The Commercial Processes

The hardwood distillation industry was developed early in the United States on account of the large supplies of good quality hardwoods available and on account of the high excise tax on ethyl alcohol which forced the use of wood alcohol for solvent purposes where the cheaper ethyl alcohol might otherwise have been used. Even since the denatured alcohol legislation of 1906 which made available the cheaper denatured ethyl alcohol for solvent purposes, the industry has expanded due to the increased demands for methyl alcohol as a chemical raw material and for the other products, acetate of lime and charcoal.

The early industry used either the small horizontal cylindrical retorts or the brick by-products kilns for the distillation of the wood, but for many years 110 new plants have been built with either of these types of distillation apparatus. Many of the old retort or kiln plants have been replaced by the modern type of plant but some of them are still in operation. The main objection to the retorts was the high labor costs for charging the wood and removing the charcoal, these operations being carried out entirely by hand. The kilns which gave low yields of the chemical products were used only in. connection with iron furnaces where large quantities of charcoal were required and the chemical products were of secondary importance. When the chemical products became of more importance no more kiln plants were built. In the following general description of the plant and operation only the modern type of distillation apparatus, the rectangular oven, will be included.

Wood for Distillation

The wood used for distillation is generally cordwood 52 inches long and with a minimum diameter of 3 to 5 inches varying with the conditions of supply and demand. Sticks of greater diameter than about 8 inches are usually split. Some sawmill slabs or other wood wastes are used in which case smaller dimensions in length and thickness are allowed. Small blocks, chips, shavings or sawdust are not_ used^ The source of the wood varies; in some cases the wood distillation plant is run in connection with a sawmill and the 13

tes such as tops and limbs are gathered and cut for In other cases the distillation wood is the main product tn the forest and the sawmill operation may be secondary l

:ntirely. This usually takes place where the timber is 1 growth or of poor quality for lumber. Most of the or control their own wood supply and cut and collect ■ood or have a contract with a sawmill for waste wood, the smaller plants purchase a part of their wood already cut. i is not generally selected by species except that softwoods i. All the hardwoods which naturally occur in the for- ;

:he wood is cut are commonly collected, since the main \

satisfactory and the others occur in too small quantities I

:h effect on the results. In the north the main species f irch and maple, but other species which may occur with .s elm, ironwood, oak, ash, etc., are used. In the south *

story are the favorite species and some selection may be in order to eliminate such undesirable hardwoods as d is dried for six to eighteen months either in the woods >r in storage yards at the plant. It is not required to sod completely dry and "air dry" wood with 15 to 20 isture is considered satisfactory. If the wood is dried in /here cut the drying is slow and there is a good chance ition by rotting and danger from fire, whereas if dried yard there is an extra expense of handling to and from n either case the interest charge on the investment in a y of wood is an important item of expense. In order ese difficulties connected with air drying a system of 'ing has been developed and adopted by several plants, ries much faster when in short lengths and so for artificial iticks are preferably cut into , blocks 6 to 10 inches long, s are run by conveyor from the saws to the cars in ire distilled and no further handling is required. It is a that the smaller the pieces of wood which go to make e less wood can be contained in a given space, but when hes long are dropped from a hopper into a car a greater ood is put in than when 52 inch,sticks are piled by hand car. This is another advantage of the artificial drying le cars and the wood being hot at the time they enter lso has a little effect on the time and fuel required for ficial drying is accomplished by the waste heat from the retort furnaces. The cars of wood are placed in closed and the flue gases from the retorts are blown into the ■ect contact with the wood. Care must be taken that the

Fig. i.—Drawing of Oven in Brick Setting.

temperature of the incoming gases is not too high and that sparks do not carry through to the wood. Otherwise, the process is nearly automatic. Commonly three charges of wood are in the driers at one time, the wood from the hot end of the drier being drawn direct to the retort while a fresh charge of green wood is introduced at the cold end.

