93-1315 Deoiling Manual

(1)

Report EP 93

-

1315 November 1993 Confidential

DEOlLlNG MANUAL

Revision 1.1

Author: A. C. Lawrence, EPDi421 Reviewed by: K. M. Perrin, EPDi421 Approved by: G. 0. Hajek, EPD/42

This dociinient I S confidential Neither the whole nor any part of this document may he ilisclosed to any third party without the !mor written consent of

Shell Internationale Petroleum Maatschappij B.V., The Hague. the Netherlands.

The copyright of this docuiiieiit is vested in Shell lriteriiationale Petroleum blaatschappi] B V.. The Hague. the Netherlands All rlyhts reserved. Neither the whole nor any pan of this docuinent m a y he reproduced, stored in airy retrieval systciii ot traiisniitled In aiiy form or hy any means (electronic, mechanical. reprographic.

recording or otherwise) withoot the prior writlnri consent of the copyright owner

SHELL INTERNATIONALE PETROLEUM MAATSCHAPPIJ B.V., THE HAGUE

(2)

Revision

history

By Revision Draft Revision details Revision 1 .O Revision 1.1 _____ Issue date 14 May 1993 16 Aug 1993 S Nov 1993

REVISION HISTORY

Allan Lawrence EPD/42I Allan Lawrence EPD/42 1 Allan Lawrence EPD/42 1

First draft of revised manual. Developed from existing SIPM DehydratiodDeoiling Manual EP 89-1 50. Update based on literature searches of SlPM EP report database, in-house files, OWTC test work and a questionnaire circulated to operating companies. Diagrams not included. Updated based on comments received on the Draft. Significant changes included the inclusion of all diagrams, a section introducing sludge treatment, clarification of iso-kinetic and isoenergetic sampling, inclusion of tables summarising the performance of equipment in tests, trials and operating installations and an appendix summarising the equations used to predict hydrocyclone performance.

Updated with remaining comments received from Revision 1 .O issue. Equations in Appendix A and B corrected. Sizing of sampling quill added to

(3)

Corrections

From: Company: Location:

CORRECTIONS

-

ADDITIONS

-

SUGGESTIONS

Date: Indicator: TeVFax:

This Deoiling Manual will only remain accurate and relevant if feedback is received from operating companies and all other users.

Users are invited to forward any relevant information such as corrections to information given in this manual, additional information on equipment installations, performance trials or new developments and any suggestions for improvements to this manual such as changes in structure or emphasis, new sections or topics for inclusion etc.

This page can be photocopied and completed, including attachments as required, and forwarded to the address given below.

Forward to: SIPM, Posthus 162, 2501 AN Dcn

Haag,

The Netherlands

-

Attention EPD/42

(4)

Table

of

1 .

INTRODUCTION

...

1-1

1.1. 1.2. 1.3. 1.4. 1.5. 1.6. 1.7. 1.8. GENERAL

...

1-1 PURPOSE OF MANUAL

...

1-1 DISTRIBUTION

...

1-1 ORDER OF DOCUMENT PRECEDENCE

...

1-1 STRUCTURE OF DEOlLlNG MANUAL

...

1-2 INFORMATION BASE

...

1-3 EQUIPMENT BRANDS AND SUPPLIERS

...

1-3 ABBREVIATIONS

...

1-3

2 .

CHARACTERISATION OF WASTE WATER

...

2-1

2.1. GENERAL

...

2-1 2.2. SOURCES OF WASTE WATER

...

2-1

2.2.1. Produced water

...

2-1 2.2.2. Ballast water

...

2-1 2.2.3. Process water

...

2-2 2.2.4. Drainage water

...

2-2 2.2.5. Other waters

...

2-3

2.3. CONTAMINANTS OF WASTE WATER

...

2-3

2.3.1

.

Hydrocarbons derived from production streams

...

2-3 2.3.2. Treatment chemicals

...

2-5 2.3.3. Suspended solids

...

2-5 2.3.4. Heavy metals

...

2-6 2.3.5. Salinity

...

2-6 2.3.6. pH

...

2-7 2.3.7. Hardness

...

2-7 2.3.8. Thermal contamination

...

2-7 2.3.9. Dissolved oxygen

...

2-7 2.3.10. Organic carbon

...

2-8

2.4. DROPLET SIZE DISTRIBUTIONS

...

2-8

3 .

EMULSIONS

...

3-1

3.1. DEFINITIONS

...

3-1 3.1

.

1. General

...

3-1 3.1

.

2. Distribution of phases

...

3-1 3.1

.

3. Stability

...

3.1.4. Size distributions

...

3.2. FORMATION OF EMULSIONS

...

3-2 3.2.1. Formation locations

...

3-2 3.2.2. Mixing energy and droplet size distributions

...

3-2

3.3. STABlLlSATlON OF EMULSIONS

...

3-2

3.3.1, Immiscible liquids

...

3-2 3.3.2. Surface tension

...

3-2

(5)

Table of

...

3-3 3.3.4. Contact angles and wetting of solids

...

3-3 3.3.5. Emulsion stabilisers

...

3-4

3.4. TREATMENT OF EMULSIONS

...

3-6

3.4.1. General

...

3-6 3.4.2. Emulsion breaking chemicals

...

3-6 3.4.3. Other methods of emulsion treatment

...

3-8

4 .

WATER DISPOSAL

...

4-1

4.1. 4.2. 4.3. 4.4. 4.5. GENERAL

...

4-1 SURFACE DISPOSAL

...

4-1

4.2.1. Regulatory discharge limitations

...

4-1 4.2.2. Setting of environmental discharge standards

...

4-3

WATER INJECTION

...

4-3

4.3.1. General

...

4-3 4.3.2. Water compatibility

...

4-3 4.3.3. Permeability impairment

...

4-4 4.3.4. Secondary waste water streams

...

4-5

DISPOSAL INJECTION

...

4-5

4.4.1. General

...

4.5 4.4.2. Water quality constraints

...

4-5

SECONDARY (REJECT) STREAM DISPOSAL

...

4-6

4.5.1

.

General

...

4-6 4.5.2. Disposal options

...

4-6 4.5.3. Sludges

...

4.7

5 .

WASTE WATER SAMPLING

...

5-1

5.1. 5.2. 5.3. 5.4. 5.5. 5.6. 5.7. GENERAL

...

5-1 SAMPLE POINTS

...

5-1

Orientation of sampling points

...

5-2 5.2.1.

5.2.2. 5.2.3.

Location of sampling points

...

5-1

Design of sampling quills

...

5-2

SAMPLE CONTAINERS

...

5-3

5.3.1. Materials

...

5-3 5.3.2. Cleaning of sample containers

...

5-4

SELECTION OF SAMPLING METHOD

...

5-4 ROUTINE SAMPLING

...

5-5

5.5.1. Introduction

...

