Design of Drumless Condenser Overhead Systems

1. Purpose

This procedure describes the guidelines to be followed in designing a fractionator overhead air cooled condensing system without the use of an overhead receiver.

2. General

A drumless condenser is a fractionator overhead system without a receiver. The receiver is replaced by a large piece of pipe (condenser pot) to separate non-condensables. Additional instrumentation is also required for this modification to operate the column more efficiently.

Conventional Drumless Condenser

The use of the drumless condenser design can reduce hydrocarbon inventory in the overhead system almost 50%, making it inherently safer. It also reduces capital cost, plot space, energy, and maintenance of overhead systems.

This procedure is limited to atmospheric overhead systems that meet the following criteria:

• Total condensing using an air cooled condenser
• Essentially water free material in the overhead
• Fractionation systems without high purity specifications
• Fractionation system where receiver is not required to provide surge time for downstream reactor system

Inflection Point Engineering has specified drumless condenser systems for several Aromatics Complexes. Included were Parex Extract, Raffinate and Finishing Columns, Light Reformate Splitter and Reformate Splitter columns.

3. Design Guidelines

3.1 Air Cooled Condenser Guidelines

a. Specify variable speed motors for 50% of the condenser fans. This will allow for good temperature control of the overhead material and result in energy savings. Control of the condenser outlet temperature is crucial to maintaining a liquid level in the condenser pot and to keep the composition of the overhead material constant.

b. Design the condenser surface at 110% of the required condensing surface to account for the surface area covered by the flooded liquid at the outlet end.

The 110% is expected to be in addition to any other design contingencies required for the condenser. See Total Condensing Systems section of Procedure .

c. Specify a 1 inch vent located at the end of the condensation section. One vent is required for each tube bundle.

d. Elevate the condenser/condenser pot system sufficiently to satisfy pump NPSH requirements based on an empty condenser pot. Also, ensure that the bottom of the condenser shell is a minimum of 20 feet above grade.

e. Add a spillback line to protect the overhead pump during startup and in cases of severe column upset. Connect the spillback to the condenser outlet rather than the inlet to avoid the possibility of hammering. The spillback will be normally, no flow.

f. Since the nitrogen purge system is typically used only at startup, a pressure let down system is not necessary.

3.2 Condenser Pot Guidelines

The condenser pot is a vertical pipe that replaces the conventional horizontal overhead receiver and separates non-condensables in the overhead from the liquid. The non-condensables, which are not normally present can be manually vented to the flare. The pot holds approximately 90% less hydrocarbons than a typical receiver.

To allow identification of nozzle locations, generate a 301 specification for the pot.

a. Placement

Place the condenser pot downstream of the overhead condenser and upstream of overhead pumps. The condenser run down line must be indicated as free draining from the outlet of the condenser to the condenser pot on the P&ID. See figure below:

Placement of Air Condenser and Condenser Pot

b. Sizing

The length of the condenser pot is determined by mechanical constraints. The minimum distance from the top of the condenser to the inlet of the condenser pot is determined by the size of the condenser and the placement of the condenser outlet header. This distance is estimated at 8’0" (2400 mm). The total length of the condenser pot should be approximately 13’ (4000 mm).

Size the diameter of the pot based on a liquid velocity of 1 foot per second maximum.

c. Construction

(1) Pump suction lines up to and including 3" diameter shall not have a liquid velocity in excess of 1.5 feet per second. Four to eight inch lines shall be limited to 2.25 feet per second. For ten inch diameter and larger sizes, the liquid velocity shall not exceed 3.5 feet per second.

(2) Both ends of the pot shall be flanged.

(3) An anti-vortex baffle (grating type) shall be installed in the condenser pot. See Standard Drawing 3-322.

(4) The top of the condenser pot will have a 2" non condensable vent line.

(5) The vent line from the condenser pot is not normally open to the flare.

(6) The condenser pot shall be equipped with pipe column and gauge glasses covering the entire length.

(7) The line from the condenser pot to the overhead pumps shall have no pockets and have a minimum run of horizontal pipe.

Condenser Pot (Typical)

3.3 Instrumentation Guidelines

The drumless condenser design requires proper instrumentation to maintain control. The following summarizes the major process control systems. (See Section 3.3, f).

a. Pressure Compensated Temperature Composition Control

The composition of the overhead material on straight temperature control normally controls the net overhead product rate. Use pressure compensated temperature control on drumless condenser design to provide more accurate composition control, and stable column operation.

b. Reflux/Net Overhead Pump Spillback Control System

The system is designed to keep a minimum flow going though the overhead pump. Sum the net overhead, the reflux and the spillback flows and use as the process variable for the spillback flow controller. Normally, design the system for no flow spillback.

c. Heat Input Control

Due to the reduced inventory in the overhead system, there is a need for more stable heat input control of the column. For hot oil reboiler systems, the heat input control system directly controls the heat to the column, eliminating variations due to the temperature of the heating medium. In addition, a lead-lag controller is provided to compensate for changes in feed flowrate to the column. This improves the stability of column operation.

d. Column Pressure Control

Pressure control is achieved by adjusting the reflux flowrate. Basically, whatever material that does not leave the column on composition control is returned to the column as reflux. When the reflux is reduced, the condenser tubes will start to flood thus increasing the column pressure.

Loss of the reflux pump will cause the condenser to rapidly flood. A signal from the pressure controller to reduce hot oil flow at the reboiler, reduces the potential for extensive vapor relief.

e. Condenser Pot Level Controller

A level controller with a low liquid level setpoint, acts through a high signal selector, to provide an override of the pressure control signal to prevent large amounts of vapor going to the overhead pumps.

f. Design Sketch

NC: normally closed

3.4 P & ID Modules

Column: F430001E

Condenser Pot: F430005E