100 125 150 175 200 TEMPERATURE, °F

FIGURE 2 Black liquor viscosity

Liquor containing up to 50% solids is reasonably easy to pump, but allowance must be made for viscosity effects. In noting the values in Figure 2, it should be remembered that the plant must often start up cold, so cold liquor with a higher viscosity may have to be pumped. The liquor-specific gravity rises during evaporation from about 1.1 to 1.25. Group B pumps are recommended.

Black Liquor with Total Solids of 50 to 65% This is often referred to as heavy black liquor because the specific gravity rises to 1.35. The liquor is formed by further evaporation, either in the multiple-effect units or by contact evaporators using hot flue gas.

From a pumping standpoint, this liquor is probably the most difficult of all liquids to pump satisfactorily in pulp and paper mills. Continuous operation requires careful attention to pump sealing and maintenance. Steaming out at regular intervals of the evaporator and piping is particularly important to prevent solids buildup affecting NPSHA to the pumps.

No accurate figures are available for the viscosity of liquids with a solids concentration above 55% because there is wide variation in the liquors produced from different wood species and also in the liquors from the same wood of different ages. Hardwood species produce a more viscous liquor, especially eucalyptus, as well as more liquor per ton of pulp produced. Black liquor produced from straw pulping is even more viscous and, in addition, causes the deposition of silica on the walls of pumps and piping. An approximation of the viscosity of straw mill heavy black liquor may be determined from published figures, which give viscosities up to 2000 centistokes. This is probably at least 50% higher than liquor from normal long-fibered softwood.

During recovery, liquor is sprayed into the furnace for evaporation to dryness and burning. Prior to this, the make-up chemical (sodium sulfate or salt cake) is added and reduced to Na2S in the reducing atmosphere of the furnace.

Little is known with certainty about heavy black liquor, but it does not seem to be very corrosive, and carbon steel is often used for pipework, although stainless steel pumps are fairly common. The pumps are subjected to severe duties—notably high heads, lumpy material, high temperature and pressure, and continuous service. Group B pumps are almost universally specified, often with casings of more wear-resistant material such as Alloy 20 or duplex stainless steels. In some cases, mills making straw pulp have not found suitable centrifugal pumps and have had to resort to gear pumps because of the very high viscosity of the liquor.

green liquor Green liquor is a solution of sodium carbonate and sodium sulfide plus other elements and compounds. One of these other compounds is iron sulfide in a colloidal form, which produces a greenish color. The liquor is formed by dissolving smelt from the causticizing process. Severe erosion takes place in green-liquor pumps, primarily because of the violent action inside the dissolving tank but also because of the gritty matter always present. Green liquor also builds up on the walls of pumps and piping, causing high fric-tional losses. The specific gravity is usually about 1.2, and an allowance of about 20% should be made for viscosity. Group B pumps are recommended for this service.

lime slurries In causticizing, various solutions and slurries are present that, apart from causing excessive wear in standard pumps, do not cause any problems. Thus, any normal slurry pump should prove satisfactory. In sulfate mills, the lime mud formed during green liquor causticizing presents the most serious problem, for approximately 1000 lb (500 kg) of mud may be formed for each ton (1000 kg) of pulp produced. Solid loads above 35% can occur, and frequent blockages are likely unless pumps are selected for minimizing downtime. For mild slurry duty, group B pumps with duplex stainless steel construction are satisfactory. For harsher applications, hard iron pumps like those used in the mining industry are employed.

bleach plant liquor Most bleached pulp mills today use at least four stages of bleaching, and often six or more. Bleaching is used to remove residual lignins or to convert them to compounds that are stable regarding color and heat. The stages used include chlorina-tion, either by hypochlorite, gaseous chlorine, (both becoming obsolete) or chlorine diox ide (usually two stages), with an alkali extraction washing stage between. On occasions oxygen is also used. Bleach plant chemicals are usually prepared in the mill so solutions such as chlorine water, sulfuric acid, sodium chlorate, sodium chloride, sodium hydroxide, calcium hypochlorite, and chlorine dioxide all have to be pumped.

