Module 8: Introduction to Process Integration Tier III

Transcription

1 Program for North American Mobility In Higher Education Program for North American Mobility in Higher Education (NAMP) Introducing Process Integration for Environmental Control in Engineering Curricula () Module 8: Introduction to Process Integration Tier III Created at École Polytechnique de Montréal & Universidad de Guanajuato Rev:2.3

2 Project Summary Objectives Create web-based based modules to assist universities to address the introduction to Process Integration into engineering curricula Make these modules widely available in each of the participating countries Participating institutions Two universities in each of the three countries (Canada, Mexico and the USA) Two research institutes in different industry sectors: petroleum (Mexico) and pulp and paper (Canada) Each of the six universities has sponsored 7 exchange students during the period of the grant subsidised in part by each of the three countries governments 2

3 Structure of Module 8 What is the structure of this module? All Modules are divided into 3 tiers, each with a specific goal: Tier I: Background Information Tier II: Case Study Applications Tier III: Open-Ended Design Problem These tiers are intended to be completed in that particular order. r. Students are quizzed at various points to measure their degree of o understanding, before proceeding to the next level. Each tier contains a statement of intent at the beginning and a quiz at the end. 3

4 Purpose of Module 8 What is the purpose of this module? It is the intent of this module to cover the basic aspects of Process Integration Methods and Tools,, and to place Process Integration into a broad perspective. It is identified as a pre- requisite for other modules related to the learning of Process Integration. 4

5 Tier III Open-ended problem 5

6 Tier III: Objective Tier III: Statement of intent The goal of this tier is to solve a real-life life application of Process Integration, in which the student must interpret the results obtained from a range of Process Integration tools. At the end of o Tier III, the student should be able to identify the following: Benefits of the use of Process Integration tools Potential cost saving opportunities from the use of Process Integration tools Environmental impact reduction resulting from the application of Process Integration tools How the application of Process Integration tools can be used to obtain an operable process 6

7 Tier III Problem Statement The Kraft pulping process The basic features of a Kraft pulping process are shown on the next n slide. Wood chips (containing 50% water) are conveyed from a surge hopper to a presteaming unit to facilitate subsequent impregnation of the chips with chemicals. A high-pressure feeder transfers the chips from the presteaming vessel to the digester. In the digester, the wood chips are cooked using white liquor (a mixture of cooking chemicals including NaOH, Na 2 S, Na 2 CO 3 and water) to solubilize the lignin in the wood chips. In the cooking process, methanol is produced. Following digestion of the lignin, the cooking chemicals are washed out of the pulp. A countercurrent multistage washing unit is utilized to minimize the carryover of chemicals with the pulp. The residual chemicals from m the pulping process are called the weak black liquor. The black liquor contains sodium salts s (hydroxide, sulphide, carbonate, chloride, sulphite and sulphate), dissolved lignin, methanol m and water. Before the outlet to the digester is fed to the washers, the cooked pulp p and liquor are passed to a blow tank where the pulp is separated from the weak black liquor which is fed to a recovery system for conversion to white liquor. The first step in recovery is concentration of the weak black liquor via multiple effect evaporators. The concentrated solution is sprayed in a recovery furnace. The evaporation process results in the generation of a large amount of combined condensate which is classified as a wastewater r stream and of gaseous waste whose primary pollutant is H 2 S. The smelt from the furnace is dissolved in water to form green liquor which is reacted with lime (CaO( CaO) ) to produce white liquor and calcium carbonate mud.. The recovered white liquor is mixed with make-up materials and recycled to the digester. The calcium carbonate mud is thermally decomposed ed in a kiln to produce lime which is used in the causticizing reaction. There are several gaseous wastes emitted from the process, some of which can be used for steam generation or cogeneration. Reference: El-Halwagi Halwagi,, M. M., Pollution Prevention through Process Integration: Systematic Design Tools.. Academic Press,

