A hydrate in natural gas system is a physical combination of water and other small molecules to produce a solid which has an “ice-like” appearance but has a different structure than ice. It resembles dirty ice but has voids into which gas molecules will fit. Most common compounds found in gas hydrate are water, methane, and propane, or water, methane, and ethane.
Natural gas dehydration unit is one of important facilities in natural gas processing. Purpose of water vapor removal in natural gas are to prevent gas hydrates formation and to reduce corrossion in pipeline. There are several methods of gas dehydration, which are absorption with liquid desiccant, adsorption with solid desiccant, and cooling below initial dew point. Selection of the method is depend on water content in feed gas and required dew-point in dry gas.
One of the most popular methods of natural gas dehydration is absorption process employing diethylene glycol or triethylene glycol as dehydration medium or desiccant. It is a mature process having been used for over fifty years. Triethylene glycol has emerged as the most popular chemicals in recent years due to several favorable desiccant characteristics. Those characteristics are high affinity for water vapor, non-corrosive, ease of generation, and low chemical losses. Read More
In this post, I want to share you about natural gas dehydration using desiccant using adsorption principle and how to size it.
In common commercial use, desiccant can be classified into three categories, which are gels (alumina or silica gels), alumina, and molecular sieves.
When used for natural gas dehydration, silica gel will give outlet dewpoints of approximately -70o to -80oF. As for alumina, outlet dewpoint is appoximately -100oF. Molecular sieves produced the lowest water dewpoints, as low as -150oF. For gas going into cryogenic processing, the only adsorbent that can obtain the required dehydration is a molecular sieve.
Table below shows characteristics of several type of desiccants . Read More
In almost every gas processing plant, there will be natural gas dehydration unit. Sometimes, it is called dehydration unit (DHU) or dehydrator. When, I was working in Tripatra and involving in Senoro Gas Development Project, the facilities consisted of dehydration unit as one of them.
Natural gas dehydration unit is an important facilities in onshore and offshore gas processing plant. Its function are:
- To mitigate risk of water condensation, which leads to flow capacity issues (pipeline clogging and blocking)
- To prevent hydrate formation or to minimize corrosion
- To ensure smooth operation in downstream facilities. For example, gas pipeline is usually required 4-7 lb/MMSCF water content (87.2-152.6 ppm). For cryogenic unit (to produce LNG), water content in gas shall be less than 1 ppm. For CNG plant, before entering compressor unit, water content shall be reduced to maximum 3 lb/MMSCF to meet product specification.
Line sizing is a part of every engineering activity. Although it is not major part, line sizing is required when we prepare piping and instrumentation diagram. In this post, I want to share you how to do natural gas line sizing (actually the method is applicable for all kind of gases).
I usually use process simulator when it came to natural gas line sizing. Long time ago, I tried to size manually. But the result is not the same as from simulator. But, I tried again and yesterday I got a satisfied results.
Natural gas line sizing calculated in this post will be from manual calculation and from process simulator. API RP 14E is used as reference.
Based on API RP 14E, single-phase gas lines should be sized so that the resulting end pressure is high enough to satisfy the requirements of the next piece of equipment. So, the point is the end pressure. The velocity is also a noise problem if it exceeds 60 ft/s. However the velocity of 60 ft/s should not be interpreted as an absolute criteria. Higher velocities are acceptable when pipe routing, valve choice and placement are done to minimize or isolate noise.
API RP 14E mentioned several approach to calculate pressure drop, but in this post I will use general pressure drop equation.