Concentrated Sulfuric Acid Production from Non-Condensable Gases and Its Effect on Alkali and Sulfur Balances in Pulp Mills
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CONCENTRATED SULFURIC ACID PRODUCTION FROM NON-CONDENSABLE GASES AND ITS EFFECT ON ALKALI AND SULFUR BALANCES IN PULP MILLS Andrés Mahecha-Botero1, Isabel M. C. L. Sêco2, Igor Aksenov1, C. Guy Cooper1, Jon Foan1, Rohan Bandekar1, Kam Sirikan1, Jim Wearing1 1NORAM Engineering and Constructors Ltd., 200 Granville Street, Suite 1800, Vancouver, BC, V6C1S4, Canada, Phone: +1 604 681 2030, [email protected] 2Now with Altri, SGPS, S.A. SUMMARY The pulp and paper industry often encounters challenges that require process improvements to remain competitive. These challenges may include the requirement to meet more stringent environmental regulations, stricter energy policies, or the need to improve product quality, increase production capacity and profitability. As a result, the pulp mills of today have to focus on becoming more efficient by possessing an effective chemical recovery system and reducing chemical losses. The high degree of closure is beneficial for environment, water consumption and mill economy but can upset the Na/S balance and increase the build-up of non-process elements in the system. Installing an acid plant to convert the sulfur containing Non Condensable Gases (NCG) into sulfuric acid will eliminate the NCG as a sulfur input to the recovery cycle, eliminate purchases of sulfuric acid, reduce caustic purchases, and produce additional steam that will positively impact the mill’s heat balance. This paper provides an overview of the technology required to produce sulfuric acid in a pulp mill from NCG, presents some of the unique challenges related to feed variability, and discusses some of the technical features of NORAM’s sulfuric acid process technology and equipment. Keywords: Non-condensable gases, sulfur-containing gas, sulfuric acid plants. INTRODUCTION Kraft process is a chemical pulping process in which the cellulose fibers are extracted from wood by using aqueous white liquor which is composed of the chemicals NaOH and Na2S. The sodium and sulfur balances impact pulp quality. The right Na/S balance in Kraft mills at first was achieved by replacing the chemical losses with sodium sulphate and sodium hydroxide. The typical chemical losses included brown stock washing losses, liquor spills, boiler and kiln emissions and purging of ESP ash or lime mud for NPE control. The process modifications over the years such as improved brownstock washing, partial or total recovery of bleach plant effluent, recovery boiler precipitator dust purification processes to selectively remove chloride and potassium, etc., have significantly reduced the typical chemical losses in a pulp mill. Furthermore the make-up chemicals used now, apart from NaOH and Na2SO4, include ClO2 generator by-product, sulfur input from a lignin extraction process, sodium and sulfur input from tall oil acidulation process, etc. Mills that recycle bleach plant effluent to the kraft recovery system often see an excess of sodium/sulfur in the chemical balance of the mill. The alkali and sulfur compounds (NaOH, H2SO4, SO2) charged in the bleach plant effectively replace the make-up chemicals typically used in a scenario with no bleach plant effluent recycle. This results in a surplus of alkali and/or sulfur which then have to be controlled by purging ESP ash or ClO2 generator by-product. Table 1 below gives examples of 1 sodium sulfur balance in mills that do and don’t recycle bleach pant effluent [1]. It’s apparent from the data that mills that recycle bleach plant effluent have to purge their ESP ash or invest in a technology that can deal with a situation of excess chemicals. Table 1: Sodium/ Sulfur Balances [1] No Bleach Total Bleach Total Bleach Total Bleach Effluent Recycle Effluent Recycle Effluent Recycle Effluent Recycle (PAA sequence) (Peroxide (Ozone sequence) sequence) Input Na, S, Na, S, Na, S, Na, S, Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Wood, etc 0 0.2 0 0.2 0 0.2 0 0.2 Chem. O stage 0 0.5 - - - - - - Bleaching - 0 14 2 14 5 14 7 Chemicals Make-up and 9 0 0 0 0 0 0 0 Scrubber akali Tall oil plant 0 2.8 0 2.8 0 2.8 0 2.8 Total 9 3.5 14 5 14 8 14 10 Output Na, S, Na, S, Na, S, Na, S, Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Pulp 4.5 1 1.5 0.5 1.5 0.5 1.5 0.5 Bleached 4.5 1.5 4.5 4.5 4.5 1.5 4.5 1.5 Rejects, Sludges, Accidental Losses, Condensates, etc S gas - 1.0 - - - 0.5 - 0.5 Purges Na, S, Na, S, Na, S, Na, S, Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt Kg/ADt ESP ash - - 5 3 8 5.5 8 5.5 Soda (G.L) - - 3 - 0 - 0 - S purge - - - 0 - 0 - 2 (Liquor Heat Treatment) Total 9 3.5 14 5 14 8 14 10 Similarly in a softwood mill, an excess amount of sulfur usually