Designing a lean energy process for resin production, by alternative treatment of process water using freeze concentration or direct vapour incineration

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CPD NR

3325

Conceptual Process Design

Process Systems Engineering DelftChemTech - Faculty of Applied Sciences

Delft University of Technology

Subject

Designing a Lean Energy Process for Resin Production,

by Alternative Treatment of Process Water

using Freeze Concentration or Direct Vapour Incineration

Authors (Study nr.) Telephone

Dirk-Jan Boerman 9086132 06-23454786

Ronald van Dijk 1098888 06-42077490

Mark van der Heijden 1098934

06-13309322

Pieter Pickhardt 1099000 06-20113866

Shems Vouwzee 9909472

0 6-24112143

Keywords

Freeze Concentration, FC, Direct Vapour Incineration,

DVI, powder coating resins, energy saving, Process

Water, DSM

Assignment issued :

13 September 2005

Report issued

:

06 December 2005

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Designing a Lean Energy Process for Resin Production, by Alternative Treatment of Process Water CPD3325

-i- Preface

The Conceptual Process Design (CPD) project is part of the curriculum of the studies Chemical Engineering (CE) and Biochemical Engineering (BE). Our group comprises three CE students and two BE students. The goals of the CPD projects are to familiarize the students with the problems associated with design engineering, creativity or “out of the box thinking”, and working in teams. Modern engineers must have, besides engineering skills, the ability to use creativity and function in teams. The duration of the project is 12 weeks, during which the progress of the group is monitored by a supervisor and a creativity coach. Approximately one monthafter initiation of the project, the group is required to submit a preliminary Basis of Design (BOD) report in which the “solution pathway” is outlined, the division of tasks within the group is described, and proposed solutions are presented. The group is required to give an oral presentation about the contents of the BOD report, which is the foundation of this report. This report and an oral presentation at the end of the project are the final requirements.

The nature of the CPD assignment is determined by research groups within the (Bio)Chemical Engineering Department or by industrial companies. The fundamental difference between these types of assignments is that with the latter a third party is involved. That third party is then a representative of the company in question, which makes the project in our view more interesting, but the communication can be more difficult due to larger distances and busy schedules. In addition, the assignment is more defined because it concerns a real life problem and the solutions must fit in the company’s vision and constrains. Our group experienced it as more challenging and better related to the things that will be expected from us in the future.

The third party involved in our project is DSM. When the company was founded in 1902 the main product was mining coal. However, over time the products changed from mining coal to bulk chemicals, and nowadays the products are more and more specialized. Today’s production can be grouped in three clusters; life science products, performance materials, and industrial chemicals. The division related to our project is DSM Coating Resins, which is a part of the performance materials cluster.

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-ii-Executive Summary

The objective of this CPD assignment is to design a lean energy process for the DSM Coating Resins plant Schoonebeek III with special attention to the natural gas usage. The design should not involve alterations of the reactor section or production rate; the treatment of the process waste water stream is the most energy inefficient part of process. This report comprises the description and design of two proposed alternatives for the treatment of the waste stream; Freeze Concentration (FC) and Direct Vapor Incineration (DVI). The anticipated natural gas reduction potential of these alternatives compared to the current process are determined at 12% for FC and 13% for DVI. The total energy reduction is determined at 5% for FC and 8% for DVI. The CO2 emission is reduced in both cases. However, application of the DVI option is recommended due to relatively low investment costs (€ 152,000), short pay back time (3.5 years), the absence of expensive utilities, the absence of additional health and safety hazards, easy adaptation to increased production rates, and relatively simple operation of the required equipment.

An important aspect of the DVI process is that it is operated in batch mode, without the use of the buffer vessel in the first phase of the process. The process vapour stream is lead to the incinerator when the batches are running, which demands a predetermined batch schedule. In addition, at this time DVI is not considered proven technology, i.e. information on design and operation are limited.

Another aspect is that the economical feasibility changes significantly when combining energy reduction with reactant recovery. However, the boundaries of this aspect are not fully explored in this report, but rough analysis is given on the possibility of eutectic freeze concentration. In addition, it is recommended to investigate the possibility of applying moving bed chromatography to the reactors individually.