The Distillation of the Wood

The large rectangular retorts commonly used for the distillation of the wood are frequently called ovens or "Jumbos" to distinguish them from the older, smaller, cylindrical retorts. These ovens are commonly made of flange steel thick with somewhat thicker metal on the bottom where the temperature is highest. They are 6' 3" wide and 8" 4" high and vary in length according to the capacity desired, the standard 10 cord oven being 56' long. The oven doors open the full cross section and are fastened by steel wedges. Since the doors are the only surfaces of the retorts not directly heated they are usually provided with outside "storm doors" to prevent radiation and condensation of tar on their inner surfaces. On account of the great length of the oven and the difference in expansion with heat between steel and brickwork the ovens can not be supported at the bottom on brick but are suspended from hangers along the top of the sides. (See Fig. I.) The ovens are heated from fire boxes at one end in the case of the shorter ovens or at both ends in the case of the longer ones. In order to distribute the heat evenly throughout the length of the ovens special arches must be provided. These brick arches in the case of two fire boxes run from the fire boxes nearly to the center of the oven and prevent direct contact of the flames on the bottom of the oven. By properly spaced openings in the arches the flames or the hot flue gases are distributed lengthwise and sidewise so as to distribute the heat over the bottom of the retort as evenly as is practicable. The flue gases are then guided around the sides of the retort by suitable baffles, finally passing over the top and thence up the stack. Even with this arrangement the equalization of the heating is not complete and the bottom of the retort commonly gets the highest temperature. The ovens are set in pairs with the brickwork for the two continuous although there is no connection between the lire boxes. The space between each pair is thus utilized for condensers, piping, etc.

The wood is loaded on iron cars or "buggies" with slatted ends and sides, the latter sometimes reaching only about two-thirds of the height of the piled wood and serving to retain the charcoal after carbonization. The sides are also removable for loading the wood and unloading the charcoal. The cars very nearly fill the cross section of the retort and are long enough to hold about cords, four cars making one charge for a 10 cord retort. The cars are sometimes moved by hand but usually with a power cable or a small locomotive.

As soon as the wood is inside the retort the doors are closed and the heating started. The first distillate is almost entirely water and it is not until about the fourth hour that the liquor slowly darkens and contains increasing amounts of acid. The tar first appears at the eighth to tenth hour and from this time onward very little more heat is required to finish the distillation at about the twenty-second hour. For about two hours the retort is allowed to cool and ^ then the charcoal is removed and a new charge of wood run in, making a complete cycle in 24 hours. The time for a complete cycle may vary at different plants depending on the moisture condition of tWe wood,

the kind of fuel used or the desire of the operator, but a 24 hour cycle is common. One plant operated for some tune on a ib-ia hour cycle and thus greatly increased the capacity of the retorts.

Various methods are used to control the firing, the appearance of the distillate and the temperature in the vapor outlet from the retort being the most common signs for determining the status oi the distillation. It is usually required to finish the distillation m a certain definite time and without too rapid heating or too high a temperature during the stage when the valuable products are distilling. To accomplish this the charge is heated rapidly at the start and then at the proper time, usually eight to ten hours from the start, the firing is cut down so that the distillation will not "run away" or become uncontrollable.

Tar and gas from the plants' own products are generally used for retort fuel and besides these only a small amount of other fuel is required, If the tar or gas is used for boiler fuel other fuel as coal, natural gas or waste wood is used. One plant has used hogged saw mill waste almost entirely as a retort fuel by modifying the ordinary fire box into a small Dutch oven for the more efficient use of this kind of fuel. The wood tar and natural gas make excellent fuel because they both give a quick, hot fire and yet can be readily regulated.

