NATURAL GAS DEHYDRATION IN OFFSHORE RIGS : COMPARISON BETWEEN TRADITIONAL GLYCOL PLANTS AND INNOVATIVE MEMBRANE SYSTEMS

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1 1 NATURAL GAS DEHYDRATION IN OFFSHORE RIGS : COMPARISON BETWEEN TRADITIONAL GLYCOL PLANTS AND INNOVATIVE SYSTEMS F.Binci 1,a*#, F.E. Ciarapica 1,b, G.Giacchetta 1,c 1 Department of Energetics, Faculty of Engineering, University of Ancona via Brecce Bianche, 6131, Ancona, Italy a Tel ; Fax ; filbin@supereva.it b Tel. +39 () ; Fax ; f.ciarapica@unian.it c Tel. +39() ; Fax ; g.giacchetta@unian.it Abstract Natural gas dehydration is a treatment that removes most of the water vapor content before forwarding the gas to pipelines. For decades, the most widely-used technology has been to absorb the water with a liquid solvent flowing in countercurrent inside bubble cap trays columns. In the last ten years, research has been developing new dehydration systems based on selective membranes in order to lower plant costs, increase separation efficiency and reduce emissions. The present work draws a technical and economic comparison between traditional glycol absorption plants and an innovative system using polymer membranes. The membrane system proved more cost-effective for low feed gas flow rates and more environment-friendly. 1. Introduction Dehydration treatment to remove water vapor from natural gas is needed to prevent hydrate formation and pipeline acid corrosion, and to guarantee compliance with the dewpoint required by the standard specifications. About 95% of existing offshore installations currently use TEG (triethylene glycol) technology, where dehydration is achieved inside an absorber column where natural gas bubbles in countercurrent through a highly hygroscopic solvent (i.e. triethylene glycol) capable of absorbing the water molecules contained in the gas. The use of membranes for natural gas dehydration began only ten years ago and these systems are still in the experimental stage. For the time being, there are just a few onshore installations, while nothing is known about their use offshore: all the information available come from experimental data and from the few pilot plants working onshore. Such installations are nonetheless likely to be the next step in natural gas dehydration technology. Their large-scale application will solve most of the old problems related to TEG plants, particularly reducing the hazardous environmental effects of BTEX (benzene, toluene, ethyl benzene, xylene) and VOCs (volatile organic compounds) emissions. The experimental results obtained make this technology in the field of offshore plants even more attractive and cost-effective than for onshore plants because membrane modules enable major savings in terms of the weight and size of the installations, leading to considerable platform CAPEX (capital expenditure) savings as a result. 2. Plant description For the analysis of traditional dehydration plants, we considered a TEG system with bubble cap trays column. The novel membrane plant taken for comparison is based on the use of PRISM polymer membranes made into hollow fiber modules by AirProducts.

2 2 The gas transport performances of this membranes, in terms of permeability and stability, are shown in fig.1 and fig.2 [1]. These devices have been used at the ENI s Hera 98, Lacinia onshore gas field (CR Italy) working on a feed gas rate of 6, Sm 3 /day 1 and the plant layout at said installation was considered 96, for the present analysis (fig.3)[2]. H2O% Removal 94, Days of Operation Fig. 1 PRISM membrane stability CH 4 N 2 O 2 CO H 2 H 2 O, Fig. 2 PRISM membrane permeability Feed Gas Compressor Natural Gas Feed The most widely-known advantages of the membrane technology over TEG plants are as follows: no emissions are released into the atmosphere and no solvents are used; a reduction in the size and weight of the installations. Cooler Liquids Prism Dehydrator Heater Permeate Fig. 3 Membrane plant layout Permeate compressor Sweep Dry Natural Gas 3. Background hypothesis for the comparison The economic assessment compared traditional TEG plants with bubble cap trays versus PRISM polymer membrane plants on the assumption of the following: feed gas rates varying between 5, and 4,5, Sm 3 /day; -year plant lifetime; upstream conditions: water-saturated methane gas at a pressure of bar and a temperature of C; downstream conditions: dehydrated methane gas at a pressure of 7 bar and a dew point of - C. The TEG plants considered were classified on the strength of the operating conditions shown in Table 1. TEG reference scheme Case Description (High pressure column) HIGH-PRESSURE (7 bar) COLUMN: the gas has to be compressed and cooled before entering the column. TEG LPC (Low pressure column) LT HT LOW-PRESSURE COLUMN ( bar) and LOW-TEMPERATURE ( C) FEED GAS : the gas directly enters the column. LOW-PRESSURE ( bar) COLUMN and HIGH-TEMPERATURE (5 C) FEED GAS: the gas is cooled to C, then enters the column. Table 1 TEG plant reference schemes 1 Sm 3 /day = Standard Cubic Meters per day. (Standard Conditions : T = 15 C and P = 1325 Pa.)