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-iii-Table of Contents

Preface ...i

Executive Summary ... ii

Table of Contents ... iii

1. Introduction...1

2. Process Options & Selection ...3

3. Basis of Design...8

3.1. Description of Design ...8

3.2. Process Concept Chosen...8

3.3. Block Schemes...8

3.3.1. Current Process...8

3.3.2. Freeze Concentration ...12

3.3.3. Direct Vapour Incineration ...13

3.4. Thermodynamic Properties and List of Pure Component Properties ...14

3.5. Basic Assumptions ...14

3.6. Margins...14

4. Thermodynamics ...16

4.1. Heat Capacity of the Components in the Liquid Phase ...16

4.2. Enthalpy of Reaction...17

4.3. Process Water Phase Diagram ...18

4.4. Vapour Liquid Equilibrium...19

5. Current Process ...20

5.1. Process Structure & Description ...20

5.2. Energy Balance...21

6. Freeze Concentration ...24

6.2. Process Control ...27

6.3. Mass and Energy Balances ...28

6.3.1. Mass Balances ...29

6.3.2. Energy Balances ...31

6.4. Equipment Design...32

6.4.1. Heat Exchanger E101...33

6.4.2. Scraped Surface Heat Exchanger E102...33

6.4.3. Recrystallizer V101...36

6.4.4. Wash Column C101 ...36

6.4.5. Pumps P101, P102 and P103 ...37

6.5. Wastes & Environment ...38

6.6. Health & Safety...39

6.7. Economics...42

6.7.1. FC Cost Estimation by Coulson & Richardson Method ...42

6.7.2. FC Cost Estimation by Freeze Tec ...45

7. Direct Vapour Incineration ...46

7.2. Process Control ...48

7.3. Mass and Energy Balances ...48

7.3.1. Mass Balances ...48

7.3.2. Energy Balances ...50

7.4. Equipment Design...50

7.4.1. Condenser E201 ...51

7.4.2. Piping ...56

7.5. Wastes & Environment ...59

7.6. Health & Safety...59

7.7. Economy ...60

8. Creativity & Group Process...62

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-iv-

10. Conclusions & Recommendation ...66

11. Acknowledgement...67

12. Symbol List ...68

13. List of Abbreviations ...70

14. References ...71

Appendices Table of Contents Appendices ...i

A. DSM ...1

B. Key Data and Thermodynamics...21

B.1. General Plant Information ...21

B.2. Process Water...25

B.3. Physical and Chemical Properties of the Components ...26

B.4. Figures and Tables on Thermodynamics...31

C. Freeze Concentration ...37

C.1. Equipment Specification Sheets...37

C.2. FC Process Structure with Concentrate Recycle ...45

C.3. Stream Summary ...45

C.4. HAZOP Tables ...46

C.5. MathCAD Calculations ...49

D. Freeze Tec Reports...57

D.1. Freeze Tec Pilot Test for DSM Coating Resins Report...57

D.2. Freeze Tec Cost Estimation FC Report...62

E. Direct Vapour Incineration...64

E.1. Equipment Specification Sheets...64

E.2. Stream Summary...66

E.3. HAZOP Tables ...67

E.4. MathCAD Calculations ...70

E.4.1. Condenser Phase I ...70

E.4.2. Condenser Phase II ...75

E.4.3. Total Condenser ...80

F. Catalytic Wet Oxidation...85

F.1. Process Structure & Description...85

F.2. Design of the Packed Bed Reactor ...87

F.3. Process Control ...89

F.4. Mass Balances ...90

F.5. Economic Evaluation...91

G. Reactor Heat-Integration System ...93

G.1. BOD on R-HIS ...93

G.2. Process Control ...95

G.3. Mass and Heat Balances (Energy reduction potential) ...97

G.4. Equipment Design ...99

G.5. Wastes ...99

G.6. Economy...100

G.7. Review on R-HIS ...102

H. Project Approach & Literature Origin...104

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-1- 1. Introduction

The performance of the CPD project comprises the design of an integrated process for the reduction of energy. The design includes the process flow sheet (PFS), material balance, energy balance, stream overview, equipment design, safety, health, environment, and economical evaluation. The advancement of the assignment is monitored by the supervisor, creativity coach, and principal during the following meetings and events;

• The Kick-off meeting; the team’s introduction to the problem statement and planning.

• The Basis of Design (BOD) report; the starting-point and the team’s design approach for the design are presented in this report.

• The creativity meetings; during these meetings the creative aspects of the design and group processes are discussed. All reports include a summary of the creative process.

• The meetings with the supervisor

• The final report

• The oral presentation for university and contractors.

This project, the design of a lean energy process for resin production, is a continuation of the study carried out by students of the Technical University Eindhoven (TU/e) [b]. The report stated that the incineration of the process water stream is the part of the process with the highest energy demand. The objective of this project is to reduce the energy (provided by natural gas) input of the process by 50%. The report included a list of alternatives for the treatment of the process water. DSM included two main leads with the assignment; Catalytic Wet Oxidation (CWO) in either vapour or liquid state, and Freeze Concentration (FC). Naturally, any proposed solution should have minimal effect on the local ecosystem and should not negatively affect the product price.

There was an extension in sustainability projects within DSM in 2002. DSM has two motivations for their interest in this project; firstly, the price for natural gas is steadily increasing. Secondly, the incinerator at the plant site is ready for replacement. DSM aims for an extension of the leadership position by minimizing of the costs product, while maintaining product quality and minimization of the environmental impact.

On the Schoonebeek III production location more than 120 different powder coatings resins are produced. New R&D projects, such as such as UV curable powder coatings, must lead to improved product performance, new applications and innovations in application technology. Powder coatings are nowadays mainly applied to the metal appliance industry for in- and outdoor applications:

• White goods such as washing machines, refrigerators and dishwashers.

• Electrical equipment such as computer housings, audio equipment and vacuum cleaners.

• Metal furniture: office furniture, chairs, desks.

• Facades: aluminium and steel window and doorframes.

• Automotive parts such as wipers, radiators and wheel suspensions.

• Machinery for industrial and agricultural equipment [I].

Compared to the solvent containing paints the (solvent free) powder coatings are much more environmental friendly, because no solvents are released into the environment. The powder coatings qualify not for a sustainable material because its raw materials are not produced from non fossil fuels. The demand for power resins is steadily increasing due to the fact that the use of solvents has to be decreased to meet requirements set by legislation. Each product is produced in

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-2-

a different batch process, resulting in a waste stream with variable composition. Often the client specifies the product qualifications and the resins are used for a variety of applications [a].

The profit of the whole DSM Coating Resins department is approximately 400 million euros a year of which 60% in Europe, 20% in the USA, and 20% in the rest of the world. The world market for resins consists of half decorative and half industrial resins. The return of DSM Coating Resins consists for 88% of industrial products (including protective coatings) and for 12% of decorative products. DSM is one of the leaders on the powder coating market with a market share of about 25%. The total global consumption in 2000 was estimated at 23 million tonnes [II]. The main raw material for the powder coating is neopentyl glycol and is produced by BASF [III] and Perstorp [IV].

The main competitors of DSM on powder coating resins are Bayer [V] and Cray Valley [VI]. Dow is also produces powder coating resins, but these are epoxy powder coating resins. DSM did not provide information on business intelligence. On the other hand detailed information on competitors has a minor relevance for this project.

The plant comprises five batch reactors of different sizes (1 of 16 m3, 3 of 24 m3, 1 of 36 m3), to which the preheated reactants are fed. The product is drained from the reactors and is then cooled on cooling belts. The obtained resin is pulverized, packed and stored. The water produced in the reactors is fed to incinerator and evaporated; contaminations in the water are burned. The function of the incinerator is on one hand the heating of the heating oil on the other hand the combustion of unpleasant smelling compounds (ethers and esters) released in the process picked up by the process water steam, the vent gas, air and natural gas. In the incinerator the burning of the organics takes place.

Even at the current energy market situation, energy reduction programs are most of the time quite costly and have a much smaller impact on the cost price than projects on production yield. The freeze concentration process improvement proposal illustrates that, because the payback time cuts down very much, so it is very interesting if the raw material can be saved at the same time. The proposed alternatives should not have much influence on the market.

In chapter 2 the decisions made throughout the CPD project are described. The Basis of Design is given in chapter 3. Thermodynamics that are relevant for this project are presented in chapter 4. The current process structure and concerning energy balances of the DSM resin production plant Schoonebeek III are described in chapter 5 in detail. The two redesign options for a lean energy process, FC and DVI are described in chapters 6 and 7 in detail. The creativity and group process applied to this CPD project are discussed in Chapter 8. Chapter 9 comprises a comparison between these alternatives and the current process. Conclusions and recommendations are given in chapter 10.

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-3- 2. Process Options & Selection

Scope and Design Basis

Investigation of a problem requires the equationtion of the Scope and Design Basis. This is roughly depicted in Figure 2.1, where it is important to notice that the products and reactants are displayed. This is done, because they are predefined and take no part in the design assignment.

Figure 2.1: Schematic representation of the Scope and Design Basis.

There are some restrictions and objectives in which the design has to be carried out. They are equationted as follows;

• The current production has to be maintained

• The natural gas (NG) consumption has to be reduced

• The organics and especially smelly substances (ethers) have to be removed from the waste water stream

As a result of the first demand, the reaction, stoichiometry, catalysts, and reaction kinetics are not to be considered during this project. Most of the available information was provided in the form of a report produced earlier by students of the TU/e [b]. This project was initiated by evaluation of the alternatives proposed in this report, which are summarized in Table 2.1.