5-5 5.5.2. lsokinetic conditions

...

5-5 5.5.3. Procedure for routine sampling

...

5-6

SAMPLING TO MEASURE DROPLET SIZE DISTRIBUTIONS

...

5-6

5.6.1. Introduction

...

5-6 5.6.2.

5.6.3. Effect of process pressure on sampling and droplet size distributions Sampling procedure to determine droplet size distribution

...

5-7 5-7

SAMPLE PRESERVATION AND STORAGE

...

5-8

5.7.1. Introduction

...

5-8 5.7.2. Chemical Preservation

...

5-9 5.7.3. Sample storage

...

5-9

(6)

Table

of

6 .

WASTE WATER ANALYSIS

...

6-1

6.1. 6.2. 6.3. 6.4. 6.5. GENERAL

...

~1

HYDROCARBONS

.

INFRARED ABSORPTION ANALYSIS

...

6-1

6.2.1

.

Introduction

...

6-1 6.2.2. Sample preparation

...

6-2 6.2.3. Hydrocarbon extraction

...

6-2 Removal of polar. compounds

...

6-3 6.2.5. Infrared analysis

...

6-3 6.2.4.

HYDROCARBONS

.

OTHER METHODS

...

6-5

6.3.1

.

Dispersed and dissolved hydrocarbons

...

6-5 6.3.2. Gravimetric analysis

...

6-5 6.3.3. Visible spectrum colourimetry

...

6-6 6.3.4. Gas chromatography

...

6-6 6.3.5. Others

...

6-6

ON-LINE OIL-IN-WATER ANALYSERS

...

6-7

6.4.1

.

General

...

6-7

MEASUREMENT

OF

PARTICLE AND DROPLET SIZES

...

6-8

6.5.1

.

General

...

6-8 6.5.3. Potential problems in measuring droplet size distributions

...

6-8 6.5.4. Droplet size distribution analysers

...

6-8 6.5.5. Particle size analyser trials

...

6-10 6.5.2. Sampling

...

6-8

7 .

EQUIPMENT SELECTION AND SYSTEM INTEGRATION

...

7-1

7.1. INTRODUCTION

...

7-1 7.2. EQUIPMENT SELECTION AND SYSTEM DESIGN

...

7-1

7.2.1. Introduction

...

7-1 7.2.2. Sources and magnitude of water streams (1)

...

7-1 7.2.3. Identify contaminants in the waste water stream (2)

...

7-2 7.2.4. Identify treated water quality requirements (3)

...

7-2 7.2.5.

7.2.6.

7.2.7. Select the number of treatment stages (6)

...

7-3 7.2.8. Select the suitable deoiling equipment (7)

...

7-4 7.2.9. Treatment or disposal of secondary streams (8)

...

7-5 7.2.10.

Select a suitable process location for the water treatment system (4)

...

7-3 Identify upstream methods of improving ease of water treatment (5)

...

7-3

System optimisation and integration (9)

...

7-5

7.3. SYSTEM OPTlMlSATlON AND INTEGRATION CONSIDERATIONS

...

7-7

7.3.1. Introduction

...

7-7 7.3.2. Maximising droplet size

...

7-7 7.3.3. Minimising the hydrocarbon content in the feed stream

...

7-7 7.3.4. Production separators

...

7-8 7.3.5. Stable feed streams

...

7-8 7.3.6. Recycle streams

...

7-8 7.3.7. Mixing of water streams

...

7-8 7.3.8. Treatment chemicals

...

7-8

7.4. EXAMPLES

OF

WATER TREATMENT SCHEMES

...

7-9 7.5. WATER CHARACTERISATION CONSULTANCY SERVICES

...

7-9 7.6. PILOT PLANT TRIALS

...

7-9

8 .

DISPERSED HYDROCARBONS

...

8-1-1

...

8-1-1

8.1

.

1.

(7)

Table of

_..

...

8-2-1 8.2.1. Deoiling efficiencies

...

8-2-1 COALESCERS

...

8-3-1 8.3.1. General

...

8-3-1 8.3.2. Definitions

...

8-3-1 8.3.3. Performance variables

...

8-3-1 8.3.4. Installation/Configuration

...

8-3-2 8.3.5. Coalescer designs

...

8-3-2 FLOCCULATION

...

8-4-1 8.4.1. General

...

8-4-1 8.4.2. Floc separation

...

8-4-1 8.4.3. Operation considerations

...

8-4-2

SKIMMING TANKS AND VESSELS

...

8-5-1

8.5.1. Introduction

...

8-5-1 8.5.2. performance variables

...

8-5-1 8.5.4. Design guidelines

...

8-5-1 8.5.5. Control configuration

...

8-5-2 DISCHARGE CAISSONS

...

8-6-1 8.6.1. Introduction

...

8-6-1 8.6.3. Caisson internals

...

8-6-1 8.6.4. Operational considerations

...

8-6-2 API SEPARATOR

...

8-7-2 PLATE INTERCEPTORS

...

8-8-5 STATIC HYDROCYCLONE

...

8-9-1 8.9.2. Definitions

...

8-9-1 8.9.4. InstallationlConfiguration

...

8-9-8 8.9.5. Control configuration

...

8-9-1 1 8.1 0

.

ROTARY HYDROCYCLONE

...

8-1 0-1 8.1 0.1

.

Introduction

...

8-1 0-1 8.10.2. Definitions

...

8-10-1 8.1 0.3. Performance variables

...

8-1 0-1 8.10.4. Installation/Configuration

...

8-1 0-5 8.1 0.5. Design guidelines

...

8-1 0-6 8.10.6. Control configuration

...

8-10-7 8.1 0.7. Operational considerations

...

8-10-7 8.11. CENTRIFUGES

...

8-11-1 8.11.1. Introduction

...

8-1 1-1 8.1 1.3. Installation/Configuration

...

8-1 1-2 8.1 1.4. Control configuration

...

8-1 1-4 8.1 1.5. Operational considerations

...

8-1 1-4

8.12. INDUCED GAS FLOTATION

...

8-12-1

8.1 2.1

.

Introduction

...

8-1 2-1

(8)

Table

of

...

:

...

8-12-3 8.1 2.3. Installation/Configuration

...

8-1 2-7 8.12.5. Control configurations

...

8-1 2-7 8.12.6. Operational considerations

...

8-1 2-8

8.13. DISSOLVED GAS FLOTATION

...

8-13-1

8.13.1. Introduction

...

8-1 3-4 8.13.4. Design guidelines

...

8-1 3-5 8.13.5. Operational considerations

...

8-1 3-5

8.14. DEEP BED MEDIA FILTRATION

...

&-14-1

8.14.1. Introduction

...

8-1 4-5 8.14.4. Design guidelines

...

8-1 4-6 8.14.5. Control configuration

...

8-1 4-6 8.14.7. Crushed nut shell deep bed media filters

...