3 3 All the TEG plants considered operate at an internal temperature of C. In the only case LPC we examined the further condition of a feed gas temperature of 5 C with the aim to evaluate the economic effects of a cooling cycle before the dehydration column. The membrane system operates at 7 bar and 5 C, with a permeate gas flow rate of 7% of feed gas flow rate at a pressure of 2,5 bar. Depending on the predicted membrane module lifetime, three alternatives were considered : Alternative 1 : no membrane replacement during the -year lifetime; Alternative 2 : one membrane replacement after years; Alternative 3 : two membrane replacements, after 7 and 14 years. All the typical expenditures were analyzed for the above-described systems, defining the CAPEX and operating expenditure (OPEX) of each installation. Then the items were used as input data for the economic evaluation of the various technologies using the NPV (Net Present Value) and a 5% discount rate. 4. CAPEX comparison between TEG and membrane plants Direct CAPEX [M$] Platform Cost Weighting Factor Area 73 $/m 2 7% Weight 3215 $/Kg % Allocated Platform Cost [M$] ,9,8,7,6,5,4,3,2,1 Fig. 4 Direct CAPEX Table 2 Platform cost coefficients TEG,,5 1,5 2,5 3,5 4,5 Fig. 5 Allocated platform cost In comparing the CAPEX, we analyzed the expenses incurred for the construction and installation of the various systems. We studied the direct cost items (costs of separation equipment, ancillary equipment and installation) and the indirect cost items (engineering and allocated platform cost). The cost of separation and ancillary equipment was obtained directly from manufacturers. The installation costs were evaluated as a percentage of the amount of separation and ancillary equipment costs (35% and 25% for TEG and membrane plants respectively). The engineering cost was evaluated as a percentage of 15% of the amount of direct costs [8]. The results for direct CAPEX are shown in fig. 4. The analysis attributed fundamental importance to the assessment of the allocated platform cost, which represents the cost of the part of platform occupied by the dehydration plants, because the sizes and weights of the systems being compared were very different and their reduction should always be pursued. The coefficients used in the analysis are shown in Table 2. The results, processed on the strength of data from specialist companies and the existing literature, are shown in fig. 5 [3] [4] [5] [6] [7]. The analysis of the allocated platform cost (depending on the feed gas rate) shows that membrane plants enable a considerable saving in terms of the weight and size of the installation for low feed gas rates. As the plant s size grows, there is a corresponding growth in the number of membrane modules that have to be installed in parallel, leading to a greater increase in the size and weight of the

4 4 membrane skid than in the case of a TEG plant. So, in terms of allocated platform cost, the advantage of the membrane plant is considerable for small platforms, where the space available for dehydration systems is limited. This advantage decreases as the feed gas flow rate grows until it reaches the break even point for a feed gas flow rate of 4.. Sm 3. Over this value of feed gas flow rate TEG plants become advantageous (fig. 5). As for the total CAPEX, by comparison with the traditional TEG plant, the initial outlay for a membrane plant is (fig. 6): lower only for feed gas rates up to 85, Sm 3 /day; comparable, depending on the operating conditions, between 85, and 1,6, Sm 3 /day; higher for feed gas rates over 1,6, Sm 3 /day. The greater rise in the CAPEX for larger membrane systems versus TEG plants is due to the need to buy larger numbers of dehydration modules the higher rate of the feed gas being processed. Direct and indirect cost items, for a feed gas flow rate of 1,5, Sm 3 /day are shown in fig. 7. CAPEX [M$] Fig. 6 CAPEX CAPEX [M$] Feed gas flow rate : 1.5.Sm3/day TEG LPC- HT Allocated platform cost Engineering cost Installation cost Separation and ancillary equipment cost Fig. 7 CAPEX cost items 5. OPEX comparison between TEG and membrane plants As for the OPEX, we analyzed the expenditure involved in operating the plant properly throughout its lifetime. We studied the following cost items: manpower; chemical agents; routine maintenance; extraordinary maintenance; energy. The calculation of the first three items was made with reference to the hypothesis shown in Table 3 and the results obtained are shown together in fig. 8 [8]. Membrane systems were found to enable an evident saving on the whole range of feed gas flow rates considered, with a greater absolute advantage the higher the feed gas flow rate. Manpower + routine maintenance + chemical agents cost [M$] 2,5 2 1,5 1,5 TEG LPC -LT TEG LPC -HT M EM BRANE Fig. 8 Manpower, current maintenance and chemical agents costs