Process analysis: Gather knowledge / information

Knowledge of the currently used process is indispensable to be able to develop and evaluate new ideas. Basic understanding of the entire process and the process water treatment section in particular is required to come to innovative solutions. The different process alternatives are investigated in literature and the outcome is discussed in brainstorm sessions. The different literary sources are depicted in Figure 2.2. The key words used for searches in electronic databases and journals are listed in Appendix H.

Process:

Lean Energy Process for Resin Production

Energy (gas, electricity)

Reactants Product

Waste + Emissions Process:

Lean Energy Process for Resin Production

Energy (gas, electricity)

Reactants Product

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Table 2.1: Selection of alternatives of the TU/e Report presented with the applied criteria.

Alternative Criteria Rejected

Catalytic wet oxidation -Good applicability after literature study

-DSM main lead

No Biochemical treatment of waste water -Problems with low concentrations but there might be possibilities

so first discuss with expert

No

Membrane technology -Seems to be possible, but problems with plugging

-Not DSM’s favourite

Yes

More efficient incinerator -Would not satisfy specifications, but is logical if incinerator is

needed

Yes

Mol sieves -Literature study

-Rejected after discussion with DSM at Kick-off meeting

Yes

Multi-effect vaporizers -In Appendix XIII of the TU/e report dhr. Leegwater indicated that

it would not be a solution for DSM Schoonebeek

Yes

Distillation of the waste stream -Expensive and would not solve the problem at hand Yes

Adjustment reactor size -Not significant in relation to the problem

-5 reactors are already available

Yes

Better Catalyst -Not significant in relation to the problem Yes

Microwave theory -Would increase necessary energy supply

-DSM was not enthusiastic, after studying literature

Yes Heating/cooling of reactor with liquid

propane

-Would increase necessary energy supply due to high propane pressure

Yes

Using steam to heat reactor -Heat integration is always an option but will not solve the

problem, but will always be applied

Yes

Change of acid alcohols in reactor -This will affect the product stream Yes

Leave out the condenser -No reflux, so not possible

-It might be used in combination with a membrane

Yes Continuous instead of batch operation -Would probably reduce the yield

-Lower concentrations so more waste water

Yes

Nitrogen recovery -Not significant in relation to the problem

-Recovery might be used for drying

No

Application of a chemical solvent -Incinerator always necessary

-Low concentration in water so huge streams needed

Yes

Freeze concentration -After studying some articles it looks quite interesting and

applicable

No

Gather Knowledge

Pre-Reading DSM informat ion [a] TU/e Report [b]

Production

Design Alternatives

DSM informat ion [a] TU/e Report [b]

Databases Journals Coulson & Richardson [14]

Douglas [40] Seader & Henly [39] Gather Knowledge

Pre-Reading DSM informat ion [a] TU/e Report [b]

Production

Design Alternatives

DSM informat ion [a] TU/e Report [b]

Databases Journals Coulson & Richardson [14]

Douglas [40] Seader & Henly [39] Figure 2.2: Information resources used during this process.

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-5- Process Analysis: Decomposition

The resin production process can be subdivided in four functional blocks; reactor section, incineration, process water treatment, and product treatment. The process water treatment block in Figure 2.3 is grey, because that is the part of the process that will be worked on during this assignment. Although the content of this block in the block scheme can be changed, it must not affect the reaction section or end products. The dotted line drawn from the process treatment block to the incineration block illustrates the fact that some process alternatives require the use of the incinerator, and some not. However, the incineration function remains a necessity, because it provides the energy for the reaction section and the hot oil system.

Lean energy process for Resin Production

Reaction

Section

Incineration

Process water

Treatment

Product

Treatment

Natural Gas

Reactants

Product

Waste Water

Flue Gas

Lean energy process for Resin Production

Reaction

Section

Incineration

Process water

Treatment

Product

Treatment

Natural Gas

Reactants

Product

Waste Water

Flue Gas

Figure 2.3: Block scheme of the currently used process. Alternative process configuration

The given constrains limit the number of possible solutions. The production process and thus the content of the process waste water stream cannot be changed. The proposed treatment of this stream will have to reduce the natural gas usage of the plant and at the same time remove the unwanted substances. Figure 2.4 is the process of initiation of the design procedure for each process alternative, represented schematically. Currently, the process water treatment section is operated continuously. However, the production process is operated batch wise. Therefore, operating the process water treatment batch wise seems to be a viable option. The options applicable to the treatment itself are limited due to the low concentration (ppm level) of the smelly substances that has to be removed. Complete oxidation meets this requirement and seems a promising option, although the energy input is rather high. Some alternatives using separation technology are satisfactory, but even after purification treatment there will still be some unwanted compounds left in the process water stream.

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Figure 2.4: Initiation process of the design procedure for each alternative.

Alternatives to Preliminary Basis of Design (BOD)

All of the restrictions and qualifications described above taken into account, seven options remained open for discussion. The first two options were given with the assignment as main leads to be investigated further by DSM. This choice was based on the content of the TU/e report.

• Catalytic wet oxidation (liquid or vapour phase and with or without subsequent biotreatment)

• Freeze concentration

• Desublimation (in combination with a partial condenser)

• Direct vapour incineration using a partial condenser (with or without a more efficient incinerator)

• Biological treatment

• Moving bed chromatography

• Drying (using the N2 of the process, but always in combination with other solutions) These options were evaluated on industrial applicability and economics. The desublimation option is rejected due to the properties of the components in the process water stream and the lack on industrial applicability. Biotreatment is rejected as a unique solution to the problem due to the toxicity of the components, the difficulty of bioconversion, and the concentration fluctuations of the waste water stream. However, a combination of CWO with biotreatment is a widely applied process for the treatment of process water; therefore, the option using this combination was not rejected. A literature study of technologies using moving bed chromatography learned that this is a very promising technology, but not yet ready for industrial applications. At this moment, only small plants using this technology obtain satisfactory results and the high throughput of the DSM plant would certainly become a problem. In addition, multi-component operation is complicated, especially when the composition varies, as is the case here [1, 2]. Drying is found not feasible, because the N2 flow is insufficient and unsuitable.

CWO and FC are the main leads given by DSM and are the only options from the list that seemed viable. The foundation of the third possible solution laid in the group and is best described as Direct Vapour Incineration (DVI).

