (see Table 5). The turpentine content for Pinus species is highest compared to other tree species, namely 6-16 kg/T pulp compared to 2-3 kg/T pulp for fir or spruce trees (Gscheidmeier and Fleig. 1996).
Turpentine has long been associated with "naval stores", which refers to rosin, turpentine, tall oil and more, all produced from pine trees. Turpentine, formerly the most widely used paint thinner, is still employed in paints (both household and artist) as well as in other coatings. The use of turpentine has diminished recently due to the availability of less expensive petroleum-based solvents (Cronin. 1979, Gscheidmeier and Fleig. 1996).
Currently, turpentine continues to be used as a solvent or diluent for various products, such as natural or modified binders, resins, including alkyd resins, oils, paints, and polishes. In oil-based paint and coating formulations, peroxidation of the terpenes in turpentine accelerates the drying of oils and other film formers.
Even though its use as a solvent has decreased, turpentine has attracted tremendous interest and use as a raw material for the chemical industry (Gscheidmeier and Fleig. 1996). In 1988, 209 million pounds of derivatives were produced from sulfate wood turpentine (Chinn. 1989). Terpenes and other compounds derived from turpentine are used as raw materials or submaterials for products such as tires, plastics, adhesives, flavors and fragrances, cosmetics, paints, and pharmaceuticals. Separation by process-scale chromatography can yield Î±-pinene (purity up to 99%) and Î²-pinene, 3-carene, and monocyclics (Î±-terpinene, limonene, and phellandrene). Steam distillation of the residue separates out the higher boiling fractions (terpene alcohols, sesquiterpenes, and diterpenes) (Gscheidmeier and Fleig. 1996). Turpentine derivatives are essential ingredients in the manufacture of fragrance chemicals. The value of turpentine reflects about 25% of the value of all aroma chemicals produced both for sale and for internal use each year (Plocek. 1998). Some of the chemicals derived from turpentine along with their uses are listed in Table 6.
Table 6: Chemicals derived from turpentine and their uses
Flavor ingredient, insect attractant
Pharmaceutical, plasticizer, fragrance
Esters of pinic acid
Flavor ingredient and perfume fragrance
Cleaning agent, solvent, flavor and fragrance additive
Polymerization accelerator for rubber
Solvent, textile auxiliaries, flotation aids, cleaning and disinfecting products
Varnish resin, melt adhesive
Disinfectant, textile auxiliaries, fragrance
Terpenes containing sulfur
Lubricating oil additives
Fuel additives, oil field chemicals, fragrances
MSDNA & Biodegradability of Turpentine
Turpentine is a natural product and is completely biodegradable. Below the solubility limits, turpentine does not represent a hazard to biological wastewater-treatment plants (Gscheidmeier and Fleig. 1996). However, the biological and chemical oxygen demand for turpentine is exceptionally high (Irwin. 1997) and therefore effluent discharges are regulated (40 CFR 454.22 and 40 CFR 454.32). Environmental releases of turpentine may occur at production facilities where faulty equipment or spills occur. Facilities are required to use best management practices (BMP) to reduce the amount of turpentine released to the air during turpentine production processes (40 CFR 63.446; 40 CFR 430.3).
The Clean Air Act of 1990 did not classify terpenes, constituents of turpentine, as air polluting substances. Turpentine released into the environment is completely degraded by natural processes within a few days. The rate of degradation depends on the concentration of turpentine, temperature, availability of air, and presence of bacteria. Turpentine has been ranked as having zero potential as an ozone depleting substance or for global warming (Gscheidmeier and Fleig. 1996).
Background, Processes for Producing Turpentine
The Kraft Process
The aim in chemical pulping is to liberate the fibres from the wood matrix by delignifying the
wood. A typical kraft process scheme is shown in Figure 10. Wood chips are cooked at an elevated temperature (150-180Â°C) in an aqueous digestion liquor (NaOH, Na2S, Na2CO3, also called white liqour). The sodium sulfide in the cooking liquour while the sodium hydroxide is consumed by reaction with the lignin and carbohydrates in the wood. The cooking is done in large pressure vessels at 7-13 bar, for 1-5 h. There are two main cooking procedures, batch-wise or continuous, using different digesters and equipment. Batch-wise cooking contribute to some advantages like (Monica et al. 2009):
More flexible production -easy to start / stop allowing stand-by time -possibility to quick changes between, different pulp qualities or softwood / hardwood
More efficient turpentine recovery
An ordinary pulp mill may be equipped with a set of four digesters or more, with a size of 150-400 m3 each. Once the cooking is complete, the wood is broken down into two phases: a soluble phase containing the lignin and alkali-soluble hemicellulose and an insoluble phase containing the alpha cellulose or pulp.
