Chapter 3. Processing and plant description


Introduction
Olustee process for the production of turpentine and rosin
Production of turpentine and rosin in Portugal
Scale of operations and labour requirements


Introduction

The separation of resin into its component parts, rosin and turpentine, involves two basic operations: cleaning and distillation. Cleaning is necessary to remove all extraneous material from the resin, both solid and soluble. This includes forest debris such as bark, pine needles and insects, which may have fallen into the cup during its period on the tree, and which require removal by filtration. Water-soluble impurities carried into the cup by rain water are removed by washing the filtered resin with water. The approximate composition of crude resin, as it is received at the plant for processing, is 70% rosin, 15% turpentine and 15% debris and water. Small amounts of iron produced by the corrosive action of excess sulphuric acid on galvanized iron guttering and cups may also contaminate the resin. As the presence of iron would lead to a darker, lower-grade rosin, it is removed by adding oxalic acid prior to filtration. Iron contamination has become less of a problem as the use of acid paste rather than spray has become more widely adopted. The use of cups made of plastic or other non-ferrous material eliminates the risk of iron contamination from this source.

The cleaned resin is then ready to be distilled, or, to be precise, steam-distilled; the older type of direct-fired still has given way, almost universally, to a still in which steam is used both to heat the resin and to facilitate the distillation by co-distilling with the turpentine vapours. Designs of equipment, and the procedures followed, vary somewhat between producing countries. The Olustee process, developed and used in the United States and adopted elsewhere, is described first. The methodology is well documented, and since the differences between this and any other system of processing are likely to be matters of detail rather than principle, a description of the process serves as a useful guide to any prospective processor of crude resin. The final design of plant can be tailored to suit local preferences and requirements in terms of scale. A description is then given of Portuguese methods which are based on the same principles as the Olustee process, but which differ in the layout of equipment and the relative capacity of some of the units. Processing methods used in other producing countries are not described. To a greater or lesser degree they all follow the same basic principles, namely, filtration of the hot, diluted resin, usually including a washing stage, and steam-distillation.

Olustee process for the production of turpentine and rosin

The scheme of processing is illustrated in Figure 2. Barrels of resin arriving at the plant are immediately weighed and upturned over an iron grill covering a large concrete or mild steel dump vat. The barrels are placed over steam outlets to remove the last of the adhering semisolid resin. 'Scrape', the solidified resin which is taken off the face of the tree at the end of the season and which yields a poorer quality rosin, is emptied into a separate compartment for separate processing.

Figure 2. Scheme of resin processing in the United States of America

Source: Based on McConnell (1963)

In order to facilitate the flow from one unit to another, the resin has to be diluted with turpentine and heated. As well as making the resin more fluid, dilution lowers its specific gravity, so that in the later washing stage it will form a two-phase system water more readily. The resin is transferred first from the dump vat to a blow-case, and then from the blow-case to a melter, by the use of steam pressure. Filter aid (diatomaceous earth, 0.5-0.6 kg/tonne of resin) and oxalic acid (0.6-1.2 kg/tonne) are added at either of the two units. Turpentine (from a previous distillation) is added to bring the turpentine content of the resin to between 30% and 40% the precise amount added depends on whether good quality resin or 'scrape' is being processed. The temperature inside the melter is raised to 85-100C by steam, the exact temperature again being dependent on the quality of the resin. Steam pressure is then used to force the hot resin first through a metal screen at the bottom of the melter to remove the larger sized solid matter, and then through a filter to remove all remaining solids. The filter is of the horizontal or vertical plate type and consists of about 12 plates backed with filter paper or cloth; filtration is assisted by the filter aid added previously. The resin passes directly from the filter to the bottom of a wash tank containing hot water. Each tank holds 1500-2000 litres of water which is sufficient for washing up to 20000 litres of resin (about seven charges from the melter). After washing, the mixture is allowed to settle for at least 4 hours and preferably overnight. The bottom aqueous layer is then run off to waste, an intermediate layer of unbroken emulsion ('muck') is run off to be returned to the low grade dump vat for reprocessing, and the top layer, which consists of washed resin, is drained and pumped to a charge tank in preparation for distillation.*

* Simple batch, rather than continuous or vacuum distillation, is most commonly used and is described here, although the other methods offer some advantages in terms of product quality and steam consumption if there is sufficient throughput of resin to justify their use.

The still is filled with resin from the charge tank. The temperature is then raised by means of steam coils to about 110C at which point live steam is gradually introduced through sparger valves. As the temperature continues to rise, distillation proceeds and the sparger steam inflow is increased until, at the end of the distillation, the temperature has reached 160-170C. The rate of increase in temperature, and therefore the time taken for the distillation, is dependent on the steam pressure used; the higher the pressure within the range 8.8-10.5 kg/cm2 (125-150 psi), the faster the distillation. For still capacities of about 4-5 tonnes, distillation times vary between 90 and 150 minutes. If the steam pressure is too low, it will be more difficult to remove the last of the turpentine (particularly if there are appreciable amounts of high-boiling components) and there will be an inordinately long residence time for the hot rosin in the still; both these factors have an adverse effect on the quality of the rosin. The turpentine and steam vapours pass through an entrainment trap to remove any entrained resin and then condense in a water-cooled condenser. Completion of distillation is indicated by a minimal level of turpentine in the distillate (which, by experience, is found to correspond to a particular temperature). A small proportion of the turpentine coming over at the beginning and end of the distillation may be collected separately as slightly lower-quality turpentine, and used for diluting the next batch of resin at the melter. Otherwise, there is no fractionation.