Changing the way the process waste water is treated would probably be insufficient to obtain the objective of reducing 50% of the NG energy consumption. During the visit to DSM Schoonebeek two additional methods came to light to achieve the desired energy reduction. The goals of the project are therefore expanded to:

Oxidation Separation

Incineration Other Form of oxidation Concentration Purification

Level 1 Batch Continuous

Level 2 Level 3 Alternatives Operation Mode Treatment Oxidation Separation

Incineration Other Form of oxidation Concentration Purification

Level 1 Batch Continuous

Level 2 Level 3

Alternatives

Operation Mode

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• Alternative process water treatment

• More efficient use of heat oil systems

• More efficient use of other utilities The decision process is summarized in Figure 2.5.

B ra in s to rm T U /e R e p o rt S e s s io n s 7 ideas

•Catalytic Wet Oxidation •Freeze Concentration •Direct Vapour Incineration Process Water

DSM Visit •Better batch planning •Minimal use of cooling towers •Heat integration between reactors

B .O .D . B ra in s to rm T U /e R e p o rt S e s s io n s 7 ideas

•Catalytic Wet Oxidation •Freeze Concentration •Direct Vapour Incineration Process Water

DSM Visit •Better batch planning •Minimal use of cooling towers •Heat integration between reactors

B

.O

.D

.

Figure 2.5: Decision tree concerning the ideas at the moment of the preliminary BOD.

Another considered option is the installation of a reactor heat-integration system (R-HIS). It turned out that the anticipated reduction of energy usage was lower than expected, while the investment and operating costs were higher. Application of this system certainly requires the installation of software for batch planning, which results in additional costs, and the pay back time is relatively long. The mode of operation used on the site contradicts the energy efficient operation necessary for this principle.

Table 2.2: Comparison of the three process water treatment alternatives.

Treatment alternative Energy Reduction

Potential Investment Costs Data Availability FC + - + DVI + 0 + CWO 0 - -

In addition to the preliminary BOD report, an additional report was produced containing the Energy Reduction Potential (ERP) calculations and the economic evaluation of all alternatives. In order to carry on the design assignment for CWO and FC, additional experimental data was required. FreezeTech [d] performed some experiments on the process water, so for FC this data could be obtained. However, the data required for the CWO is not available at this time. The anticipated reduction of the natural gas usage is equal for FC and DVI and lower for CWO. In addition, application of CWO required the highest investments, as can be seen in Table 2.2. This report was discussed with the representatives from DSM and it was decided not to continue investigating CWO and R-HIS. However, all work done studying these alternatives is included in Appendix F and G. This report is concerned with the FC and DVI alternatives only.

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-8- 3. Basis of Design

The Basis of Design is the first step in the creation of a conceptual process design. The design is based on the general available data and can be used by others to understand the actual design and the decisions that were made. A description of the current process is given in section 3.1. The process options that are chosen are reviewed in the following section. The block schemes of the current process and these options will be worked out in section 3.3, followed by the thermodynamic properties in section 3.4. The basic assumptions will be listed in section 3.5 and this chapter ends with treating the economic margins in section 3.6.

3.1. Description of Design

The assignment given by DSM was to reduce the energy consumption of the Schoonebeek III resin production plant by 50%, with the main focus on the natural gas reduction. The main leads given by DSM were focused on process water treatment. This process water is evaporated in an incinerator, which is the most energy consuming part of the process. At the moment, the Schoonebeek plant consumes 169 TJ/a of which 107 TJ/a is natural gas energy [a]. The high-energy demand of the production process itself might become a problem in staying competitive on the market due to the annual raise of the energy price. Aside from the economical aspects, reduction in the energy demand of the process would be friendlier to the environment due to reduced CO2 emissions. However, after analysis of the current process the original given goal of 50% natural gas reduction seems to be unrealistic by only changing the process water treatment as can be seen later in this report.

3.2. Process Concept Chosen

Many options were considered during the starting phase of the project, four of which remained, that were explored for the preliminary basis of design. One of these options (R-HIS) had nothing to do with treatment of the process water stream. This idea was a result of the visit to the DSM plant site. This process was described in Chapter 2. The feasibility of two of them seemed quite low after performance of the calculations of the energy reduction potential and economic evaluation. This resulted in two options ready for further calculations to come up with a design. This means that FC and DVI are also treated here in the BOD and that the work performed until that moment on CWO and R-HIS is included in the appendix F and G.

3.3. Block Schemes

Block schemes are suitable to obtain a quick comprehensive overview of the process. Three block schemes will be treated in this section. The one of the current process, and those of the two alternatives, freeze concentration and direct vapour incineration, respectively.

3.3.1. Current Process

In addition to the data provided by DSM (Appendix A), mass and energy balances are set up and the mass flows of the currently used process are calculated. In Figure 3.1 a block scheme of this process is shown; where streams 3, 4, 5, 8 and 10 will be regarded constant for all processes. The general calculations and assumptions are presented in this section; additional assumptions regarding the process alternatives will be mentioned in the chapter concerned with the alternative.

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Designing a Lean Energy Process for Resin Production, by Alternative Treatment of Process Water CPD3325 -9- Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Incinerator 1 bar 900°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73)

7a. Process water 5,163 t/a (0.16)

8. Product stream 32,831 t/a (1.00)

10. Product (20,1) 32,831 t/a (1.00) 1a. Natural gas (20,1.2)

2,322 t/a (0.07)

9a. Flue gas (230,1) 52,067 t/a (1.59) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6. Vent / vacuum 1,587 t/a (0.05) 2a. Air (20,1) 42,995 t/a (1.31) Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Incinerator 1 bar 900°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73)

7a. Process water 5,163 t/a (0.16)

8. Product stream 32,831 t/a (1.00)

10. Product (20,1) 32,831 t/a (1.00) 1a. Natural gas (20,1.2)

2,322 t/a (0.07)

9a. Flue gas (230,1) 52,067 t/a (1.59) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6. Vent / vacuum 1,587 t/a (0.05) 2a. Air (20,1) 42,995 t/a (1.31)

Figure 3.1: Block scheme of the current process.

In the first mass calculations (equation 3.1 to 3.3), a balance is made over the reactor, where the data provided by DSM is used. The given annual raw material input of 38 kton is used as a basis. A typical product preparation is listed in Table 3.1. The process water composition is given in Table 3.2, where the assumption is made that the composition of “others” is 0.1 g/kg instead of 2.9, to prevent that the total composition exceeds unity.