8-1 4-7

8.15. CARTRIDGE FILTERS

...

8-15-1 8.15.1. Introduction

...

8-1 5-2 8.16. PRE-COAT FILTRATION

...

8-16-1 8.1 6.1

.

Introduction

...

8-1 6-1 8.1 6.2. Performance variables

...

8-1 6-2 8.1 6.5. Operational considerations

...

8-1 6-2 8.17. MEMBRANES

...

8-17-1 8.1 7.1

.

Introduction

...

8-1 7-1 8.1 7.2. Performance variables

...

8-1 7-3 8.1 7.4. Membrane trials

...

8-1 7-3

9 .

DISSOLVED HYDROCARBONS

...

9-1-1

...

9-1-1 9.2. LEVELS OF DISSOLVED HYDROCARBONS IN WATER

...

9-2-1 9.3. GAS STRIPPING

...

9-3-4 9.4. STEAM STRIPPING

...

9-4-1 9.4.1

.

Introduction

...

9-4-2 9.5. BIOLOGICAL TREATMENT

...

9-5-1 9.5.2. Aerobic processes

...

9-5-1 9.5.3. Anaerobic processes

...

9-5-4

(9)

...

9-6-1 9.6.1. Introduction

...

9-6-1 9.6.2. Performance variables

...

9-6-1 9.6.3. Installation/Configuration

...

9-6-2 9.7. SOLVENT EXTRACTION

...

9-7-1 9.7.1

.

Introduction

...

9-7-1 9.7.2. Co-current extraction

...

9-7-1 ~~ ~~~

10 .

NEW TECHNOLOGY

...

10-1

10.1. GENERAL ... 0-1 10.2. OZONEILILTRAVIOLET

...

10-1 10.3. ADSORPTION ON MODIFIED ZEOLITES

...

10-1

10.4. MEMBRANE LIKE MATERIAL

...

10-1

10.5. PERVAPORATION ... 0-1

10.6. MEMBRANES FOR DISSOLVED HYDROCARBONS

...

10-2

Appendix

A

.

Emulsification and coalescence

...

A-I

A.l. Introduction

...

A-1 Prediction of droplet size

...

A-1

Mixing energy in piping

...

A-2 A.l

.

1

A.1.2 A.1.3

Mixing intensity

...

A-1

A.2. Optimum Mixing Intensity For Coalescence

...

A-3

Appendix

B

.

Sampling t o determine droplet size

...

8-1

B.l. Introduction

...

B-1 8.2. Calculation of the mixing intensity in the process line

...

B-1 8.3.

8.4. 8.5.

Pressure drop in the sampling system

...

8-2 Sizing the sample quill for isokinetic conditions

...

8-5

Contrast with iso-kinetic sampling

...

B-6

Appendix

C

.

Published infrared analysis procedures

...

C-1

C.l List of IR analysis procedures

...

C-1 C.2 Key characteristics of identified IR analysis procedures

...

C-2

Appendix

D

.

Oil-in-water monitor trials

...

D.1

D.l Oil-In-Water Monitor Testing

.

Phase I

...

D-1

D.l

.

1

.

General

...

D-1

D.1.2. Monitors tested

...

D-1

D.1.3. Variables investigated

...

D-1

D.1.4. Summary of results

...

D-2

(10)

.

Phase It

...

D-2

D.2.1. General

...

D-2 D.2.2. Monitors tested

...

D-2 D.2.3. Variables investigated

...

D-3

D.2.4. Summary of results

...

D-3 D.3 Oil-In-Water Monitor Testing

.

Phase 111

...

D-4

Appendix E

.

Particle size analyser trials

...

E-1

E.l. OWTC particle size analyser trials

...

E-1

E.l.l. General

...

E-1 E.1.2. Analysers tested

...

E-1 E.1.3. Variables investigated

...

E-1 E.1.4. Summary of results

...

E-1 E.2. Galai CIS-1000 trials at KSLA

...

E-2 E.2.1. Results of trial

...

E-2 E.2.2. Conclusions

...

E-2

Appendix

F

.

Stokes law

...

F-1

F.l. Derivation

...

F-1 F.2. Variables In Stokes Law

...

F-2 F.2.2 Gravitational acceleration

...

F-2 F.2.4 Density difference

...

F-2 F.2.1 Droplet size

...

F-2 F.2.3 Viscosity

...

F-2 F.3. The Effect Of Temperature On Stokes Law

...

F-2

Appendix

G

.

Coalescer equipment and trials

...

G-1

G.l. Introduction

...

G-1 G.2. Knitmesh Dusec

...

G-1 G.2.1 Description

...

G-1 G.2.2 Packaging

...

G-1 G.2.3 Performance

...

G-2 G.3. Knitmesh DC coalescing mesh

...

G-2 G.4. Hydrocyclone coalescer

...

G-2 G.4.2 Packaging

...

G-2 G.5. MPE SP Pack

...

G-3 G.5.2 Packaging

...

G-3 G.6. Monosep Spiralsep Coalescer

...

G-3 G.6.2 Packaging

...

G-3 G.7. Sulzer Mellapak

...

G-3 (3.7.2 Packaging

...

G-3 G.8. Upflow Filter/Coalescers

...

G-4

(11)

Table

of

...

G-4 G.10. Skimovex lnverto Filter/Coalescer

...

G-5 G.11. Elf Anvar Filter/Coalescer

...

G-5 G.12. Plate interceptors

...

G-6 G.13. Natco Performax

...

G-6

Appendix

H

.

Plate pack sizing

...

H-1

H.l. Introduction

...

H-1 H.2. Reynolds number

...

H-1 H.3. Plate pack dimensions for droplet interception

...

H-2 H.4. Sizing a plate pack

...

H-2 H.5. Effect of Reynolds number

...

H-3 H.6. Standard Shell CPI design

...

H-4

Appendix

I

.

Predicting hydrocyclone performance

...

1-1

1.1 Introduction

...

1-1 1.2 Cut size diameter

...

1-1 1.3 Cut size diameter correlations

...

1-1

1.3.1 Equilibrium orbit theory

...

1-1 1.3.2 Residence time model

...

1-2 1.3.3 Empirical model

...

1-2

1.4 Migration probability

...

1-2 1.5 Empirical migration probability correlations

...

1-3 1.6 Empirical pressure drop correlations

...

1-3

1.7 Application of correlations to hydrocyclone trials

...

1-3

1.7.1 North Cormorant trials

...

1-3

1.7.2 OWTC trials

...

1-3 1.8 Nomenclature

...

1-4

Appendix

J

.

Predicting levels of dissolved hydrocarbons

...

J-1

J.1, Introduction

...

J-1 J.2. Theory

...

J-1 J.3. Comparison with experimental and field results

...

J-2 5.4. Parameters affecting solubility

...

5-3

Appendix

K

.

