Vapour recovery units Guidance on preventing and controlling temperature excursions in carbon beds

Transcription

1 Vapour recovery units Guidance on preventing and controlling temperature excursions in carbon beds 2nd edition

2 VAPOUR RECOVERY UNITS GUIDANCE ON PREVENTING AND CONTROLLING TEMPERATURE EXCURSIONS IN CARBON BEDS Second edition March 2008 Published by ENERGY INSTITUTE, LONDON The Energy Institute is a professional membership body incorporated by Royal Charter 2003 Registered charity number

3 The Energy Institute (EI) is the leading chartered professional membership body supporting individuals and organisations across the energy industry. With a combined membership of over individuals and 300 companies in 100 countries, it provides an independent focal point for the energy community and a powerful voice to engage business and industry, government, academia and the public internationally. As a Royal Charter organisation, the EI offers professional recognition and sustains personal career development through the accreditation and delivery of training courses, conferences and publications and networking opportunities. It also runs a highly valued technical work programme, comprising original independent research and investigations, and the provision of IP technical publications to provide the international industry with information and guidance on key current and future issues. The EI promotes the safe, environmentally responsible and efficient supply and use of energy in all its forms and applications. In fulfilling this purpose the EI addresses the depth and breadth of energy and the energy system, from upstream and downstream hydrocarbons and other primary fuels and renewables, to power generation, transmission and distribution to sustainable development, demand side management and energy efficiency. Offering learning and networking opportunities to support career development, the EI provides a home to all those working in energy, and a scientific and technical reservoir of knowledge for industry. This publication has been produced as a result of work carried out within the Technical Team of the Energy Institute (EI), funded by the EI s Technical Partners. The EI s Technical Work Programme provides industry with cost-effective, value-adding knowledge on key current and future issues affecting those operating in the energy sector, both in the UK and internationally. For further information, please visit The EI gratefully acknowledges the financial contributions towards the scientific and technical programme from the following companies: BG Group BHP Billiton Limited BP Exploration Operating Co Ltd BP Oil UK Ltd Chevron ConocoPhillips Ltd ENI E.ON UK ExxonMobil International Ltd Kuwait Petroleum International Ltd Maersk Oil North Sea UK Limited Murco Petroleum Ltd Nexen Saudi Aramco Shell UK Oil Products Limited Shell U.K. Exploration and Production Ltd Statoil (U.K.) Limited Talisman Energy (UK) Ltd Total E&P UK plc Total UK Limited Copyright 2008 by the Energy Institute, London: The Energy Institute is a professional membership body incorporated by Royal Charter Registered charity number , England All rights reserved No part of this book may be reproduced by any means, or transmitted or translated into a machine language without the written permission of the publisher. ISBN Published by the Energy Institute The information contained in this publication is provided as guidance only and while every reasonable care has been taken to ensure the accuracy of its contents, the Energy Institute cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The Energy Institute shall not be liable to any person for any loss or damage which may arise from the use of any of the information contained in any of its publications. Further copies can be obtained from Portland Customer Services, Commerce Way, Whitehall Industrial Estate, Colchester CO2 8HP, UK. Tel: +44 (0) sales@portland-services.com Electronic access to EI and IP publications is available via our website, Documents can be purchased online as downloadable pdfs or on an annual subscription for single users and companies. For more information, contact the EI Publications Team. e: pubs@energyinst.org.uk

4 CONTENTS Page Foreword...iv Acknowledgements...vi 1 Introduction Scope Carbon adsorption vapour recovery Activated carbon Process description Processes causing temperature increases in carbon beds Heat of adsorption Heat of combustion Possible causes of temperature excursions Excessive carbon activity Oxidation reaction occurs Poor heat removal Precautions against high temperature excursions Carbon pre-conditioning Model sequence for dealing with temperature excursions References Annex A Decision tree for dealing with temperature excursions Annex B Recommendations for temperature monitoring systems on new vapour recovery units iii

