Jan-2012

Using cold boiler feed water for energy recovery

Feeding membrane deaerated cold boiler feed water to appropriate units will enable waste heat to replace substantial steam duty in a refinery

AliS¸an DoGˇan
Turkish Petroleum Refineries Corporation

Article Summary

Steam at different pressure levels is used for many purposes in refineries, including power production (steam turbines), heating, steam tracing, stripping, atomising and deaeration. Steam is produced from fired utility boilers, cogeneration units (gas turbine HRSGs), furnace waste heat boilers, product rundowns, column refluxes and so on by adding heat to supplied boiler feed water. Boiler feed water is conventionally supplied by deaerators, where steam is used to heat water to saturation conditions at a certain pressure to strip dissolved oxygen, with the aim of preventing corrosion in steam production units. In this article, the benefits of providing cold boiler feed water from membrane deaerators to steam-producing or water-heating waste heat streams will be explained, with some typical examples for an oil refinery.

Membrane deaerators
Membrane deaerator technology is used for degassing liquids around the world. They are widely used for removing oxygen from water, as well as for carbon dioxide removal. They have displaced the vacuum tower, forced draft deaerator and oxygen scavengers for over 20 years. Membrane contactors are used extensively for the deaeration of liquids in the microelectronics, pharmaceutical, power (boiler feed water), food and beverage, industrial, photographic, ink and analytical markets. A trial study of the scope of a heat recovery project has been carried out within a refinery, where the aim of the project was to decide whether this technology should be used extensively for the deaeration of boiler feed water, together with existing conventional steam deaerators.

Membrane deaerator systems consist of membrane contactors combined in series, parallel or both, designed according to the water flow, pressure drop limitations and oxygen concentration needed at the outlet of the system. The contactors work on the basic principle of letting only gas (oxygen) molecules pass to the other side of membranes, where a vacuum is applied via a vacuum pump and sweep gas (high-purity nitrogen) is supplied. Oxygen molecules in the water side have a high partial pressure compared to the vacuum side, so they tend to pass through the hydrophobic membranes. Here, high-purity sweep gas is introduced to the vacuum side to prevent oxygen from concentrating in the vacuum side, which sustains the mass transfer efficiency (partial pressure difference).

The maximum oxygen concentration requirement for boiler feed water in this scenario is 7 ppb. However, 1 ppb was targeted when selecting the configuration for this project (to be on the safe side). The purity of the sweep gas nitrogen has critical importance when selecting the best configuration because it obviously has to contain a minimum amount of oxygen. In this case, the refinery has a high-purity nitrogen ring (99.99 vol%) which is mainly consumed by reformers and other processes that need high-purity nitrogen. Nitrogen consumption by the membranes is very low (~10 Nm3/h, of course depending on water flow and oxygen concentration targeted) when compared to process needs (in the range of several thousand Nm3/h) and therefore does not have a negative effect on the refinery nitrogen balance.

Nitrogen is purchased by the refinery and delivered by trucks, which periodically supply high-purity nitrogen to the main tanks in the refinery nitrogen system (ring). In a refinery without an available nitrogen ring, a nitrogen tank, sized according to the capacity of the system, will be needed. This can be periodically filled with the nitrogen provided by trucks. The other needs for the system are a small amount of electricity and cooling water for the vacuum pump system, which are easily accessible in the refinery configuration.

Water temperature at the membranes is another point of importance in the system’s configuration. The level of oxygen dissolved in water depends on the temperature of the water. The solubility of gases decreases with increasing temperature. Usually, at atmospheric temperatures, ~6 ppm (6000 ppb) of oxygen is dissolved in water. Depending on the water temperature, solubility can be between 5 and 8 ppm. Therefore, depending on the water temperature, the membrane system load changes. In this project’s scenario, demineralised water is heated by condensate drum flash vapour and the temperature to the membrane deaerator system will change between 30 and 50°C, depending on the ambient temperature. A high temperature is good for efficient oxygen removal; however, another important point is that temperatures above 60°C are not desirable for the membranes, as they may be damaged at such temperatures, depending on the operating pressure. The demineralised water system pressure in this case is 6–8 kg/cm2g, which can easily be decreased to 5 kg/cm2g or less with the appropriate valves.   

The advantages of the membrane deaerator system, including low investment and operating costs and relatively small size, make it an appropriate selection for the cold boiler feed water heat recovery project. Membrane deaerator systems can be purchased from various OEM firms.
 
Case 1
Energy recovery from a hydrocracker hydrogen production unit
Waste heat from the hydrocracker hydrogen steam reformer furnace is one of the main steam producers from a furnace waste heat boiler. Approximately 90–120 t/h of 38 kg/cm2g steam is produced from furnace waste heat, depending on the unit’s working capacity. Boiler feed water is supplied at 55 kg/cm2g and 125°C, and this is pumped from the deaerator at the utility production unit. Boiler feed water is first heated by the shift converter outlet raw hydrogen stream, which has impurities such as water vapour (over 40 wt%), CO, CO2, CH4 and N2. After heating boiler feed water, this hydrogen stream is than cooled down further by air and cooling water to get rid of water and dissolved gases. Condensate is taken from hot and cold condensate drums, then the steam is stripped of its dissolved gases and sent back to the utility production unit. The boiler feed water heating scheme prior to the hydrogen reformer furnace steam generator inlet is shown in Figure 1.

For the most part, the latent heat of condensation of water vapour in the raw hydrogen stream is given to the boiler feed water to boost steam production. A portion of condensate recovered from this used heat is taken from the hot condensate drum, while the rest of the heat is wasted to air and cooling water. In this layout, 7.4 Gcal/h is recovered by heating boiler feed water, while 17.4 Gcal/h is wasted.

In the current project, cold boiler feed water (30–50°C) will be supplied from the membrane deaerators to this unit through existing feed water pumps. The driving force for heat transfer will be increased in exchanger E-203 (see Figure 1) and more latent heat of condensation will be recovered. In this way, steam used in the deaerators is saved and water is heated by waste heat.