Dissolved hydrocarbons equipment comparison

...

K-1

K.l. Introduction

...

K-1 K.2. Design basis

...

K-1

(12)

Table

of contents

K.3. Results

...

K-2

Appendix

L

-

Membrane trials

...

1-1

L.l. Introduction

...

1-1 L.2. Alcoa Petrolox

...

1-1 L.3. SDI Extran

...

1-2 L.4. X-Flow

...

1-2 L.5. Stork-Wafilin

...

1-2 L.6. Schelde-Delta

...

1-2 L.7. WL Gore

...

1-2 L.8. Hoogovens

...

1-2 L.9. Zenon

...

1-3

..

Bibliography

...

Bibliography-1

1

.

Introduction

...

Bibliography-1 2

.

Literature searches

...

Bibliography-1 3

.

Index

...

Bibliography-1

(13)

THIS PAGE HAS BEEN

INTENTIONALLY LEFT BLANK

(14)

1. Introduction

1.1. GENERAL

The production of hydrocarbons is usually associated with the generation of a waste water stream which requires treatment to allow it to be disposed in a safe and environmentally acceptable manner.

Over the four years since the last revision of this manual both the technical aspects of waste water treatment and the philosophies governing waste water treatment have undergone significant changes, especially in the light of increasing environmental awareness in the community and in industry.

New information and developments that have been incorporated into this update of the Deoiling Manual include;

The use of droplet size distribution as tool in optimising the design and operation of waste water treatment equipment.

The results of independent equipment testing programs conducted at the Orkney Water Test Centre.

Procedures for the sampling of oil-in-water dispersions.

The use of more advanced deoiling equipment such as hydrocyclones and centrifuges. New waste water treatment technologies under development.

The removal of dissolved hydrocarbons.

1.2. PURPOSE OF MANUAL

This updated Deoiling Manual has been designed to satisfy the following major objectives.

To provide background information on all aspects of water deoiling including water sampling, analysis and characterisation, hydrocarbon-in-water emulsions and the disposal of treated water. To provide users with an understanding of the design principles and performance characteristics of currently available equipment for the removal of dispersed and dissolved hydrocarbons. To provide guidelines for deoiling equipment selection and facilities design.

To provide a listing of reports, documents, papers and equipment trials relevant to the selection, testing or operation of deoiling equipment.

It should be emphasised that this Deoiling Manual should not be considered to be a fixed set of rules or methods governing deoiling and associated topics. It should serve as an introduction and a foundation to these subjects, but not hinder further developments and advancements.

1.3. Dl STRIB UTlON

Unless otherwise authorised by SIPM, the distribution of this document is confined to companies forming part of the Royal Dutch/Shell Group or managed by a Group company, and to contractors nominated by them.

1.4. ORDER OF DOCUMENT PRECEDENCE

This Deoiling Manual forms the first stage of the updating of the SlPM Dehydration/Deoiling Manual (EP 89-01 50, January 1989). All information relevant to deoiling from the current Dehydration/Deoiling Manual has been included or updated into this Deoiling Manual.

Information more closely related to dehydration has not been included in this manual, but will be updated into a new Dehydration Manual in 1994.

This update procedure will eventually result in the creation of the following two documents, which together will supersede the existing SlPM Dehydration/Deoiling Manual.

(15)

1. Introduction

Deoiling Manual (This document) Addressing the removal of hydrocarbons and other contaminants from water streams.

Addressing the removal of water from hydrocarbon streams. Dehydration Manual

However, until the new Dehydration Manual is completed, this document (Deoiling Manual) will coexist with the current Dehydration/Deoiling manual.

In general, the order of document precedence should be as follows;

This document (Deoiling Manual) should be used for all information relevant to deoiling.

The current Dehydration/Deoiling manual (EP 89-0150) should be used for information specific to dehydration.

In the event of conflict, this document (Deoiling Manual) shall take precedence.

1.5. STRUCTURE OF DEOlLlNG MANUAL

This Deoiling Manual has been divided into the following structure. Chapter 1

-

Chapter 2

-

Chapter 3

-

Chapter 4

-

Chapter 5

-

Chapter 6

-

Chapter 7

-

Chapter 8

-

Chapter 9

-

Chapter 10

-

Introduction

-

Forms the preamble to the Deoiling Manual, outlining the objectives, scope and organisation of the document.

Characterisation of waste water

-

Discusses the elements defining the character of waste waters such as the source of the waste water, typical contaminants, the distribution and nature of contaminants and measurements of water quality.

Emulsions

-

Discusses the characteristics, formation, stability and treatment of hydrocarbon in water emulsions.

Water disposal

-

Discusses the technical and environmental considerations governing the methods of water disposal, including both surface and subsurface disposal.

Waste water sampling

-

Discusses the sampling procedures required to obtain representative samples of waste water, particularly for water containing dispersed hydrocarbon droplets.

Waste water analysis

-

Discusses the analysis methods required to determine the hydrocarbon content in waste waters, the measurement of dispersed and dissolved hydrocarbons, the performance of on-line oil-in-water analysers and the measurement of dispersed droplet size distributions.

System design and integration

-

Discusses the selection and application of deoiling equipment with emphasis on system integration. This approach recognises that the deoiling equipment forms only one part of the production process and must be integrated with other systems. Topics covered include the location of the deoiling equipment in the process train, the minimisation of emulsifying forces, the influence of direct and indirect addition of treatment chemicals on the deoiling process and the integration of recycling hydrocarbon reject streams into the process.

Deoiling equipment

-

Dispersed hydrocarbons

-

Examines proven deoiling equipment in detail. Information provided includes design principles, process design requirements, deoiling performance, typical unit dimensions and weights and basic sizing guidelines. Where applicable, tables are given summarising the performance of the equipment in a range of Shell installations, pilot plant tests and field trials.

Deoiling equipment

-

Dissolved hydrocarbons

-

Examines the range of processes available for the removal of dissolved hydrocarbons from water streams.

New technology

-

Briefly discusses a selection of new technologies currently under development for the treatment of waste waters.

Appendices

-

The Appendices cover a wide range of topics which cannot be easily incorporated into the text of the manual, particularly calculation methods such as mixing intensity calculations, plate pack sizing and methods for predicting the levels of dissolved hydrocarbons in

(16)

I. Introduction

water.

Bibliography

-

The Bibliography lists all the references used for the compilation of this Deoiling Manual. If not available through local sources, these references can be requested from the SlPM Report Library EPD/52 or through SlPM EPD/4.

1.6. INFORMATION BASE

The starting point for this Deoiling Manual has been the 1989 SlPM Dehydration/Deoiling Manual (EP 89-0150). The deoiling sections of EP 89-0150 are now superseded by this updated Deoiling Manual. The additional information incorporated into this update of the Deoiling Manual has been taken from the following sources.

i) Literature survey of the SlPM STAIRS document database for reports related to deoiling and associated topics. Details of the keyword searches conducted are included in the Bibliography. Input from operating companies including;

Results of test work conducted at the Orkney Water Test Centre and experimental work conducted at KSLA.

ii)

Requests for relevant references and documentation.