It cannot be emphasized too strongly that materials of construction are of vital importance in the chemical preparation area of the bleach plant.

In addition to the standard chemicals, some of the common pulp mill bleach substances, together with some chemical preparation systems, are as follows.

chlorine This is usually delivered to the mill in tank cars but is always vaporized to a gas before use.

chlorine water (HYPOCHLOROUS and hydrochloric acid) Concentrations cover the range from pH 2 to 10 for bleaching pulp. In some cases, the gas is mixed directly with pulp in special mixers. Group C lined pumps are essential.

sodium hypochlorite and calcium hypochlorite This mixture is made in the mill by permitting chlorine to react with either sodium or calcium hydroxide concentrated caustic (70%) diluted to 5 to 6% before chlorination. Calcium hypochlorite is made from a 10% solution of slaked lime at temperatures up to 150°F (66°C), but not normally exceeding 70°F (21°C). These liquors are corrosive to steel, and group C or lined pumps are necessary when handling solutions to the bleach plant; after bleaching the filtrate may still have residual hydrochloric acid.

chlorine dioxide This is the most common bleach solution used because it gives an excellent brightness to the pulp and, despite corrosion problems, is usually cheaper than other bleach solutions.

After generation of the gas, during which absolute cleanliness is vital, the gas is stripped in a packed tower as an aqueous solution and stored in plastic tanks made of special resins that resist chemical attack. In modern plants, increasing use is made of glass-reinforced plastic with selected resins for piping, valves, and pump linings. This is sometimes a cheaper alternative than the use of exotic metals, such as titanium, for pumps. Pumps must be group C, and stainless steel is not satisfactory. Solution strengths of up to 8 g/liter are used.

sodium peroxide and hydrogen peroxide These are used for bleaching groundwood pulp. Typical solutions contain sodium silicate (5%), sodium peroxide (2%), and sulfuric acid (1.5%). The latter controls the pH of the liquor. Concentrations of bleach liquors are up to 15%. Temperatures are usually less than 90°F (32°C). Group C pumps are necessary.

wash liquors In general, the filtrate from bleach washing stages will exhibit at least some of the properties of the stage immediately before washing, owing to slight excesses of chemical present. Filtrates are collected in corrosion-resistant pipes and vessels, usually made from glass-reinforced plastic, and the pumps used will be either group B or C, depending on the stage in question. The filtrate from the chlorine dioxide stages should be pumped with a super austenitic stainless steel case and trim pump because the filtrate is not as corrosive as the bleach solution.

Spent acid from chemical preparation plants is also highly corrosive, and usually stainless steel is not satisfactory for use with it.

Effluent from the bleach plant, on the other hand, is usually a mixture of several liquors, and experience has shown that 317 stainless steel is a suitable material for pumps that handle it.

chlorine dioxide preparation; sodium chlorate Chlorine dioxide is produced by permitting sodium chlorate to react with sulfuric acid and hydrochloric acid in a vessel into which a reducing agent such as NaCl, SO2, or methanol is introduced in controlled quantities. Sodium chlorate solutions are usually from 43 to 46%, at which strength the specific gravity is about 1.38. Stainless steel pumps may be used, but epoxy-resin-lined pumps are superior.

foul condensate This arises from the evaporation of water from black liquor at the multiple-effect evaporators, as these units flash vapor from the liquor in one stage and use this to evaporate the liquid in the next stage. The vapor when condensed contains some carry-over from the black liquor, and thus the condensate is contaminated and corrosive. When a nickel cast iron casing and stainless trim are used, group A pumps should be satisfactory. Some liquors produce very corrosive vapors, and a stainless casing pump may prove necessary. Group B pumps are recommended.

Stock Stock is the term applied to the suspension of cellulose fiber in water. It first appears either after grinding (in the case of mechanical pulp) or after the blow tank (in the case of chemical pulp). Stock production rates may be converted to pump flow rates with the following formula:

Production (air dried short tons per day) X 15

Consistency (% oven dried)

After the separation of chemicals or impurities by washing and screening, the stock is given a mechanical treatment known as either beating or refining, depending on the nature of the treatment. This enhances the sheet properties. Additives such as starch, clay fillers, alum, and size are introduced to impart special characteristics, depending on the end use of the product.

