Distillation, heat recovery meet RVP specification for stabilized crudeBefore being exported from a producing field, crude oil flowing from the wellhead must be stabilized to comply with the Reid vapor pressure (RVP) production specification. Prosernat and IFP say they have developed an alternative solution for crude oil stabilization, called Direc (distillation with heat recovery). This is a compact equipment assembly designed to achieve the final crude RVP specification with lower capital and operation expenditure requirements than conventional equipment.

Oil stabilization processes usually involve an oil-gas fractionation step to adjust the bubble point of the oil produced so as to make it thermodynamically stable under storage conditions. The specification a stabilized crude must meet is a true vapor pressure (TVP) or a Reid vapor pressure (RVP), in psi. Depending on local conditions, the RVP commonly ranges from 8 psi to 12 psi. For a given RVP and crude composition, the maximum theoretical efficiency of a stabilization process can be achieved by selectively removing the lightest compounds from the crude in order of decreasing volatility - nitrogen, methane, ethane, carbon dioxide, hydrogen sulfide, propane and so on. The performance of a given process can be compared to this maximum efficiency (the worse the separation, the worse the yield of the process).Limited oil recovery

Crude can be stabilized using various process schemes, depending on the investments and the oil recovery efficiency required. The simplest involves heating and flashing down the oil to successive pressure levels. This process arrangement is inexpensive and reliable but does not allow high oil recovery efficiencies, especially for light crudes, since the gas removed in each stage contains medium-volatility constituents. Moreover, it is only able to treat slightly sour crudes. These two limitations can be explained by the poor separation efficiency of the Joule-Thomson expansion.The gas produced in each stage can be used as a fuel gas or sent to the flare (early production facility developments). Recompression of the gas produced on each expansion with partial condensation by cooling can significantly improve oil recovery efficiency and produce the associated gas at high pressure for easy recovery.This configuration, however, increases the investment and operating costs (compressors are necessary), and a cooling source is also required to partially liquefy the compressed gas. The low temperature is another important parameter of the process (the more complete the liquefaction, the higher the quality of separation). However, even with gas recompression, this process scheme is ineffective for treating significantly sour crudes.Another common way of stabilizing a crude is to use a stripper at about 10 bar after high and medium pressure drums. This arrangement leads to a lower machinery investment (only one compressor is needed) and a lower utilities consumption (less compression and heating power). At the same time, the column improves the separation efficiency of the whole process, with two consequences:

The quantity of oil recovered is increased Acid crudes can be treated (the separation is more selective).

The main drawback of this scheme is the cost and size of the column.System principleTo reduce the duty of the conventional stabilization column and make it more compact, IFP conceived and patented a new technology, called Direc, in 1991. This is a compact equipment assembly, with a vertical counter-flow shell and tube heat exchanger flanged to the top of a kettle reboiler. The basic principle is as follows:

1. Unstabilized crude oil, at ambient temperature and approximately 145 psi (10 bar), flows down the inside of heated tubes to the reboiler bottom. The reboiler bottom is subjected to external heat, raising the crude oil temperature to approximately 390°F, depending on the crude oil composition. Hot stabilized oil exits the reboiler by flowing upward on the shell side of the exchanger and heats the incoming crude. With this heating, the light hydrocarbons are vaporized and rise to flow counter-current against the downward-flowing unstabilized crude oil.

2. A heat exchange stripping action occurs along the entire length of the vertical tubes, as the downward-flowing, cooler, unstabilized crude oil absorbs the heat from the upflowing, heated vapors. This heat exchange process also condenses the heavier fractions of the hydrocarbon vapor given off, similar to the process in the rectification section of a distillation column. A multi-staged and multi-reboiled distillation is then achieved within the heat exchange column.

Validation

Extensive studies on the system have been carried out since 1991 in order to validate the concept and define the parameters needed for the design of the industrial unit. A 3-meter high pilot unit was built and operated at IFP with mixtures containing crude from the North Sea, propane and butane.Among other parameters, the height equivalent to one theoretical plate (HETP) and heat exchange coefficients were determined. Simulations of the experimental results were performed with PRO/II software and served to finalize modeling of the unit. The design method for the system is now validated and available for industrial use.It appears that a distillation column and Direc operated at the same pressure are equivalent in terms of oil recovery efficiency (oil recovery=75%). Direc technology however, allows significant reduction of the heating duty. A comparison was carried out between the following two process schemes:

Classical stabilization process using three successive flashes (operating at 435 psi, 100 psi, and 22 psi) with recompression of the associated gas

Similar scheme where the low-pressure stage is eliminated and the medium-pressure stage is replaced by a Direc unit.

The results show that for a similar oil recovery efficiency for the two schemes (63%), the compression power and the heating duty can be decreased by 60% and 50% respectively when the system is used when the IFP-Prosernat system is used.EconomicsIn a conventional flash cascade stabilization, the IFP-Prosernat system replaces the low-pressure and medium-pressure drums and the compression stages needed to compress the associated gas from atmospheric to medium pressure. This arrangement leads to savings in capital investment and operating costs, as shown by an economic study carried out for the following three process schemes:

Scheme 1: Three successive flashes operating at 435 psi, 97 psi, and 22 psi. The oil is heated before the last drum in order to adjust the RVP of the final product.

Scheme 2: Same as scheme 1, but the gas produced in low-pressure and medium-pressure stages is compressed, partially condensed, and recycled to the upper-pressure drum. The gas is produced at high pressure (435 psi).

Scheme 3: Same as scheme 2, but low-pressure and medium-pressure stages are replaced by a Direc unit operating at 97 psi. This study was done with a North Sea crude oil (GOR=549), an inlet temperature of 140°F and a flow rate of 20,000 b/d. The RVP specification on the product was taken equal to 12 psi.

Following estimates of the various investments involved (ISBL, engineering, offsites), it appears that the IFP-Prosernat system allows nearly 40% savings in capital investment compared to scheme 2, for an equivalent oil recovery in the two cases (±0.5% for Direc). Operating costs were also calculated and found to be 40% less for Direc than for scheme 2. Compared to scheme 1, where no gas compression is used, Direc can increase the oil recovery by 6%, leading to a payback on capital investment within 1.5 years, against eight 8 years for scheme 2.When a conventional stripper is used, the IFP-Prosernat system replaces the column and preheater and decreases by 50% the power of the cooler on the stabilized crude. An estimate was performed this year by an engineering firm to compare the two options. This study was carried out with a 52° API crude, an inlet flow rate of 14,400 b/d, and an RVP specification of 10 psi. Both schemes were designed to give the same oil recovery and the stripper was equipped with a preheater to recover the heat of the product. The FOB equipment cost was found to be more than 20% less for the IFP-Prosernat system and the heating duty was reduced by 50%