5 FOREWORD This second edition, has been prepared by the EI s Vapour Recovery Working Group, in consultation with vapour recovery unit (VRU) suppliers and the UK Health & Safety Executive. The first edition arose from a workshop held in 2000, where it was agreed that common procedures were required for monitoring bed temperatures or dealing with abnormal conditions in gasoline VRUs. This followed a number of instances of VRUs experiencing increases in carbon bed temperatures greater than would be expected in normal operation. These so called 'temperature excursions' had been up to, and in a few instances exceeded, 120 C in which case a 'hot spot' is said to have occurred. The main conclusions from that workshop are still valid: High temperature excursions are not a serious safety critical issue: VRUs are designed to shut down safely and automatically in the event of a hot spot. There have not been any reported instances world-wide of carbon bed fires in VRUs recovering gasoline vapour, with the major supplier of this type of VRU having an accumulated unit run time exceeding 85 million hours. Experience outside of the UK has confirmed that it is possible for a gasoline VRU to experience a high temperature excursion in a carbon bed. Since the first edition of these guidelines, published in January 2001 [1], no hot spot has been reported to have formed in a VRU installed in the UK. This second edition takes into account experience gained in the practical use of the guidance during the control of temperature excursions on operating VRUs. In addition, since 2001 two Research Reports have been published by the EI which have provided more background information. These concern the use of CO monitors for hot spot detection [2] and tests on activated carbon, including analyses of the residual hydrocarbons (the 'heel') adsorbed on both newly installed and aged carbons [3]. The main change in the guidance is in the model sequence for dealing with temperature excursions (Annex A). Step 2 in the first edition of the guidance has been deleted. This step was initiated if the temperature reached 80 C and involved isolating the unit and running in manual mode to regenerate, and thus attempt to cool, the beds. However, experience has shown that the greatest risk of a temperature excursion occurring is during the first few months of VRU operation when the heel on the carbon is less stable and is more easily stripped off due to excessive regeneration [3]. The original step 2 could exacerbate this problem, as running in manual mode may cause further desorption of the heel, resulting in the unit experiencing another temperature excursion once put back into service. The original step 3, which involves cooling with nitrogen circulation, is now initiated at 80 C instead of 100 C. This action at an earlier stage will assist in preventing a temperature excursion increasing above 120 C, at which point a hot spot can develop resulting in an exothermic temperature increase. A response before this can occur will reduce the likelihood for the need to water flood a bed and thus decrease the potential VRU downtime. iv

6 The majority of VRUs in the UK were installed between 1998 and 2004 to meet the deadlines imposed by the European Directive 94/63/EC [4]. This publication has been written taking into account the minimum level of temperature measurement equipment likely to be installed on those VRUs. Recommendations for temperature monitoring systems on new vapour recovery units are provided in Annex B. Although it is anticipated that this publication will assist those involved in the operation of VRUs, the information contained in this publication is provided as guidance only. While every reasonable care has been taken to ensure the accuracy of its contents, the EI, and the technical representatives listed in the acknowledgements, cannot accept any responsibility for any action taken, or not taken, on the basis of this information. The EI shall not be liable to any person for any loss or damage which may arise from the use of any of the information contained in any of its publications. The above disclaimer is not intended to restrict or exclude liability for death or personal injury caused by own negligence. Suggested revisions are invited and should be submitted to the Technical Department, Energy Institute, 61 New Cavendish Street, London, W1G 7AR. v

7 ACKNOWLEDGEMENTS This second edition of the guidance was prepared by the Energy Institute's (EI) Vapour Recovery Working Group, comprising: S Alderson D Coker R Harris M Hunnybun P Lambeth R May T Ramsey M Randall J Robson B Smithers A Sykes Vopak Consultant Consultant EI BP Oil UK BP Oil UK ExxonMobil Shell UK Oil Products Roplex Engineering Ltd Consultant (chair) EI The EI wishes to record its appreciation of the work carried out by them and also the following specialists who provided comments on drafts of this publication: Harold Dinsmore Stuart Hamilton John Hazeldean Mark Jordan Ties Mulder Simon Shipley John Zink Company LLC Health & Safety Executive Health & Safety Executive Jordan Technologies Inc CarboVac SARL Aker Kvaerner Cool Sorption A/S Technical editing and project co-ordination was carried out by Martin Hunnybun (EI). vi

8 1 INTRODUCTION The implementation of gasoline vapour emission control legislation [4] in Europe has resulted in the installation of VRUs using adsorption of the vapours onto activated carbon as the most common control technology. These units operate on a cyclic basis, with one vessel ('bed') containing activated carbon in adsorption mode, whilst a second bed is being regenerated by applying a vacuum. During normal operation the temperature of the carbon in the bed in adsorption mode can increase by up to 25 C due to the release of the heat of adsorption. During regeneration, however, the desorption of hydrocarbons from the carbon results in a cooling effect which reverses the majority of this temperature increase. Under conditions of prolonged heavy loading of the VRU the build-up of residual heat can result in bed temperatures of more than 25 C above ambient. However, the only time that there should be a significant increase in bed temperature is during initial controlled pre-conditioning of the carbon when the VRU is being commissioned. A 'temperature excursion' is defined as any increase in temperature of a carbon bed that is greater than would be expected in normal operation. Heat generation due to adsorption is self-limiting. As the temperature of carbon increases, its ability to adsorb hydrocarbons decreases. Experience has shown that adsorption of gasoline vapours, even onto virgin carbon during pre-conditioning, does not generate enough heat to cause the carbon bed temperature to increase beyond about 120 C. If the temperature increases beyond this limit then it is due to causes other than solely adsorption, and the bed is said to experience a 'hot spot'. A hot spot is caused by catalytic oxidation within the carbon bed. This can be initiated when the bed operates at above normal operating temperatures, and thus maintaining bed temperatures as low as possible is an important consideration. A hot spot is not self-limiting and, if not detected and controlled correctly, could lead to an exothermic temperature increase. Page 1

9 2 SCOPE These guidelines apply to VRUs using carbon beds regenerated by a vacuum as the primary technology to control gasoline vapour emissions. They may not be appropriate for emission control systems where the carbon beds are regenerated by other means (e.g. steam), where the carbon beds are used to control vent emissions from a primary control device using a different technology, or for emissions from volatile products other than automotive gasolines. Page 2