chemical engineering

Tuesday, February 10, 2009

FREE Chemical Engineering Digital Issue for Feb 2009

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FREE Chemical Engineering Digital Issue for Feb 2009 has just been released !

Chemical Engineering magazine has just released Feb 2009 issue. If you are the subscriber of Chemical Engineering, you should have received similar notification.

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Interesting articles for this month :

A Primer On Coal-to-Liquids
Converting coal to liquid fuels is one option China and the U.S. are pursing

Selecting a Conveyor
Characteristics of flexible screw, aero-mechanical, vacuum and pneumatic conveyors are discussed here

Facts At Your Fingertips - Pipe Sizing
This one-page guide provides the formulas needed to approximate friction factors, discharge, pressure drop, and pipe diameter.

Plate Heat Exchangers : Avoiding Common Misconceptions
A solid understanding of the critical areas presented here will insure good performance

Compact Heat Exchangers : Improving Heat Recovery
These units offer distinct advantages over shell-and tube heat exchangers, as quantified by the example presented here

Eye-and-Face Personal Protective Equipment
Protecting the eyes and face in the workplace is imperative to preventing the estimated 10–20% of work-related eye injuries that result in temporary or permanent vision loss

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TIPS
If you are subscriber, you may access previous digital releases (Jan 2008, July 2008 - Jan 2009). Learn more in "How to Access Previous Chemical Engineering Digital Issue".

If you yet to be subscriber of Chemical Engineering, requested your FREE subscription via this link (click HERE). Prior to fill-up the form, read "Tips on Succession in FREE Subscription".

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Monday, February 9, 2009

Why Control Valve Operate Around 60-70% Opening ?

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Control valve capacity (Cv) for particular application is determined by the use of recognized valve sizing equations. This valve equation can be found in several handbooks i.e. Fisher, Masoneilan, etc as discussed in "Useful Documents Related to Control Valve".This article presented a simple idea why a control valve is normally operate at around 60-70% valve opening and it associated impact such as increase signal dead band effect and affect optimum controller settings-wider proportional band and faster reset. It recommended a new way to overcome the impacts by introducing VARIMAX. Read more...

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Sunday, February 8, 2009

Dynamic Modeling - Process Control and Design Applications

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Dynamic modeling is a common tool for evaluation of plant operating conditions and control strategies. It can be used during design, prior to start-up, campaign changes, normal plant operation and prior to shutdown to understand the performance. It prepare design and operator to understand the potential problem and get ready to handle and manage the situation. High fidelity dynamic simulators has been developed for the process industries provide a solid basis for accurate modeling of the dynamic transitions. Nevertheless, development of custom components (for specific features of the process and measurement system) is often needed to provide a realistic model of the plant.

This paper presents a development approach and application of a dynamic model for the plant off-gas system, characterized by the complex structure and strong interaction of the production units with fast dynamics and sharp unexpected changes of the process pressure.

The model was developed in several phases using a HYSYS dynamic simulator. Initially, the model for the existing plant configuration, in nominal operational mode, was created; i.e. the plant emergency pressure relief devices that required custom modeling were not included. This model was validated using the plant data historian and was used for evaluation of the process dynamics and improvement of the existing control system. Then the model for the thermal oxidizer unit (a redesign option to decrease plant emissions) was added. This model was used in the design of the advanced control strategies for the modified process and thermal oxidizer itself. These strategies were tested for various scenarios of plant events and the expected plant unit interactions were evaluated. Finally, as the confidence of Plant Manufacturing grew, the model was extended with the custom pieces of the pressure relief devices (existing and new projected ones) and was used for development and justification of the plant vent system redesign.

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There is a Dynamic Simulation manual available in ASPEN HYSYS. You may obtain your copy. Read more in "Useful Documentation for HYSYS ...".

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Saturday, February 7, 2009

Control Valve at Inlet or Outlet of HEX ?

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Product fluid in reactor involve exothermic process is commonly hot. It is then sent to distillation and separation system for catalyst and raw material recovery. Separated product is then cooled by plant wide Cooling Water (CW) before it is sent to storage tank. The product temperature will have to be maintained.

How this temperature is controlled ?