The water-turpentine distillate is led immediately to a separating tank; the upper turpentine layer overflows and passes first down to the base of the dehydrator and then upwards through a bed of rock salt to remove all traces of water. The dry turpentine is then fed to holding tanks for subsequent storage in bulk or in galvanized steel drums. The hot rosin from the body of the still is discharged from the bottom into suitable containers which are set aside for the rosin to cool and solidify.

Yields of rosin and turpentine obtained by US producers were about 700 kg and 160 litres (140 kg), respectively, from one tonne of resin. 'Scrape' yields less turpentine than normal resin. Specifications for both products are given in Appendix 2. The general appearance of the rosin should be hard, clear and bright, pale yellow-brown in colour, and with no visible sign of foreign matter or turbidity due to the presence of moisture. Packaging options for turpentine and rosin are described below, but more detailed packaging and labelling requirements are given in Appendix 4.

The materials used for the construction of the plant are important. The dump vat is concrete or mild steel and the blow-case is mild steel. In order to avoid corrosion by the acidic material, any area where hot resin is handled should be stainless steel; the melter, filter (where it comes into contact with the resin), charge tank, still and all pipework are all therefore made of stainless steel. The wash tank may be aluminium or stainless steel. The condenser, separator and dehydrator are also usually of stainless steel.

In the United States, melter capacity varied between 2.5 and 5 tonnes of resin when there were a number of independent processors. There were usually two or more wash tanks, each capable of holding up to 22 tonnes of resin and each providing material for four distillations. Operation of two stills, or double shifts, allowed up to 10000 tonnes of resin to be processed in a year.

Production of turpentine and rosin in Portugal

The scheme of processing followed in Portugal is based on the same principles which led to the development of the Olustee process in the United States but some of the units which make up the plant differ in design and capacity. The process lay-out is illustrated diagrammatically in Figure 3.

Metal drums containing resin are unloaded at the dump vat. To facilitate the emptying of the drums (which is the most labour-intensive part of the whole process), a rectangular section (measuring about 25 cm x 15 cm) is cut out of the side before use. The section is then replaced but can easily be removed subsequently as required. On receipt, the drums are rolled on their sides to the vat opening and the resin is forced out with large spatulas. Resin from the dump vat is fed directly into a mixer (the equivalent of the melter in the previous scheme) with no intermediate transfer to and from a blow-case. The mixer, rather than being just a containing vessel like the melter, incorporates a stirrer so that the contents, including the added turpentine and oxalic acid (if used), can be thoroughly mixed as well as heated.

Another significant difference is the addition of washing water in the form of live steam (up to 10%) at this stage rather than in liquid form at a later stage. The hot mixture is next passed through a metal screen to take out the larger solid impurities; this occurs in a separate vessel rather than at the bottom of the melter as in the Olustee process.

Figure 3. Scheme of resin processing in Portugal

Source: Based on Gama (1982)

A slurry of diatomacaeous earth is added from another vessel and the screened mixture then passes immediately through a fine filter as before. The hot, filtered mixture passes to one of several decanters in which the aqueous portion is allowed to settle out, usually overnight; one charge from the mixer is sufficient to fill one decanter (unlike the Olustee system where one wash tank takes four to seven charges from the melter).

Both batch and continuous distillations are carried out in Portugal and although batch stills are predominant, the larger throughput which is possible with continuous distillation means that a significant proportion of Portuguese production is obtained in this way. Batch stills in Portugal are relatively small, with a capacity of 0.5 or 1 tonne, and distillation times are short, about 20-30 minutes. On completion of the distillation, the rosin is often drained from the still into a wagon; this can then be pushed between two lines of steel drums spaced out on a concrete floor, and rosin can be discharged into them by lowering an overflow arm. Alternatively, paper sacks may be filled.

Stainless steel is again the main construction material. Mixer and decanter capacities are each about 5-7 tonnes, and there are usually at least four decanters to provide a constant supply of material for distillation. Nominal plant capacities range from a few thousand tonnes up to about 10000 tonnes/year.

Scale of operations and labour requirements

The target production is determined by the availability of crude resin, the throughput which can be sustained, and the size of the market for the products. In some cases, the resin supply may be limited by the number of suitable trees available for tapping. The annual production capacity required to meet the target is largely dictated by the still size, the number of stills, and the shifts worked. The smallest sized plant might have a single still with a capacity of one tonne and be capable of performing three distillations per day (single shift). The number of working days available each year will be governed by the length of the tapping season, but assuming a 260-day year, an annual throughput of around 800 tonnes of resin could be achieved.

The factors to be considered when choosing between the different processing systems include relative plant costs, availability and cost of technical expertise, the costs of maintenance and spare parts, steam and water requirements, and the relative advantages or disadvantages of using larger or smaller batch sizes. Estimates of plant costs are discussed in Chapter 4. Data necessary to make an accurate comparison of steam and water requirements for the different types of plant are not available. The use of larger stills means fewer distillations are needed to process a given amount of resin. However, a small still allows for greater flexibility if interruptions in steam, water or raw material supplies are anticipated. As the stills designed for distillation of pine resin are not suitable for distilling harvested plant material to produce essential oils, this is not an option for using spare capacity should it occur. In any case, the risk of cross-contamination and taints would be too high.

The labour requirements are comparatively small for a plant capable of handling up to 1000 tonnes/year of resin. Only four or five skilled workers and a greater number of general labourers are needed; one person is normally responsible for operating the still, two or three others assist and operate the other pieces of equipment, and one is in charge of the boiler. The emptying of the barrels or drums of crude resin is the most labour-intensive and time-consuming part of the whole processing operation and at least six labourers are required for unloading, loading and similar work. A storeman, and office and transport staff, are also required. The total labour requirements for a larger plant do not increase proportionately as the same number of workers are needed to operate the specialized pieces of equipment. More general labourers will be required, though, to handle the greater quantities of resin and its products.