exists and hence mills often tend to sewer the ClO2 by-product or recovery boiler ESP ash. In the process, sodium is lost which is usually made up by using expensive NaOH. Furthermore the ClO2 by-product has to be neutralized before being purged to the environment. ESP ash purge to the environment can be an important issue in the future when more stringent environmental regulations come into place. There are several methods that 2 have been previously proposed to handle a situation of high sulphidity. The following examples illustrate some of the options: Tall oil recovery with carbon dioxide. The use of carbon dioxide to replace a portion of the sulfuric acid required can provide significant benefits to the mill but sulfuric acid would still have to be purchased to achieve the low pH required for the process [2]. Electrolytic salt splitting to generate acid or a base from neutral salt using membrane electrolysis. This technology can be potentially applied to ESP catch from the recovery boiler to produce sulfur free alkali and sulfuric acid simultaneously. However the relatively low costs of sulfuric acid and caustic today make the operating and capital costs difficult to justify on economic grounds alone, except under certain circumstances [3]. Production of sulfuric acid from NCG. These gases are rich in sulfur and normally they are burned in an incinerator, lime kiln or recovery boiler. NCGs burning forms SO2 which can be sent to a sulfuric acid plant for acid production [4]. Generator acid purification (GAP) technology uses a short-column resin-bed technology to separate sodium sesquisulphate by-product from a ClO2 generator into sulfuric acid and sodium sulphate. This operation allows a reduction of caustic makeup while the sulfuric acid produced replaces the merchant acid in the bleach plant [5]. Bleaching with a combination of ozone, oxygen, hydrogen peroxide and chlorine dioxide to reduce the spent acid or saltcake produced in chlorine dioxide generation [2]. Adoption of improved chlorine dioxide generation technologies to reduce the saltcake production per ton of ClO2 produced [2]. The management of the chemical balances of a modern Kraft pulp mill is a challenging task due to the multiple variables to be controlled in the mill. There is increased pressure to reduce losses of chemicals to the environment by improving the efficiency of the chemical recovery system to comply with more stringent environmental regulations for emissions to air and water, and to reduce the make- up of chemicals. The high degree of closure is beneficial for environment, water consumption and mill economy but can upset the Na/S balance and increase the build-up of non-process elements in the system. There is a significant economic and environmental incentive in-situ sulfuric acid production. This paper provides an overview of sodium sulfur balance in pulp mills when integrated with in-situ sulfuric acid production. Moreover the technology required to produce sulfuric acid in a pulp mill is reviewed. Mill Balances and System Integration It is important to optimize the sulfur and sodium balances in the pulp mill. There are several disadvantages of running the mill at high sulphidity such as increased emission of NCGs throughout the recovery cycle, upsets in recovery boiler operation, increased corrosion throughout the pulping and recovery cycle, etc. For mills whose sulfur inputs must be limited or sulfur would need to be purged: the byproduct from the chlorine dioxide generator cannot be fully used as make-up chemical in the recovery cycle or part of the electrostatic precipitator ash from the recovery boiler would need to be dumped. Either way this represents a sodium loss in the recovery cycle that needs to be replaced by NaOH or Na2CO3 and therefore the cost of controlling the Na/S increases. In the Kraft mill the chemical balance is of interest, both to minimize the production costs, and to control the process conditions. The Na/S balance has an influence on the pulping yield and pulp quality and emissions of sulfurous gases. Table 2 presents an example of a Na/S balance for a Kraft pulp mill before and after conversion 3 Table 2: Na/S balance to recovery cycle – Example for Mill A with and without a H2SO4 system from NCGs Case at Mill A Base Case With H2SO4 System chemical, kg/t Na, kg/t S, kg/t chemical, kg/t Na, kg/t S, kg/t Inputs Wood 0.2 0.2 ClO2 byproduct 7.3 2.4 1.6 10.4 3.4 2.4 Purchased Na2SO4 0.0 10.2 3.3 2.3 Caustic 10.0 5.8 0.0 2.5 1.4 0.0 Totals 8.1 1.8 8.1 4.8 Outputs Sulfur in CNCG’s 3.0 Other Losses 8.1 1.8 8.1 1.8 Totals 0 8.1 1.8 8.1 4.8 Because the sulfur balance should be kept undisturbed, the generation of sulfuric acid in the mill could be a solution to keep the Na/S ratio in balance and give a positive economic outcome to the pulp mill.