Retrofit of heat exchanger networks with pressure recovery of process streams at sub-ambient conditions

This paper presents a new mathematical programming model for the retrofit of heat exchanger networks (HENs), wherein the pressure recovery of process streams is conducted to enhance heat integration. Particularly applied to cryogenic processes, HENs retrofit with combined heat and work integration is mainly aimed at reducing the use of expensive cold services. The proposed multi-stage superstructure allows the increment of the existing heat transfer area, as well as the use of new equipment for both heat exchange and pressure manipulation. The pressure recovery of streams is carried out simultaneously with the HEN design, such that the process conditions (streams pressure and temperature) are variables of optimization. The mathematical model is formulated using generalized disjunctive programming (GDP) and is optimized via mixed-integer nonlinear programming (MINLP), through the minimization of the retrofit total annualized cost, considering the turbine and compressor coupling with a helper motor. Three case studies are performed to assess the accuracy of the developed approach, including a real industrial example related to liquefied natural gas (LNG) production. The results show that the pressure recovery of streams is efficient for energy savings and, consequently, for decreasing the HEN retrofit total cost especially in sub-ambient processes.

Effects of ocean warming and acidification on the early stages of marine fishes

Tese de doutoramento, Ciências do Mar, Universidade de Lisboa, Faculdade de Ciências, 2016

(Table 1) Thermal conductivities of lithified core samples from ODP Leg 166 sites

The geothermal regime of the western margin of the Great Bahama Bank was examined using the bottom hole temperature and thermal conductivity measurements obtained during and after Ocean Drilling Program (ODP) Leg 166. This study focuses on the data from the drilling transect of Sites 1003 through 1007. These data reveal two important observational characteristics. First, temperature vs. cumulative thermal resistance profiles from all the drill sites show significant curvature in the depth range of 40 to 100 mbsf. They tend to be of concave-upward shape. Second, the conductive background heat-flow values for these five drill sites, determined from deep, linear parts of the geothermal profiles, show a systematic variation along the drilling transect. Heat flow is 43-45 mW/m**2 on the seafloor away from the bank and decreases upslope to ~35 mW/m**2. We examine three mechanisms as potential causes for the curved geothermal profiles. They are: (1) a recent increase in sedimentation rate, (2) influx of seawater into shallow sediments, and (3) temporal fluctuation of the bottom water temperature (BWT). Our analysis shows that the first mechanism is negligible. The second mechanism may explain the data from Sites 1004 and 1005. The temperature profile of Site 1006 is most easily explained by the third mechanism. We reconstruct the history of BWT at this site by solving the inverse heat conduction problem. The inversion result indicates gradual warming throughout this century by ~1°C and is agreeable to other hydrographic and climatic data from the western subtropic Atlantic. However, data from Sites 1003 and 1007 do not seem to show such trends. Therefore, none of the three mechanisms tested here explain the observations from all the drill sites. As for the lateral variation of the background heat flow along the drill transect, we believe that much of it is caused by the thermal effect of the topographic variation. We model this effect by obtaining a two-dimensional analytical solution. The model suggests that the background heat flow of this area is ~43 mW/m**2, a value similar to the background heat flow determined for the Gulf of Mexico in the opposite side of the Florida carbonate platform.

Mineral and chemical composition of hydrothermally altered rocks and sediments from the Middle Valley, ODP Leg 139 data

