Diethylene Glycol (DEG) Bulk Exporter: Unsaturated Polyester & Gas Dehydration

Diethylene glycol, CAS 111-46-6, is a linear aliphatic diol containing an ether bridge, supplied in bulk marine parcels and ISO tank containers for unsaturated polyester synthesis and gas dehydration. Export-grade material is controlled to a purity of 99.5 wt% minimum, water 0.05 wt% maximum, and Pt-Co color 10 maximum under ASTM D1209. The compound has a molar mass of 106.12 g/mol, a boiling range of 244–245 °C at 101.3 kPa, and a freezing point near -10.45 °C. At 20 °C, the dynamic viscosity is approximately 35.7 mPa·s when measured by rotational viscometry. The structural hydroxyl functionality is two, and the theoretical hydroxyl value is approximately 1057 mg KOH/g; that value is used in resin formulation calculations to balance stoichiometry against dibasic acid anhydrides. These properties define the logistics envelope for bulk loading: cargo tanks require heating coils because the liquid freezes below -10 °C in cold ports, and the hygroscopicity of DEG requires closed-loop transfer with dry nitrogen padding. The same ether oxygen that imparts chain flexibility in unsaturated polyester also raises the equilibrium water content in lean glycol dehydration loops and must be managed through regeneration temperature limits.

When High-Humidity Transfer Alters Export-Grade DEG

Bulk transfer of DEG through ship manifolds, shore tanks, and road tankers is a moisture-sensitive operation. At ambient relative humidity above 60%, an open hatch or unblanketed holding tank can shift the water content of technical-grade DEG toward 0.10 wt% within a single shift, invalidating the esterification feedstock specification. Transfer systems therefore use nitrogen pads with a dew point below -40 °C and positive pressure of 3–5 kPa. Loading pumps are sized for a flow velocity below 2.5 m/s in carbon steel piping to avoid static charge accumulation, and in-line cartridge filters with 5 µm retention are placed ahead of the shore tank to remove rust and particulates.

Parameter	Test method	Bulk export limit
DEG purity	ASTM E2409	99.5 wt% min
Water	ASTM E203	0.05 wt% max
Pt-Co color	ASTM D1209	10 max
Density at 20 °C	ASTM D4052	1.115–1.120 g/cm³
Acidity as acetic acid	ASTM D1613	0.005 wt% max

Certificates of analysis for bulk export lots use the matrix above. Water content is the most unstable parameter during transit; a nitrogen-padded tank is required for voyages longer than 15 days through humid equatorial waters. Published data for precise moisture uptake rates under marine transit conditions is limited, so exporters rely on sealed sampling ports and avoid line clearing with wet air.

In unsaturated polyester resin synthesis, DEG is charged with maleic anhydride and phthalic anhydride into a glass-lined or 316L stainless-steel reactor fitted with a partial condenser, total condenser, and decanter. The molar feed ratio is typically set so that maleic anhydride represents 0.30–0.45 mol per mole of total dibasic acid, leaving the balance as phthalic anhydride or isophthalic acid. The esterification is operated at 180–220 °C under a nitrogen sparge; the overhead partial condenser is controlled to return glycol while passing water of reaction to the total condenser and decanter. Xylene is used as azeotrope carrier at 2–5 wt% of the reactor charge when atmospheric distillation is used; vacuum dehydration at 10–30 kPa absolute is an alternative for low-color resins. Acid number is driven to 20–35 mg KOH/g as determined by ISO 2114:2000, and the melt is cut with styrene monomer to 35–45 wt%. Hydroquinone or tert-butylcatechol is added at 50–150 ppm; the resin is cooled below 45 °C through a heat exchanger before drumming.

The ether bridge in DEG changes the mechanical response of the cured network. When post-cured at 80 °C for 2 h, DEG-rich castings are evaluated by ASTM D638-14 for tensile strength, ASTM D790-17 for flexural modulus, and ASTM D648-18 for heat deflection temperature at 1.82 MPa. The heat deflection temperature is generally lower than that of a propylene glycol control at equivalent styrene content because the ether oxygen reduces the glass transition temperature of the cured polyester. Water uptake after immersion is measured by ISO 62:2008; end users compare this value against the laminate specification because DEG-modified resins may show higher equilibrium water absorption than aromatic glycol resins. Barcol hardness is measured according to ASTM D2583.