In the case of batch digesters, air trapped with the chips and gases formed during digestion are relieved intermittently during cooking. In softwood pulping, turpentine is obtained by venting the digester and then separating the fibres and black liquor from the water and turpentine in a cyclone separator. The vapour mixture is then piped to a condenser and then to a separation tank, where the aqueous and turpentine phases separate due to their density difference. The aqueous
underflow is piped off, and the Crude Sulfate Turpentine (CST) overflow is also piped off to storage tanks. Because of the corrosive effects of CST, piping and storage tanks are made of mild steel, and all other components of stainless steel. The various non-condensibles (i.e. unable to be condensed
under the operating conditions used) are passed to a scrubbing column where they are cleansed of undesirable compounds by caustic solution (to form non-volatile salts).
Crude sulfate turpentine is condensed from the vapors of wood digestion. Sulfur compounds (methanethiol, dimethyl sulfide) are oxidized with sodium hypochlorite solution at 60Â°C to less volatile sulfonic acids, sulfoxides, or sulfones. These are removed by a variety of methods. This product has a characteristic musty odor. At the end of the cook, the digester contents are transferred to an atmospheric tank, called the blow tank. Gases leaving the blow tanks pass through a condenser to remove moisture, and the uncondensed gases are incinerated in a combustion device.
Black liquor is spent cooking liquor containing the dissolved organic substances and the used inorganic cooking chemicals. In order to transform the inorganic substances back to the active cooking chemicals the black liquor is evaporated and burnt, resulting in a smelt. After dissolution in water, the smelt is turned into green liquor. Causticising converts the green liquor into white liquor.
The degree of delignification is measured by determining the kappa number of the pulp. The
kappa number value gives an estimate of the lignin content in pulp. Lower kappa number equals
lower amount of lignin in pulp. The total yield of the cook is determined as the amount of pulp
produced compared to amount of wood charged. The pulp viscosity can be related to the degree
of polymerisation of the carbohydrates and can be used to monitor the degree of carbohydrate
degradation. The lower the viscosity value, the more the carbohydrates have been degraded resulting in shorter carbohydrate chain length (Monica et al. 2009).
A batch-wise production process is more suitable (with respect to continuous production) for turpentine recovery as it is more efficient (Monica et al. 2009). The most common batch digester in use is the stationary vertical cylinder with a conical or spherical bottom.
Figure 5: Schematical overview of kraft pulping process (Buonicore and Davis. 1992)
The delignification of wood (i.e. the removal of the structural polymer lignin from the wood) in aqueous alkali proceeds rapidly provided that the cooking liquor also contains hydrosulphide ions. It was early recognized that the delignification rate increased with the charge of sodium sulphide as shown in Figure 6.
The selectivity of dissolution of carbohydrates and lignin during a kraft cook proceeds in three
distinct phases with the first one merely being an extraction of both types of components (initial
Figure 6: Dissolution of lignin from spruce wood at a cooking temperature of 160 Â°C showing the influence of sulphide charge (Hagglund. 1951)phase). When around 20 % of both carbohydrates and lignin have gone into solution, the kinetics
Figure 7: Selectivity in the dissolution of carbohydrates and lignin on kraft pulping of softwood (Monica et al. 2009)changes dramatically and a rather selective lignin dissolution takes place until approximately 90 % of all lignin has been dissolved (bulk phase). The final portion of the lignin can, however, only be removed with great difficulty and at the expense of a large carbohydrate loss (final phase). In practice, the cook is interrupted at the transition point to the final phase in order not to lose in pulp quality or yield (Figure 7). The predominant loss of carbohydrates in kraft pulping is due to the fact that most of the hemicellulose components are degraded and dissolved in the alkaline liquor. Due to the differences in wood morphology and lignin structure, the rate of delignification is higher in hardwood species as compared to softwood (Monica et al. 2009).
Typical values for the total changes in component yields as the result of a kraft cook are shown in Table 7 where the large loss of carbohydrates is further illustrated.