Requests for comments on the existing Dehydration/Deoiling manual.

A questionnaire to identify the performance of existing deoiling equipment installations. iii)

iv) Discussions with equipment vendors.

1.7. EQUIPMENT BRANDS AND SUPPLIERS

The commercial or proprietary nature of many items of deoiling equipment often requires the mention of specific equipment brand names or equipment suppliers. In some cases, a specific equipment brand may be used to illustrate the characteristics of a generic class of equipment.

It should be clearly noted that the inclusion or exclusion of any process equipment, manufacturer or supplier from this manual should not be interpreted as representing a preference for a particular equipment design, manufacturer or supplier.

To ensure a competitive market for deoiling equipment, alternatives to the equipment discussed in this manual should be actively sought wherever possible.

1.8. ABBREVIATIONS

The following abbreviations have been used in the Deoiling Manual. AMS APHA API ASTM BOD BTEX CCD COD CPI DC DAF DG F DOC E & P EPA GBS GOR HSE IGF IP IR C-H

Amsterdam Method Series (Analytical methods developed and documented by KSLA) American Public Health Association

American Petroleum Institute

American Society for Testing and Materials Biological Oxygen Demand

Benzene, Toluene, Ethyl benzene, Xylene Carbon-Hydrogen

Charge Coupled Device Chemical Oxygen Demand Corrugated Plate Interceptor Dissolved Carbon

Dissolved Air Flotation Dissolved Gas Flotation Dissolved Organic Carbon Exploration and Production

Environmental Protection Authority (USA)

Gravity Based Storage Gas to Oil Ratio

Health, Safety and Environmental Induced Gas Flotation

Institute of Petroleum Infrared

(17)

1. Introduction

ISF IS0 KSEPL

KSLA

MLM OWTC PPI SIPM SMS TC TDS TOC TOD TPI TRC

Induced Static Flotation

International Organisation for Standardisation

Royal Dutch/Shell Exploration and Production Laboratory Royal Dutch/Shell Laboratories Amsterdam

Membrane Like Material Orkney Water Test Centre Parallel Plate Interceptor

Shell lnternationale Petroleum Maatschappij B.V., The Hague Shell Method Series

Total Carbon

Total Dissolved Solids Total Organic Carbon Total Oxygen Demand Tilted Plate Interceptor Thornton Research Centre

(18)

2. Characterisation of waste water

Number of platforms measured

2.1. GENERAL

Oil platforms Gas platforms

12 15

Each waste water stream is unique, with characteristics defined by a wide range of variables such as the water source, processing operations and directly or indirectly added chemicals.

Correct characterisation of the waste water stream is essential to ensure the appropriate selection, design and operation of any deoiling system. This chapter discusses a number of the elements which define the characteristics of a waste water stream.

smVd m3/d 2.2. SOURCES

OF

WASTE WATER

75,600 49,100,000

63,713 612

(475,500 bopd) (1,734 MMscfd)

Waste water streams may originate from a number of sources, each giving the water stream particular characteristics. The selection of the correct treatment process for the waste water requires that the characteristics of all the waste water sources are taken into consideration.

Total hydrocarbon discharge (11

Dispersed oil concentration Dissolved oil concentration Polar hydrocarbon concentration 2.2.1. Produced water

tonne/y 500 216

mgn 15.3 483.9

mgn 6.2 481.9

msn 165.9 230.1

The major source of waste water from

E

& P activities is produced water, particularly associated with the production of oil.

The main component of produced water is normally the formation water that is initially present in the reservoir. Formation water should be regarded as being uniquely different from other waters, having existed for millions of years in contact with rock minerals and hydrocarbons, typically under conditions of high temperature and pressure.

The produced water may also contain fluids associated with enhanced recovery schemes, particularly injection water. Breakthrough of injection water or other fluids associated with enhanced recovery schemes (e.g. polymers) to the producing wells will result in a change in the character of the produced water.

The largest volumes of produced waters are associated with oil production, with gas production having significantly lower rates of produced water. Table 2.1 illustrates the magnitude of the waste water streams produced from a selection of oil and gas platforms in the North Sea.

It

can be seen that the waste water production from the oil platforms is several orders of magnitude greater than that of the gas platforms.

Table 2.1

Typical hydrocarbon discharges from a selection of North Sea oil and gas production platforms (1989)

2.2.2.

Total oil or gas production Total produced water

Ballast water

Ballast water is a waste water stream generally associated with the storage of oil. The major sources of ballast water are;

(19)

2. Characterisation

of

waste water

Oil tankers

In many oil tankers, ballast water is carried in the cargo tanks after the vessel is unloaded to maintain the stability of the vessel. The ballast water will become contaminated by contact with the oil remaining in the cargo tanks and must be discharged to ballast water cleaning facilities before the vessel is loaded with the next cargo of oil. The waste water stream generated during the cleaning of storage tanks on oil tankers is also often classed as ballast water and may be treated in the same facilities.

Tankers using Segregated Ballast Tanks (SBT) minimise the risk of contact between the oil and water by designating separate tanks which are only used for ballast water.

Gravity Based Storage (GBS) platforms.

Sea water is displaced from the storage cells as they are filled with produced oil, while sea water displaces oil from the cell when the oil is exported. Oil contaminates the water through a number of mechanisms including bulk mixing as the cells are filled and discharged, contact between the water and the oil coating the walls of the storage cell, residual oily water separating from the oil and through hydrocarbons dissolving and diffusing into the water phase.

Ballast waters which have been in contact with hydrocarbons must be regarded as waste water and will usually require some treatment before they are suitable for discharge into the environment.

Ballast waters can often be treated with relatively simple deoiling equipment. Residence times in storage tanks are generally quite long while shear forces are low which promotes the coalescence and separation of dispersed hydrocarbons.

However ballast waters have a number of characteristics that do require special consideration.

0 0 0 There levels some

Ballast waters are relatively cold in comparison with production waters with temperatures typically ranging from 5°C to 20°C.

The hydrocarbon concentration can vary widely, ranging from clean water to slugs of hydrocarbons.

Depending on the ballast water intake location and local environmental conditions, ballast water may contain significant quantities of suspended solids such as mud, sand, silt, organic and animal matter.

are several cases of GBS structures which discharge displaced ballast water with hydrocarbon below 40 mg/l without any deoiling treatment. However provision should generally be made for form of treatment of displaced GBS ballast waters to provide protection against accidental discharges of hydrocarbons. Passive methods such as buffer water ceils could be -used to provide suitable protection.

