You cannot redirect from Global.asax. If you want to redirect on Session end you need to do in the following way
protected void Page_Load(object sender, EventArgs e)
{
HtmlMeta meta = new HtmlMeta();
meta.HttpEquiv = "Refresh";
meta.Content = Convert.ToString(Session.Timeout * 60) + ";url=LogOut.aspx";
this.Page.Header.Controls.Add(meta);
}