Thermal Degradation and Corrosion of Amines for CO2 Capture

This report examines the thermal degradation and corrosion of various amine solvents as they apply to amine scrubbing for CO2 capture. Amines were placed in stainless steel cylinders and heated in convective ovens to simulate the stripping conditions inside a scrubbing unit. Samples were measured for remaining amine concentration, to test for degradation, and metals concentration, to estimate corrosion of the cylinder. The maximum stripping temperature of a particular compound, a measure of resistance to thermal degradation, strongly correlated with amine chain length. The linear amines studied had the following max temperatures: EDA (116 °C), PDA (124 °C), DAB (126 °C), BAE (130 °C), HMDA (140 °C), MEA (116 °C), MPA (129 °C), and DGA® (134 °C). The SHA/PZ blends had the following weighted max temperatures: AMP (143 °C), AMPD (135 °C), TRIS (130 °C), tBuAE (150 °C), PM (97 °C), and PE (129 °C). The linear amines follow initial first-order degradation curves, consistent with literature mechanisms. EDA, PDA, BAE, and AMP degraded significantly more slowly under acid conditions, suggesting that the degradation mechanisms do not incorporate CO2. Acid loaded DAB degraded at a similar rate to CO2-loaded conditions. MEA corroded 15 times faster than MPA; MAE corroded 3 times faster than EAE; DMAE-PZ corroded qualitatively faster than DMAP-PZ. These three pairs support the hypothesis that two-carbon chains corrode more than three-carbon chains. EDA corroded 40 to 80 times more than PDA according to older studies, seen in Figure 38, but more recent tests show similar corrosion rates where EDA is only 1.2 times faster (Figures 32 and 33). Corrosion and amine concentration correlate strongly; corrosion does not correlate strongly with temperature or CO2-loading. Corrosion and formate generation appear to correlate, supporting corrosion mechanisms proposed in literature.Chemical Engineerin

Star formation in Perseus. IV. Mass dependent evolution of dense cores

In our SCUBA survey of Perseus, we find that the fraction of protostellar cores increases towards higher masses and the most massive cores are all protostellar. In this paper we consider the possible explanations of this apparent mass dependence in the evolutionary status of these cores, and the implications for protostellar evolution and the mapping of the embedded core mass function (CMF) onto the stellar IMF. We consider the following potential causes: dust temperature; selection effects in the submillimetre and in the mid-infrared observations used for pre/protostellar classification; confusion and multiplicity; transient cores; and varying evolutionary timescales. We develop Core Mass Evolution Diagrams (CMEDs) to investigate how the mass evolution of individual cores maps onto the observed CMF. Two physical mechanisms -- short timescales for the evolution of massive cores, and continuing accumulation of mass onto protostellar cores -- best explain the relative excess of protostars in high mass cores and the rarity of massive starless cores. In addition, confusion both increases the likelihood that a protostar is identified within a core, and increases mass assigned to a core. The observed pre/protostellar mass distributions are consistent with faster evolution and a shorter lifetime for higher-mass prestellar cores. We rule out longer timescales for higher-mass prestellar cores. The differences in the prestellar and protostellar mass distributions imply that the prestellar CMF (and possibly the combined pre+protostellar CMF) should be steeper than the IMF. A steeper prestellar CMF can be reconciled with the observed similarity of the CMF and the IMF in some regions if a second opposing effect is present, such as the fragmentation of massive cores into multiple systems.Comment: 11 pages, 5 figures. Accepted by A&

Molecular line contamination in the SCUBA-2 450 {\mu}m and 850 {\mu}m continuum data

Observations of the dust emission using millimetre/submillimetre bolometer arrays can be contaminated by molecular line flux, such as flux from 12CO. As the brightest molecular line in the submillimetre, it is important to quantify the contribution of CO flux to the dust continuum bands. Conversion factors were used to convert molecular line integrated intensities to flux detected by bolometer arrays in mJy per beam. These factors were calculated for 12CO line integrated intensities to the SCUBA-2 850 {\mu}m and 450 {\mu}m bands. The conversion factors were then applied to HARP 12CO 3-2 maps of NGC 1333 in the Perseus complex and NGC 2071 and NGC 2024 in the Orion B molecular cloud complex to quantify the respective 12CO flux contribution to the 850 {\mu}m dust continuum emission. Sources with high molecular line contamination were analysed in further detail for molecular outflows and heating by nearby stars to determine the cause of the 12CO contribution. The majority of sources had a 12CO 3-2 flux contribution under 20 per cent. However, in regions of molecular outflows, the 12CO can dominate the source dust continuum (up to 79 per cent contamination) with 12CO fluxes reaching \sim 68 mJy per beam.Comment: Accepted 2012 April 19 for publication in MNRAS. 21 pages, 13 figures, 3 table

