04 IV Process Flow and Control

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Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-1 
A Honeywell Company 
This information is confidential and must not be used for any purpose, duplicated, or disclosed to others without UOP's written permission. 
IV Process Flow and Control 
A. PROCESS FLOW 
The Guara One-Stage Membrane unit consists of the major components shown in 
the diagram below. 
 
 
Typical Flow Diagram One-Stage Separex™ Membrane System 
 
 
The One-stage SeparexTM Membrane System can be designed to process various 
flow rates of feed gas with a broad CO2 content range at a design minimum feed 
temperature and minimum feed operating pressure. For the multiple cases of 
operation, the Separex Membrane System will produce sales gas from the 
Membranes with a maximum sales CO2 content. Membrane permeate pressure 
control, the number of trains brought on line, and preheaters are adjusted as the 
membranes flow rate, condition, and composition requires or as the membranes 
age. The rated feed case membrane train configurations, rates, compositions and 
conditions are presented in Table 1 below. For Detailed Process Flow and Control 
Sketches, refer to UOP’s S660M Specification. 
 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-2 
A Honeywell Company 
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B. MEMBRANE PREHEATER CONTROLS 
Treated gas leaving the Dew Point Control Unit (DPCU) enters the Separex 
Membrane System and is sent to the Membrane Preheater(s) where the gas is 
heated if required. Feed to the membranes is warmed by passing a fraction of the 
feed gas through the membrane preheaters. The membrane feed temperature is 
controlled by TIC-2710-02 through split range control that first opens TV-2710-02A, 
the heating media valve (0 to 100% opening for a controller output of 0 to 50%), 
and then closes TV-2710-02B, the feed bypass valve (100 to 0% opening for a 
controller output of 50 to 100%), to send more flow through the preheat 
exchangers. Manual valves (8” D-2RRC-283-007, 8” D-2RRC-283-014 and 8” D-
2RRC-283-016 on the feed gas and 3” D1H-2RRC-283-033, 3” D1H-2RRC-283-
037 and 3” D1H-2RRC-283-041 on the heating media) are closed to stop the flow 
through the preheat exchangers. 
 
 
C. MEMBRANE TRAIN CONTROLS 
From the Membrane Preheaters the gas is fed to the Membranes. The 
Membranes separate the CO2 from the feed gas to the required sales gas 
specification and collect in the permeate piping as controlled via a pressure 
controller at low pressure. As the hydrocarbon components of the feed gas 
permeate much slower than CO2, these components are concentrated in the high-
pressure residual stream. Since the membrane is not a perfect separator, some 
methane will pass into the low pressure permeate stream. The permeate gas from 
these Membranes is sent to a CO2 Reinjection Compressor. In case this 
downstream compressor is not operational, the CO2-rich permeate will be flared. 
 
Each individual membrane train is capable of turn-up or turndown. Multiple trains, 
operated in parallel, are typically required to produce the design export gas. The 
trains allow for operational flexibility for a wide range of feed flow rates and 
compositions. As the flow rate and/or the CO2 concentration is/are increased, more 
membrane banks are needed on line to meet the CO2 removal requirements. 
Membrane maintenance activities can be performed by removing a train from 
service. At reduced export gas demand or the reduced feed gas rates, membrane 
trains can be taken out of service as needed to maintain product quality and 
hydrocarbon recovery. Figure 1 demonstrates the boundaries within which the unit 
can be operated with the feed CO2 concentration being between 8 – 35%. This 
figure has been generated using the design specification of the unit in keeping the 
desired product specifications. The number of trains (that is A, B & C) to be put 
online is directly related to the membrane area required (calculated as number of 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-3 
A Honeywell Company 
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tubes online) for the entire process. It is important to realize that each train 
contains varying number of tubes to allow flexibility in their individual or 
combined CO2 removal capacity. Trains A, B and C have 24, 36 and 48 tubes 
respectively. For example, if the feed gas flow rate is 3.40 MMSCMD with a CO2 
concentration of 8%, then from the figure below, it can be determined that 60 tubes 
will be required for this particular scenario. Hence, a combination of Trains A and 
B will be required ideally to be put online for the unit to meet the desired product 
specifications. Similarly, if feed gas flow rate is 3.40 MMSCMD with a CO2 
concentration of 32%, then the number of tubes required would be 84 (as the solid 
line passes closer to 84 tubes rather than 72 tubes) which would imply that Trains 
B and C will have to be online. Note that in the graph below, the feed temperature 
of the gas to the membranes may be different in each case. 
 
Figure 1 
 
 
Each of the cases have been defined in the Table 1 in the following page. The data 
points without any case numbers are intermediate points generated by the 
simulation model in UOP. In each of these cases, the feed tempereature was 35°C 
and the permeate pressure was 401 kPa (a). 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-4 
A Honeywell Company 
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Table 1 
 
 
 
 
 
 
 
 
 
 
 
X = online 
 
The overall system performance can be adjusted by manipulating a combination of: 
 
• Membrane permeate pressure via Pressure Control Valve on the Membrane 
Skid or Compressor Suction Pressure Control (Out of Scope and 
downstream) 
• Number of Membrane Trains used. 
• Membrane feed gas temperature. 
 