Table 3.1: Typical batch example and annual consumption of resin production DSM Schoonebeek III [a]. Component Typical batch example (kg) Annual consumption (ktons) Terephthalic acid 520 17.0

Neopentyl glycol (stream 3) 430 14.1

Isoftalic acid 100 3.3 Catalyst + additives 0.65 0.0 Isoftalic acid 60 2.0 Adipinic acid 40 1.3 Additives 10 0.3 Nitrogen (stream 5) 0.05 1.580 TOTAL IN 1,160 39.6

Table 3.2: Typical batch example, process water composition, and annual production [a]. Component Typical batch example (kg) Process water composition (kg/kg) Annual production (ktons) Product (10) 1,002.76 32.831

Water from reaction 150 0.95 4.911

Neopentyl glycol 0.035 0.181 5,5-dimethyl-1,3-dioxane 0.0053 0.027 2-isopropyl-5,5-dimethyl-1,3-dioxane 0.0073 0.038 Methanol 0.0021 0.011 Isobutanol 0.0002 0.001 Others 0.0001 0.001 Nitrogen 0.04 1.580 TOTAL OUT 1,160 39.6 7.9

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-10-It is given that each typical batch example produces 150 kg water, which amounts to 95% of the total process water. A mass balance is used to calculate the organic mass flow.

out water organics product

φ

=

φ

+

φ

+

φ

(3.1) water organics organics water x x

φ

= ⋅ (3.2)

where φi denotes a specific mass flow and xi the mass fraction of component i. The annual mass flow values are obtained by normalizing the mass balance for 1 ton of product. The results are listed in the last column of Tables 3.1 and 3.2.

Some additional assumptions have to be made for stream 6 (vent);

• The concentration of organics in the process vapour flowing to the vent is the same as in the process water

• The water content in the vent is calculated using the saturation pressure of the water at the condensation temperature (35°C), which is 5621.5 Pa [3]. Equations 3.3 to 3.7 are applied to this calculation

2 , 2 2

N V N N

φ

=

φ

⋅

ρ

(3.3)

The volume flow of N2 is taken from [b] (1.40⋅106 m3/a). The density of N2 is 1.13 kg/m3. This results in a N2 mass flow of 1,580 ton/a (stream 5).

, 35 , o vap T C molar pw condenser P y P = = (3.4)

The pressure in the condenser is (without pressure drop) 8.3 bar in the first phase of the reaction, resulting in a molar fraction of water of 0.0068. The following relation can be used to convert this to mass fraction, where Mi is the molar weight of compound i.

(

2

)

2 2 , , , 1 , molar pw H O mass pw molar pw H O molar pw N y M y y M y M ⋅ = ⋅ + − ⋅ (3.5)

The total vent mass flow (6) can now be calculated with the nitrogen mass flow given by DSM (see Table 3.2).

(

2

)

2 , , 1 N vent mass pw N mass pw y y

φ

= ⋅ +

φ

− (3.6) , , pw pw total w vent

φ

=

φ

−

φ

(3.7)

The process water mass flow (5,163 ton/a, stream 7a) is calculated with equation 3.7, where the flow is corrected for the mass flow of water in the vent. The annual natural gas consumption (stream 1) for the treatment of the process water is calculated for each different process option. This means that the natural gas needed for the heating of the reactors via the

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-11-thermal oil and steam system is excluded, but this amount is included at the end of the calculation. For this calculation the CP values for the liquid and vapour phase for all components are needed (See appendix B.4). The thermodynamic properties for these mixtures are estimated with the following relations.

, . , p liq i p i c =

∑

x c⋅ (3.8) , . , p vap i p i c =

∑

y c⋅ (3.9) , vap i vap i H x H ∆ =

∑

⋅∆ (3.10)

The necessary heat is provided by the combustion of natural gas. The annual natural gas consumption can be obtained from the following relation.

(

)

(

)

, . , .

, ,

total p liq b in total vap total p vap out b

heat NG C NG C NG c T T H c T T Q H H

φ

= = ⋅ ⋅ − + ⋅∆ + ⋅ ⋅ − ∆ ∆ (3.11)

where Tin, Tb and Tout denote the temperatures of respectively the feed, boiling point, and chimney. φtotal represents the total mass flow (process water / vent) to the incinerator and ∆HC,NG denotes the combustion energy of natural gas (∆HC,NG=31.65 MJ/kg) [b].

Air (stream 2) provides the oxygen needed for both the combustion of NG and organics. The values for the combustion of the organics are approximated using the values for neopentyl glycol. The combustion energy of organics is estimated with molecular bonding energies [4]. This method uses average values resulting in a rough estimation for the combustion energy. The obtained value with this method is 30 MJ/kg. Supposing the worst case the combustion energy for the organics in the process water is assumed lower at 20 MJ/kg (∆HC,org). Assuming the air contains 20% oxygen, the air mass flow to the incinerator can now be calculated. Complete combustion to carbon dioxide and water is assumed.

4( ) 2 2( ) 2( ) 2 2 ( )

CH g + O g →CO g + H O l (3.12)

5 12 2( ) 7 2( ) 5 2( ) 6 2 ( )

C H O s + O g → CO g + H O l (3.13)

The obtained value for the natural gas flow has to agree with the value for the energy consumption provided by DSM. The model for the calculated NG flow has to be corrected by adding an amount of NG needed for the heating of the oil. By iteration the calculated NG energy content should be 22.7% of the total NG gas consumption of the incinerator given by DSM. The remaining energy is assumed to be used for heating the hot oil (77.3%), this value is added to the calculated value to be in agreement with the given value. The natural gas usage for the current process (NG,CP) is divided as follows:

NG for process water incineration 20.0 TJ/a

NG for hot oil (constant for all processes) 68.2 TJ/a NG total incinerator calculated for current process 88.2 TJ/a DSM value (total NG to incinerator 2004) 88.2 TJ/a

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-12-The calculated and the real NG consumption are agreeable; therefore, the applied model is accurate enough to determine the energy reduction potentials of the different process options. By this model each process alternative results in an annual NG mass flow. The NG reduction potential (RP) of the different processes (i) can than be calculated by

CP i i CP RP

φ

− = (3.14)

The total energy, cost and CO2 reduction potentials, presented later on in this report, are calculated in the same manner.

3.3.2. Freeze Concentration

The first option for the treatment of the process water is Freeze Concentration (FC). The main objective of this process is to withdraw a pure water stream from the process waste water stream. This results in higher concentrated contaminated stream, which is fed to the incinerator. Compared to the currently used process, the mass flow to the incinerator is decreased and the caloric value of this stream is increased. Both aspects contribute to the reduction of the amount of natural gas used by the incinerator. However, the reduced amount depends to large extent on the degree of concentration in the FC unit. Practically, waste water streams are concentrated to approximately 25 weight percent [5]. Concentration to higher values is undesirable, because it results in impurities in the water stream by contamination of the pure ice crystals. Therefore, the process waste water stream is concentrated with factor 4.5, which is further described in section 4.3. The FC set up requires the installation of three units; the cooling unit for the formation of the crystals, the stirred vessel in which the crystals grow, and the unit for high purity solid-liquid separation. Application of the FC process to the current process structure does not require large modifications. Figure 3.2 is a block scheme of the resin production process with the freeze concentration section integrated in the structure.