2.2.3. Process water

Process water is secondary water generated as a result of the processing activities undertaken during the production of hydrocarbons. Process water can originate from a wide range of sources including;

0 Wash water used for crude oil desalting.

Condensed vapours from glycol regeneration equipment.

Water condensed during the compression of water saturated gas streams. Water associated with the injection of treatment chemicals.

The importance of the process water streams is dependent upon their magnitude relative to other water streams. In the case of gas platforms which often have low production rates of produced water, the process water can be a significant fraction of the waste water and may dominate the characteristics of the overall waste water stream.

2.2.4. Drainage water

The drain system associated with E 8, P production sites collects water from a wide variety of sources.

(20)

2. Characterisation of waste water

0 Closed drain systems will normally collect produced or process water, typically from the draining

of process vessels.

Open drain systems will typically collect drainage from surface area drainage and tundishes etc. The majority of the water stream in the open drains system will be intermittent run off from rain or hosing.

Drainage water will typically exhibit the following differences from either production or process water; Flow rates are intermittent, resulting from operations such as vessel draining, rain water, hosing and housekeeping activities.

In addition to hydrocarbons and contaminants associated with the production stream, open drains water may contain other contaminants such as lubrication oils and greases from rotating equipment, diesel oil spillage and any other fluids or materials entering the drains by design or accident.

Open drains waters will contain dissolved oxygen. Mixing of oxygenated drainage water with other waters may result in scaling or corrosion.

Due to the relatively short contact time, hydrocarbons associated with rain water or wash down water are more likely to be dispersed rather than dissolved and thus potentially easier to treat. Information on typical drain system loads due to rainwater, hosing, deluge systems etc. and guidelines for the sizing and design of drainage systems are given in DEP 34.1 4.20.31 -Gen., “Drainage and Primary Treatment Facilities”.

2.2.5. Other waters

Other waste water streams which may be discharged from E & P operations include cooling water, sewage effluent, waste cuttings and drilling mud. The treatment of these categories of waste waters is not addressed in this manual.

2.3. CONTAMINANTS OF WASTE WATER

When considering waste water from E & P operations, the concentration of produced hydrocarbons is normally the focus of both the system design and environmental monitoring requirements. Produced hydrocarbons in waste waters are generally considered to encompass the non-polar mineral hydrocarbons present in the production well-stream.

However in addition to the non-polar mineral hydrocarbons, waste water streams typically contain a wide range of contaminants, all of which may have a potential environmental impact. In line with the Group commitment to identification, management and minimisation of all waste streams, all the constituents of waste water streams should be identified and accounted for.

It should be stressed that the relative importance of contaminants in discharge waters is strongly dependent upon the receiving environment, particularly the differential between the waste water and the receiving environment. An effective example of this is water salinity. Saline production waters may be relatively benign when discharged into a sea water environment, but may have a significant environmental impact when discharged into a fresh water environment.

A number of references are made to ASTM standard methods (American Society for Testing and Materials). These standards are readily available through most technical library services or can be requested from SIPM.

2.3.1. Hydrocarbons derived from production streams

A typical waste water stream from an E &

P

operation will contain a wide variety of hydrocarbons derived from contact with the hydrocarbon production stream. The hydrocarbons in the waste water stream can be broadly grouped as dispersed, dissolved and polar hydrocarbons. To date, normally only the insoluble dispersed hydrocarbons have been considered when selecting deoiling equipment.

Table 2.1 gives an indication of the relative contribution of each group of hydrocarbons in the waste waters discharged from a selection oil and gas production platforms in the North Sea.

2.3.1

.l. Dispersed hydrocarbons

The treatment of waste water streams from many E & P operations is focused on the removal of

(21)

2. Characterisation

of waste water

ALKANES

dispersed hydrocarbons. Dispersed hydrocarbons are relatively insoluble in water and are present as discrete hydrocarbon droplets within the continuous water phase.

Crude oil streams are usually largely composed of aliphatic hydrocarbons which have a low solubility in water. Thus the hydrocarbons in waste waters from oil production operations are typically present as dispersed hydrocarbons.

One of the critical characteristics of dispersed hydrocarbons is the droplet size distribution of the dispersed hydrocarbon phase. The performance of

most

deoiling equipment will be ultimately limited by the smallest hydrocarbon droplet size that can be efficiently removed from the water stream. Droplets of dispersed hydrocarbons smaller than the minimum cut off size will not be removed. Droplet size distributions are discussed in more detail in section 2.4.

AROMATICS

The majority of deoiling equipment currently installed in E & P operations is only capable of removing dispersed hydrocarbons from waste waters.

cycio

2.3.1.2.

Dissolved hydrocarbons

1 Ring

Some hydrocarbons are partially or completely soluble in the water phase. Table 2.2 lists the solubility of various pure hydrocarbons in fresh water at _15"C,illustrating that the lower molecular weight hydrocarbons and aromatic hydrocarbons have the highest solubility.

Table 2.2

Solubility of hydrocarbons in water (mgll at 15°C)

Number of carbon atoms 1 2 3 4 5 6 7 8 9 10 11 Notes: Normal CnH2n+2 25 75 100 100 60 12 2.6 0.6 0.1 0.02

___-_--

1. These solubilities are lor binary systems of a single hydrocarbon and fresh water and are only indicative for produced waters. Solubility levels in produced waters may be different due to lactors such as:

a) The effect of the temperature and pressure in the reservoir where the water is in contact with the hydrocarbons.

b) The presence of other species dissolved in the water phase such as inorganic salts and soluble organic species such as organic acids.

c) Preferential partitioning of the hydrocarbons to the hydrocarbon phase when both hydrocarbon and water phases are present.

Source: Superseded SlPM Dehydration/Deoiling manual, EP 89-0150

2.

Crude oil streams are typically composed of higher molecular weight aliphatic and aromatic hydrocarbons which have a low solubility in water. Thus the dissolved oil content in the waste water from oil production operations is normally relatively low. This usually allows the waste water streams from crude oil operations to be treated to typical discharge standards by removal of the dispersed

hydrocarbons alone.

For gas production operations the dissolved oil content may be more significant. Gas hydrocarbon streams are composed of low molecular weight hydrocarbons and may also contain significant quantities of relatively soluble low molecular weight aromatic hydrocarbons such as benzene, toluene and xylene. The higher solubility of hydrocarbons associated with gas production is illustrated in Table 2.1. It can be seen that the typical level of dissolved hydrocarbons from the gas platforms is 482 mg/l, compared to

only 6 mg/l for the oil platforms.

For some gas processing installations, a significant quantity of the aromatic hydrocarbons in the waste PUXC 2-4 SlPM Deoilinji Miiniml, EP 93.1315, Rev 1.1. Nov 1993. File nctme = 2CHAP.DOC

(22)

2. Characterisation

of

waste water

water may be derived from the glycol regeneration system. Aromatic compounds are absorbed from the gas by the glycol in the gas dehydration contactor. Water and aromatics are then stripped from the glycol in the glycol regenerator, condensed from the regenerator overheads stream and the resulting watedaromatics mixture added to the waste water stream.

