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Apr-2013

Energy-efficient vacuum systems

Case analysis of the techniques available to reduce energy consumption in vacuum systems reveals the potential for cost savings

C Chandra Sekhara Reddy and S V NAIDU, Andhra University
G P RANGAIAH, National University of SingaporeV
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Article Summary
Vacuum technology is used in petroleum refineries to facilitate the distillation of heavy ends at low temperatures, to prevent coking and degradation of products and for other applications. Currently, steam jet ejectors and steam ejector-liquid ring vacuum pump (LRVP) combinations are the most common methods for vacuum generation in petroleum refineries. Although steam jet ejectors are very reliable, they are highly inefficient. Due to increasing energy costs and environmental concerns, it is essential to reduce the energy required for vacuum generation. Petroleum refineries discard a lot of waste heat to the environment, which could be used to reduce energy consumption for vacuum generation. There are publications illustrating the merits and limitations of vacuum generation methods1,3,4,6 and chilled/refrigerated water generation.2 However, there is a need for an integrated approach covering all aspects of vacuum generation and its energy reduction possibilities.

The main objective of this study is to analyse various methods for developing energy-efficient vacuum generation systems in petroleum refineries. Three case studies are presented to enhance understanding in the selection process.

Vacuum generation in refineries
Steam ejectors and LRVPs are generally used in petroleum refineries. A review of various vacuum generation equipment capacities, operating ranges and efficiencies is available.6 Steam ejectors may have one or more stages in series or a series-
parallel combination, with pre- and/or interstage condensers, depending on the level of vacuum required and the utility optimisation and operational flexibility sought for various plant loads. Steam ejectors are highly reliable, and the availability of steam in petroleum refineries makes ejectors the natural choice. However, they are highly inefficient6 (<10%), mainly due to a lack of moving parts to convert fluid velocity to pressure efficiently.4

LRVP most commonly uses water as a seal liquid since it can be separated and reused safely. They 
are generally more expensive compared to steam ejectors. However, they do not require large heat exchangers to condense the vapour at their outlet, and the operating costs of LRVPs are generally lower than steam jet ejectors. For better operating cost savings, a steam ejector–LRVP combination is sometimes used to replace the last one or two stages of a multistage steam ejector system.

Design principles and utility requirements
This section presents useful design principles and tools for estimating the utility requirement for steam jet ejectors and LRVP. Use of pre- and interstage condensers can reduce both capital and operating costs for the vacuum system. The vacuum produced is limited by the temperature of the cooling water; the colder the temperature of the cooling water, the lower the vacuum produced.

Steam requirement for ejectors can be estimated based on the dry air equivalent (DAE) of suction gases (including air, water vapour and other gases). As per the HEI (Heat Exchange Institute) procedure for calculating DAE8, water vapour in the suction gases is considered separately and all other gases (including air) are treated as a mixture, in accordance with this mixture’s molecular weight. HEI has published curves to convert suction gas streams to DAE using molecular weight and temperature entrainment ratios. Molecular weight entrainment ratio (MWc) is defined as the ratio of the weight of suction gas to the equivalent weight of air. Temperature entrainment ratio is defined as the ratio of the weight of air (or water vapour) at actual suction temperature to the weight of air (or water vapour) at 21.1°C.

The following equations are derived from HEI curves8 for temperature entrainment ratios (TCa and TCw) and MWc. These are convenient for use in computer programs:

TCa = -4 * 10-10 T3 + 3 * 10-7 T2 - 0.0005 T +
1.0131                (1)

TCw = -1 * 10-13 T4 - 7 * 10-12 T3 + 8 * 10-8 T2 -
0.0006 T + 1.015            (2)

For M = 0 to 60, MWc = 1 * 10-5 M3 - 0.00013 M2 + 0.0642 M + 0.016                 (3.1)

For M = 60 to 150, MWc = -2 * 10-5 M2 + 0.0077 M + 0.9464                                      (3.2)

Water vapour and other components in the suction gas can be converted to DAE using the correction factors from equations 1, 2, 3.1, 3.2 and the following equation:
DAE of suction            WOG    +             Ww                              
gas or vapour =  TCa * MWCOG  TCW * MWCWV(4)

The amount of motive steam required to compress (from suction pressure to discharge pressure) the unit DAE mass of suction gas/vapour in a steam ejector is defined as Ra (kg of motive steam/kg of DAE equivalent of load gas). Values of Ra are available4 as curves, with suction pressure on the abscissa and discharge pressure on the ordinate. For rough estimation of an ejector’s steam consumption, one can simply use the following equation:9

Ra = >Pd (0.434 - 1.338 + 0.000475 Pa) - 0.187H
         Pa                  Pa          

(1.2 - Pv - 10.2)   
             20                                             (5)

Steam requirement for ejectors can be estimated by multiplying DAE and Ra values.

For condenser calculations, involving air and water mixtures, the overall heat transfer coefficient, U W/(m2K), can be estimated using the following developed equations.4 For a gas vapour mixture with non-condensable vapour mole percentage, NC, from 1% to 50%:

U = 5.678 (220.0417 + 1.6919 ln(NC) -
2.67975 [ln(NC)]2 - 1.5465 [ln(NC)]3)      (6)

For a gas vapour mixture with NC from 50% to 95%:

U = 5.678 (-245 896 + 233 845.3 ln(NC) -
83 300.5 [ln(NC)]2 + 13 183.62 [ln(NC)]3    - 782.58 [ln(NC)]4)                                            (7)

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