Figure 3.2: Block scheme of the resin production process with process water freeze concentration. Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Freeze Conc. Section 1 bar - 15°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73) 7b. Process water 5,163 t/a (0.16) 8. Product stream 32,831 t/a (1.00) 10. Product (20,1) 32,831 t/a (1.00) 1b. Natural gas (20,1.2) 1,979 t/a (0.06) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6. Vent / vacuum 1,587 t/a (0.05) Incinerator 1 bar 900°C 11. Concentrate 1,147 t/a (0.03) 12. Water (0,1) 4,015 t/a (0.12) 9b. Flue gas (230,1) 41,704 t/a (1.37) 2b. Air (20,1) 36,992 t/a (1.13) Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Freeze Conc. Section 1 bar - 15°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73) 7b. Process water 5,163 t/a (0.16) 8. Product stream 32,831 t/a (1.00) 10. Product (20,1) 32,831 t/a (1.00) 1b. Natural gas (20,1.2) 1,979 t/a (0.06) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6. Vent / vacuum 1,587 t/a (0.05) Incinerator 1 bar 900°C 11. Concentrate 1,147 t/a (0.03) 12. Water (0,1) 4,015 t/a (0.12) 9b. Flue gas (230,1) 41,704 t/a (1.37) 2b. Air (20,1) 36,992 t/a (1.13)

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-13-As discussed earlier, the reaction section and the product treatment of the current process remain unchanged. The process waste water stream (7b) from the reaction section is fed to the FC section, resulting in a pure water stream (12) and a concentrated stream (11). Stream 11 is fed to the incinerator with the vent/vacuum stream (6). According to ref [5] the pure water stream (12) contains approximately 50 ppm organic, which is pure enough to be used as utility in the plant. For example, the stream can be used for stream production. The advantage is that this stream does not have to be demineralised as the tap water. The FC unit also provides additional cooling utilities due to the low operating temperature. Due to the withdrawal of water from the process water stream FC reduces the natural gas consumption (stream 1b) needed for the treatment of waste water with 12% to 1,979 ton/a.

The design, the control, and the process structure of the FC unit are discussed in detail in Chapter 6. The mass and energy balances that define the internal streams are given; produced wastes and their environmental impact are described. Safety aspects related to operating the unit are analyzed and integrated into the design. In conclusion, the feasibility of the application of this alternative to the current process is evaluated.

3.3.3. Direct Vapour Incineration

The main difference of Direct Vapour Incineration (DVI) with the currently used process is alternative use of the condenser and a new piping system. In the current process a lot of energy is lost by vaporization of the water in the incinerator, while a vapour phase is formed in the reactor. Currently the whole stream is condensed, a part is refluxed and the remainder is process water and is fed to the buffer tank.

Figure 3.3 is a block scheme of DVI. The process vapour (17) and vent / vacuum (6d), which is assumed to contain only nitrogen, are directly fed to the incinerator. The water formed in the second phase (vacuum) is totally condensated, because mass flow exceeds vacuum pomp capacity. The capacity of the current vacuum pomp is 250 m3/h [a] (0.016 kg/s) and the total process water mass flow in the second phase is 0.10 kg/s. The formed liquid (stream 7d, process water) is treated the same as in the old process. Another option could be to introduce a new vacuum pomp with a larger capacity. The required vacuum pump is modelled as a compressor in Aspen [c] (isentropic, model UNIFAC). Application of this compressor to the previously defined piping system leads to a pressure change from 0.1 to 8.3 bar. The duty of the compressor will be 34.5 kW. However, a problem was encountered while modelling the compressor; the outlet stream would have a temperature of over 1000 °C, which is undesirable regarding the pipe system. A multistage compressor with cooling might be an option, but a lot of energy will be lost. Therefore, the alternative using the compressor is rejected and the choice is made to condense the entire stream during the second phase of the process.

The design, the control, and the process structure of the DVI system are discussed in detail in Chapter 7. The mass and energy balances over the condenser are given; produced wastes and their environmental impact are described. Further attention is given to the new piping system. Safety aspects related to operating the unit are analyzed and integrated into the design. In conclusion, the feasibility of the application of this alternative to the current process is evaluated.

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-14-Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Incinerator 1 bar 900°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73) 17. Process vapour 4,653 t/a (0.14) 8. Product stream 32,831 t/a (1.00) 10. Product (20,1) 32,831 t/a (1.00) 1d. Natural gas (20,1.2) 1,952 t/a (0.06) 9d. Flue gas (230,1) 45,224 t/a (1.38) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6d. Vent / vacuum 1,580 t/a (0.05) 7d. Process water 517 t/a (0.02) 2d. Air (20,1) 36,524 t/a (1.11) Reaction Section 0.1 - 8.3 bar 260°C Product Treatment 1 bar 180-20°C Incinerator 1 bar 900°C 3. Neopentyl glycol (180,1) 14,078 t/a (0.43) 4. Other reactants (20,1) 23,922 t/a (0.73) 17. Process vapour 4,653 t/a (0.14) 8. Product stream 32,831 t/a (1.00) 10. Product (20,1) 32,831 t/a (1.00) 1d. Natural gas (20,1.2) 1,952 t/a (0.06) 9d. Flue gas (230,1) 45,224 t/a (1.38) 5. Nitrogen (20,6.5) 1,580 t/a (0.05) 6d. Vent / vacuum 1,580 t/a (0.05) 7d. Process water 517 t/a (0.02) 2d. Air (20,1) 36,524 t/a (1.11)

Figure 3.3: Block scheme of the process using direct vapour incineration.

3.4. Thermodynamic Properties and List of Pure Component Properties

The thermodynamics are treated more explicit in chapter 4. The most important properties used are the specific heat capacity, the reaction enthalpy, solubility and VLE diagrams. For the CP a group contribution model, according to Misenard [6] is used. The reaction enthalpy has to be estimated, because different products are produced. Also no ‘typical’ value for condensation polymerisation could be found, this requires assumptions and estimations.

A phase diagram is required for the design of the freeze concentration. One side of the diagram was constructed using the data provided by Freeze Tec (Appendix D). The other side is estimated by a group contribution method and one literature value.

No data was available for the vapour liquid equilibrium of the process water required for DVI design. Therefore, the VLE diagram was constructed in Aspen [c], using the thermodynamic model UNIFAC. The pure component properties are summarised in Appendix B.3.

3.5. Basic Assumptions

The plant capacity and location are no variables in this CPD project. The production of the Schoonebeek III plant of DSM is defined and fixed, as is the location. The reaction section is not specifically fixed, but DSM is not pleased with adaptations to this section. The possible alternatives must be found after the rectification column.

The production and therefore the reactant and product streams are fixed. The only ingoing streams that can be changed are the natural gas and air streams and other utilities. Limitations on the outgoing streams are not specified, but environmental issues have to be considered.