Cure response is measured on the production floor with ASTM D2471. A typical room-temperature cure uses methyl ethyl ketone peroxide at 1.0–2.0 phr and cobalt naphthenate at 0.2–0.5 phr of a 6% cobalt solution. The gel time and exotherm peak are recorded; thick sections may exceed 180 °C peak exotherm if cobalt and methyl ethyl ketone peroxide are dosed at the upper limit. On production lines, resin viscosity is maintained between 250–500 mPa·s at 23 °C for spray-up and filament winding; DEG-rich formulations at the lower end of this range may require fumed silica at 0.5–1.5 wt% to prevent drainage on vertical laminate schedules.

Manufacturing lines using DEG-rich resin observe two recurring bottlenecks. The first is phthalic anhydride sublimation in the overhead vapor line if the partial condenser skin temperature falls too low; the second is batch-to-batch acid number drift when vacuum control exceeds ±2 kPa during final dehydration. These are controlled by maintaining partial condenser skin temperature above 130 °C and by automating acid number sampling. Published data for specific DEG-rich unsaturated polyester mechanical properties are limited, but the degradation chemistry of the ether linkage is well documented in polymer science literature.

What Limits the Use of DEG in Lean Glycol Dehydration Loops?

Gas dehydration with DEG uses a closed loop comprising a high-pressure contactor, rich-glycol flash drum, solids filter, activated carbon filter, lean/rich heat exchanger, and regenerator. Wet gas enters the bottom of a vertical absorber and rises counter-current to lean DEG distributed over 6–10 bubble-cap trays or structured packing. The rich glycol at the bottom leaves at 30–55 °C and is flashed at 0.3–0.6 MPa to reject dissolved methane and natural gas liquids. The flash gas is routed to fuel gas; the liquid is filtered to 5 µm before preheating and regeneration.

The thermal ceiling is the central restriction. DEG degradation becomes measurable near 164 °C, so the reboiler bulk temperature is limited to 160–165 °C. Without stripping gas, this produces a lean DEG concentration of 96–98 wt%. Lean glycol cannot be regenerated above this without risking organic acid formation; the acid products reduce pH and can corrode the regenerator overhead and reboiler tubes. Triethylene glycol, by contrast, can be regenerated at 200–204 °C to 99.0–99.5 wt%, which enables lower outlet water dew points.

Operating parameter	DEG-based system	TEG-based system
Atmospheric boiling point	244–245 °C	285–288 °C
Maximum reboiler bulk temperature	160–165 °C	200–204 °C
Regenerated lean concentration without stripping gas	96–98 wt%	99.0–99.5 wt%
Dew-point depression at 3.5–10.0 MPa	20–30 °C	40–55 °C

Because dew-point depression is controlled by equilibrium water partial pressure over the lean glycol, the lower lean concentration restricts DEG units. At absorber pressures of 3.5–10.0 MPa and inlet gas temperatures of 25–35 °C, DEG systems typically achieve a dew-point depression of 20–30 °C. The higher vapor pressure of DEG also increases solvent losses from the absorber overhead; an overhead demister or wash tray is required to keep glycol loss within the emission limit specified by the plant air permit. Circulation rate is normally set between 15–25 L of glycol per kg of water removed for trayed contactors; higher rates do not materially change the dew point and only increase reboiler fuel gas demand. The water content of the inlet gas should be measured with an online chilled-mirror hygrometer or computed from ISO 18453; this value plus the required outlet water dew point determines the necessary lean DEG concentration and circulation rate.

At regenerator temperatures above 165 °C, trace oxygen accelerates DEG degradation to formic and acetic acids, which can lower the regenerator overhead pH below 4.5 and initiate carbon steel corrosion. Rich DEG filters are specified at 5 µm absolute to remove iron sulfide and salt particles that stabilize foam in the regenerator; activated carbon filters downstream remove dissolved hydrocarbons and degradation products. In unsaturated polyester service, DEG storage should not be combined with amine-based additives, because alkaline pH accelerates color development and reduces inhibited resin stability. Glycol contactors that are shut down for more than 72 h should be blanketed with fuel gas at a dew point below -40 °C to prevent wet glycol stagnation and under-deposit pitting.