5 5 (Manpower + Chemical Agents + Routine Maintenance) TEG % of (Direct CAPEX + Engineering) (Manpower + Chemical Agents + Routine Maintenance) 4% of (Direct CAPEX + Engineering) Table 3 Manpower, chemical agents and routine maintenance costs calculation Conversely, for the cost of the energy needed to work the compressors, the analysis showed that the presence of the permeate compressor makes the new membrane systems clearly more expensive than TEG plants over the whole feed gas flow rate range considered, with a gap that widens as the feed flow rate increases (fig. 9). The energy cost was calculated assuming to use gas-driven type compressors and evaluating their natural gas consumption. The extraordinary maintenance costs were assessed by calculating the expenses incurred at regular intervals for absorber and still column inspections by auditing authorities (for the TEG plants) and for the periodical replacement of the dehydration modules assuming the progressive loss of efficiency declared by the manufacturer (for the membrane plants). The extraordinary maintenance of membrane modules was assumed to be negligible during the whole lifetime of the module. Energy cost 2,5 2, 1,5 1,,5, Fig. 9 Energy cost,5 1,5 2,5 3,5 4,5 6. Economic evaluation of the alternatives through NPV method (Net Present Value) Applying the NPV method to the aboveillustrated costs enabled us to compare traditional TEG plants and new membrane systems on the assumption of different membrane module lifetimes. We analyzed the economic advantage of each of the three previously-mentioned membrane alternatives vis-à-vis each TEG configuration variable with the operating conditions of the plant. Membrane - Alternative 1 This always proved better than all possible TEG configurations for feed gas flow rates varying between 5, and 2,, Sm 3 /day. Between 2,, and 3,35, Sm 3 /day it appears to be less cost-effective than the, but more so than the TEG HPC or. Between 3,35, and 4,35, Sm 3 /day it was only better than the. Over 4,35, Sm 3 /day it was never more cost-effective than any of the TEG system configurations (fig. ). Membrane - Alternative 2 This was always a good choice vis-à-vis any Fig. NPV membrane alternative 1 of the TEG configurations for feed gas flow rates varying between 5, and 1,6, Sm 3 /day. Between 1,6, and 2,5, Sm 3 /day it was less cost-effective than the TEG NPV [M$ NPV [M$ MEM ALT.1 MEM ALT.2 Fig. 11 NPV membrane alternative 2

6 6 LPC-LT, but more so than the or TEG LPC-HT. Between 2,5, and 3,25, Sm 3 /day it was only better than the TEG LPC- HT. Over 3,25, Sm 3 /day it was never better than any of the TEG plants (fig.11). Membrane - Alternative 3 This alternative was always more costeffective than the TEG configurations for feed MEM ALT.3 gas flow rates varying between 5, and 1,, Sm 3 /day. From 1,, to 1,9, Sm 3 /day it lost its edge over the TEG Fig. 12 NPV membrane alternative 3 LPC-LT, but was still better than the and. Between 1,9, and 2,4, Sm 3 /day it still performed better than the. Over 2,4, Sm 3 /day it was always less cost-effective than any of the TEG plants (fig.12). NPV [M$ Conclusions The economic assessment showed that membrane dehydration plants could be better value for money than the TEG only depending on predicted membrane lifetimes, on feed gas flow rates and on the operating conditions of the plant. Assuming the most likely membrane life span is years, selective membrane systems can only claim to be the most cost-effective technology for offshore rigs of limited dimensions (processing up to 1,6, Sm 3 /day), while they definitely offer no economic advantage for medium to large rigs (over 3,25, Sm 3 /day). As for all the possible situations falling within these limit conditions, the advisability of installing a dehydration plant based on selective membrane modules needs to be evaluated case by case, in the light of the operating conditions that the system will be required to work with. References [1] WILLIAM M. POPE Jr. : AirProducts Natural Gas Dehydration Sales Presentation January 1 personal correspondence. [2] ENI S.p.a. DIVISIONE AGIP : Impianto per la disidratazione del gas naturale mediante membrane polimeriche da installarsi presso la centrale gas Hera Lacinia nel comune di Crotone Relazione tecnico-illustrativa e prevenzione incendio, 1. [3] S. De Donno, S. Biagi, G. De Ghetto : Disidratazione gas naturale con sistemi a membrane selettive Studio di pre-fattibilità per un impianto da 6. Sm 3 /giorno per la Centrale di Hera Lacinia AGIP-RIIN, [4] C. Richard Sivalls : Glicol dehydration design Laurance Reid Gas Conditioning Conference, University of Oklahoma, February 1. [5] ENI AGIP E&P Division Research & Development Department : personal correspondence. [6] SiiRTEC NIGI S.p.a. : personal correspondence. [7] Michael P. Quinland, Linda W. Echterhoff, Dennis Leppin, Howard S. Meyer : Costcutting for offshore sulfur recovery process studied Oil & Gas Journal, 21 July [8] R. D Agostini, M. Calvarano, L. Ciccarelli : Progetto M.E.D.E.A. MEmbrane di Disidratazione Estensione Applicabilità Comparazione tecnico economica tra impianti di disidratazione gas naturale con TEG e membrane Relazione N TEIM-52-, 1998.