8 Gaseous Waste Tier III Problem Statement Reference: El-Halwagi Halwagi,, M. M., Pollution Prevention through Process Integration: Systematic Design Tools.. Academic Press, Wood Chips Strong Black Liquor MULTIPLE EFFECT EVAPORATORS Weak Black Liquor Steam Steam DIGESTER Off Gas BLOW TANK Gases Pulp to further processing Recovered White Liquor WASHERS Off Gas Condensate RECOVERY FURNACE Smelt DISSOLVING TANK Water Flue Gas Green Liquor Air LIME KILN Gases Lime Calcium Carbonate SLAKING & CAUSTICIZING SETTLING & FILTRATION 8

9 Tier III Problem Statement Wastewater treatment in the Kraft pulping process Pulp and paper mills employ high levels of fresh water that lead to the generation of a significant amount of aqueous effluent. Therefore, the objective of optimizing water usage and wastewater discharge presents a major challenge to the industry. Due to the direct contact of water with various species, the aqueous streams are laden with various compounds ounds including methanol, non-process elements and organic and inorganic species. Methanol is classified as a high priority pollutant for the pulping industry. In addition, it may provide a source of revenue if properly recovered. Methanol can be found in most wastewater streams of the Kraft pulping process particularly in the condensate leaving the multiple effect evaporators and the condensers c used to condensate the steam from the presteaming unit before the wood chips are taken to the digester. All the wastewater streams are treated using biotreatment and then discharged to the river. Any stream discharged to the river should not have a methanol composition which w exceeds 15 ppmw.. The following information is available for the biotreatment facility: acceptable methanol composition entering biotreatment < ppmw average outlet methanol composition = 15 ppmw biotreatment operating cost = 0.11*M *G where M is the mass load (kg/h) of methanol and G is the flowrate of wastewater (kg/h) Reference: El-Halwagi Halwagi,, M. M., Pollution Prevention through Process Integration: Systematic Design Tools.. Academic Press,

10 Tier III Problem Statement Wastewater treatment in the Kraft pulping process (2) The amount of methanol in the wastewater could be reduced using air stripping and recovered from aqueous streams to provide methanol sales that are higher than t recovery costs. The flowrate of air is determined as follows: L = 0.5*ƒ*G Where L and G are the mass flowrates (kg/h) of air and wastewater, respectively, and ƒ is the fractional mass removal of methanol from water by stripping. The operating cost for air stripping is given by the following relationship: Operating Cost (US$/h) = 0.003*L (kg air/h) This cost includes air compression and methanol condensation. The wastewater treatment plant operator also has problems predicting when the treatment process will go from one operating regime to another or when the process will produce water with above permitted limits concentrations of methanol and other pollutants. He disposes of the treatment facility s s last three years of operating data but does not know how to interpret such amounts of information. Reference: El-Halwagi Halwagi,, M. M., Pollution Prevention through Process Integration: Systematic Design Tools.. Academic Press,

11 Tier III Problem Statement Wastewater treatment in the Kraft pulping process (3) Along with methanol as one of the main pollutants found in Kraft pulp mill aqueous effluents, other organic and inorganic compounds are found. These include chloroform, c formaldehyde, phenol and others, depending on the mill and process used. Phenol l is of concern primarily because of its toxicity, oxygen depletion and turbidity. In addition, phenol can cause objectionable taste and odour in fish flesh and potable water. Several techniques can be used to separate phenol. Three external l technologies are here considered for the removal of phenol. These processes include adsorption using activated carbon, ion exchange using a polymeric resin and stripping using air. The operating costs for each method comprise cost of make-up and cost of regeneration. For activated carbon, steam is used to regenerate the mass-separating separating agent while caustic soda (NaOH)) is used for the regeneration of the ion exchange resin. In the case of air stripping, the gaseous stream leaving the mass-exchange unit cannot be discharged to the atmosphere owing to air-quality regulations. Hence, the air leaving the separation unit is fed to a phenol-recovery unit in which a refrigerant is used to condense phenol. The operating cost related to each technology is thus US$, US$ and US$ per kg of removed phenol for activated carbon, ion exchange resin and air stripping respectively. Reference: El-Halwagi Halwagi,, M. M., Pollution Prevention through Process Integration: Systematic Design Tools.. Academic Press,