Temperature Control Methods
There are several ways to maintain the product temperature :

(i) Provide a product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) with fixed CW flowrate
(ii) Provide a CW bypass across the Cooler, control valves on CW bypass line and CW inlet to Cooler with full product flow across cooler
(iii) Provide a CW bypass across the Cooler, control valves on CW bypass line and CW outlet to Cooler with full product flow across cooler
(iv) Provide a Control valve at the Cooler inlet with full product flow across cooler
(v) Provide a Control valve at the Cooler outlet with full product flow across cooler

Generally the product flow is fixed by operator based on production plan and the product flow shall not be controlled. Thus, it is always not recommended to provide a control valve at the inlet and outlet of product line for product temperature control.

Disturbance of CW Network Balance
Cooling water is in a network supplying to many heat exchanger through out the plant for cooling purpose. It is normally supplied by a set of centrifugal pump. As centrifugal pump head will be affected flow across, any changes in the CW demand will affect the CW balance in network. This will further affect the pressure in the network and hence the CW flow into other heat exchangers. Thus, it is always recommended not to throttle the CW flow as much as possible to avoid CW balance.

Scaling
Throttling CW flow into heat exchanger would potential lead to low CW flow into heat exchanger, high film temperature at on CW side and promote scaling. The option (ii) and (iii) are always recommended IF throttling on CW side is chosen.

Potential affecting Production
Controlling product fluid temperature with product bypass across the Cooler, control valves on Product bypass line and Outlet line (Split range control) and fixed CW flowrate (option i) is one of the common way in temperature control for product cooling. As it minimize the impact to CW network. Nevertheless, there is still concern about manipulating product fluid or mal-operation (controller failure) of control valves would potentially lead to production lost, the option (ii) and (iii) are always the recommended option.

CW Pressurise or Non-Pressurise
Option (ii) and (iv) compare to option (iii) and (v), the difference is the location of main CW line control valve (either at the inlet or the outlet). Providing a control valve at the outlet will have the following advantages :

a) Maintain high pressure in the heat exchanger and higher pressure will results higher heat transfer

b) CW at high pressure will minimise potential of boiling

c) CW at high pressure will minimise potential release of dissolved gases in CW , trap in heat exchanger and reduce heat transfer

d) In event of Control valve failure (failed to full close position), not further Cooling. CW in the heat exchanger will be heated and potentially lead to heat exchanger overpresure due to thermal expansion and/or boiling. The CW will be relieved via Pressure Relief Device provided on the heat exchanger. Providing control valve at the outlet would allow continue CW feeding into the heat exchanger, this minimise the potential of sudden temperature increase and cause heat exchanger due to thermal shock. The downside is release CW into disposal network.

Considering above advantages, it is always recommended to provide control valve on CW line at the outlet IF throttling CW side is chooses.

CONTROLLING SHELL AND TUBE EXCHANGERS
"Controlling Shell & Tube Heat Exchanger", an excellent article by Walter Driedger discussed about all type of control schemes around heat exchanger. Check out.

Thursday, February 5, 2009

Control Around Heat Exchanger

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CONTROLLING SHELL AND TUBE EXCHANGERS
Shell and tube heat exchangers are among the more confusing pieces of equipment for the process control engineer. The principle of operation is simple enough: Two fluids of different temperatures are brought into close contact but are prevented from mixing by a physical barrier. The temperature of the two fluids will tend to equalize. By arranging counter-current flow it is possible for the temperature at the outlet of each fluid to approach the temperature at the inlet of the other. The heat contents are simply exchanged from one fluid to the other and vice versa. No energy is added or removed.

Since the heat demands of the process are not constant, and the heat content of the two fluids is not constant either, the heat exchanger must be designed for the worst case and must be controlled to make it operate at the particular rate required by the process at every moment in time. The heat exchanger itself is not constant. Its characteristic changes with time. The most common change is a reduction in the heat transfer rate due to fouling of the surfaces. Exchangers are initially oversized to allow for the fouling which gradually builds up during use until the exchanger is no longer capable of performing its duty. Once it has been cleaned it is again oversized...