The hot charcoal from the retorts catches fire as soon as it comes in contact with the air and it must, therefore, be cooled out of contact with the air. Closed charcoal coolers are provided into which the cars of charcoal are run as rapidly as possible after the retorts are opened, a spray of water being used to keep down the fire a little during the transfer. The charcoal stays in this cooler until another distillation cycle is finished and then it is moved on to the next. It

remains there for 24 hours and then is allowed to stand in the cars in the open air for another 24 hours before it will be received for shipment by the railroads. The coolers are of the same shape and size as the ovens but are built of lighter metal and sometimes are without a bottom, being banked with sand to prevent access of air.

The best layout for the wood yard, retort house, coolers and charcoal sheds is one in which the progress of the cars is in one direction throughout the operation. (See Fig. 4.) In some cases lack of space makes this layout impossible and the retorts may be charged and discharged at the same end. It is almost a necessity, however, that the first coolers be in line with the retorts so that the hot charcoal may be moved rapidly.

The vapors are taken from the retort through one to three outlets, depending on the size of the retort. Ordinarily only two outlets are used, even on the 10 cord retorts. The vapor outlets are on the sides of the retorts near the top and pass through the brickwork to the condensers. The condensers are of the vertical tubular type and

THE COMMERCIAL PROCESSES 19

they must have a capacity to condense and cool the vapors during the peak load, although this may cover only a small part of the distillation period. The inside of the condenser tubes should also be readily accessible, since it is necessary frequently to clean out the tubes to remove deposits of pitch and tar which greatly decrease the efficiency of the condenser. At many plants the retort condenser tubes are cleansed regularly once a week.

At the condenser outlet the liquid distillate is separated from the gas by means of a simple gooseneck or trap which allows the liquid to flow through but stops the gas. The gas is taken off at the top of the trap on the end toward the condenser and piped to the retort fire box or to the boilers. No storage is provided for the gas and it is burned as it is produced. A valve in the gas line from each condenser is usually closed when no gas is coming off from the retort.

Refining Processes

The liquid distillate, consisting of tar and a dark brown watery liquid called pyroligneous acid, is pumped to wooden settling tanks where the tar settles to the bottom. These tanks are usually arranged in series with connecting pipes at gradually lower levels from the first, where the mixture of tar and pyroligneous acid enters, to the last, where the clear acid is drawn off for further refining. The tar is drawn off as required from the bottom of each tank, most of it naturally coming from the first.

Although most of the valuable products, wood alcohol and acetic acid, is contained in the pyroligneous acid, yet there are small quantities contained in the tar which is distilled with steam for their recovery. A wooden still is used for this operation, since copper is so rapidly corroded by the hot tar. The still is provided with a closed steam coil of copper and also with a steam jet. The water in the tar is driven off by heating with the closed coil and then live steam is blown through the tar. The volatile pils from the tar are distilled with the steam and are separated from the water layer by settling. The first oils distilled are lighter than water, but if the distillation is carried far enough oils heavier than water will distil. Sometimes these oils are kept separate as part of the marketable products, but frequently they are mixed with the residue left in the tar still and used for fuel. The water distillate from the tar still contains acetic acid and alcohol and is mixed with the similar distillate from the next operation.

The pyroligneous acid from the settling tanks, although it has been settled free from suspended tar, still contains some tar in solution which must be separated in the first refining operation, This is'

accomplished by complete distillation of the pyroligneons acid, leaving the tar behind in the still. A copper still equipped with closed steam coils is commonly used for this operation, although triple effect evaporators are sometimes used. Since it is simply the separation of a volatile from a non-volatile liquid no special precautions are required. A small amount of oil distils with the water and this is separated before the next operation. The residue of tar is allowed to accumulate in the still for several days and then it is given a steam distillation, like the settled tar, either in the copper still or after removal to the wooden tar still.