12 Tier III Problem Statement Energy in the Kraft pulping process The Kraft pulping process is a very energy-intensive process: electricity end-uses common to all pulp and paper mills include pumping, air-handling, and lighting. In addition, steam needs and the large number of process streams makes this sector of the industry a good candidate for improved heat integration. Black liquor concentration is usually the biggest single steam using operation in a Kraft pulp mill. Evaporators installed in the 1960s and 1970s were built with four or five effects, whereas most Kraft mills today use five or six effect evaporators, with a concentrator to further increase solids content. Firing the recovery boiler with the black liquor at higher solids content improves overall boiler performance and is a general trend in the industry. To counter this energy consumption problem, a Kraft pulp mill uses biomass. In fact, in addition to being the feedstock for pulp and paper production, biomass is also a major energy resource for the industry. The industry also has access to residues of pulpwood harvesting, some of which can be removed from the forest on a sustainable basis. All black liquor and most mill residues are used at mill sites to fuel cogeneration systems, providing steam and electricity for on-site use. Cogeneration also known as Combined Heat and Power (CHP) is the simultaneous production of electricity and useful heat from the same fuel or energy. A typical cogeneration system consists of an engine, steam turbine, or combustion turbine that drives an electrical generator. A waste heat exchanger recovers waste heat from the engine e and/or exhaust gas to produce hot water or steam. 12

13 Tier III Problem Statement Energy in the Kraft pulping process (2) Cogeneration produces a given amount of electric power and process heat with 10% to 30% less fuel than it takes to produce the electricity and process heat h separately. Facilities with cogeneration systems use them to produce their own electricity, and use the unused excess (waste) heat for process steam, hot water heating, space heating,, and other thermal needs. They may also use excess process heat to produce steam for electricity production. In the chemical recovery, steam plant and cogeneration areas, pulping liquor l solids, purchased and self-generated woodwaste,, primary clarifier sludge from the wastewater treatment plant, and knots are burned to recover cooking chemicals and to produce energy. Spent pulping liquors account for over 70% of the biomass-derived fuels used in the pulp and paper industry today. In the recovery process, the resulting strong black liquor from the evaporators is sprayed into the recovery boiler where the organic content in the liquor is burned, b releasing energy and producing steam for use in the mill. Upon combustion, the inorganic nic portion of the strong black liquor produces a flue gas. The electricity-to to-heat production ratio for a conventional back-pressure steam turbine cogeneration system ranges from kwh/gj, which is relatively well-matched to the steam and electricity needs at older Kraft mills. Much higher electricity ity-to-heat ratios are possible using biomass and black liquor cogeneration technologies based on gas turbines rather than steam turbines. Commercially-aimed aimed development of technologies for converting black liquor or biomass residues into combustible fuel gas is ongoing, along with h the cleanup systems that would be needed to enable use of the gas in gas turbine cycles. 13

14 Tier III Questions Question 1. Wastewater treatment in the Kraft pulping process Which Process Integration tools could be used to address all the issues presented in the methanol related slides? Define the steps in the methodology ogy you would use to answer the following points: (A) Methanol minimization in the wastewater streams as well as reduced water usage and reduced wastewater discharge (B) Trade-off between minimization of operating costs related to the elements stated in (A) and benefits resulting from the recuperation of methanol (C) Interpretation and use of process operating data to help the treatment plant operator obtain better control of the operation of the wastewater r treatment plant Question 2. Wastewater treatment in the Kraft pulping process (2) Using your knowledge of Process Integration tools, describe the methodology that could be used to choose the best mass-separating separating agent to treat the waste streams of phenol in this Kraft pulp and paper mill. 14

15 Tier III Questions Question 3. Energy in the Kraft pulping process With the knowledge of Process Integration acquired over the last two tiers, propose a methodology that would help identify the energy savings s possibilities as well as the potential for cogeneration in a Kraft pulp mill. Elaborate on each of the steps taken to conduct such a study and remember to include in your y proposition the impact of your solution on the environment. 15

16 End of Tier III This is the end of Module 8. Please submit your report to your professor for grading. We are always interested in suggestions on how to improve the course. You may contact us as process-integration.tamu.edu/ 16