"Controlling Shell & Tube Heat Exchanger", an excellent article by Walter Driedger discussed about all type of control schemes around heat exchanger. You may download here.

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Source : www.driedger.ca

Tuesday, February 3, 2009

Problems and Measures for Condensate Recycle Control Valve

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Steam is commonly used in oil & gas, refinery, petrochemical and power plant for heating and power generation. Steam is condensed in equipment for heating and in turbine for power generation. Condensate is then collected in common collector before it is sent to condensate drum. Condensate from drum is then pumped to Boiler for steam generation via a Boiler Feed Water (BFW) pump. BFW is normally a centrifugal type and a minimum flow recirculation line is provided on BFW discharge for pump protection.

Minimum flow control can be

a flow meter on pump discharge with control valve on recycle line
a flow-Delta P and flow meter on pump discharge with control valve on recycle
an automatic Recirculation Valves (ARC) valve

as discussed in "Centrifugal Pump Minimum Flow Control Strategies". Restriction orifice option is not normally used due to energy saving, avoidance of continuou noise and vibration.

Problems
There are several problems assocaited with these valves in condensate recycle line :

i) Erosion - flashing and cavitation results trim and body erosion
ii) Severe noise and vibration - flashing and cavitation
iii) Leakage - energy loss

Recommendation
Several recommendations to miniminse above mentioned problems :
i) Harden trim to resist erosion cause by flashing and cavitation

ii) Correct material i.e. alloy selection to avoid erosion-corrosion

iii) Anti-cavitation trim to minimise / avoidance of cavitation.

iv) Multi-stage anti-cavitation trim for small valve

v) Multi-hole anti-cavitation trim for large valve

vi) Multi cage anti-cavitation trim for high pressure recovery (FL) valve

vii) High lift (more than 20% lift) valve to increase trim life

viii) Large body valve to minimise velocity (high velocity lead to high erosion) in the valve inlet and outlet chambers. [Tips : Body erosion proportional to 3-5 power of velocity]

ix) Elevate condensate drum to increase back pressure to the valve (if possible)

x) Provide restriction orifice downstream of control valve to increase back pressure. One shall take note at low flow, the pressure drop acrosss RO is negligible. Majority pressure drop (energy being "killed") still occurred at valve

xi) Tight shut off (class V) valve to avoid leakage and hence energy loss.

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Sunday, February 1, 2009

Relate NORMAL to STANDARD Volumetric Flow

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In gas processing industry, gas flow has been referred to STANDARD (i.e. Sm3/hr, MMSCFD, etc) and NORMAL condition (i.e. Nm3/h). Vendor catalog in many event can be in Sm3/h or Nm3/h.

What is the useful factor to convert Nm3/h to Sm3/h or vice versa ?

Definition
As discussed in "Avoid Confusion In "Standard" Flow Definition", definition is one of the most important factors to avoid discrepancies. First far most important task is to provide a correct definition of STANDARD and NORMAL condition. In "general",

NORMAL condition : 1.01325 bara @ 0 degC
STANDARD condition : 1.01325 bara @ 15 degC

Conversion
From this post,

Q₂ = (z₂/z₁) x (T₂/T₁) x (P₁/P₂) x Q_{1 .....[1]}

where
Q₁ & Q₂ are Volumetric Flow in m3/h for condition 1 & 2
P₁ & P₂ are Pressure in bar abs for condition 1 & 2
T₁ & T₂ are Temperature in K for condition 1 & 2
z₁ & z₂ are compressibility factor for condition 1 & 2

Condition 1 : 1.01325 bara @ 0 degC (NORMAL Contractor manual)
Condition 2 : 1.01325 bara @ 15 degC (STANDARD)
Assume z₁ = z₂ = 1
Q₁ = 1 Nm3/h

From [1],
==> Q₂ = (z₂/z₁) x (T₂/T₁) x (P₁/P₂) x Q₁
==> Q₂ = (288.15 / 273.15) x 1
==> Q₂ = 1.055 Sm3/h

Therefore,

1 Nm3 = 1.055 Sm3

when
NORMAL condition : 1.01325 bara @ 0 degC
STANDARD condition : 1.01325 bara @ 15 degC

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