3.6. Margins

The margin as defined in the CPD handout is not very suitable for this CPD project. For all process option the same amount of product is made and the same amount of reactant is used. There is only a small change in waste streams, so more or less flue gas or a waste water stream. More important in this project are savings on the natural gas consumption. The processes

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-15-designed must be compared to the reduced costs for natural gas consumption and the investment costs that have to be made. The objective is to look at the rate of return for the different processes. The pay back time (PBT) can be calculated with equation 3.15 .

Investment PBT

Annual savings

= (3.15)

Industrial pay back times for process adjustment related to energy consumption are around 10 years. This means that the margins of our design depend on the time in years the investment must be paid pack, which is dependent on the annual savings and the total investments of the treatment.

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-16-4. Thermodynamics

In this chapter the relevant thermodynamics for this project will be treated. These thermodynamic properties are used in the calculations in the report, unless mentioned differently. 4.1. Heat Capacity of the Components in the Liquid Phase

The values for the heat capacity (CP) of the components in the liquid phase are estimated semi-empirically. The liquid phase is composed of neopentyl glycol (NPG), teraphtalic acid (TA), isophtalic acid (IA) and adipic acid (AA). Experimental data of the relevant molecular structures and the Misenard group contribution method are used for the estimation [6]. The following equation is used for the contribution to CP value of the groups in the molecule:

( )

, ( ) , p i l j p j c T =

∑

k c⋅ T (4.1) where i compound i j groups in compound i

k number of groups j in compound i

These contributions are temperature dependent. Therefore, quadratic extrapolation is used to obtain the group contribution in the operation window of 298 ≤ T ≤ 533 K. These extrapolations are shown in Figures B.1 and B.2 in Appendix B. According to [6], it is custom to use the quadratic fit for polynomial fits up to the fourth order. Third and fourth order fits resulted in a shape that could be expected for a CP-curve. The assumption is made that the CP-values for the dissolved solids equal those obtained for the liquid state.

In order to obtain more accurate values for the CP, the values of molecules that are somewhat similar to the compounds in the liquid phase are calculated using the Misenard group contribution method. Somewhat similar means that the compounds lack one or two functional groups, from hereon they will be referred to as “look-like” compounds (LL-compounds). The experimental CP-values at room temperature for these compounds are known. Table 4.1 lists the LL-compounds chosen to represent the compounds in the liquid phase.

Table 4.1: Compounds involved in CP calculation for NPG, TA, IA and AA

Compound Look-like compound

NPG Neopentane (NP)

TA Benzene

IPA Benzene

AA Pentane Acid (PA)

The temperature correction on the CP-value for the LL-compounds is applied as follows:

( )

(

)

, , , ,

*

( ) ( )

p LL p LL p LL room p ref room

c T =c T + c T −c T (4.2) where

( )

, * p LL

c T Corrected CP-value of the LL-compound as function of temperature

, ( )

p LL room

c T CP-value of the LL-compound at room temperature

, ( )

p ref room

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-17-The semi empirical (se) CP-values are calculated as follows:

( )

(

2

( )

3

( )

)

, * , , , , 2 2 p NPG l se p OH p CH p CH p NP l c T =c T + ⋅c T + ⋅ c T −c T (4.3)

( )

(

2

( )

3

( )

)

* , , 2 , 2 , , p TA p benzene l p COOH p CH p CH c T =c T + ⋅c T + ⋅ c T −c T (4.4)

( )

(

( )

)

, 2 3 * , , , , 2 2 p IPA se p COOH p CH p CH p benzene l c T =c T + ⋅c T + ⋅ c T −c T (4.5)

( )

(

( )

)

, 2 3 * , , , , 1 1 p AA se p COOH p CH p CH p PA l c T =c T + ⋅c T + ⋅ c T −c T (4.6)

where NP(l) represents neopentane in liquid state.

The term

(

( )

)

2 3

, ,

p CH p CH

c T −c T is added to correct that first H-atoms have to be removed from NP before the OH or COOH groups can be added. The resulting CP-curves, the mathematical description, and the calculated Aspen [c] curve (NLVL model) are shown in Figure B.3 – B.6 in Appendix B. The CP-value of the polymer is assumed to equal that of AA, since it should have a value comparable to that of PET [a]. PET has a CP at 298 K of 1.9 kJ/(kmol·K) [4],

CP of AA at this temperature is 1.88 J/(mol·K). The CP values for each temperature interval is calculated as follows: 2 1 1,2 2 1 T P T P intervalT T c dT c T T = −

∫

(4.7) 4.2. Enthalpy of Reaction

The data required to calculate the reaction enthalpy of a typical batch is unavailable at the moment. Therefore, the reaction enthalpy is estimated using a value for a comparable esterification reaction; the reaction between the co-polymer biso-(o-Aminophenyl)-2,2’-dibenzylamidazoleoxide with terephtalic acid, which is also used in the reaction [7]. The obtained ∆Hr data are given in Appendix B.4.

The calculated reaction enthalpy ∆Hrcalculated at 250°C and 260°C are given in Table 4.2.

Table 4.2: reaction enthalpy at 260 and 250°C.

260 250 °C

13 12 kJ/mol H20

737 688 kJ/kg H2O

110 103 GJ/kton

The calculated value for the reaction enthalpy should be corrected for the phase transition of water above 373K;

real calculated water

r r vap

H H H

∆ = ∆ + ∆ (4.8)

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-18-4.3. Process Water Phase Diagram

The phase diagram of the process water is essential in determining the maximum degree of concentration and thus the feasibility of the FC process. The phase diagram is not included in the DSM database and is not found using other source. In addition, the phase diagram of neopentyl glycol in water is unavailable, which could be used as a substitute. However, a study was done by FreezeTech on the application of FC to the DSM process [d]. The data used from this study is summarized in Table 4.3. The concentration is measured in the unit brix, which was converted into mass fraction.

Table 4.3: Data from Freeze Tec report [d]. Concentration [Brix] Mass fraction [kg/kg] Melting Temperature [°C] 2.7 0.050 -1.1 5.6 0.104 -2.3 9.3 0.172 -3.4 11.1 0.206 -4.2 15.8 0.293 -6.0

This data can be used to construct the water rich (left) side of the phase diagram of neopentyl glycol - water. The other side is constructed using the solubility window of neopentyl glycol in water. Solubility diagrams of neopentyl glycol in water are unavailable at this time. However, the solubility as a function of temperature is estimated using the separate group contributions. This is known as the AQUAFAC method [8] and results in the following expression;

logS_u =0.8−

∑

f_i−0.01(T_m−T) (4.9)

where fi denotes the contribution of each group of the neopentyl glycol molecule. These values are given in Table 4.4. Tm is the melting temperature of neopentyl glycol (Tm= 120°C).

