Stalling of Condensate in Reboilers of Solvent Recovery Plants

Chemical and Pharmaceutical (bulk drug) plants generate a lot of effluent whose treatment is crucial. The process adopted for reducing the COD (Chemical Oxygen Demand) from effluent is called as Effluent Treatment Process (ETP). This effluent also contains some percent of solvent in it. This solvent needs to be recovered for reuse. Efficient solvent recovery helps to reduce the demand for new solvents / chemicals that can be reused in production, and help manufacturers meet regulatory requirements or process standards.

Typical solvents with boiling point <100°C found in the effluent stream are listed below.

Efficient condensate evacuation plays a critical role in ensuring the optimum steam consumption for a defined feed flow rate. As this process uses low pressure steam and a PID based temperature control valve, stalling of condensate affects equipment efficiency. Stall is the inability to evacuate condensate effectively from the heat exchanging equipment.

The conventional steam traps cannot remove the condensate unless the trap by-pass valves are kept open. The opening of trap bypass valve results in higher steam consumption. A properly sized Steam Operated Pump Trap (SOPT) ensures condensate removal during stall condition and helps to lower the steam consumption. This eliminates the need to open the trap bypass and leads to a steam savings of 6-10% at an equipment level. It results in enhanced productivity due to better steam to feed ratio.

Condensate load of the process equipment defines the trapping capacity and computation of stall load based on the operating parameters defines the required pumping capacity. Another important factor to consider during the selection of SOPT is the available installation head, which is a site specific parameter. A range of SOPT variants are available to give a reliable solution for stall in varying condensate flow rates, stall loads and installation heads.

Case Study : Methanol Recovery Column in SRU

Methanol inlet temperature: 30°C

Boiling point of Methanol: 64.7 °C

Steam supply pressure: 1.5 barg

Back pressure on steam trap: 0.5 barg

In sample isolation valves for steam applications, valve parts are exposed to high temperature differences. To avoid sample leakages through glands it is recommended to use metallic braided graphoil packing rings. These glands must be packed in welded bonnets with top pressure plate on glands. The pressure plate can be tightened using bolts to ensure leak tight glands once valves exposed to steam.

The Stall Chart is plotted on the basis of % heat load on the X- axis and temperature on the Y – axis. In the above stall chart the blue line indicates the rise in Methanol (secondary fluid) temperature from 30°C to 64.7°C. The red line joins the saturation temperature of steam at 1.5 barg (127°C) to the 64.7°C (boiling point of Methanol). The black line is the saturation temperature corresponding to the back pressure (0.5 barg) on the steam trap i.e. 111°C.

The point of intersection of the back pressure temperature line and the steam inlet saturation temperature line is “stall point”. When a line is plotted vertically downward from the stall point, the intersection with the secondary fluid temperature (39°C) enables determining the temperature of the secondary fluid at which stall will occur. The intersection of the vertical line with the heat load indicates the % stall.

Here 74% of the total steam load is held up in the process equipment that impacts the critical equipment efficiency parameter. Typically on the SRU column it is observed that plants install ball float steam traps or inverted bucket type steam traps. As explained earlier; these traps fail to remove condensate when the column is under stall condition. To address this waterlogging, trap bypass valve is opened so as to facilitate condensate evacuation.

The Steam Operated Pumping Trap offers a reliable solution for efficient condensate removal for this application.