3 Jan, 2017

Non-Refluxed Split-Feed Condensate Stabilizer Column-Part 4

One option for stabilizer configuration is a split-feed design where a portion of the feed is pre-heated by heat exchange with the bottoms product. The remainder of the feed is fed to the top tray, similar to a standard “cold feed” stabilizer. Figure 1 presents an example of split-feed design. Split-feed provides heat recovery by pre-cooling the stabilized condensate upstream of the product cooler (not shown on this schematic), and reduces the required column reboiler duty (see also page 352 of Reference [1]).

This tip is the follow up of  December 2016 tip of the month (TOTM) [2] which investigated the benefits of having a water-draw and its optimum location in a non-refluxed condensate stabilizer column. That tip simulated the performance of an operating condensate stabilizer column equipped with a side water-draw tray to remove liquid water and a stripping sweet gas stream to reduce the H2S content of a sour condensate. It determined and reported the possible locations of water-draw tray based water partial pressure along the column.

Figure 1. A non-refluxed stablizer column with feed-split, side water-draw and stripping gas

In order to lower the reboiler duty, a portion of the cold feed is heated by the hot stabilized condensate in a pre-heater. In addition, a free water knockout the upstream of the preheater is used to remove water and flashed gas from raw condensate. This tip will perform computer simulation to study the benefit of the upstream free water knockout drum and the pre-heater used in the split-feed design.

Specifically, this tip will determine if a liquid water draw tray is needed and how much reboiler duty is reduced by splitting the feed. This tip will consider stabilizing a sour condensate for Reid Vapor Pressure (RVP) specifications of 7, 7.5, and 8 psi (48, 52, 55 kPa).  The tip will study the impact of the top feed-split % on the reboiler duty, bottom temperature, stabilized condensate H2S content and True Vapor Pressure (TVP). The tip will present a summary of the computer simulation results and the key diagrams for the same plant.

Case Study:

Table 1 presents the raw condensate, containing about 21 mole % H2S, and stripping gas compositions, rates and conditions. Figure 1 presents a simplified process flow diagram equipped with the preheater, stripping gas and water-draw tray for stabilization of this raw condensate. The 3-phase separator upstream of the preheater removes essentially all excess/free water. The tip utilizes   a feed splitter to heat up a portion of feed in the preheater by the hot stabilized condensate.

Table 1. Feed and stripping gas compositions, rates and conditions

The stripping sweet gas stream lowers the H2S content of the stabilized oil and achieves the desired condensate vapor pressure at the specified reboiler temperature. For each case, the boil-up ratio was adjusted to meet the RVP specification. Table 2 presents the specified column variables.

Based on the data in Tables 1 and 2, and the process flow diagram of Figure 2, the tip performed simulation using the Soave-Redlich-Kwong (SRK) equation of state [3] in ProMax [4] software.

Table 2. Condensate stabilizer column specifications

Simulation Results:

The tip adjusted the boil-up ratio in the reboiler to meet the specified RVP of the stabilized condensate. The resulting boil-up ratio as a function of top feed-split is presented in Figure 2. This figure indicates that to achieve lower RVP at a specified boil-up temperature, higher boil-up ratio is required.

Table 3 also presents the summary of results for three specified RVPs of 7, 7.5, and 8 psi (48, 52, 55 kPa). This table indicates that decreasing the specified RVP decreases the stabilized condensate rate. Lower RVP requires higher vaporization of lighter compounds. The simulation results (not shown in this table) indicated that overhead vapor temperature of about 100 °F (37.8 °C) is practically independent of specified RVP and top-feed split. In addition, the results presented in Table 3 are independent of top-feed split which varied from 80 to 100 % with an increment of 2%.