The removal of dissolved hydrocarbons is more difficult than the relatively simple phase separation required for removal of dispersed hydrocarbons. With the exception of biological treatment systems, the

majority of the deoiling equipment used in E 8 P operations is not designed for the reduction of the dissolved hydrocarbon content of waste water.

Appendix J presents calculation methods for estimating the levels of dissolved hydrocarbons in water and discusses the effect of a number of external variables such as water salinity and temperature on the hydrocarbon solubility.

Guidelines on the use of physical property predictive routines for modelling hydrocarbodwater systems within simulation packages (e.9. Pro/lI) are given in DEP 20.00.10.10-Gen., “Basic Data and Phase Behaviour Methods”.

2.3.1.3.

Polar hydrocarbons

Waste water streams may contain a significant quantity of polar hydrocarbons such as organic acids which due to their polar characteristics are relatively soluble in water. It can be seen from Table 2.1 that in the waste water from oil platforms the concentration of polar hydrocarbons is nearly eight times greater than the combined dispersed and dissolved oil content.

It should be noted that the polar compounds are generally not reported as part of the hydrocarbon content of the water. The analysis procedures used to determine the hydrocarbon concentration in water often includes an adsorption step where the polar hydrocarbons are removed by an absorbent such as alumina or Florosil. The hydrocarbon content is then measured after the polar compounds have been removed.

In addition, most hydrocarbon analysis procedures require the extraction of the hydrocarbons from the water phase using a solvent. Polar hydrocarbons are not efficiently extracted by some solvents and are thus not included in the subsequent hydrocarbon analysis.

However, some analysis methods (e.g. Total Organic Carbon) will measure polar compounds and will often indicate a hydrocarbon concentration far higher than that measured through conventional analysis methods.

As for dissolved hydrocarbons, the majority of deoiling equipment currently installed in E & P operations is not capable of reducing the polar hydrocarbon content of waste water.

2.3.2. Treatment chemicals

A wide variety of treatment chemicals may be present in waste water streams. Typical examples include corrosion inhibitor, scale inhibitor, demulsifiers, hydrate inhibitors, biocides, flotation aids etc. Other incidental chemicals may also be present such as detergents used for wash down purposes.

The effect of such chemicals can be significant in the selection and design of deoiling equipment as; Many treatment chemicals are surface active and may stabilise small hydrocarbon droplets in the water phase. The corrosion inhibitors used in gas production operations and the demulsifiers used to assist oil dehydration can sometimes result in such stabilisation. The small droplets may be very difficult to separate with conventional deoiling equipment.

Some treatment chemicals may be incompatible with other chemicals. Examples are demulsifier chemicals interfering with subsequent deoiling chemicals, or deoiling chemicals reacting with dilute polymers present in the water as a result of enhanced recovery schemes.

Efforts should be made to reduce the use of treatment chemicals whenever possible on the basis of minimising both operating costs and the ingress of additional chemicals into the environment.

2.3.3. Suspended solids

Suspended solids in waste water can originate from a number of sources such as formation solids, corrosion and erosion products, precipitates and as by-products of biological processes within the process facilities. Solids may also originate from process equipment such as the degradation of molecular sieve or packing materials. The treatment and discharge of drilling cuttings and drilling mud is

(23)

2. Characterisation of waste water

not addressed in this manual.

From an environmental viewpoint, suspended solids may have a number of potential impacts, including;

0

Some solids may be toxic themselves, contain toxic elements or have radioactive constituents. Solids may trap or collect other contaminants. For example, solid particles may be oleophilic, attracting and trapping hydrocarbons which will be discharged with the solids.

Discharged solids may accumulate as mud or silt in the local environment.

0 Discharged solids may result in turbidity in receiving environments with poor dispersion

characteristics. Such turbidity may have an environmental impact as well as being undesirable visual pollution.

Suspended solids may have a significant impact on the design of deoiling equipment. Erosion and the effects of solids accumulation in deoiling equipment must be considered. In some cases equipment will be specifically designed to allow the accumulation of solids which can be periodically removed.

The "Sand Wash Design Manual" EP 93-1270, due to be issued in the 4th quarter of 1993, contains additional information on topics including:

The transportation, deposition and accumulation of solids in production systems. The design of equipment and jetting systems for the collection and removal of solids. The handling and disposal of solids contaminated with hydrocarbons.

2.3.4. Heavy metals

The presence of heavy metals in waste waters can be of concern due to their potential toxicity and propensity for bio-accumulation. However not all heavy metals are regarded as dangerous. Table 2.3 gives a guide to the relative toxicity of a range of heavy metals and inorganic compounds.

Table 2.3

Relative toxicity of heavy metals and inorganic compounds Heavy metal or inorganic compound Manganese Mercury Molybdenum Nickel Phosphorus Potassi u m Silicon Sodium Strontium Sulphate Vanadium Zinc Heavy metal or inorganic compound Aluminium Barium Boron Cadmium Calcium Chromium Cobalt Copper Iron Lead Lithium Magnesium Rankings: 1 ... 1 Environmental Ranking 1 3 2 3 1 1 1 1 1 1 2 3 Environmental Ranking 1 1 1 3 1 3 1 3 1 3 1 1 critical

2

...

Toxic, but rare or having low solubility 3

...

Highly toxic and relatively accessible-

Source: Shell Expro's Wastes, 1989, Safety and environmental affairs department, ref

2.3.5. Salinity

The salinity of water is typically expressed as the concentration of Total Dissolved Solids (TDS), indicating the quantity of dissolved inorganic salts present in the water.

The salinity of the waste water should be considered for the following reasons;

For surface disposal, the salinity of the discharge water should not be significantly different from the salinity of the receiving environment. This should encompass both the discharge of saline water into fresh waters and the discharge of fresh water into saline waters (e.g. inshore).

P(ixC 2-6

_ _ ~ ~

(24)

2. Characterisation

of

waste water

Salinity affects the density of the water which will in turn influence the design of gravity based separation equipment and the dispersion characteristics of disposed waste water.

Changes in the composition or process conditions of saline waters (e.g. temperature changes) may lead to the precipitation of inorganic salts and scale formation.

The salinity and aeration of the water will add to the corrosion potential of the water stream, influencing the selection of materials for process equipment.

2.3.6. pH

The pH determination of water is a relatively reliable indication of its acidic or alkaline tendency. However it is not a measure of the actual acidity or alkalinity in a water sample. ASTM D 1293-78 discusses the measurement of pH while ASTM D 1067 discusses the measurement of actual acidity or alkalinity. The pH of the waste water is subject to regulation in some countries which typically require the pH of discharged waste waters to be between 5.5 and 9.0

Alkaline waste waters with a pH in the range of 8-10 may react with components of the hydrocarbon stream to form surfactant type chemicals. These surfactants may assist in stabilising emulsions.