Over the range of stock in normal use, the specific gravity may be considered constant for all practical purposes, with a value equal to that of water at the appropriate temperature.

Cellulose fibers have a specific gravity slightly greater than water, and constant agitation is required to ensure that stratification does not occur in storage. Agitation, however, can also introduce air, to the detriment of the stock.

The pH of stock varies over a wide range—from as low as 1.0 during some bleaching processes to 11.0 with others. In the paper machine room, the pH of the stock will usually range from 4 to 8. Thus from a corrosion viewpoint washed stock does not usually present special problems except when high-grade bleached products are produced. Stains will be caused by iron sulfides or oxides, and therefore stainless steel must be used—frequently 304 for washed stock, but 316 or 317 within the bleach plant before washing or where bleach liquor is likely to be present with the stock.

Unbleached paper mills generally do not experience corrosion with washed stock, except in the case of groundwood mills, where the pH is usually lower than in chemical pulp mills.

fiber characteristics Stock made from softwoods will have a predominance of fibers 2.8 to 3.5 mm long and 0.25 to 0.3 mm wide; fibers from hardwoods will be about 1.0 to 1.3 mm long and 0.1 mm wide. Straw fiber will be still shorter—0.75 mm on the average—but flax can have fibers up to 9.0 mm long. These figures are typical and are of interest because of their effect on pump performance.

consistency This is the amount of dry fiber content in the stock, expressed as a percentage by mass. Typical values will vary from about 0.1% for the feed to the headbox of a special paper machine to 16% for stock between some bleaching stages or in high-density towers. The critical stock consistency in the selection of pumps is 6%. Up to the 6% level, pumps may be selected on the basis of their water performance.

freeness When stock is beaten, or refined, it acquires an affinity for water, and the longer the stock is beaten, the longer the water retention period. The retention of water by the stock increases the friction factor of the flow of stock.

FIGURE 3 End suction stock pump. (Courtesy ITT / Goulds Pumps, Inc.)

Freeness is often measured by an instrument called the Canadian standard freeness tester. The range of values covers a scale from 0 to 900, with a higher freeness value indicating a less refined stock and thus a lesser affinity for water. This instrument measures the amount of water drained from a sample of stock under a regularly decreasing head. Its use is recommended by the Technical Association of the Pulp and Paper Industry (TAPPI), and it is commonly employed in North American mills.

stock pumps In stock pumps, consistency is not a major problem until a value of about 6% is reached. The essential requirement is to get the stock to the pump impeller, and every effort should be made to keep the piping as large and straight as possible. A typical open impeller, end suction stock pump is shown in Figure 3.

Above 6% consistency, special pumps are required, and they can be of the positive displacement screw type or centrifugal type. Air entrainment in the stock will reduce pump output. Air entrainment occurs from agitation in the chests, from flow over weirs, and from flow through restricted openings. How air entrained in water and in stock affects pump performance is shown in Figure 6.

piping arrangement Piping should be as straight and short as possible. This is particularly important on the suction side of the pump to prevent dewatering of the stock. The diameter of the suction piping should be at least one pipe size larger than the diameter of the pump suction and should project into the stock chest. The inlet end of the suction pipe should be cut at an angle, and the bottom of the pipe should be at least 11 pipe diameters from the bottom of the chest. With the long side of the pipe on top, the probability

FIGURE 5 Fan pump. (Courtesy ITT / Goulds Pumps, Inc.)
FIGURE 6 General effect of entrained gas on pump performance. (Courtesy ITT / Goulds Pumps, Inc.)

of drawing air into the suction of the pump through vortices is reduced. Some manufacturers provide a lump breaker or screw feeder at the suction side of the pump for pumping stock above from 6% to 8% consistency.

size OF pumps It is important to estimate the performance requirements of stock pumps as accurately as possible. Oversizing of centrifugal pumps will cause an unbalanced radial thrust on the impeller resulting in excessive shaft deflection and reduced bearing and seal life. Oversizing can also result in impeller recirculation and the accompanying cavitation-like noise and damage to pump components.

Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

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