Table 4.4: AQUAFAC group contribution values. Group f-value Occurrence Total

X4-C 0.019 1 0.019

X2-CH2 0.545 2 1.090

X-CH3 0.706 2 1.412

X-OH (1°) -2.298 2 -4.596

Total f value for neopentyl glycol -2.075

Inserting these values in equation 4.9 leads to the following expression

0.8 2.075 0.01(120 )

10 T

NPG

S = + − − (4.10)

There is some deviation when comparing the values found with equation 4.10 with those in literature (SNPG(T=20°C)=830 g/L) [XII]. Therefore, the solubility curve is corrected for this deviation to construct the right side of the phase diagram, Figure 4.2.

The left side boundary is, as mentioned before, obtained from Table 4.4, added with the melting temperature of pure water (0 °C). The trend in the data points is supposed to be curved but the data does subscribe to a linear trend.

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-19-Phase diagram neopentyl glycol - water

-10 -5 0 5 10 15 20 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Weightfraction neopentyl glycol [kg/kg]

T e m p e ra tu re [ C ]

Experimental data FreezeTech Corrected AQUAFAC model

Figure 4.2: Phase diagram constructed with experimental data and corrected AQUAFAC solubility model. Freeze concentration is limited by the liquid composition at the eutectic-point. This point is positioned where the left boundary crosses the right boundary. Here, the organic solution has the lowest melting temperature. It is impossible to concentrate the process water over the organic concentration of this point. Figure 4.2 shows that the eutectic-point of neopentyl glycol in water is at a mass fraction of 0.255, the organic solution then has a melting temperature of -5°C. In theory the process water can be concentrated. This represents a concentration factor of 5.1. However, slight temperature deviations could than cause the organics to crystallize, resulting in contamination of the pure water stream. To prevent this in practice a concentration factor of 4.5 is taken, resulting in a concentrate organics concentration of 0.225 with a melting point of -4.6°C. 4.4. Vapour Liquid Equilibrium

As mentioned earlier, neopentyl glycol is used as the model compound. Therefore, the vapour-liquid equilibrium of the water-organic system is substituted by the water-neopentyl glycol system. Aspen uses a VLE-diagram, presented in figure B.8, to determine the composition of the mass streams. This diagram has been modelled using the UNIFAC method. The choice for this model is based on the determination table given in the Aspen course [e]. The models NRTL and UNIQUAC were insufficient because binary parameters are not available in Aspen. The phase diagram is given in Appendix B.4.

liquid

liquid + solid water

liquid + solid

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-20-5. Current Process

This chapter contains a description of the current process. The process structure is described in section 5.1. The mass and heat balances are explained in section 5.2.

5.1. Process Structure & Description

The production reaction can be described as an esterfication of a alcohol and a di-carboxylacid, where water is produced as a by-product. The removal of the produced water is essential to influence the reaction equilibrium profitably. The reaction is endothermic; heat has to be added to the reaction mixture. The general reaction mechanism is

1 2 1 2

- - ( ) - - ( ) -[ - - - - 2- -] -n ( ) ( )

HO R OH l +HOOC R COOH l ↔COOH O R O CO R CO OH l +H O g (5.1)

The process can be divided into two phases. An overview of all steps in the batch sequence is given in Appendix B.1. The di-ol (e.g. neopentyl glycol) is kept at 150°C in the silo by the use of steam (171oC). The other reactants are dissolved in the liquid neopentyl glycol. In the first phase the production of water, formed during the condensation reaction, causes an elevated pressure of 8.3 bar (absolute pressure). The reactants in the reaction vapours are rectified back into the reactor by a distillation column (to be more accurate a rectification column). The remaining vapour is condensed totally and the water rich condensate is partially refluxed. The equilibrium shifts to the right due to the water removal, of which 90% is removed in the first phase. The removed water (the process waste water) is collected in a buffer tank. The end of the first phase can be determined by a certain acid value, after which the phase is ended by a pressure reduction to 0.4 bar.

The second phase is initiated by the addition of acids to the reaction mixture, after which the temperature is lowered slightly from 260 to 250°C. After 1 hour the pressure is lowered to 0.1 bar to remove the last fractions of water to complete the reaction. The additives are added to the reaction mixture when the preferred conversion is obtained. The product mixture is cooled by the cold oil system to a temperature of approximately 180°C, after which the reactor is drained and the product is fed to the cooling belts for product treatment.

Nitrogen is used to make the system inert and prevent oxidative reactions during the first phase of the process. The nitrogen gas flow is lead to the incinerator and is burnt there. The nitrogen flow is stopped during the second phase when the reactor is operating under vacuum. Many of the streams in the process are burnt in the incinerator; the process water, vent gas, nitrogen, air, vacuum, and natural gas. The process water contains approximately 5% of organic waste. Steam is added to nebulize the process water to increase the heat transfer efficiency in the incinerator. The heat released by incineration of the streams is used to heat the heating oil system. One of the reasons why the process water is incinerated is that it contains some components that have an unpleasant odour. These chemical cause inconveniences in their direct environment even at ppm level; phase separation by a settler would be insufficient. A schematic representation of the currently used process is given in Figure 5.1.

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-21-Figure 5.1: Schematic representation of the current process (not created according to CPD guidelines) [a]. 5.2. Energy Balance

The mass balance of the current process is the starting point of all calculations and is given in the Basis of Design, chapter 3. The use of energy within the plant should be determined in order to design a lean process. In this chapter an attempt is made to determine the energy balances of the currently used process, for as far that it possible with the available data. The starting point of the calculations is the overall energy balance of the process, which is defined as follows;

Pr

Consumed oduct Loss

E =E +E (5.2)

Two figures are required for the determination of the balance; the energy investment in the product starting from the raw materials (see Appendix G.3) and the total energy supply. The total energy supply is calculated using the meter readings of the plant in 2004, which are summarized in Table 5.1.

Table 5.1: Total energy usage in the Schoonebeek III plant

Measured amount Containing energy [TJ] Fiscal amount [TJ]

Natural gas usage 2004 3,393,005 m3 107.4 107.4

Electricity usage 2004 6,368,981 kWh 22.9 57.3

Organics equivalent - - 4.5

Total - 130.3 169.2

The energy amount given to tax is listed in the right column of Table 5.2. 3.6 MJ/kWh is the conversion factor normally used for the conversion of kWh to MJ. However, this factor is

Vacuum to incinerator Vent to incinerator Oil in Oil out Condensate tank

Inert gas Distillation_column

Process water to incinerator

Process Water

Organics Combustion Air

Steam + Natural Gas

O2 Chimney Incinerator 2 Vacuum to incinerator Vent to incinerator Oil in Oil out Condensate tank

Inert gas Distillation_column

Process water to incinerator

Process Water

Organics Combustion Air

Steam + Natural Gas

O2 Chimney