Star formation in Perseus: III. Outflows

We present a search for outflows towards 51 submillimetre cores in Perseus. With consistently derived outflow properties from a large homogeneous dataset within one molecular cloud we can investigate further the mass dependence and time evolution of protostellar mass loss. Of the 51 cores, 37 show broad linewings indicative of molecular outflows. In 13 cases, the linewings could be due to confusion with neighbouring flows but 9 of those sources also have near-infrared detections confirming their protostellar nature. The total fraction of protostars in our sample is 65%. All but four outflow detections are confirmed as protostellar by Spitzer IR detections and only one Spitzer source has no outflow, showing that outflow maps at this sensitivity are equally good at identifying protostars as Spitzer. Outflow momentum flux correlates both with source luminosity and with core mass but there is considerable scatter even within this one cloud despite the homogeneous dataset. We fail to confirm the result of Bontemps et al. (1996) that Class I sources show lower momentum fluxes on average than Class 0 sources, with a KS test showing a significant probability that the momentum fluxes for both Class 0s and Class Is are drawn from the same distribution. We find that outflow power may not show a simple decline between the Class 0 to Class I stages. Our sample includes low momentum flux, low-luminosity Class 0 sources, possibly at a very early evolutionary stage. If the only mass loss from the core were due to outflows, cores would last for 10^5-10^8 years, longer than current estimates of 1.5-4 x 10^5 years for the mean lifetime for the embedded phase. Additional mechanisms for removing mass from protostellar cores may be necessary.Comment: 26 pages, 21 figures. Version with full colour figures from http://www.astro.ex.ac.uk/people/hatchell/RecentPapers/hatchell07_outflows.pd

JCMT in the new era

This is the final version of the article. Available from OUP via the DOI in this record.Star-formation studies continue at the James Clerk Maxwell Telescope under new management, as Jennifer Hatchell and Derek Ward-Thompson report from an RAS meeting in March

What can the SEDs of first hydrostatic core candidates reveal about their nature?

The first hydrostatic core (FHSC) is the first stable object to form in simulations of star formation. This stage has yet to be observed definitively, although several candidate FHSCs have been reported. We have produced synthetic spectral energy distributions (SEDs) from 3D hydrodynamical simulations of pre-stellar cores undergoing gravitational collapse for a variety of initial conditions. Variations in the initial rotation rate, radius and mass lead to differences in the location of the SED peak and far-infrared flux. Secondly, we attempt to fit the SEDs of five FHSC candidates from the literature and five newly identified FHSC candidates located in the Serpens South molecular cloud with simulated SEDs. The most promising FHSC candidates are fitted by a limited number of model SEDs with consistent properties, which suggests the SED can be useful for placing constraints on the age and rotation rate of the source. The sources we consider most likely to be in FHSC phase are B1-bN, CB17-MMS, Aqu-MM1 and Serpens South candidate K242. We were unable to fit SerpS-MM22, Per-Bolo 58 and Chamaeleon-MMS1 with reasonable parameters, which indicates that they are likely to be more evolved.Comment: 26 pages, 28 figures. Accepted for publication in MNRA

CS Lines Profiles in Hot Cores

We present a theoretical study of CS line profiles in archetypal hot cores. We provide estimates of line fluxes from the CS(1-0) to the CS(15-14) transitions and present the temporal variation of these fluxes. We find that \textit{i)} the CS(1-0) transition is a better tracer of the Envelope of the hot core whereas the higher-J CS lines trace the ultra-compact core; \textit{ii)} the peak temperature of the CS transitions is a good indicator of the temperature inside the hot core; \textit{iii)} in the Envelope, the older the hot core the stronger the self-absorption of CS; \textit{iv)} the fractional abundance of CS is highest in the innermost parts of the ultra-compact core, confirming the CS molecule as one of the best tracers of very dense gas.Comment: 17 pages, 5 figures, 1 table, In press in Ap

The JCMT dense gas survey of the Perseus Molecular Cloud

We present the results of a large-scale survey of the very dense gas in the Perseus molecular cloud using HCO+ and HCN (J = 4 - 3) transitions. We have used this emission to trace the structure and kinematics of gas found in pre- and protostellar cores, as well as in outflows. We compare the HCO+/HCN data, highlighting regions where there is a marked discrepancy in the spectra of the two emission lines. We use the HCO+ to identify positively protostellar outflows and their driving sources, and present a statistical analysis of the outflow properties that we derive from this tracer. We find that the relations we calculate between the HCO+ outflow driving force and the Menv and Lbol of the driving source are comparable to those obtained from similar outflow analyses using 12CO, indicating that the two molecules give reliable estimates of outflow properties. We also compare the HCO+ and the HCN in the outflows, and find that the HCN traces only the most energetic outflows, the majority of which are driven by young Class 0 sources. We analyse the abundances of HCN and HCO+ in the particular case of the IRAS 2A outflows, and find that the HCN is much more enhanced than the HCO+ in the outflow lobes. We suggest that this is indicative of shock-enhancement of HCN along the length of the outflow; this process is not so evident for HCO+, which is largely confined to the outflow base.Comment: 25 pages, 14 figures, 9 table

Signatures of inflow motion in cores of massive star formation: Potential collapse candidates

Using the IRAM 30 m telescope, a mapping survey in optically thick and thin lines was performed towards 46 high mass star-forming regions. The sample includes UC H{\sc ii} precursors and UC H{\sc ii} regions. Seventeen sources are found to show "blue profiles", the expected signature of collapsing cores. The excess of sources with blue over red profiles ([$N_{\rm blue}$ -- $N_{\rm red}$]/$N_{\rm total}$) is 29% in the HCO$^+$ $J$=1--0 line, with a probability of 0.6% that this is caused by random fluctuations. UC H{\sc ii} regions show a higher excess (58%) than UC H{\sc ii} precursors (17%), indicating that material is still accreted after the onset of the UC H{\sc ii} phase. Similar differences in the excess of blue profiles as a function of evolutionary state are not observed in low mass star-forming regions. Thus, if confirmed for high mass star-forming sites, this would point at a fundamental difference between low- and high-mass star formation. Possible explanations are inadequate thermalization, stronger influence of outflows in massive early cores, larger gas reserves around massive stellar objects or different trigger mechanisms between low- and high- mass star formation