 
D. DISTRIBUTED CONTROL SYSTEM AND INSTRUMENTATION 
The Separex Membrane System is controlled by a Distributed Control System 
(DCS). Key controls for membrane performance are via feed temperature controls 
on the preheater, and permeate pressure controls, limited by the minimum suction 
capacity of the membrane permeate re-compressor. The Emergency Shutdown 
and Process Control Systems are capable of operating as stand-alone systems 
that initiate the shutdown and blow down of the Membrane System during upset 
conditions. Process upsets and shutdowns are annunciated through the DCS. The 
Pretreatment and Membranes Train’s Instrumentation consist of transmitters for 
temperature, level, flow, pressure, and analytical control. The alarm set points for 
the membrane system will occur when one of the alarm switch set points exceeds 
its preset limit. A Process Shutdown of the membrane system will occur when any 
one of the shutdown switch set points exceed preset limits, or can be operator 
initiated remotely from the control room. Both the feed temperature and the feed 
flow alarm settings will be discussed below to demonstrate logic used to program 
RATED CASE FLOWS, CONDITIONS, AND TRAIN USAGE 
Parameter Case 1 2 3 4 5 6 
FEED Mol% CO2 8.3 11.9 18.0 20.7 35.7 30.7 
FEED FLOW MMSCMD 2.81 4.43 4.47 4.67 1.44 4.73 
A → 24 Tubes ; B → 36 Tubes ; C → 48 Tubes 
A X X X 
B X X X X X TRAINS ONLINEC X X X X X 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-5 
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the alarm system in the DCS. Some of the membrane train shutdown triggers will 
also be discussed below. 
 
1. Feed Gas High Temperature Alarm 
 
The high temperature alarm (TAH) set point is constant at 60°C. The 
alarm is labeled TAH 2710-02. Refer to the S623M-1 specification. 
 
2. Feed Gas Low Temperature Alarm 
 
The feed gas temperature alarm low (TAL) TAL-2710-02 has been set 
using a combination of two linear relationships with respect to the CO2 
concentration in the feed gas. Two relationships exist for feeds above and 
below the 20% feed CO2 concentration level. Refer to the S623M-1 
specification for these correlations. 
Figure 2 
 
3. Feed Flow Alarm Low (FAL) 
 
The feed flow alarm low setting is based on linear behaviors generated 
from the simulation model as a function of the feed CO2 concentration. 
Figure D.3 shows two separate linear trends at 8% (lower limit of design 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-6 
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specification) and 35% (upper limit of design specification) CO2 
concentration in feed. Each of these trends represents the varying 
combination of trains that could possibly be online at any given point of 
time during regular unit operations. The stars between the lines represent 
the low flow alarm setpoints as a function of sections online and CO2 
concentration in feed. The correlations are provided in the S623M-1 
specification. 
 
Figure 3 
 
 
These trends will be programmed into the Local Control Panel (LCP) as a 
part of the logic system. However, it is important for the operator to 
understand that the alarm set points change with the changing CO2 
concentration in the feed gas. 
 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
uop No. 956457, Rev. 1 IV-7 
A Honeywell Company 
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4. Feed Flow Alarm High (FAH) 
 
The CO2 Membrane feed gas high flow alarm limit is a function of the 
number of membrane tubes online. Because the trains have different 
number of tubes, the FAH depends on which trains are online. 
 
The following table shows the FAH alarm set points as a function of the 
number of tubes online. 
 
Trains Online FAH (MMSCMD) 
A 1.6 
B 2.4 
C 3.2 
A + B 4.0 
A + C 4.9 
B + C 5.7 
A + B + C 5.8 
 
The LCP will make the required alarm adjustments for the trains starting-
up or online which will be manually entered by the operator using a 
selector switch (HS-2705-02x). The selection will be checked with the 
position signals from the membrane train feed valves (ZI-2705A-01, ZI-
2705B-01, and ZI-2705C-01) 
 
5. Feed Treatment Section Emergency Shutdown 
 
 An ESD of the upstream feed treatment system will result in an ESD of 
 the membrane system as well. This prevents untreated feed gas from 
 reaching the membranes. This ESD is outside the scope of this manual. 
 
6. Low Membrane Feed Flow Rate Shutdown 
 
 The setting for FSLL-2705-01A is a function of the CO2 content in the feed 
 (AI-2705-01A) and the number of membrane trains online and initiates 
Separex™ Membrane System – Operating Manual Process Flow and Control 
 
 
 
 
 
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A Honeywell Company 
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 a shutdown of the membrane system. See formulas in the UOP S660M 
 Specification. 
 
7. Low Membrane Feed Inlet Temperature Shutdown 
 
 The membrane feed temperature measurement range for TI-2705-01A is 
 from 0 to 70 °C. The setting for TSLL-2705-01A is a function of the CO2 
 content in the feed (AI-2705-01A) and initiates an ESD of the membrane 
 systems. See formulas in the UOP S660M Specification. 
8. High Membrane Feed Inlet Temperature Shutdown 
 
 The setting for TSHH-2705-01A is 65°C and initiates an ESD of the 
 membrane system. Refer to UOP’s S660M Specification.