The distilled tar-free pyroligneous acid to which is added the water distillate from the tar still is now ready for neutralization with lime. A thick milk of lime is used for the neutralization and the operation is carried out in a covered wooden tub provided with a stirring apparatus. The neutral point is usually determined by the color change in the solution itself which becomes very much darker as the neutral point is reached. In fact, the color is so dark that the use of outside indicators is difficult. A dark wine-color is the common neutral point and this must be attained with considerable accuracy to avoid difficulties in the future evaporation of the acetate solution. During the neutralization an insoluble sludge is formed which settles partly in* the neutralizing tub and partly in wooden tanks provided for the storage of the neutralized liquor. The sludge must be occasionally removed from the neutralizing tub which should be designed for accomplishing this readily. The wet sludge retains a considerable amount of acetate liquor and is frequently washed or filtered in a filter press for its recovery.

The neutralized settled solution contains the acetic acid in nonvolatile form and the alcohol and other volatile products can be separated from the acid by distillation. Since the solution is now neutral an iron still, commonly called the "lime-lee" still, can be used for this distillation. The lime-lee still is provided with a closed steam coil of copper and no direct steam is used in this operation. The distillation is carried on until the gravity of the distillate is unity showing that all the alcohol has been removed. A small amount of oil also collects with the distillate from this operation and should be separated before the next distillation. The residue in the still is called the acetate liquor and the" distillate, the alcohol liquor.

_ The acetate liquor is commonly evaporated in shallow, steam-jacketed 11-011 pans although single or multiple effect evaporators are frequently used. In the iron pans the first separation of the solid acetate of lime should take place on the surface of the liquor, since if it starts on the bottom of the pan a hard dense deposit forms which reduces the effective heating surface and is very difficult to remove Starting on the surface the separation of the solid acetate proceeds downward, the steam escaping through small craters in the crust. The

Fig. 4.—Layout of Hardwood Distillation Plant.

evaporation is stopped when the mass has reached the consistency of a thick mud and the rest of the drying takes place elsewhere. For the final drying of the acetate drying floors are provided on top of the ovens where the heat of the flue gases can be used. The acetate mud is spread in a thin layer on the floor and frequently raked to uncover the wet acetate as the top surface dries. The drying acetate is sometimes rolled with a light roller to prevent the formation of too large lumps. The acetate is not decomposed by any temperature likely to be reached on the floor and no special precaution needs to be taken. This is the last operation for the preparation of the finished acetate of lime and when dry it is bagged for shipment.

When multiple effect evaporators are used for evaporating the acetate liquor another method for drying is introduced. The evaporators carry the concentration almost to the point where separation of the solid acetate begins and then the solution is run to a shallow pan into which dips a slowly revolving steam-heated cylinder. The cylinder is provided with automatic scrapers which remove the deposit at proper intervals. Usually the speed of the cylinder, the steam heat and the scraping are coordinated so that the material scraped off has been dried to a consistency of a thick mud. The mud drops to a V-shaped trough in which it is picked up 011 an endless wire belt. The belt passes through heated chambers where the drying is completed and the dried acetate is removed from the belt by the belt running over rolls which change the direction at sharp angles and by an automatic shaking or beating device. This method of drying is very efficient in the way of saving labor costs, since the acetate is dried and delivered to the storage hoppers without being touched by hand.

The alcohol liquor distilled from the lime-lee still is concentrated to a crude wood alcohol of about 82 per cent strength as determined by the gravity. This concentration can not be reached by a single distillation from an ordinary still without some kind of fractionating column. For a long time the column commonly used was the "Burcey pan," a special type of still head used practically entirely by the hardwood distillation plants. Many other types of columns are now used more in line with modern fractional distillation apparatus. The distillation is controlled by the gravity of the distillate and if the apparatus is not efficient enough to produce the required strength of alcohol in one operation the last part of the distillate is kept separate and returned to the alcohol liquor tank for further treatment. The distillation is stopped when the gravity of the distillate shows no more alcohol distilling and the residue is run to waste. A small amount of oil distils with the alcohol and is separated before further refining. -

When evaporators and roll dryers are used for preparing the acetate the tone-lee still is usually cut out and a continuous column still substituted which is able to prepare a concentrated alcohol direct

WOOD Distilled in retorts

HOT CHARCOAL Cooled in coolers I

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