The raw material for synthesizing polyurethane is polyester polyol, typically produced by reacting organic polycarboxylic acids with organic polyols. The production process mainly consists of five steps: pre-production verification, feeding materials, heating, catalyst addition, and further heating.

I. Process Overview Polyester polyols are essential raw materials for the synthesis of polyurethane materials. As precursors to polyester resins, they primarily consist of hydroxyl-terminated polyester compounds, also known as polyester polyols. In industrial applications, low-molecular-weight polyester polyols—typically with molecular weights ranging from 1,000 to 3,000—are required for alkyd resins and unsaturated polyesters. Adipic acid-based polyester polyols exhibit broad applicability; they can be used to produce flexible and rigid polyurethane foams, rubber adhesives, synthetic leather, coatings, and other products. By optimizing process conditions and experimenting with feed ratios, we synthesized adipic acid-type polyester polyols with molecular weights around 1,500–2,000. These polyols, when processed via casting techniques, enable the production of polyurethane materials with a wide range of properties. Through extensive experimentation, we refined the conventional nitrogen‑stripping dehydration process. By precisely controlling a specific equilibrium temperature, enhancing vacuum levels, and adjusting the appropriate holding time, we shortened reaction times, reduced energy consumption, lowered production costs, and ensured consistent product quality.

II. Before production, confirm that the reactor is empty, the bottom valve is closed, and raw material reserves are sufficient; verify that the correct batch and quantity of materials to be charged are available. Ensure that all pipelines have been properly purged—pipeline purging is critical, as it can affect subsequent dehydration and may also impact product color. Close all feed line valves for each reactor; confirm reactor pressure and agitator operation are normal, and cooling water circulation is functioning properly. For the distillation column, close both the SV valve and manual valve on the bypass (vacuum line), close the vacuum control valve, and open the SV valve and manual valve on the distillation column’s main line. The drain line must be fully open for draining; ensure the drain line vent and vapor-phase equalization valve are open. Recovered alcohol has been discharged from the recovery tank, and all valves have been returned to their normal operating positions. Boiler temperature and pressure are normal, and the boiler system status is normal. Hot oil inlet and outlet valves are in proper working order.

3. After charging and purging are complete, open the drain line vent valve and the pressure equalization valve, then open the manway to further confirm that the reactor is empty. Begin feeding the low‑molecular‑weight alcohol by first recording the flow rate on its flow meter. Verify that the feed manual valves for all other reactors are closed, then open the manual valve for the designated reactor. Set up the program on the computer: reset the cumulative total to zero, enter the target amount of alcohol to be fed, and confirm that the program matches the order. Start the feeding sequence using the standalone computer program. First, check whether the pump in the filling area has started; in cold winter weather, the pump may fail during feeding, preventing further operation and disrupting production. Next, verify the instantaneous flow rate, which should be between 6–8 m²/h. Inspect the alcohol stream: if bubbles persist for more than 30 minutes, stop the program, discharge the material, recalculate, and restart feeding. Once feeding is complete, close the manual valve, record the data, and perform a final check to ensure the fed quantity matches the order; if insufficient, arrange for an additional charge. Monitor the liquid level in the reactor, weigh out 14 gOB‑1, and add it to the reactor primarily for whitening purposes—do not add excessive amounts, as this could affect the product’s shade. Start stirring and maintain agitation for approximately 30 minutes. Then add adipic acid according to the order specifications; before dosing, recount the adipic acid to confirm sufficient quantity. If insufficient, refrain from adding and contact your supervisor. To add adipic acid: place a funnel atop the reactor, use an overhead crane to position the bag over the funnel, ensuring minimal swinging during movement. Cut open the bottom of the bag with a knife. To guarantee product quality and compliance with shade specifications, do not use bags containing contaminated material. After adding the adipic acid, securely tie off the bag and double-check the measured dose to avoid under- or over‑dosing, both of which can impact downstream processes. Finally, seal the opening by tightly closing the inlet cover, tightening the bolts diagonally, and pressurizing the system to 0.015 MPa. Shut off the nitrogen supply and perform a leak test using soapy water. Inspect for leaks; if present, re‑tighten any loose connections. If sealing issues stem from improper gasket installation, replace the gasket promptly. Airtight integrity is critical; inadequate sealing can cause internal material to spray outward during heating, adversely affecting production.