The Middle Valley segment at the northern end of the Juan de Fuca Ridge is a deep extensional rift blanketed with 200-500 m of Pleistocene turbiditic sediment. Sites 857 and 858 were drilled during Ocean Drilling Program Leg 139 to determine whether these two sites were hydrologically linked end members of an active hydrothermal circulation system. Site 858 was placed in an area of active hydrothermal discharge with fluids up to 270°C venting through anhydrite-bearing mounds on top of altered sediment. The shallow basement of fine-grained basalt that underlies the vents at Site 858 is interpreted as a seamount that was subsequently buried by turbidites. Site 857 was placed 1.6 km south of the Site 858 vents in a zone of high heat flow and numerous seismically imaged ridge-parallel faults. Drilling at Site 857 encountered sediments that are increasingly altered with depth and that overlie a series of mafic sills at depths of 460-940 m below sea floor. Sill margins and adjacent baked sediment are highly altered to magnesian chlorite and crosscut with veins filled with quartz, chlorite, sulfides, epidote, and wairakite. The sill interiors vary from slightly altered, with unaltered plagioclase and clinopyroxene in a mesostasis replaced by chlorite, to local zones of intense alteration and brecciation. In these latter zones, the sill interiors are pervasively replaced by chlorite, epidote, quartz, pyrite, titanite, and rare actinolite. The most complete replacement is associated with brecciated horizons with low recovery and slickensides on fracture surfaces, which we interpret as intersections between faults and the sills. Geochemically, the alteration of the sill complex is reflected in significant whole-rock depletions in Ca, Sr, and Na with corresponding enrichments in Mg, Al, and most metals. The latter results from the formation of conspicuous sulfide poikiloblasts. In contrast, metamorphism of the Site 858 seamount includes incomplete albitization of plagioclase phenocrysts and replacement of sparse mafic phenocrysts. Much of the basement alteration at Site 858 is confined to crosscutting veins except for a highly altered and veined horizon at the contact between basaltic basement and the overlying sediment. The sill complex at Site 857 is more highly depleted in 18O (d18O = 2.4 per mil - 4.7 per mil) and more pervasively replaced by secondary minerals relative to the extrusives at Site 858 (d18O = 4.5 per mil - 5.5 per mil). There is no evidence of significant albitization of the plagioclase at Site 857, suggesting high Ca/Na in the pore fluids. Fluid-inclusion data from hydrothermal minerals in altered mafic rocks and veins at Sites 857 and 858 show a consistency of homogenization temperatures, varying from 245 to 270°C, which is within the range of temperatures observed for the fluids venting at Site 858. The consistency of the fluid inclusion temperatures, the lack of albitization within the Site 857 sills, and the apparently low water/rock ratio collectively suggest that the sill complex at Site 857 is in thermal equilibrium and being altered by a highly evolved Ca-rich fluid similar to the fluids now venting at Site 858. The alteration evident in these two deep crustal drillsites is a result of the ongoing hydrothermal circulation and is consistent with downhole logging results, instrumented borehole results, and hydrothermal fluid chemistry. The pervasive alteration of the laterally extensive sill-sediment complex at Site 857 determines the chemistry of the fluids that are venting at Site 858. The limited alteration of the Site 858 lavas suggests that this basement edifice acts as a penetrator or ventilator for the regional hydrothermal reservoir with much of the flow focussed at the highly altered and veined sediment-basalt contact.

(Table T1) Permeability, electrical resistivity and density of ODP Hole 193-1188A and Site 193-1189 minicores, PACMANUS field

Magmatic fluids, heat fluxes, and fluid/rock interactions associated with hydrothermal systems along spreading centers and convergent margins have a significant impact on the genesis of major sulfide deposits and biological communities. Circulation of hydrothermal fluids is one of the most fundamental processes associated with localized mineralization and is controlled by inherent porous and permeable properties of the ocean crust. Heat from magmatic intrusions drives circulation of seawater through permeable portions of the oceanic crust and upper mantle, discharging at the seafloor as both focused high-temperature (250°-400°C) fluids and diffuse lower-temperature (<250°C) fluids. This complex interaction between the circulating hydrothermal fluids and the oceanic basement greatly influences the physical properties and the composition of the crust (Thompson, 1983; Jacobson, 1992, doi:10.1029/91RG02811; Johnson and Semyan, 1994, doi:10.1029/93JB00717). During Ocean Drilling Program (ODP) Leg 193, 13 holes were drilled in the PACMANUS hydrothermal system (Binns, Barriga, Miller, et al., 2002, doi:10.2973/odp.proc.ir.193.2002). The hydrothermal system consists of isolated hydrothermal deposits lined along the main crest of the Pual Ridge, a 500- to 700-m-high felsic neovolcanic ridge in the eastern Manus Basin. The principal drilling targets were the Snowcap (Site 1188) and Roman Ruins (Site 1189) active hydrothermal fields. Samples from these two sites were used for a series of permeability, electrical resistivity, and X-ray computed tomography measurements.

(Table 1) Stable carbon and oxygen isotope ratios of benthic foraminifera of ODP Hole 184-1148A

Changes in intermediate and deep ocean circulation likely played a significant role in global carbon cycling and meridional heat/moisture transport during the middle Miocene climate transition (~14 Ma). High-resolution middle Miocene (16-13 Ma) benthic foraminifer stable isotope records from the South China Sea reveal a reorganization of regional bottom waters, which preceded the globally recognized middle Miocene ~1 per mil d18O increase (13.8 Ma) by 100,000 years. An observed reversal of the benthic foraminifera d13C gradient between ODP Sites 1146 (2092 m) and 1148 (3294 m; 13.9-13.5 Ma) is interpreted to reflect an increase in the southward flux of low d13C deep (> 2000 m) Pacific Ocean waters (Flower and Kennett, 1993, doi:10.1029/93PA02196; Shevenell and Kennett, 2004). Large-scale changes in Pacific intermediate and deep ocean circulation, coupled with enhanced global carbon cycling at the end of the Monterey Carbon Isotope excursion, likely acted as internal feedbacks to the Earth's climate system. These feedbacks reduced the sensitivity of Antarctica to lower latitude-derived heat/moisture and facilitated the transition of the Earth's climate system to a new, relatively stable glacial state.