2.3.7. Hardness

Hardness in water is generally caused by the presence of calcium and magnesium ions, though any polyvalent cation can contribute to hardness. Hardness in water can result in scale formation in process equipment. The release of dissolved CO, during depressurisation may also alter the hardness characteristics of water.

2.3.8. Thermal contamination

Although not strictly a contaminant, the temperature of discharged waste waters may have an impact on the environment. The impact will be dependent on factors such as the temperature differential to the receiving environment and the physical extent of the temperature gradient.

The temperature will also affect the density and viscosity of the waste water. These variations must be accounted for in the design of waste water treatment equipment and when modelling the dispersion characteristics of waste water discharges.

The temperature of waste water discharges may be subject to regulation, especially in the case of discharges into inland water bodies such as rivers and lakes.

2.3.9. Dissolved oxygen

From an engineering standpoint, the presence of oxygen in a water stream is very important due to its potential role in the corrosion of process piping and equipment and in the resultant corrosion products in effluent waters. Produced waters do not normally contain any dissolved oxygen, however oxygen may be introduced into the process through open equipment and tanks or through the use of aerated waters such as fresh water used for desalting.

From an environmental viewpoint, the levels of oxygen in discharged water can have a significant impact on the receiving environment. Discharge of oxygen free water into surface waters may result in an oxygen deficiency in the local aquatic environment. Some countries have regulations governing the quantity of dissolved oxygen that must be available in waste water discharges.

Conversely, the presence of contaminants in the waste water can affect the level of dissolved oxygen in the receiving environment by consuming oxygen through biological and/or chemical oxidation of the contaminants. The oxygen demand of a waste water stream is an indication of the oxygen depletion that may be imposed on the receiving environment. Oxygen demand is measured and expressed in several different ways.

2.3.9.1.

Biological oxygen demand (BOD)

The biological oxygen demand is defined as the quantity of oxygen (expressed in mg/l) consumed by micro-organisms degrading organic material under aerobic conditions. An example is the test method from the American Public Health Association (APHA), "Standard methods for the examination of water and waste water", p483, 15th ed.

(25)

2. Characterisation of waste water

Complete biological oxidation of all the contaminants present in waste waters can potentially take from 20 to 100 days. As this time period is too long for analytical purposes a shorter test, termed the “five day BOD” (BOD,), which measures the amount of oxygen consumed in the first 5 days of biological oxidation at 20°C is commonly used.

BOD is an important guide to the amount of micro-organic activity that can be supported by the organic elements present in the waste water. Discharging waters with a high BOD may result in an oxygen deficiency in the local aquatic environment.

2.3.9.2. Chemical oxygen demand (COD)

The chemical oxygen demand is defined as the quantity of oxygen (expressed in mg/l) consumed by chemical oxidation of organic and inorganic material in waste water, corrected for the presence of chlorides. The COD test was developed to give a faster estimate of the oxygen demand than the BOD procedure. Although theoretically the COD should equal the ultimate BOD, in practice this is not always the case for waste waters. However it is oflen possible to develop a relationship between COD and BOD which allows the COD to be used as an indicator for controlling BOD.

An example of a COD test method is ASTM D-1252-78 which is based on the reduction of a dichromate solution under specified conditions. However this method is not suitable for waste waters containing more than 1 g/l of chloride ions, which precludes its use for many saline produced waters.

2.3.9.3. Total oxygen demand (TOD)

The total oxygen demand is the amount of oxygen required to convert the elements present in waste water contaminants to their most stable oxidised forms. An example of a TOD test method is ASTM D 3250-77 which measures the TOD by oxidation of the water stream at 900°C. The TOD method can be used for brackish waters and brines.

2.3.1 0. Organic carbon

A number of measurements of the quantity of carbon in a water sample are possible. When a sample is homogenised such that both inorganic and organic carbon is measured, the result is known as the Total Carbon (TC). Eliminating the inorganic carbon response from the measurement gives the Total Organic Carbon (TOC). When particulate carbon is removed from the sample, the net measurement is the Dissolved Carbon (DC), or if the inorganic carbon response is eliminated, the Dissolved Organic Carbon (DOC).

ASTM D 2579-78 discusses test methods for measuring total and organic carbon and references additional information relating the TOC to other measures of water quality such as BOD and COD. It should be noted that carbon measurement encompasses all forms of carbon including polar hydrocarbons, carbonates and other forms of carbon which are not normally classified as part of the “mineral hydrocarbon” content of a water stream.

2.4. DROPLET SIZE DISTRIBUTIONS

The droplet size distribution is one of the most crucial characteristics governing the removal of suspended solids or hydrocarbon droplets from water. Many designs of deoiling equipment have a limit on the smallest hydrocarbon droplet size that can be efficiently removed from the water stream.

Figure 2.1 illustrates a typical droplet size distribution curve in terms of hydrocarbon volume. The size distribution is typically categorised by a mean droplet size, however the mean droplet size must always be defined on an associated basis, e.g. numerical mean, volume/weight mean, surface area mean etc. When reporting mean droplet sizes the basis for the mean should always be clearly stated.

A recommended method for expressing mean droplet sizes is the mean volume droplet size D(v,o.5). This is defined as the droplet diameter such that half the volume of hydrocarbons present is encompassed in smaller droplets and half the volume is contained in larger droplets. Graphically, as illustrated in Figure 2.1, the mean volume droplet diameter divides the volume distribution such that half the area under the curve is on either side of the mean, in this case the mean volume diameter being 26.6 pm.

The volume based mean droplet diameter can be more readily determined by plotting the cumulative percentage volume as indicated in Figure 2.2.

The numerical mean, defining the point where half the number of droplets are either larger or smaller is oflen not as useful for indicating the distribution of dispersed hydrocarbons. The volume

of

hydrocarbon contained in the smaller droplets may be insignificant when compared to the larger droplets. For

(26)

2. Characterisation

of

waste water

example, a single 50pm droplet contains 1000 times the volume of a 5pm droplet.

The numerical mean for the distribution illustrated in Figure 2.1 was separately determined to be 2.3 pm,

significantly lower than the volume based mean. Thus, even though half the number of droplets are

smaller than 2.3 pm, from Figure 2.2 we can see that volume of hydrocarbons contained in these droplets is less than 5% of the total hydrocarbon droplet volume.

10

9

8

0.1 1 10 100

Particle size (urn)

Figure 2.1

Example of a particle or droplet size distribution

1000 100 90 80 70 60 50 40 30 20 10 0 0.1 1 10 100

Particle size (urn)

Figure 2.2

Cumulative volume based droplet size diameter

1000

(27)

2. Characterisation

of

waste water

THIS PAGE H

INTENTIONALLY

K