Figure 2. Effect of top feed-split on the reboiler boil-up ratio for three RVP specifications

Table 3. Summary of simulation results for three specified RVP

Similarly Figure 3 presents the required reboiler duty as a function of top feed-split and the specified RVP. This figure indicates that decreasing RVP increases reboiler duty and the reboiler duty increases linearly with the top feed-split %. The installation of the preheater reduced the required reboiler duty by about 18% for the case of top feed-split of 80%.

Figure 4 presents the variation of stabilized condensate temperature as a function of top feed-split. Decreasing the top feed-split from 100 to 80%, the bottom product temperature decreases by about 1.8, 2.1, and 2.4 °F (1, 1.2, and 1.3 °C) for RVP of 7, 7.5, and 8 psi (48, 52, 55 kPa), respectively.

Figure 5 presents the variation of stabilized condensate true vapor pressure (TVP) as a function of the top feed-split. Decreasing the top feed-split, increases the stabilized condensate TVP by about 2.7, 3.0 and 3.3 psi (19, 21, and 23 kPa) for RVP of 7, 7.5, and 8 psi (48, 52, 55 kPa), respectively.

Figure 3. Effect of top feed-split on reboiler duty for three RVP Specifications

Figure 4. Effect of top feed-split on bottom temperature for three RVP specifications

Figure 5. Effect of top feed-split on stabilized condensate TVP for three RVP specifications

Figure 6. Effect of top feed-split on stabilized condensate H2S conetent for three RVP specifications

Figure 6 presents the variation of H2S content of the stabilized condensate as a function of top feed-split. This figure indicates that decreasing the top feed-split from 100 to 80%, the stabilized condensate H2S content increases by about 56, 80, and 104 ppm for RVP of 7, 7.5, and 8 psi (48, 52, 55 kPa), respectively.

This figure also indicates that for the specified RVP of 7.5 and 8 psi (52, 55 kPa), the H2S content exceeds the limit of 60 ppm at the top feed-split of 84 and 87 %, respectively. At the higher RVP, the bottom product temperature is cooler and there is not enough heat or stripping gas available to vaporize H2S.

It should be noted that since the raw feed condensate leaves the feed tank (three phase seprator) saturated with water but with no free water, simulation resuts showed no liquid water being trapped in the column. Therefore, no water was removed by the water-draw tray in all cases studied in this tip. This is contrary to the previous tip in which free water was allowed into the column and the traped water was removed by the water-draw tray.  

Conclusions:

This tip investigated the impact of the feed-split on the performance of a non-refluxed stabilization column by variating the top feed-split from 80 to 100% by an increment of 2 % for three RVP specifications. Based on the simulation results, this tip presents the following observations:

  1. Lower specified RVP, requires higher vaporization ratio at a given boil-up temperature. (Figure 2).
  2. Decreasing the top feed-split from 100% to 80 %, decreases the required reboiler duty by about 18% (Figure 3).
  3. the top feed-split from 100% to 80%, decreases the bottom product temperature (Figure 4).
  4. the top feed-split, increases the stabilized condensate TVP (Figure 5).
  5. Decreasing the top feed-split from 100% to 80%, increases the stabilized condensate H2S content (Figure 6).

To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Advanced Applications in Gas Processing),  and PF4 (Oil Production and Processing Facilities), courses.

PetroSkills offers consulting expertise on this subject and many others. For more information about these services, visit our website at http://petroskills.com/consulting, or email us at consulting@PetroSkills.com.

 

By: Dr. Mahmood Moshfeghian


Reference:

  1. Campbell, J.M., Gas Conditioning and Processing, Volume 2: The Equipment Modules, 9th Edition, 2nd Printing, Editors Hubbard, R. and Snow–McGregor, K., Campbell Petroleum Series, Norman, Oklahoma, 2014.
  2. Moshfeghian, M., December 2016 TOTM, PetroSkills | John M. Campbell, 2016.
  3. Soave, G., Chem. Eng. Sci. 27, 1197-1203, 1972.
  4. ProMax 4.0, Bryan Research and Engineering, Inc., Bryan, Texas, 2016.