4. Heating: Begin by using steam to raise the temperature. The lower steam temperature ensures that the adipic acid in the reactor melts completely without scorching on the heating tubes. Once the temperature reaches 128°C and water is visible in the drain sight glass—indicating that the material has melted—shut off the steam, verify that the thermal oil pressure is correct (typically around 0.15 MPa), then start the thermal oil circulation. Set the automatic temperature program to 128°C, with the upper limit determined by the specific reaction conditions. After maintaining this temperature for one hour, proceed to the next program step. Dehydration Heating: Initiate the dehydration heating program. During this phase, closely monitor the distillation column’s overhead temperature, ensuring it remains within the specified upper and lower limits; exceeding these limits can cause alcohol to carry over into the product stream, adversely affecting the acid value. Perform frequent inspections during dehydration, observing the liquid flow in the sight glass and the pressure indicated on the process diagram. Peak pressure should generally not exceed 0.001 MPa. If pressure fluctuates during heating, a blocked pipeline may be the cause; clear the line with steam. When temperatures become elevated, consider venting nitrogen at the reactor top as a temporary measure. Additionally, appropriately reducing the heating rate can help mitigate flooding at the column top.

5. After the acid value of the added catalyst meets specifications, add an appropriate amount according to the material grade. Adding too much or too little will affect the reaction and result in a nonconforming product. During this period, drain the recovered alcohol produced in the previous reactor; evacuating the system takes about ten minutes. Start the vacuum pump: close the bottom valve of the pump’s buffer tank and the vent valve above it, then turn on the circulating water—do not set it too high; typically open it to about one-third. Start the vacuum pump, check that the current is normal, and verify that the pressure in the buffer tank is within acceptable limits.

6. After the cooling sample passes inspection, begin cooling: close the hot oil outlet valve, open the cold oil outlet valve, open the cold oil inlet valve, start the cold oil pump, adjust the outlet temperature to 40°C, and monitor the pressure, which should be around 0.2 MPa. Check whether the hot oil temperatures in each reactor have changed. After cooling, shut down the system.

VII. Conclusion

1. Extending the holding time, i.e., the high-temperature reaction time, can reduce the total amount of extractables, and the overall reaction time is also correspondingly extended.

2. Raising the temperature is beneficial for the alcohol–acid polycondensation reaction; it can reduce the acid value before vacuuming, shorten the vacuuming time, increase the amount of water removed, and decrease the amount of material extracted. However, the reaction temperature should not be too high, as excessively high temperatures can easily lead to decomposition reactions and the loss of carboxyl groups.

3. The amount of acid and the amount of ethylene glycol both affect the acid value before and after vacuum removal. In a simple alcohol–acid polycondensation reaction, appropriately adding organic acid anhydrides is beneficial for controlling the degree of branching; it increases water yield and shortens the overall reaction time. Consequently, incorporating organic acid anhydrides so that the reaction proceeds at a specific degree of branching influences the vacuum time, the amount of water removed, and the temperature of the discharged water.

4. Influence of Hydroxyl Value The hydroxyl value significantly affects polyester synthesis and polyurethane elastomers. At higher hydroxyl values, the pre‑vacuum acid value is lower and the amount of material evacuated is greater; the actual hydroxyl value of the polyester increases as the theoretical calculated value rises. Both the evacuation time and the polyester viscosity are markedly influenced by the hydroxyl value when the branching ratio is 1/1; increasing the hydroxyl value leads to a significant decrease in both evacuation time and polyester viscosity. When the branching ratio is 1/2, raising the hydroxyl value also results in a lower water‑evaporation temperature.