(142b) Practical Considerations in Debottlenecking of Sour Water Stripper Units for Revamps

Introduction

With increasing demand of refined oil products and production of heavier crudes in some world markets, refineries are upgrading heavy crude processing facilities to increase throughput.  Along with this change is the need to similarly increase the capacity of supporting units, in particular sulfur treatment facilities such as Sulfur Recovery Units (SRUs), Tail Gas Treating Units (TGTUs), Amine Treating Units, and Sour Water Strippers (SWS).  These units are not necessarily revenue-earning for the refinery, so the capacity-increase costs, both explicit and implicit, would ideally be minimized to increase the margin earned per incremental barrel of processed crude in the plant expansion. One economically attractive option is to revamp these units for the larger throughput, not only to minimize costs, but also to reduce execution time.  This paper is intended to focus specifically on the Sour Water Stripper, which would be required not only at heavy oil processing facilities, but any facility that processes even a slightly sour crude slate.  Herein, potential modifications to the Sour Water Stripper are identified that would accommodate a crude throughput expansion.  The modifications are intended to be low-cost and easily implementable solutions relative to the construction of new units.  Furthermore, a case study proposing a 50% throughput increase to a sour water stripper facility is undertaken to provide specific quantitative results illustrating the benefits of the proposed modifications.

Sour Water Stripper Process Overview

Specifics of the Sour Water Stripper process will be provided in the full report.  The sour water stripper will typically treat the collected sour water streams from various locations in the refinery down to specifications of 10 ppmw NH3 and 30-50 ppmw H2S.  Because NH3 is much more soluble in water than is H2S, the Sour Water Stripper design parameters will be based on meeting the NH3 specification.  When increasing throughput of heavy crude components, produced sour water volumes will increase, and operational issues, including not meeting the stripped water specification may occur from a combination of the following factors: undersized reboiler, increase phenol presence in column, undersized exchangers, and excessive column pressure drop.  These factors, among others, are explored in more detail in the following sections.

Why is Debottlenecking Necessary

Three main reasons to undertake a debottlenecking study of an existing Sour Water Stripper when increasing capacity of upstream units are the following:

1. Minimize capital cost

2. Reduce turnaround time

3. Reduce complexity of construction

Both engineering firms and vendors allow for design margins in existing equipment, which may be exploited during unit capacity increases.  Furthermore, equipment spares are included in original units designs, and could be run in parallel with previously identified “operating” equipment.  Although these options reduce unit flexibility and may require more operator intervention to assure high run time, these are ways that site managers can avoid expending excessive capital.

Another advantage of running the unit up to its physical limits would be to reduce turnaround time associated with implementing modifications for debottlenecking.  Modifications that require removing the stripper column from service will serve as the “bottleneck” for a unit shutdown.  Rather, the time required to make modifications in the column and perform required testing will be longer than that needed for implementing other changes.

Lastly, the debottlenecking modifications will likely be easier to implement from a constructability standpoint.  Associated construction tasks such as installing new foundations and performing hot taps may be done during normal operation before the scheduled shutdown, and even may be done in stages with limited staff. Existing plant equipment is typically located in close proximity to other plant infrastructure to optimize process hydraulics and even allow for modular construction (when larger plant “building blocks” are constructed off-site so that only tie-ins to neighboring modules are required during site construction).

Equipment Methodology and Evaluation

Sour Water Feed Pump to Stripper Column Inlet

The site’s sour water will be accumulated in a dedicated vessel, from where it will be pumped to the Stripper inlet nozzle via the Feed Water / Treated Water Exchanger.

The increased water rate will have the following effects on this section:

-Pressure drop will increase – this can be illustrated by a system curve update

-Temperature will be cooler at Stripper column inlet, which will increase the column’s heat duty demand

Potential modifications include the following:

-May be able to use spare pumps in parallel with operating pumps

-Could recommend changing out impeller

-May need flow control valve change out for larger C_v

Stripper Column Internals and Hydraulics

The sour liquid in the column is contacted with stripping steam to release H2S and NH3 from aqueous phase to vapor phase.

The increased water rate will have the following effects on this section:

-Pressure drop will increase across the trays, potentially causing flooding

-Higher pressure at the column inlet will effect the inlet pump hydraulic calculation, as well as cause a marginal increase in column heating duty

Potential modifications include the following:

-Internal trays could be replaced with ones with greater open area

-New inlet nozzle may be installed at a lower position along column

-Reboiler duty or amount of stripping steam rate to column may be increased

-Provide caustic injection to assist in treating water for NH3

Stripper Column Heating

Heating duty to create stripping steam in the column is provided by a reboiler, typically using MP steam, but stripping steam may also be directly injected into the column either by itself or in tandem with a reboiler.

The increased water rate will have the following effects on this section:

-Vapor in reboiler inlet line may not self-vent with excessive liquid velocity, thus choking the exchanger

-Pressure drop from chimney tray through Reboiler and back to column could prevent natural circulation

-Increased heating duty is required to meet same treated water specification

Potential modifications include the following:

-Increase size of Reboiler downcomer and/or riser, or use two-phase internal distributor

-Install nozzles in column for steam injection

-Increase heating steam pressure, and thus, temperature to Reboiler

Stripper Column Refluxing and Overhead

The vapor from the Column’s stripping section rises into a cooling section, which will consist either of a water pumparound or reflux cooler and knockout drum.  The overhead sour gas is then routed to a downstream unit, likely an SRU or polishing Sour Water Stripper unit.

The increased water rate will have the following effects on this section:

-Greater cooling duty is required to prevent overloading downstream units

Potential modifications include the following:

-Flood reflux exchanger with cooling water to provide additional duty

-Recycle reflux water to plant’s accumulation vessel

Treated Water from Stripper Column to Discharge Point

You’ve met your treated water specifications!  Now the water is pumped to the discharge point via the Feed Water / Treated Water Exchanger and other trim cooling exchangers.

The increased water rate will have the following effects on this section:

-Pressure drop will increase – this can be illustrated by a system curve update

-The cooling duty will increase to achieve a set temperature at the discharge point

Potential modifications include the following:

-Set the Feed / Treated Exchanger’s bypass to manual, and open fully during operation to accommodate increased volume

-Use water spray in trim air cooler fans to increase cooling duty

-Implement pump changes previously mentioned for the sour water line

Case Study – 50% Throughput Increase SWS

A case study will be run to evaluate the effects of an increase of sour water to the Stripping Unit from 200 Sm3/h to 300 Sm3/h to accommodate upstream processes throughput increases.  The sour water feed to the Stripping Unit will be 0.3% H2S and 0.1% NH3, whereas the treated water specifications will be 10 ppmw and 30 ppmw, respectively.

The results of the case study will show the proposed modifications along with capital cost estimate and constructability and schedule implications.