Table 7: Typical yield values (% on wood) for the individual wood components after kraft cooking of pine.
Values for wood within brackets (Monica et al. 2009)
The delignification chemistry encountered in kraft a pulping has been thoroughly investigated. It has been clearly shown that a fragmentation of the polymeric lignin and the introduction of hydrophilic groups are necessary prerequisites for the dissolution. The cleavage reactions take place in the predominant chemical linkage connecting the phenylpropane units together, the Î²-O-4 linkage. By the action of hydrosulphide ions, the phenolic Î²-O-4 structures are to a large extent fragmented. If no hydrosulphide ions are present, as in soda pulping, the delignification efficiency is poor and the rate of delignification becomes much lower (Monica et al. 2009). The chemistry of cleavage of phenolic Î²-O-4 structures is shown in Figure 8.
The chemistry of lignin dissolution in a kraft cook is not only dependent on the cleavage of Î²-O-4 structures. A low H-factor, i.e. a high charge of hydroxide and hydrosulphide ions, is beneficial thus indicating that a prolonged pulping time, in addition to an increased degree of Î²-O-4 cleavage, also results in an increase of non-desirable reactions able to prevent or to slow down the overall lignin dissolution.
As delignification is a very complex subject and the objective of this report is to produce turpentine (terpenes), the other chemical reactions fall beyond of the scope of this report. For an overview of the chemistry in kraft pulping see Monica et al. 2009.
Figure 8: Reaction scheme for the cleavage of phenolic Î²-O-4 structures in lignin during kraft pulping conditions. Competing reactions are also indicated in the figure. L denotes a lignin residue (Monica et al. 2009)
Design of turpentine recovery process
Basic Philosophy Behind the Design
As described in Section 2, the sulfate turpentine amounts to approximately 70 percent of the total annual turpentine production, and gum turpentine (tapping of living trees) amounts to approximately 30 percent. The kraft process is a complicated process and is designed for the purpose of producing pulp for paper manufacturing, although it does produce sulfate turpentine as a by-product. The main objective for the process of tapping of living trees is the production of turpentine and additional products (e.g. rosin). It is also a much simpler process focused more on the production of turpentine. Thus, using the kraft process as a basis for this turpentine production design does not give purpose to the pulp being produced. And, as the requirement for this design is to use softwood (and not living trees!) as raw material, both of the above options for the production of turpentine and terpenes seem unviable for the design in this report.
The most obvious choice would therefore be to return to the traditional ways of producing turpentine, i.e. steam-distilled (wood) turpentine. In this report, the method for recovering the turpentine will be based on extraction of the products from dead wood and/or waste wood like stumps by using the steam and solvent process which was first used in the beginning of the 20th century (Palmer. 1934), and this production method peaked in the 60's and 70's (Haneke. 2002).
The steaming part of the process consists of steam being introduced in the bottom of the vessel that holds the wood chips, the steam and oil vapors pass out of the top to condensers, and the condensate runs to automatic gravity separators. The oils that are recovered in this distillation consist essentially of turpentine and pine oil, and therefore holds the most of the terpenes. Steam-distilled pine oil contains a few per cent of terpene hydrocarbons boiling above turpentine but is principally tertiary terpene alcohols, such as alpha-terpineol, and also contains the secondary terpene alcohols, fenchol and borneol, together with a small amount of a phenol ether, methyl chavicol (Palmer. 1934).
The next (optional) step is the extraction of the rosin from the steamed chips. The contact with steam serves to heat the wood thoroughly and to draw to the surface a portion of the rosin, thus making it available for easy extraction. The rosin consists of different resin acids, especially abietic acid. Different types of solvent are used including naphtha, benzene, toluene, gasoline (Yaryan. 1909, Sherwood and Cole. 1924, Little. 1930, Black and Minch. 1953).
Design of Process
The first step in the processing is the separation of the bulk of the volatile oil present in the wood by means of steam distillation. Steam is introduced in the bottom of the vessel, the steam and oil vapors pass out of the top to condensers, and the condensate runs to automatic gravity separators. The technical control of the process begins with the steaming step. Steam may be either saturated or superheated, and the operation is conducted under low, moderate, or high pressure, depending upon the results desired. Steaming is continued to a point of steam economy by determining the proportion of oil to water in the distillate.